Lecture Slides on Nucleophilic Addition to the Carbonyl | CHEM 238, Study notes of Organic Chemistry

Download Lecture Slides on Nucleophilic Addition to the Carbonyl | CHEM 238 and more Study notes Organic Chemistry in PDF only on Docsity! Chapter 17 Aldehydes and Ketones. Nucleophilic Addition to the Carbonyl Group 17.1 Nomenclature IUPAC Nomenclature of Aldehydes H O O H O HCCHCH O Base the name on the chain that contains the carbonyl group and replace the -e ending of the hydrocarbon by -al. 4,4-dimethylpentanal 5-hexenal IUPAC Nomenclature of Aldehydes H O O H O HCCHCH O 2-phenylpropanedial (keep the -e ending before -dial) when named as a substituent formyl group carbaldehyde or carboxaldehyde when named as a suffix C H O IUPAC Nomenclature of Aldehydes CH3CH2CCH2CH2CH3 O CH3CHCH2CCH3 O CH3 H3C O Base the name on the chain that contains the carbonyl group and replace -e by -one. Number the chain in the direction that gives the lowest number to the carbonyl carbon. Substitutive IUPAC Nomenclature of Ketones Substitutive IUPAC Nomenclature of Ketones CH3CH2CCH2CH2CH3 O CH3CHCH2CCH3 O CH3 H3C O 3-hexanone 4-methyl-2-pentanone 4-methylcyclohexanone Functional Class IUPAC Nomenclature of Ketones CH3CH2CCH2CH2CH3 O O CH2CCH2CH3 CH CH2 O H2C CHC List the groups attached to the carbonyl separately in alphabetical order, and add the word ketone. The half-filled p orbitals on carbon and oxygen overlap to form a p bond Bonding in Formaldehyde 17.3 Physical Properties boiling point –6°C 49°C 97°C Aldehydes and ketones have higher boiling than alkenes, but lower boiling points than alcohols. More polar than alkenes, but cannot form intermolecular hydrogen bonds to other carbonyl groups O OH 17.4 Sources of Aldehydes and Ketones 2-heptanone (component of alarm pheromone of bees) O Many aldehydes and ketones occur naturally trans-2-hexenal (alarm pheromone of myrmicine ant) Many aldehydes and ketones occur naturally O H citral (from lemon grass oil) Many aldehydes and ketones occur naturally O H from alkenes ozonolysis from alkynes hydration (via enol) from arenes Friedel-Crafts acylation from alcohols oxidation Synthesis of Aldehydes and Ketones A number of reactions already studied provide efficient synthetic routes to aldehydes and ketones. C O R OH aldehydes from carboxylic acids RCH2OH 1. LiAlH4 2. H2O PDC, CH2Cl2 H C O R What about..? benzaldehyde from benzoic acid COH O CH O 1. LiAlH4 2. H2O PDC CH2Cl2CH2OH (81%) (83%) Example C O R H ketones from aldehydes R' C O R PDC, CH2Cl2 1. R'MgX 2. H3O+ RCHR' OH What about..? C O CH3CH2 H 3-heptanone from propanal H2CrO4 1. CH3(CH2)3MgX 2. H3O+ CH3CH2CH(CH2)3 CH3 OH O CH3CH2C(CH2)3 CH3 (57%) Example Mechanism of Hydration (base) C •• O•••• O •• H •• – Step 1: + •• HO C O •• •• •• •• – Mechanism of Hydration (base) Step 2: ••OH H •• •• HO C O •• •• •• •• – + •• ••O H •• –•• HO C OH •• •• •• Mechanism of Hydration (acid) C •• O•• Step 1: + •• H OH H + + C •• OH + •• •• H O H Mechanism of Hydration (acid) Step 2: C •• OH + + •• H O H •• C OH •• •• H O H •• + Mechanism of Hydration (acid) Step 3: + •• H O H •• C OH •• H O H •• •• •• O H •• C OH •• •• + H H O H •• + 17.7 Cyanohydrin Formation + Cyanohydrin Formation •• ••C O HCN HC ON C •• •• •• Cyanohydrin Formation •• ••C O C – N•• •• Cyanohydrin Formation – ON C C •• •• •••• H H H + O•• H H O ••••ON C C •• •••• H 2,4-Dichlorobenzaldehyde cyanohydrin (100%) Example Cl Cl CH O Cl Cl CHCN OH NaCN, water then H2SO4 Example CH3CCH3 O NaCN, water then H2SO4 CH3CCH3 OH CN (77-78%) Acetone cyanohydrin is used in the synthesis of methacrylonitrile (see problem 17.7). 17.8 Acetal Formation First stage is analogous to hydration and leads to hemiacetal acid-catalyzed nucleophilic addition of alcohol to C=O Mechanism of Acetal Formation Mechanism C O •• •• H H R + O •• Mechanism C O •• H R O •• H + •• Mechanism C O •• H + R H O •••• Mechanism C O •• O •• H + •• R H O R H •• •• Mechanism + H O R H•• C O •• O •• H •• R •• Second stage is hemiacetal-to-acetal conversion involves carbocation chemistry Mechanism of Acetal Formation Hemiacetal-to-acetal Stage C O •• O •• H •• R •• H H R + O •• Hemiacetal-to-acetal Stage H R O ••••C O •• O H •• R •• H + Hemiacetal-to-acetal Stage O •• O H •• R •• + H C Hemiacetal-to-acetal Stage CO •• R •• + O •• H H •• Hemiacetal-to-acetal Stage CO •• R •• + Carbocation is stabilized by delocalization of unshared electron pair of oxygen CO R •• +