RCHO CHR

O

CRR

O

R = alkyl or aryl group

aldehyde ketone

Special case

CHH

O

formaldehyde

(methanal)

Carbonyl group- very important functional group

Chapter 9 Aldehydes and Ketones

C O

Reactivity and properties are strongly dependent on the substituents- must identify which derivative is presentTerm "carbonyl group" is too general

CH3-C-CH3

Propanone

O

(acetone)

CH3-CH2-C-CH2CH3

O

3-pentanone

O

cyclohexanone

Nomenclature of Aldehydes- al

Nomenclature of Ketones- one

e.g.

CH3-C-H

Ethanal 3-methylbutanal

O

(acetaldehyde)

O

CH3-CH-CH2-C-H

CH3

O

CH2-CH-C-H

OHOH

2,3-Dihydroxypropanal

(glyceraldehyde)

CHO

Benzaldehyde

Common aldehydes and ketones

H-C-H

O

Polymers in pure form- used as an aqueous solution (37%)- Disinfectant + Preservative- Maufacture of Plastics etc.

CH2=CH-C-CH3

O

1234

3-buten-2-one

Rule: Chain is numbered so that the carbonyl carbon has the lowest possible number

Formaldehyde

(HCHO) Manufactured by methanol oxidation

CH3OHAg catalyst

600-700 oCHCHO + H2

Gas bp -21oC

Acetaldehyde bp 20oC - prepared from ethylene by Wacker process

2 CH2=CH2 + O2Pd-Cu

100-300oC2 CH3-C-H

O

Ethene oxidation over a palladium-copper catalyst

One of major uses- oxidation to acetic acid (ethanoic acid)O

CH3-C-H

O

CH3-C-OH

AcetoneO

CH3-C-CH3 One of the commercial routes- oxidation of 2-propanol

H3C C

OH

CH3

H

oxidationO

CH3-C-CH3

Widely used as a solvent- very important as it is miscible with water

Aldehydes + Ketones occur widely in nature- often pleasant odoursTherefore used in perfumes, soaps, air fresheners etc.

9.3 Synthesis of Aldehydes and Ketones

Oxidation of Alcohols

CH3CH2CH2OHPCC CH3CH2-C-H

O

Propanal

(CH3CH2CHO)

Propanol

1o Alcohol

CH3-CHCH2CH3

OHJones'

reagentCH3-CHCH2CH3

O

2o Alcohol Butan-2-one

Structure of the carbonyl group

C O

R

RC=O 1.24 Å

(C-O 1.43 Å in alcohols)

sp2- hybridised, structure is planar

O more electronegative than C, bond is polarised

C O

R

R

electrophilc carbon atom

C O

R

R

O is electron rich- can hydrogen bond with waterLow molecular weight aldehydes/ketones are soluble in H2O

C O

R

R

HO

H

9.6 Nucleophilic addition to the carbonyl group

C O

R

R

Electrophilic carbon susceptible to attack by nucleophiles

e.g.

Nu

C O

R

Nu

R

Tetrahedral Intermediate

C OH

R

Nu

RH2O

sp2 C sp3 C

From H2O

Aldehydes- more reactive than ketones towards nucleophiles

C O

R

R

Reaction with hydrogen cyanide (9.10)

+ HCNKOH

C OH

R

C

N

R

Cyanohydrin

H3C CH3

O

+ HCNKOH

H3C C

OH

CH3

CN

KOH + HCN K+ -CN + H2O

C O

H3C

H3C

C O

H3C

H3C

N C

N C

Cyanide anion- a carbon based nucleophile

C O

H3C

C

N

H3CH2O

C OH

H3C

C

N

H3C

Reaction with acetylide anion

O

+ Na C CH

Sodium acetylide

CO

CH

Na

H2O

CHO

CH

CH3-C-H

O

+ Na C CH CH3-CH-C CH

O Na+

H2OCH3-CH-C CH

OH

Cyanide + acetylide addition to aldehyde + ketones new C-C bond formation

C OH

CN

C O

C OH

C

CH

Nucleophilic addition of alcohols

CH3-C-H

O

CH3-C

OCH2CH3

Acetal/Hemiacetal formation

Alcohol- Oxygen based nucleophile

RO

HAcid catalysed reaction

+ CH3CH2OHH+

H

OH

Hemiacetal

In the presence of excess alcohol hemiacetals react further to acetals

CH3-C

OCH2CH3

OH

H

+ CH3CH2OH CH3-C

OCH2CH3

OCH2CH3 + H2O

H

CH3-C-H

O

CH3-C

OCH2CH3

+ 2 CH3CH2OHH+

H

OCH2CH3

All the steps are reversible- Can drive the reaction to completion by removing the wateras it forms

+ 2 CH3OH

+ H2O

Overall Reaction

H3C CH2CH3

O

H+

CH2CH3H3C

H3COH3CO

+ H2O

O

+ 2 CH3CH2OHH+

OCH2CH3H3CH2CO

+ H2O

General: O

+ 2 ROHH+

RORO

+ H2O

9.11 Nitrogen Nucleophiles

C O

R

R

+ NH2-R

1o amine

C OH

NHR

R

R

Tetrahedral Intermediate

H2OC

N

R R

R

Imine

sp2

Addition-Elimination reaction

e.g

H3C CH3

O

+ NH2CH2CH3 C

NHCH2CH3

H3C CH3

HO - H2O

H3C CH3

NCH2CH3

Imine

H3C CH3

O

+ NH2CH2CH3

C

NHCH2CH3

H3C CH3

HO

Electrophile

Nucleophile

C

NHCH2CH3

H3C CH3

O

H

proton transfer

C

NCH2CH3

H3C CH3

HO - H2O

H3C CH3

NCH2CH3

Imine

Addition step

Tetrahedral Intermediate (sp3)

Elimination step

H

e.g.

O

+ CH3NH2

HO NHCH3

- H2O

NCH3

Overall Reaction

O NCH3

R R

O

R R

N

R

Aldehydes/ketones Imines

Can use a wide range of N nucleophiles- see Table 9.1

R R

O

+ NH2-XR R

N

X

+ H2O

9.12 Reduction of aldehydes and ketones

Two common reducing agents used are: LiAlH4 (lithium aluminiumhydride) NaBH4 (sodium borohydride)

LiAlH4 is more reactive, stronger reducing agent then NaBH4

Aldehydes 1o alcohols

Ketones 2o alcohols

CH3-CH2-C-H

O(i) NaBH4

(ii) H2OCH3CH2CH2OH

PhCHO(i) NaBH4

(ii) H2OPhCH2OH

(i) NaBH4

(ii) H2O

CH3-CH=CH-C-H(i) NaBH4

(ii) H2OCH3-CH=CH-CH2OH

OH OH

O

2-butenal 2-buten-1-ol

cyclohexanone cyclohexanol

Note: Alkene not reduced by NaBH4

C O

B

H

H HH

Hydride addition

C O

H

+ BH3

C O

HB

H

HH

C O

HB

OR

ORRO

NaH2OC OH

H

C O(i) NaBH4

(ii) H2OC O

H

from NaBH4

H

from H2O

Al

H

H HH

Hydride addition

Li

" H "H3C CH3

O

C

O

CH3H3C

H C

O

CH3H3C

H

Al

H

H

H

H2O

OH

C CH3

H

H3C

9.13 Oxidation of Aldehydes

R-C-H

Ooxidising

agentR-C-OH

O

carboxylic acid

e.g.

CH3(CH2)5CHOJones'

reagentCH3(CH2)5CO2H

Heptanal Heptanoic acid

Ag2O - selective oxidising agent for aldehyde carboxylic acid

CH3-C-H

OAg2O

CH3-C-O Ag

O

Silver carboxylate (acetate)

H3OCH3-C-OH

O

CHOAg2O

CO2 AgH3O

CO2H

Alkene- not affected by Ag2O

Ketones- much less easily oxidised than aldehydes as there is noH on the carbonyl carbon

C O

R

R

C O

R

H

Easily oxidised

Difference in reactivity towards oxidation used as a test to distinguishaldehydes and ketones

Tollen's Silver Mirror Test

R-C-H

O

+ 2 Ag(NH3)2 + 3 OH R-C-O

O

+ 2 Ag + 4NH3 + 2H2O

Forms a silver mirror

9.14 Keto-enol tautomerism

Aldehydes + Ketones exist predominately in the familiar keto form

R R

O However there is a minor form which exists in equilibriumwith the keto form but in very low concentration- known as the enol form

RH

O

R

OH

Keto form Enol form

Structural isomers known as tautomers:

Enol tautomerKeto tautomer

CH3CH

O

H

Keto tautomer99.9997 %

Enol tautomer0.0003 %at equilibrium

H

CH2=C-CH3

OH

Despite the minor concentration of enol form present at equilibrium this tautomer can be very important in the reactivity of aldehydes and ketones

CH3-C-H

O

Keto

CH2=C-H

OH

Enol

Tautomers of ethanal