ALDEHYDES AND KETONES

Aldehydes and Ketones

  Most Reactive Group –  electrons + polarisation Names al – aldehydes, one - ketones Methanal (formaldehyde) Ethanal (acetaldehyde) Propanal Pentanal

Nomenclature of Aldehydes

If the aldehyde group is attached to a ring,

Nomenclature of Ketones

Oxidation

An aldehyde has a greater partial positive charge on its carbonyl carbon than does a ketone

The carbonyl carbon of an aldehyde is more accessible to the nucleophile Ketones have greater steric crowding in their transition states, so they have less stable transition states Steric factors contribute to the reactivity of an aldehyde

1. Redox reactions Reactions

2. Addition: A N Addition of water Aldehyde hydrate (ketone: ) Addition of bisulfite Bisulfite

Addition of NH 3 Aldehyde ammonia (ketone: ) Addition of HCN Cyanohydrine

3. Condensation reactions (addition followed by elimination) Ammonia derivatives Primary amine Schiff base

Hydroxylamine Hydrazine Hydrazone Oxime

Phenylhydrazine 2,4-dinitrophenylhydrazine 2,4-dinitrophenylhydrazone Phenylhydrazone

Aldol condensation Aldol

Reaction with alcohol Acetal (condensation!) Hemiacetal (addition!)

Keto–Enol Tautomerism

Oxidation of a Primary Alcohol

Reduction by Addition of a Hydride Ion and a Proton

Aldehydes and ketones react with nucleophiles to form addition products: nucleophile addition reactions

Water adds to an aldehyde or ketone to form a hydrate

Hydration of Aldehydes and Ketones HOH C O HO C O H RR' R R'

Alcohols Under Analogous Reaction with Aldehydes and Ketones R"OH C O RR' R"O C O H R R' Product is called a hemiacetal. Product is called a hemiacetal.

HCl 2CH 3 CH 2 OH + + H 2 O Benzaldehyde diethyl acetal (66%) Example CHO CH(OCH 2 CH 3 ) 2

Reaction with Primary Amines: Imines

If the nucleophile that adds to the aldehyde or ketone is an O or an N, a nucleophilic addition–elimination reaction will occur

H2NH2NH2NH2N R C O + H a carbinolamine CO HNHNHNHN R N R C + H 2 O (imine) Imine (Schiff's Base) Formation

CH 3 NH 2 CHO+ CH=NCH 3 + H 2 O N-Benzylidenemethylamine (70%) Example

Reaction with Derivatives of Ammonia H2NH2NH2NH2NG + R2CR2CR2CR2C O R2CR2CR2CR2C NGNGNGNG+ H2OH2OH2OH2O H2NH2NH2NH2NOH R2CR2CR2CR2C NOH hydroxylamineoxime

Reaction with Derivatives of Ammonia H2NH2NH2NH2NG + R2CR2CR2CR2C O R2CR2CR2CR2C NGNGNGNG+ H2OH2OH2OH2O H2NH2NH2NH2NOH R2CR2CR2CR2C hydroxylamineoxime H2NH2NH2NH2N NH 2 R2CR2CR2CR2C NNH 2 hydrazinehydrazone etc.

+ H 2 NNH 2 + H 2 O (73%) ExampleOC NNH 2 C

CCH 3 ExampleO + H 2 NNH phenylhydrazine + H 2 O CCH 3 NNH a phenylhydrazone (87-91%)

An aldol addition product loses water to form an aldol condensation product

2RCH 2 CH ONaOH RCH 2 CH OH CHCHOR Aldol Condensation

Aldol Addition of Acetaldehyde Acetaldol (50%) NaOH, H 2 O 5°C 2CH 3 CH O CH 3 CH OH CH 2 CH O

Product is called a hemiacetal. ROH, H + Hemiacetal reacts further in acid to yield an acetal R"O C O H RR' R"O C OR RR' Product is called an acetal. Product is called an acetal.

Formaldehyde formaldehyde cannot form an enolate formaldehyde cannot form an enolate formaldehyde is extremely reactive formaldehyde is extremely reactive toward nucleophilic addition toward nucleophilic additionOHCH

Acrolein H2CH2CH2CH2C CHCHO

H2CH2CH2CH2C CHCHO Acrolein

Acrolein H2CH2CH2CH2C CHCHO