Hydrocarbons A level Organic Chemistry Notes, Study notes of Chemistry

Download Hydrocarbons A level Organic Chemistry Notes and more Study notes Chemistry in PDF only on Docsity! HALOGEN DERIVATIVES There are 3 types : 1. Halogenoalkanes (alky! halides) : an alkane with one or more hydrogen atoms replaced with halogen atom(s) Haloal kanes aK \ é halogen (F-CL, Br, 1) (chlovofora) ——_arhon etrachliridé) PF Cis. CH, Bro cud, Ch, Chloremethane bromochlorometane Trichlorarngthene — TetvacNoro methane. 2. Aromatic halogen Compounds (halogenoarenes) : halogenydirectly attached to a benzepe ring Eg or Cs Hh Hs chlorobenzene. 3-chloremetbdlbencene. yh. —, Ly 3. Acid (acyl) chlorides : contains -COCI group of ones dichloraboncene, Eg: ried 0 On Cs Nl 5 ) ch etna dln Q benzay chloride. HALOGENOALKANES (AS) Classification According to how many alkyl groups or H atoms are attached to the carbon which is bonded to the halogen atom kanes : one alkyl group or 2 hydrogen atoms attached to the carbon Y Jechlorabstane (|) 3. Terti: joalkanes : three alkyl groups or no hydrogen atom on the carbon atom af as" N\- S| K-G-H mn “i carbonium ion { 1 f : M-CrC+C—h we-C-e-H carbe cation Waly aa 1 Note: to-form Z-chloromethylyropane (3) Carbonivm ion Chemical Characteristic 4 1.C~X bond is polar: The hal ens are generalh more. eltctn ‘ than carbon So the be la oy ee Tet Sh ie oper te atack by nucleophiles . Bord folly ony predict he ature OF reggext that attack. ‘ —_Haleaen “El ce [Br [ s H Tee eet he Mierence Cc Flectea negativity [ 4.01 3-01 25125175124 teater x i Z Saturated compounds therefore do not undergo addition yor Ag ter. ah oat Chayater aC, undergo substitution reaction where the halogen atom is substituted with another atom or group of atoms. @ 3. Order of reactivity : substitution involves breaking of C-X bond. The easier the bond is broken , the more reactive is the compound. Determined by bond Strowth » J resp —> bord srengh | reactivity Reason : The Tend is due to the shaved electrans being closer to the hucleus jn Fluorine than in Gding Bond sheath detersive thet der of gop ees reagivtty as C-I>C-R>C-CL>C-F Te | ANB — Supported in vate experiment “Ch Blkb o: C-Br 240 Mechanism of nucleophilic substitution C-1 2.29 Eg. Hn Kh zs a R- OX > RCH 4K H q 1, Primary halogenoalkanes : SN 2 mechanism : In the slow step, both species ( reagent and R-CH2X ) are brought together in the transition state. Hey" H Nuaz H } \ _ rhe [aT Gm H H 4 2 3. Reaction with CN" : Condition: Aqueays, alcoholic potassium gyanide Ft reflux ~ Colts Be + KCN (ag/a le) —> ColgCN + KBr Nec: A\A H important reaction - “tre & —> H- ton + tbr: extends the carbon chain H i WW i i vs by One Carbon atom Following reaction is poreentie CoVsON +20 PES Cots COOH + NHS hades) Co As CN + EH) AMS Cs Heche NH (reduction Other reactions : Prpglanine ( ) 1, Hydrolysis of halogenoalkanes with more than one halogen atom : a. two halogen atoms : i) both halogen atoms attached to different carbon atoms : each halogen undergpes the usual reactions independently H=C-C-H + 20H —H-C-E-W $0807 tod Br fr OK bi ii) both halogen atoms attached to the same carbon atom : Ceaspon the end of a chain : produces an aldehyde 4 | H WEE Bet DOH —> H-C-CoH > HE Hel ich 4 bn W (2)carbon not at the end of a chain : produces a ketone etha nal Pri ep 4 IH AIM IE Fh 6 H Br H H oH H how 2,2-dibromo prepare. Proparone. b. three halogen atoms : produces a carboxylic acid if 4 Tm han ACE Br + BOK > R-C-C-O HCH Had H br , =F oH n° ethawic acid 2. Preparation of an ether : Condition : Sodium ethoxide in Cthanol H H NK | _ a- we Nocts D> R-C-0GHs +X WW arhexide ion H Note: The same reaction takes phe with ethane! instead of sodium gttexde, but more slowly becouse ethanol does nit have negative charg so is @ poorer nucleaphile “han the ethsyide (on- IJ, Elimination reaction 1. Condition : Alcoholic petssium hydroxide. & rely 2. HX molecule eliminated to produce a\eng, 3. Substitution vs elimination reaction : Reaction type Reagent & Condition substitation Kon in water [KOH (aq) Elimination KOA in alabs) | KOH (akc)} Note : A nucleophile must be able to reach the center of electron deficiency in the molecule with which it reacts. In ethanolic alkali . the OH’ are solvated with the bigger ethanol molecules. The resulting species is too large to approach the C *' , and as a result , Substitution is impossible. Instead the OH’ acts as a base. Eg : a. primary halogenoalkane : HOH Ol, - Ce —> Neneh + 0 + 283 4 Wa Ch, H The OW jon acts as a bee and picks up a profen The proton cames fom a carbon atom next fo the one bended +0 the haligen, The yemaining electon pair moves to form a seconk pond between the carbon atins . The halide oan is ispleced Overall there (s elimination - of HBr. b. Other a ree more than one product me ur. 2-Chlorobutane Oi Ss, id oA — ns =e H t ce \y 2c cy. i “is Cis but-|-ere, Cis~ but-2-ene 2 III. Reaction wi cous 1. Reaction used to identify halogen present in R-X a. Initially free X- ion has to be produced , through either of two reactions : i) reaction with aqueous alkali or H2O R-X + NaOH —> R-OH + NaX K 4 H-G- br + Nath —> H-c-oH + NaBr H H ii) reaction with alcohol R-X + Gs 0 Nat —> R-0 “Calls + Nax 4 wu n= Conk ane 4 + H- “es ONat —>y- C-0- EH Tach a aw HH Wh b. en aqueous AgNOs , observe colour of ppt formed diets } ether X (ag) + Ay* Cag) —> AX) : Mya= White fr yt (soluble in Wis (ag)? hy Br - Cream ppt. (partially soluble ia NHs(ag)) Ay - Yellow ppt. (not solyble in NYisCaq)) 5. Environmental concern : a. CFC’s are so stable and chemically unreactive that they reach the stratosphere (upper atmosphere) unchanged. b. There , intense uv light photolyses ( breaks C-Cl bonds ) , forms chlorine radicals (and chlorine oxide ) which leads to a chain reaction which results in the destruction of the ozone layer. Note : no fluorine radicals formed c. Equations : > Chect: +E The Free radi cal catalyse he breaking we of OZone 203 30, The chine yadicals formed in phetol sis take part in a chain veaction which cometts ozone into oxygen a) Ch'ths —> Cho°+0o ) QO +0. Ch t+ a) Cho" + 0s —7 Ch, +0, 6. Effects of rise in CFC concentration : a. Depletion of ozone layer occurs hence decreasing its ability to absorb and filter uv light from the sun. More uv light reaches the earth's surface , hk Causing heaulyfic Fission of chemical Lending in molecules, producitg free vadicgls whch are reactive and unstable. ii) damage sore of the molecules of lefe, for eg. proteins and pucleic acids b. CFC’s have strong absorption of infrared light given off by earth . acts as “greenhase gases” increasing the threat of Gobel warning 7. Solutions to problem of increase in CFC concentration : a. reduce CFC usage b. use alternatives to CFC’s : egs i) SERN L Lh 2- telref fuer ethane (cfc) . rehigewst noCt it ddes pot prodece the daring chlorine reo, radials C-H relatively reactive f th hud ‘4 The presence Ot the hydragen atoms increases -the reactivity of this compound VE letive y Oc. m that it is broken es the lower atwos phere ich more rapidly D dees tot reach the __ Stratisphere - . . ii) hydrochlorofluorocarbons : C-H relatively reactive 1,1 = dich!oro- 2,2, 2- tri-Fleove ethane ropellnts, bhwi Es fi cal oe as propellats, bh ing agents Composed | BP/K [plannabiltty | opp CFA, | 207 No 4 CHsCHL CH | 23) “Yes 2 Cr,CCh iii) alkanes eeaeposiie sO No tar OT ee ees prpellart in aeresols and as ste : Plarable ard less sete (camut be ba at co iv) N20 Sea se S not lguid a} room Tenperature ) . , sty ys. Thchtero methane (chloroform) I- bome-|- chloro —2,2,2-trePlogethane. ‘euhig es’ (Fhe + hene) — Safer than Chlovot rm Note Other organic compounds containing fluorine and / or chlorine which are commercially important because of their chemical inertness, egs: PVC used in faed pack iNy Tethn/ PTFE used th non-Stick kUtches ware [For your knowledge: The ozone layer in the stratosphere protects us from harmful solar radiation. It absorbs high energy ultraviolet radiation which would otherwise damage the molecules of life eg: proteins and nucleic acid. Nearer the earth's surface, ozone is produced by a complex series of photochemical reactions involving the unbumt hydrocarbons and oxides of nitrogen from car exhausts. Ozone is poisonous and low concentrations have an irritant effect on humans. The damaging effect of ozone is put to good use in purifying drinking water, ozone 1s bubbled into water to destroy harmful bacteria.