study notes on ALDEHYDES & KETONES., Study notes of Organic Chemistry

Download study notes on ALDEHYDES & KETONES. and more Study notes Organic Chemistry in PDF only on Docsity! Aldehydes and Ketones 1255 Introduction Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond often called as carbonyl group. Both aldehyde and ketones possess the same general formula OHC nn 2 . Structure : Carbonyl carbon atom is joined to three atoms by sigma bonds. Since these bonds utilise 2sp -orbitals, they lie in the same plane and are 120° apart. The carbon-oxygen double bond is different than carbon-carbon double bond. Since, oxygen is more electronegative, the electrons of the bond are attracted towards oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial positive charge making the bond polar. The high values of dipole moment,    OC (2.3 – 2.8D) cannot be explained only on the basis of inductive effect and thus, it is proposed that carbonyl group is a resonance hybrid of the following two structures.    OCOC Preparation of carbonyl compounds (1) From alcohols (i) By oxidation. Ketone || agents oxidising Mild alcohol Secondary | '' O RCR OH RCHR   Aldehyde || agents oxidising Mild alcohol Primary 2 O HCROHCHR   Mild oxidising agents are (a) 2X (Halogen) (b) Fenton reagent ( 224 OHFeSO  ) (c)  HOCrK /722 (d) Jones reagent (e) Sarret reagent (f) 2MnO (g) Aluminium tertiary butoxide [ 333 ))(( CHCOAl  ]  When the secondary alcohols can be oxidised to ketones by aluminium tert-butoxide, AlCOCH 333 ])[( the reaction is known as oppenauer oxidation. Unsaturated secondary alcohols can also be oxidised to unsaturated ketones (without affecting double bond) by this reagent.  The yield of aldehydes is usually low by this methods. The allylic alcohols can be converted to aldehydes by treating with oxidising agent pyridinium chloro-chromate )( 355  ClCrONHHC . It is abbreviated as PCC and is called Collin's reagent. This reagent is used in non-aqueous solvents like 22ClCH (dichloro methane). It is prepared by mixing pyridine, 3CrO and HCl in dichloromethane. This is a very good reagent because it checks the further oxidation of aldehydes to carboxylic C O Carbonyl group Aldehydes and Ketones Chapter 27 C 120° 120° 120° -bond -bond O 1256 Aldehydes and Ketones acids and is suitable method for preparing ,- unsaturated aldehydes. (ii) Dehydrogenation of 1° and 2° alcohols by Cu/300° or Ag/300°C. 2 || 300/ 2 H O HCROHCHR CCu    2 || 300/ | '' H O RCR OH RCHR CCu    (2) From carboxylic acids (i) Distillation of Ca, Ba, Sr or Th salts of monobasic acids 3 || 22 2'2)'()( CaCO O RCRCaCOORCaRCOO   Thus in the product, one alkyl group comes from one carboxylic acid and other alkyl group from other carboxylic acid. Calcium salts of dibasic acid (1, 4 and higher) on distillation give cyclic ketones.      onDistillati || 2 || 2 | Ca O O CCH O O CHC                 onDistillati 52 || )( CaCOOCH O CO  (ii) Decarboxylation or Dehydration of acids by MnO/300°C. (a) This reaction takes place between two molecules of carboxylic acids. Both may be the same or different. (b) If one of the carboxylic acids is HCOOH then this acid undergoes decarboxylation because this acid is the only monobasic acid which undergoes decarboxylation even in the absence of catalyst. Case I : When both molecules are HCOOH deformaldehy || 2 300acidformic || H O CHHOHCOHCOOHOH O CH C MnO    Case II : When only one molecule is formic acid. HOHCOH O CRHCOOHOH O CR CMnO    Aldehyde 2 || 300/ acidformic acid Carboxy lic || Case III : When none of the molecule is formic acid. HOHCOR O CRHCOOROH O CR CMnO    2 Ketone || 300/ acid Carboxy lic || (3) From gem dihalides : Gem dihalides on hydrolysis give carbonyl compounds (i) Aldehyde / eGemdihalid 2 CHORCHXR HOHOH    (ii) '' || / | | R O CRR X X CR HOHOH     This method is not used much since aldehydes are affected by alkali and dihalides are usually prepared from the carbonyl compounds. (4) From alkenes (i) Ozonolysis : Alkenes on reductive ozonolysis give carbonyl compounds RCHOCHORRCHCHR ZnOH O   /(ii) (i) Alkene 2 3 '' ' ' ||||(i) / (ii) Alkene 3 2 R O CRR O CR R R CC R R O ZnOH    This method is used only for aliphatic carbonyl compounds. (ii) Oxo process CHOCHCHRHCOCHCHR COCO atmC    22 )( 300,15022 82  Oxo process is used only for the preparation of aldehydes. (iii) Wacker process (a) CHOCHCHCH ClCu HOHPdCl   3 /air / Ethene 22 22 2 (b) 3 || air / etheneAlkyl 2 22 2 CH O CRCHCHR Cl/Cu HOHPdCl   (5) From alkynes (6) From Grignard reagents O+CaCO3 Cyclopropanone Cyclohexanone O R – C  C – H H2O/HgSO4 /H2SO4 O R – C – CH3 (i) SiO2 BH3 (ii) H2O2/ OH R – CH2 – CHO  O R' – C – Cl O R' – C – R (Only ketone) HCOOC2H5 O H – C – R (Aldehyde) O R – C – R' (Ketone) R' COOC2H5 O R – C – H (i) HCN R – MgX (Excess) Aldehydes and Ketones 1259 of aldehydes and ketones are relatively lower than the alcohols and carboxylic acids of comparable molecular masses. Among the carbonyl compounds, ketones have slightly higher boiling points than the isomeric aldehydes. This is due to the presence of two electrons releasing groups around the carbonyl carbon, which makes them more polar. K322b.pt. D2.52 deAcetaldehy 3 : ..     OC H CH K329b.pt D2.88 Acetone 3 3 : ..     OC CH CH (5) Density : Density of aldehydes and ketones is less than that of water. Chemical properties of carbonyl compounds Carbonyl compounds give chemical reactions due to carbonyl group and -hydrogens. Chemical reactions of carbonyl compounds can be classified into following categories. (1) Nucleophilic addition reactions (2) Addition followed by elimination reactions (3) Oxidation (4) Reduction (5) Reactions due to -hydrogen (6) Condensation reactions and (7) Miscellaneous reactions (1) Nucleophilic addition reactions (i) Carbonyl compounds give nucleophilic addition reaction with those reagents which on dissociation give electrophile as well as nucleophile. (ii) If nucleophile is weak then addition reaction is carried out in the presence of acid as catalyst. (iii) Product of addition reactions can be written as follows, Adduct | | Addition || '' OH Nu RCRNuHR O CR         In addition reactions nucleophile adds on carbonyl carbon and electrophile on carbonyl oxygen to give adduct. (iv) Relative reactivity of aldehydes and ketones : Aldehydes and ketones readily undergo nucleophilic addition reactions. However, ketones are less reactive than aldehydes. This is due to electronic and stearic effects as explained below: (a) Inductive effect : The relative reactivities of aldehydes and ketones in nucleophilic addition reactions may be attributed to the amount of positive charge on the carbon. A greater positive charge means a higher reactivity. If the positive charge is dispersed throughout the molecule, the carbonyl compound becomes more stable and its reactivity decreases. Now, alkyl group is an electron releasing group (+I inductive effect). Therefore, electron releasing power of two alkyl groups in ketones is more than that of one alkyl group in aldehyde. As a result, the electron deficiency of carbon atom in the carbonyl group is satisfied more in ketones than in aldehydes. Therefore, the reduced positive charge on carbon in case of ketones discourages the attack of nucleophiles. Hence ketones are less reactive than aldehydes. Formaldehyde with no alkyl groups is the most reactive of the aldehydes and ketones. Thus, the order of reactivity is: deFormaldehy OC H H  > Aldehy de OC H R  > Ketone OC R R  (b) Stearic effect : The size of the alkyl group is more than that of hydrogen. In aldehydes, there is one alkyl group but in ketones, there are two alkyl groups attached to the carbonyl group. The alkyl groups are larger than a hydrogen atom and these cause hindrance to the attacking group. This is called stearic hindrance. As the number and size of the alkyl groups increase, the hindrance to the attack of nucleophile also increases and the reactivity of a carbonyl decreases. The lack of hindrance in nucleophilic attack is another reason for the greater reactivity of formaldehyde. Thus, the reactivity follows the order: deFormaldehy OC H H  > deAcetaldehy 3 OC H CH  > Acetone 3 3 OC CH CH  > ketoneisopropy l -Di 23 23 )( )( OC CHCH CHCH  > ketone buty l tert.-Di 33 33 )( )( OC CCH CCH  In general, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues. For example, benzaldehyde is less reactive than aliphatic aldehydes. This can be easily understood from the resonating structures of benzaldehyde as shown below: C O H . . : C O H . . : . .  – C O H . . : . .  – II I III 1260 Aldehydes and Ketones It is clear from the resonating structures that due to electron releasing resonance effect of the benzene ring, the magnitude of the positive charge on the carbonyl group decreases and consequently it becomes less susceptible to the nucleophilic attack. Thus, aromatic aldehydes and ketones are less reactive than the corresponding aliphatic aldehyde and ketones. The order of reactivity of aromatic aldehydes and ketones is, deBenzaldehy 56 CHOHC > neAcetopheno 356 COCHHC > neBenzopheno 5656 HCOCHC Some important examples of nucleophilic addition reactions Addition of HCN   HO HCNH O CR || nCy anohy dri | | CN OH H CR    HO HCNH O CHC  deBenzaldehy || 56 ncyanohydri deBenzaldehy | | 56 CN OH H CHC   Because HCN is a toxic gas, the best way to carry out this reaction is to generate hydrogen cyanide during the reaction by adding HCl to a mixture of the carbonyl compound and excess of NaCN.  Benzophenone does not react with HCN.  Except formaldehyde, all other aldehydes gives optically active cyanohydrin (racemic mixture).  This reaction is synthetically useful reaction for the preparation of -hydroxy acids, -amino alcohols and -hydroxy aldehydes. acid Hydroxy- |  OH COOHHCR  (If R is CH3 then product is lactic acid) OH CNCHR  | alcohol Amino- 22 |  OH NHCHHCR  aldehyde hydroxy- |  OH CHOHCR  Addition of sodium bisulphite All types of aldehydes give addition reaction with this reagent. O HCR OH NaSO HCRH O CR HCHO OHHNaHSO || oror nature in ecry stallin whiteAdduct; 3 | | || 3      Only aliphatic methyl ketones give addition reaction with sodium bisulphite. H O CR OH NaSO CHCR O CHCR HCHO OHHNaHSO     || oror product ecry stallin Colourless 3 3 | | || 3 3   This reagent can be used for differentiation between aromatic and aliphatic methyl ketones, e.g. O CHCHCCHCH 32 || 23  and O CHCCHCHCH 3 || 223  O CHCHC || 356  and O CHCCHCH 3 || 23   This reagent can be used for the separation of aldehydes and aliphatic methyl ketones from the mixture, e.g. CHOCHCH  23 and O CHCHCCHCH 32 || 23  These two compounds can be separated from their mixture by the use of NaHSO3. Higher aliphatic ketones and aromatic ketones do not react with NaHSO3. Addition of alcohols : Carbonyl compounds give addition reaction with alcohols. This reaction is catalysed by acid and base. Nature of product depends on the catalyst. Case I : Addition catalysed by base : In the presence of a base one equivalent of an alcohol reacts with only one equivalent of the carbonyl compound. The product obtained is called hemiacetal (in case of aldehyde) and hemiketal (in case of ketone). The reaction is reversible. There is always equilibrium between reactants and product.    HOCHH O CCH 3 || 3 Hemiacetal 3 | | 3 OH OCH HCCH  C O H . . : . .  – C O H IV V H2/Pt H2O/H/  (i) SnCl2/HCl (ii) HOH/ HO  Aldehydes and Ketones 1261 HOCHCH O CCH  33 || 3 Hemiketal 3 3 | | 3 OH OCH CHCCH  Hemiacetals and hemiketals are -alkoxy alcohols. Case II : Addition catalysed by acid : In the presence of an acid one equivalent of carbonyl compound reacts with two equivalents of alcohol. Product of the reaction is acetal (in case of aldehyde) or ketal (in case of ketone). OHCHH O CR 3 || 2 Acetal 2 3 3 | | OH OCH OCH HCR  OHCHR O CR 3 || 2 Ketal 2 3 3 | | OH OCH OCH RCR  (i) Formation of acetals and ketals can be shown as follows: 3 3 CHOH CHOH OC R R    OH OCH OCH C R R 2 3 3  (ii) Acetals and ketals are gem dialkoxy compounds. (iii) High yield of acetals or ketals are obtained if the water eliminated from the reaction is removed as it formed because the reaction is reversible. (iv) Acetals and ketals can be transformed back to corresponding aldehyde or ketone in the presence of excess of water. OHCH O RCROH OCH OCH RCR H 3 || (Excess) 2 Ketal 3 3 | | 2  This reaction is very useful reaction for the protection of carbonyl group which can be deprotected by hydrolysis. Glycol is used for this purpose. Suppose we want to carry out the given conversion by 4LiAlH .   4 522 || 3 LiAlH HCOOCCH O CCH OHCHCH O CCH 22 || 3  This can be achieved by protection of OC  group and then by deprotection Addition of Grignard reagents : Grignard reagents react with carbonyl compounds to give alcohols. Nature of alcohol depends on the nature of carbonyl compound. Addition of water : Carbonyl compounds react with water to give gem diols. This reaction is catalysed by acid. The reaction is reversible reaction. Ketone || ' HOHR O CR  Gemdiol | | 'R OH OH CR  Gem diols are highly unstable compounds hence equilibrium favours the backward direction. The extent to which an aldehyde or ketone is hydrated depends on the stability of gem diol. Stability of gem diols depend on the following factors: (i) Steric hindrance by +I group around -carbon decreases the stability of gem diols. +I group decreases stability of gem diol and hence decreases extent of hydration. (ii) Stability of gem diols mainly depends on the presence of –I group on -carbon. More is the –I power of the group more will be stability of gem diols. (iii) Intramolecular hydrogen bonding increases stability of gem diols. –I groups present on carbon having gem diol group increases strength of hydrogen bond. More is the strength of hydrogen bond more will be the stability of gem diol. Addition of terminal alkynes : This reaction is known as ethinylation. " ' ' | | || alky neof salt Sod. R aNO R CCCRR O CRNaCCR      HO  H  H  O (i) H – C – H (ii) HOH/H  R – CH2OH 1°-alcohol O (i) R' – C – H (ii) HOH/H  OH | R' – CH – R 2°- alcohol OH | R' – C – R' | R 3°-alcohol O (i) R' – C – R' (ii) HOH/H  RMgX Grignard reagent R–C–R O 1264 Aldehydes and Ketones (c) Oxidation by organic peracids : Organic peracids oxidise aldehydes into carboxylic acids and ketones into esters. This oxidation is known as Baeyer – Villiger oxidation. O ROCR O RCR COOOHHC |||| 56   In case of aldehyde there is insertion of atomic oxygen (obtained from peracid) between carbonyl carbon and hydrogen of carbonyl carbon. In case of ketone, insertion of oxygen takes place between carbonyl carbon and -carbon. Thus the product is ester. This is one of the most important reaction for the conversion of ketones into esters.  Vic dicarbonyl compound also undergo oxidation and product is anhydride. R O CO O CRR O C O CR COOOHHC   |||| |||| 56  Popoff's rule : Oxidation of unsymmetrical ketones largely take place in such a way that the smaller alkyl group remains attached to the CO group during the formation of two molecules of acids. This is known as Popoff's rule Example : 33 ][ 323 HOOCCHCOOHCHCHCHCOCH O  (d) Baeyer- villiger oxidation : O OHCHH O CO H O O HCH |||| | ||  OH O CCHH O CO H OH O CCH || 3 || | || 3   Reaction will be held if the oxidising agent is performic acid. (4) Reduction of carbonyl compounds (i) Reduction of group into –CH2 – group : Following three reagents reduce carbonyl group into  2CH groups: (a) // PHI (b) HClConcHgZn .// and (c)  OHNHNH /22  . (ii) Reduction of carbonyl compounds into hydroxy compounds : Carbonyl group converts into CHOH group by OHHCNaNaBHLiAlH 5244 /,, and aluminium isopropoxide. OHCHRCHOR NaBH 2 deisopropoxi Aluminium (iii) (ii) LiAlH (i) 4 4   OH RCHR O RCR NaBH LiAlH '' | deisopropoxi Aluminium (iii) (ii) (i) || 4 4   4NaBH is regioselective reducing agent because it reduced only. CHO in the presence of other reducible group. Example : alcoholCrotonyl 23 hydeCrotonalde 3 4 OHCHCHCHCHCHOCHCHCH NaBH   Hydride ion of 4NaBH attack on carbonyl carbon during reduction. Example : 32 | | 3 Butanone-2 3 || 3 2 4 CHCH OD D CCHCH O CCH OD NaBD    Butanone 2 3 || 3 CH O CCH (iii) Reductive amination : In this reduction CO group converts into 2NHCH  group amine Primary 2 / Imine 3 2 NHCH R R NHC R R NHOC R R NiH   (iv) Reduction of ketones by Mg or Mg/Hg : In this case ketones undergo reduction via coupling reaction and product is vic cis diol. (pinacol)diol Vic | | | | (ii) /g (i) || || || || cis HOH HgM OH R RC OH R CRR O R C O R CR   When this reaction is carried out in the presence of 4// TiClHgMg , the product is vic trans diol. (v) Reduction of benzaldehyde by Na/C2H5OH : Benzaldehyde undergoes reduction via coupling reaction and product is vic diol. O || – C – OH | CH3 – C – CH2 – CH3 | D OD | CH3 – C – CH2 – CH3 | H NaBD4 H2O NaBH4 D2O HI/P/ Zn/Hg/Conc. HCl    NH2 – NH2 /OH R – CH2 – R' R – CH2 – R' R – CH2 – R' O || R – C – R' (Clemmenso n reduction) (Wolff-kishner reduction) (i) Hg – Mg – TiCl4 (ii) HOH OH HO Vic trans diol O 2 Cyclohexanone Aldehydes and Ketones 1265 HOH OHHNa/C HC O H C O H CHC (ii) (i) 56 || | || | 56 52   diolvic OH HCHC OH CHHC 56 || 56  (Bouveault-blanc reaction)  Aldehydes are reduced to 1° alcohols whereas ketones to 2° alcohols. If carbon – carbon double bond is also present in the carbonyl compound, it is also reduced alongwith. However, the use of the reagent 9-BBN (9– borabicyclo (3, 3, 1) nonane) prevents this and thus only the carbonyl group is reduced Example :     222BBN9 NHCHHOCH CHOCHCH OHCHCHCH 2  If reducing agent is NaH, reaction is called Darzen's reaction, we can also use LiAlH4 in this reaction.  If reducing agent is aluminium iso propoxide 3 3 | 3 )( CH AlOHCCH  . Product will be alcohol. This reaction is called Meerwein – pondorff verley reduction (MPV reduction).  The percentage yield of alkanes can be increased by using diethylene glycol in Wolf Kishner reduction. Then reaction is called Huang – Millan conversion. (vi) Hydrazones when treated with base like alkoxide give hydrocarbon (Wolf – Kishner reduction). RCHR NHN RCR O RCR RONaNHNH     2 Hydrazone 2 |||| . '' 22 (vii) Schiff's base on reduction gives secondary amines. amine Secondary 2 / basesSchiff' ' Aldehyde 22 ' NHRCHRNRCHROCHR NiHNHR    (5) Reactions due to -hydrogen (i) Acidity of -hydrogens : (a) -hydrogen of carbonyl compounds are acidic in character due to the presence of the electron withdrawing CO group. (b) Thus carbonyl compounds having -hydrogen convert into carbanions in the presence of base. This carbanion is stabilised by delocalisation of negative charge. O RCCH || 3  stable) (more ion Enolate | 2 stable)(less Carbanion || 2 R O CCH O RCCH     (c) The acidity of -hydrogen is more than ethyne. pKa value of aldehydes and ketones are generally 19 – 20 where as pKa value of ethyne is 25. (d) Compounds having active methylene or methyne group are even more acidic than simple aldehydes and ketones. acetonephenyl - 3 || 256   CH O CCHHC  9.15pKa acetonebenzoyl - 3 || 2 || 56   CH O CCH O CHC  5.8pKa (ii) Halogenation : Carbonyl compounds having -hydrogens undergo halogenation reactions. This reaction is catalysed by acid as well as base. (a) Acid catalysed halogenation : This gives only monohalo derivative. acetone bromo 2 || 3 / Acetone 3 || 3 32     BrCH O CCH O CHCCH COOHCHBr (b) Base catalysed halogenation : In the presence of base all -hydrogens of the same carbon is replaced by halogens. Carbonyl compounds having three -hydrogens give haloform reaction. 33 || / 3 || 2 CHXRCOOCX O CRCH O CR OHOHX      (iii) Deuterium exchange reaction : Deuterium exchange reaction is catalysed by acid )( D as well as base )(  OD . In both the cases all the hydrogens on only one -carbon is replaced by D. RCD O CRRCH O CR ODOD   2 || / 2 || 2  ; RCD O CRRCH O CR DOD    2 || / 2 || 2 -Hydrogen is acidic due to strong –I group; – CO –. H O | || – C – C – | - Carbon Base  32 || 23 CHCH O CCHCH   3 | | | | 3 CH X CH O C X CHCH  3 | | || 23 CH X X C O CCHCH  X2/OH Excess – Cinnamaldehy de Cinnamyl alcohol 1266 Aldehydes and Ketones (iv) Racemisation : Ketones whose -carbon is chiral undergo Racemisation in the presence of acid as well as base. mixtureRacemic 3 52 | | || 56 or one-1-phenyl-1-methy l2 3 52 | | || 56 CH H HCC O CHC CH H HCC O CHC OH H      56 || 3 | | 52 HC O C CH H CHC  (v) Alkylation : Carbonyl compounds having - hydrogens undergo alkylation reaction with RX in the presence of base. This reaction is 2N S reaction. The best result is obtained with XCH 3 . Other halides undergo elimination in the presence of strong base.    ICHNaH CH CH C O CCH CH CH CH O CCH 3 3 3|| 3 e)(Small bas 3 3|| 3  product)(Main 3 3 3 | | || 3 CH CH CHC O CCH  ketoneisopropyl -Ethyl product)(Main 3 3|| 23 3 3 || 2 3 CH CH CH O CCHCH CH CH CH O CCH ICH    (vi) Wittig reaction : Aldehyde and ketones undergo the wittig reaction to form alkenes. oxide mPhosphoniu Triphenyl 32 alkene ketoneor Aldehyde 23 OPPhCHCOCCHPPh      2 | 1 3 || 21 3 CHRO CHRPPh O CHRCHRPPh  2 || 1 || 3 2 | 1 | 3 CHR CHR O PPh CHRO CHRPPh    (6) Condensation reaction of carbonyl compounds : Nucleophilic addition reaction of compounds having carbonyl group with those compounds which have at least one acidic hydrogen at -carbon is known as condensation reaction. In this addition reaction : Substrate is always an organic compound having a carbonyl group, e.g. , || O HCH  , || 56 O HCHC  , || O HCR  O RCR ||  etc. Addition always takes place on the carbonyl group. Reagents of the condensation reaction are also organic compounds having at least one hydrogen on - carbon and -carbon should have –I group, e.g. ,23 NOCH   , 3 | 3 CHO CH CHCH   CNCHCH   23  If substrate and reagent both are carbonyl compounds then one should have at least one -hydrogen and other may or may not have -hydrogen. Condensation reaction always takes place in the presence of acid or base as catalyst. Best result is obtained with base at lower temp. Z OH R HCCRZCH O RCR OH H    2 | | or 3 ||    Condensation is carried out at lower temperature )20( C because product of the reaction is alcohol which has strong –I group at -carbon. Such type of alcohols are highly reactive for dehydration. They undergo dehydration in the presence of acid as well as base even at 25°C. They also undergo elimination even on strong heating. ZCH OH R CR β  2 | | ZCHC R R HO    nDehydratio /  (i) Aldol condensation (a) This reaction takes place between two molecules of carbonyl compounds; one molecule should have at least two -hydrogen atoms. In this reaction best result is obtained when Both molecule are the same or One should have no -hydrogen atom and other should have at least two -hydrogens. (b) These reactions are practical when base is NaOH and reaction temperature is high )100(  . (c) The reaction is two step reaction. First step is aldol formation and second step is dehydration of aldol.                OH CHOCHCHCHCHOCHCHOCH OHNaOH 2 | 3 / 33  aldehyde dunsaturate, 3 nDehydratio    a CHOCHCHCH Due to hyper-conjugation in crotonaldehyde further condensation give conjugated alkene carbonyl compound. CH3 – CH = CH – CHO + CH3 – CH = CH – CHO NaOH OH | CH3 – CH = CH – CH – CH2 – CH = CH – CHO –H2O  CH3 – CH = CH – CH = CH – CH = CH – CHO CH3 – (CH = CH –)3 – CHO Condensed compound LDA (Bulky base) Carbanion Aldehydes and Ketones 1269 distilled with dilute sulphuric acid when pure acetaldehyde is collected.   42 ammonia deAcetaldehy 2 | 333 SOH OH NHCHCHNHCHOCH 424 deAcetaldehy 3 )( SONHCHOCH  (x) Manufacture : Acetaldehyde can be manufactured by one of the following methods: (a) By air oxidation of ethyl alcohol OHCHOCHOOHCHCH C Ag 23 300 223 222   (b) By dehydrogenation of alcohol CHOCHOHCHCH C Cu 3 300 23   (c) By hydration of acetylene CHOCHOHCHCH SOH CHgSO 3 %)40( 60%),1(, 2 42 4    (d) From ethylene (Wacker process) CHOCHOCHCH OH CuClPdCl   3 , 222 2 22 (2) Physical properties (i) Acetaldehyde is a colourless volatile liquid. It boils at 21°C. (ii) It has a characteristic pungent smell. (iii) It is soluble in water, chloroform, ethyl alcohol and ether. Its aqueous solution has a pleasant odour. In water, it is hydrated to a considerable extent to form ethylidene diol. 2323 )(OHCHCHOHCHOCH  (3) Uses : Acetaldehyde is used : (i) In the preparation of acetic acid, acetic anhydride, ethyl acetate, chloral, 1,3-butadiene (used in rubbers), dyes and drugs. (ii) As an antiseptic inhalent in nose troubles. (iii) In the preparation of paraldehyde (hypnotic and sporofic) and metaldehyde (solid fuel). (iv) In the preparation of acetaldehyde ammonia (a rubber accelerator). Table : 27.2 Comparative study of formaldehyde and acetaldehyde S.No . Reaction Formaldehyde HCHO Acetaldehyde CH3CHO Similarities 1. Addition of hydrogen (a) H2 in presence of catalyst, Ni, Pd or Pt (b) 4LiAlH (ether) (c) Amalgamated zinc + conc. HCl (Clemmenson reduction) Forms methyl alcohol OHCHHHCHO 32  Forms methyl alcohol Forms methane OHCHHHCHO 244  Forms ethyl alcohol OHCHCHHCHOCH 2323  Forms ethyl alcohol Forms ethane OHHCHCHOCH 2623 4  2. Addition of 3NaHSO solution Forms bisulphite addition product NaSOOHCHNaHSOHCHO 323 )( Forms bisulphite addition product  33 NaHSOCHOCH NaSOOHCHCH 33 )( 3. Addition of HCN Forms formaldehyde cyanohydrin CNOHCHHCNHCHO )(2 Forms acetaldehyde cyanohydrin  HCNCHOCH 3 CNOHCHCH )(3 4. Addition of Grignard reagent followed by hydrolysis Forms ethyl alcohol 3 23 CH OMgI CHMgICHHCHO  OHCHCH IOHMg OH 23 )( 2    Forms isopropyl alcohol  MgICHCHOCH 33 IOHMg OH CH HOMgICCH )( 3 | 3 2    1270 Aldehydes and Ketones 3 | 3 CH OHCHCH  5. With hydroxylamine OHNH 2 Forms formaldoxime    OH NOHHOCH 2 22 NOHCH 2 Forms acetaldoxime    OH NOHHOCHCH 2 23 NOHCHCH 3 6. With hydrazine )( 22 NHNH Forms formaldehyde hydrazone    OH NHNHOCH 2 222 22 NNHCH  Forms acetaldehyde hydrazone    OH NNHHOCHCH 2 223 23 NNHCHCH  7. With phenyl hydrazine )( 256 NHNHHC Forms formaldehyde phenyl hydrazone    OH HNNHCHOCH 2 5622 562 HNNHCCH  Forms acetaldehyde phenyl hydrazone 5623 HNNHCHOCHCH  563 2 HNNHCCHCH OH    8. With semicarbazide )( 22 NNHCONHH Forms formaldehyde semicarbazone    OH NNHCONHHOCH 2 222 22 NNHCONHCH  Forms acetaldehyde semicarbazone 223 NNHCONHHOCHCH  23 2 NNHCONHCHCH OH    9. With alcohol )( 52 OHHC in presence of acid Forms ethylal   HCl OHHCOCH 522 2 52 52 2 HOC HOC CH Forms acetaldehyde diethyl acetal   HCl OHHCCHOCH 523 2 52 52 3 HOC HOC CHCH 10. With thioalcohols )( 52 SHHC in presence of acid Forms thio ethylal  SHHCOCH 522 2 52 52 2 HSC HSC CH Forms acetaldehyde diethyl thioacetal  SHHCOCHCH 523 2 52 52 3 HSC HSC CHCH 11. Oxidation with acidified 722 OCrK Forms formic acid HCOOHOHCHO  Forms acetic acid COOHCHOCHOCH 33  12. With Schiff's reagent Restores pink colour of Schiff's reagent Restores pink colour of Schiff's reagent 13. With Tollen's reagent Gives black precipitate of Ag or silver mirror HCOOHAgHCHOOAg  22 Gives black precipitate of Ag or silver mirror  CHOCHOAg 32 COOHCHAg 32  14. With Fehling's solution or Benedict's solution Gives red precipitate of cuprous oxide HCOOHOCuHCHOCuO  22 Gives red precipitate of cuprous oxide  CHOCHCuO 32 COOHCHOCu 32  15. Polymerisation Undergoes polymerisation Undergoes polymerisation Evaporatio n H2SO4Conc. dil. H2SO4. distill Room temp. heat H2SO4Conc. dil. H2SO4. distill Aldehydes and Ketones 1271 nHCHO dehydeParaformal )( nHCHO HCHO3 dehydeMetaformal 3)(HCHO CHOCH33 eParaldehyd 33 )( CHOCH CHOCH34 eMetaldehyd 43 )( CHOCH Dissimilarities 16. With PCl5 No reaction Forms ethylidene chloride Cl Cl CHCHPClCHOCH 353  3POCl 17. With chlorine No reaction Forms chloral CHOCClClCHOCH 323 3  HCl3 18. With SeO2 No reaction Forms glyoxal CHOCHOSeOCHOCH .23  OHSe 2 19. Iodoform reaction (I2+NaOH) No reaction Forms iodoform  NaOHICHOCH 43 23 OHNaIHCOONaCHl 23 33  20. With dil. alkali (Aldol condensation) No reaction Forms aldol  CHOHCHCHOCH 23 CHOCHOHCHCH 23 )( 21. With conc. NaOH (Cannizzaro's reaction) Forms sodium formate and methyl alcohol HCOONaNaOHHCHO 2 OHCH 3 Forms a brown resinous mass 22. With ammonia Forms hexamethylene tetramine (urotropine) OHNCHNHHCHO 24623 6)(46  Forms addition product, acetaldehyde ammonia  33 NHCHOCH 2 3 NH OH CHCH 23. With phenol Forms bakelite plastic No reaction 24. With urea Forms urea-formaldehyde plastic No reaction 25. Condensation in presence of 2)(OHCa Form formose (a mixuture of sugars) No reaction Inter conversion of formaldehyde and acetaldehyde (1) Ascent of series : Conversion of formaldehyde into acetaldehyde (i) KCN PClNiH ClCHOHCHHCHO     Alc. chloride Methyl 3 alcohol Methyl 3 / deFormaldehy 52 HCl NaNONa NHCHCHCNCH    2 amineEthy l 223 Alcohol/ cyanide Methyl 3 deAcetaldehy 3 (dil.) alcoholEthyl 23 722 42 CHOCHOHCHCH OCrK SOH   1274 Aldehydes and Ketones CN OH CHCHHCNCHOCH 33  CN OH CCHHCNCOCH 2323 )()(  4. Addition of 3NaHSO White crystalline derivative NaSO OH CHCHNaHSOCHOCH 3 333  White crystalline derivative NaSO OH CCHNaHSOCOCH 3 23323 )()(  5. Grignard reagent followed by hydrolysis Forms isopropyl alcohol OMgICHCHMgICHCHOCH  2333 )( 33 2 CHOHCHCH OH   Forms tertiary butyl alcohol COMgICHMgICHCOCH 33323 )()(  COHCH OH 33 )(2  6. With hydroxylamine )( 2OHNH Forms acetaldoxime (an oxime) NOHCHCHNOHHCHOCH  323 Forms acetoxime (an oxime) NOHCCHNOHHCOCH  23223 )()( 7. With hydrazine )( 22NHNH Forms acetaldehyde hydrazone 23223 NNHCHCHNNHHCHOCH  Forms acetone hydrazone 2232223 )()( NNHCCHNNHHCOCH  8. With phenyl hydrazine )( 256 NHNHHC Forms acetaldehyde phenylhydrazone  5623 HNNHCHCHOCH 563 HNNHCCHCH  Forms acetone phenyl hydrazone  56223 )( HNNHCHCOCH 5623 )( HNNHCCCH  9. With semicarbazide )( 22NNHCONHH Forms acetaldehyde semicarbazone  223 NNHCONHHCHOCH 23 NNHCONHCHCH  Forms acetone semicarbazone  2223 )( NNHCONHHCOCH 223 )( NNHCONHCCH  10. With 5PCl Forms ethylidene chloride (Gem dihalide) Cl Cl CHCHPClCHOCH 353  Forms isopropylidene chloride (Gem dihalide) Cl Cl CCHPClCOCH 23523 )()(  11. With chlorine Forms chloral (Gem trihalide) CHOCClClCHOCH 323  Forms trichloro acetone (Gem trihalide) 33233 COCHCClClCOCHCH  12. With alcohols Forms acetal (a diether) 52 52 3523 2 HOC HOC CHCHOHHCCHOCH  Forms ketal (a diether) 52 52 235223 )(2)( HOC HOC CCHOHHCCOCH  13. With 2SeO Forms glyoxal OHSeCHOCHOSeOCHOCH 223  Forms methyl glyoxal OHSeCOCHOCHSeOCOCH 23223 )(  14. Iodoform reaction )( 2 NaOHI  Forms iodoform Forms iodoform 15. Bleaching powder Forms chloroform Forms chloroform 16. Aldol condensation with mild alkali Forms aldol CHOCHOHCHCHCHOCH 2332  Forms diacetone alcohol 322333 )()(2 COCHCHOHCCHCOCHCH  17. Polymerisation Undergoes polymerisation Does not undergo polymerisation but gives condensation reaction 18. With 3NH Forms acetaldehyde ammonia Forms diacetone ammonia Aldehydes and Ketones 1275 2 333 NH OH CHCHNHCHOCH   23323 )()( CHOCNHCOCH 32223 )()( COCHCHNHCCH 19. With conc. NaOH Forms brownish resinous mass No reaction 20. With 2HNO No reaction Forms oximino acetone NOHCOCHCHHNOCOCHCH  3233 21. With chloroform No reaction Forms chloretone 3 23323 )()( CCl OH CCHCHClCOCH  22. With alk. sodium nitroprusside Deep red colour Red colour changes to yellow on standing 23. With sodium nitroprusside + Pyridine Blue colour No effect 24. Boiling point Co21 Co56 Dissimilarities 25. With Schiff's reagent Pink colour Does not give pink colour 26. With Fehling's solution Gives red precipitate No reaction 27. With Tollen's reagent Gives silver mirror No reaction 28. Oxidation with acidified 722 OCrK Easily oxidised to acetic acid COOHCHOCHOCH 33  Oxidation occurs with difficulty to form acetic acid OHCOCOOHCHOCOCHCH 22333  Aromatic Carbonyl Compounds Aromatic aldehydes are of two types : The compounds in which CHO group is attached directly to an aromatic ring, e.g., benzaldehyde, CHOHC 56 . Those in which aldehyde )( CHO group is attached to side chain, e.g., phenyl acetaldehyde, CHOCHHC 256 . They closely resemble with aliphatic aldehydes. Aromatic ketones are compounds in which a carbonyl group )( OC  is attached to either two aryl groups or one aryl group and one alkyl group. Examples are : deBenzaldehy CHO ketone) enyl (Methyl ph neAcetopheno 3COCH ketone)(Diphenyl neBenzopheno 56 HCOC ehydeSalicylald OH Benzaldehyde, CHOHC 56 or Benzaldehyde is the simplest aromatic aldehyde. It occurs in bitter almonds in the form of its glucoside, amygdalin )( 112720 NOHC . When amygdalin is boiled with dilute acids, it hydrolyses into benzaldehyde, glucose and HCN  deBenzaldehy 562 Amygdalin | 10211256 2 CHOHCOH CN OHCHOCHC HCNOHC  Glucose 61262 Benzaldehyde is also known as oil of bitter almonds. (1) Method of preparation (i) Laboratory method : It is conveniently prepared by boiling benzyl chloride with copper nitrate or lead nitrate solution in a current of carbon dioxide. 22 deBenzaldehy 56 heat 23 or 23 chlorideBenzy l 256 22 )( )(2 2 HNOCuClCHOHC NOPb NOCuClCHHC CO   ]2[ 222 OHNONOHNO  (ii) Rosenmund reaction : CHO CHO 1276 Aldehydes and Ketones HClCHOHCHCOClHC BaSOPd   deBenzaldehy 56 xylene / chlorideBenzyl 256 4 (iii) By dry distillation of a mixture of calcium benzoate and calcium formate 3 duct)(Major pro deBenzaldehy 56 heat formate Calcium || || benzoateCalcium 56 56 22 CaCOCHOHC O O CH CH O O CaCa COOHC COOHC   (iv) By oxidation of benzyl alcohol : This involves the treatment of benzyl alcohol with dil. 3HNO or acidic potassium dichromate or chromic anhydride in acetic anhydride or with copper catalyst at Co350 . deBenzaldehyalcoholBenzyl ][ 2  O OHCH This method is used for commercial production of benzaldehyde. (v) By hydrolysis of benzal chloride : deBenzaldehy (unstable) teIntermediaChlorideBenzal )( )( 2 2 2 CHOCHCHCl OH OHCa NaOH     This is also an industrial method. (vi) By oxidation of Toluene OH CHO O CH C OV o 2 350 2 3 52   Commercially the oxidation of toluene is done with air and diluted with nitrogen (to prevent complete oxidation) at Co500 in the presence of oxides of MoMn, or Zr as catalyst. Partial oxidation of toluene with manganese dioxide and dilute sulphuric acid at Co35 , also forms benzaldehyde.     OHH OCOCH CrO OCOCHCHHCCHHC 2 23 3 / acetate eBenzyliden 2356 )(Toluene 356 )( COOHCHCHOHC 356 2 (vii) Etard's reaction :  22356 2 ClCrOCHHC deBenzaldehy 56 productaddition Brown 22356 22 CHOHCClCrOCHHC OH   (viii) Gattermann-koch aldehyde synthesis : Benzene is converted into benzaldehyde by passing a mixture of carbon monoxide and HCl gas under high pressure into the ether solution of benzene in presence of anhydrous aluminium chloride and cuprous chloride. HCl CHO HClCO AlCl   deBenzaldehyBenzene 3 (ix) Gattermann reaction    43 AlClNHCHAlClHClNHC ;   256 Benzene 56 NHCHHCNHHCHHC    42256 AlClOHNHCHHC HClAlClNHCHOHC  3356 Thus, ClNH CHO OHHClHCN AlCl 42 3   (x) Stephen's reaction : Benzaldehyde is obtained by partial reduction of phenyl cyanide with stannous chloride and passing dry HCl gas in ether solution followed by hydrolysis of the aldimine stannic chloride with water. complex Aldimine 62256 Ether / cyanidePhenyl 56 ][2 SnClHNHCHHCNCHC SnClHCl   CHOHC OH 5622  (xi) By ozonolysis of styrene   OHO CHCHHCCHCHHC 23 256 eneVinyl benz 256 – 2256 OHHCHOCHOHC  (xii) Grignard reaction 52 || deBenzaldehy 5656 || formateEthy l 52 HOC Br Mg O HCHCHBrMgC O HHCOC  Other reagents like carbon monoxide or HCN can also be used in place of ethyl formate. (xiii) From Diazonium salt NOHCHNOHHCHClNN  meFormaldoxi (2) Physical properties (i) Benzaldehyde is a colourless oily liquid. Its boiling point is Co179 . (ii) It has smell of bitter almonds. (iii) It is sparingly soluble in water but highly soluble in organic solvents. (iv) It is steam volatile. (v) It is heavier than water (sp. gr. 1.0504 at Co15 ). (vi) It is poisonous in nature. (3) Chemical properties CHO O O O OH OH benzaldehyde toluene Benzaldoxi me CHO Benzaldehy de H2O + HCl + N2 Aldehydes and Ketones 1279 green) (Malachite methanetriphenyl diaminol Tetramethy 23 23 )( Conc. anilineDimethyl 23 23 )( )( )( )( 2 42 CHN CHN CH CHN CHN H H OCH OH SOH    (i) Reaction with Ammonia : Benzaldehyde reacts with ammonia to form hydrobenzamide aldehyde other than OCH2 give aldehyde ammonia while OCH 2 forms urotropine.      56 2 2 56 56 HCCHO NHH NHH CHOHC CHOHC mideHydrobenza 56 56 56 HCCH NCHHC NCHHC    (j) Reformatsky reaction  teethylaceta Bromo 522 deBenzaldehy 56 HCOOCHCBrZnOCHHC  ester hydroxy- 522 | 56 | 52256 2  HCOOCCH OH CHHC OZnBr HCOOCCHCHHC OH   (k) Reaction of benzene ring (4) Uses : Benzaldehyde is used, (i) In perfumery (ii) In manufacture of dyes (iii) In manufacture of benzoic acid, cinnamic acid, cinnamaldehyde, Schiff's base, etc. (5) Tests : (i) Benzaldehyde forms a white precipitate with 3NaHSO solution. (ii) Benzaldehyde forms a yellow precipitate with 2 : 4 dinitrophenyl hydrazine. (iii) Benzaldehyde gives pink colour with Schiff's reagent. (iv) Benzaldehyde forms black precipitate or silver mirror with Tollen's reagent. (v) Benzaldehyde on treatment with alkaline 4KMnO and subsequent acidification gives a white precipitate of benzoic acid on cooling. Acetophenone, C6H5COCH3, Acetyl Benzene (1) Method of preparation (i) Friedel-Craft's reaction : Acetyl chloride reacts with benzene in presence of anhydrous aluminium chloride to form acetophenone. HClCOCHHCCOCHClHHC AlCl   neAcetopheno 356 chlorideAcetyl 3 Benzene 56 3 (ii) By distillation of a mixture of calcium benzoate and calcium acetate.   acetate Calcium || 3 || 3 benzoateCalcium 56 56 O CCHO O CCHO CaCa COOHC COOHC 3 neAcetopheno || 356 22 CaCO O CCHHC  (iii) By methylation of benzaldehyde with diazomethane. 23562256 NCOCHHCNCHCHOHC  (iv) By treating benzoyl chloride with dimethyl cadmium. 23562356 2)(2 CdClCOCHHCCdCHCOClHC  (v) By Grignard reagent (a) NMgBr HC CCHMgBrHCNCCH 56 | 3563  BrOHMgNHCOCHHC )(3356  (b)  acetateEthyl 3 || 2556 CH O COCHMgBrHC 52 3 || 56 HOC Br MgCH O CHC  (vi) Commercial preparation : Ethylbenzene is oxidised with air at Co126 under pressure in presence of a catalyst manganese acetate. OH COCH O CHCH C 2 3 pressure126 Catalyst 2 32 o   (2) Physical properties : It is a colourless crystalline solid with melting point Co20 and boiling point Co202 . It has characteristic pleasant odour. It is slightly soluble in water. Chemically, It is similar to acetone. (3) Chemical properties : HNO3(conc.) H2SO4 (conc.) fuming Cl2 FeCl3 H2SO4 ldehydeNitrobenzam CHO NO2 acidSulphonic deBenzaldehym CHO SO3H aldehy deChlorobenzm CHO Cl deBenzaldehy CHO H2O HCN H2NOH necyanohydri neAcetopheno 3 | | 56 CH OH CN CHC  42 entRearrangem ketoxime)ny l (Methy lphe or oxime neAcetopheno 3 | 56 SOH NOH CH CHC   eAcetanilid 356 NHCOCHHC 1280 Aldehydes and Ketones (4) Uses : It is used in perfumery and as a sleep producing drug. Benzophenone, C6H5COC6H5 (1) Method of preparation (i) From alkyl benzenes 565656256 32 HCOCHCOHCCHHC HNO   (2) Physical properties : It is a colourless, pleasant smelling solid. (3) Chemical properties : It shows the characteristic properties of keto group but does not give bisulphite compounds. (i) Reduction : carbinolDiphenyl 56565656 .2 HCHOHCHCHHCOCHC HgNa    (ii) Clemmenson reduction : OHHCCHHCHCOCHC HCl HgZn 2 methaneDiphenyl 56256 / 5656   (iii) Fusion with KOH : 6656 Fuse 5656 HCCOOKHCKOHHCOCHC   66 acidBenzoic 56 Ether Butoxide tert.Pot. 25656 HCCOOHHCOHHCOCHC    Acidified K2Cr2O7 i.e., chromic acid sulphuric acid mixture is known as Jone’s reagent. When used as an oxidising agent unlike acidified KMnO4 it does not diffect a double bond. CH2 =CHCH2OH CH2=CHCHO  Vilsmeyer reaction : this reaction involves the conversion of aromatic compounds to aldehydes in the presence of a 2o amino and formic acid.  Benzaldehyde although reduces Tollen’s reagent. It does not reduce Fehling or Benedict solution. Oxidation glyoxalPhenyl 56 COCHOHC hylbenzeneDichloroet-22, 3256 CHCClHC SeO2 C6H5COCH3 (Acetophenone) (It is relatively harmless but powerful lachrymator or tear gas and is used by police to disperse mobs.) gas) tear a as (Used chloridePhenacy l 256 ClCOCHHC Cl2 PCl5 Iodoform 356 CHICOONaHC  Iodoform reaction I2/NaO H 56 hypnotic) as used is (It Dypnone || 3 | 56 HC O CCH CH CHC  Aldol type condensation Al-ter- butoxide Nitratio n HNO3/H2SO 4 phenoneNitroaceto- 3462 m COCHHCNO conc. H2SO4 acidsulphonic neAcetopheno 3463 m COCHHCHSO K2Cr2O7/H2S O4 (CH3)2 NH + HCOOH+POCl3 Benzane CHO