Biological ChemistryFIRST YEAR ORGANIC CHEMISTRY

Lecture SevenAldehydes & Ketones

Convenor : Dr. Fawaz Aldabbagh

Aldehydes and Ketones

O 1s2, 2s2 2p2 2p1 2p1

3 sp2 orbitals

CH

RO C

R

RO

Aldehyde Ketone

CH

HO

lone Pairs

bond - two overlapping 2p orbitals

bond bond- overlapping 1s H-orbital and sp2 C-orbital

C OH3C

H118o

121oC C

H

H

H

H118o

121o

C OH3C

HC O

H3C

HResonance Structures

Most Reactive Group –

electrons + polarisation

Useful in Synthesis

Names al – aldehydes, one - ketones

C OH

HC O

H3C

HC O

CH3CH2

HC O

CH3CH2CH2CH2

H

Methanal(formaldehyde)

Ethanal(acetaldehyde)

Propanal Pentanal

OH

H O

H

H

Benzaldehydetrans-Cinnamaldehyde

Formalin, 35-40% formadehyde in waterPreservative that reacts with proteins causing them to resist decayCoelacanth, “prehistoric fish”

OH

Acrolein (2-propenal)

S

HThiopropionaldehyde (propanethiol)

- lachrymator from chopped onion- lachrymator and pleasant "odour" from barbacuing meat

Propanone(ACETONE)

Butanone

AcetophenoneBenzophenone

O

H3C CHCH2

CH3

CH3

3-Methyl-2-pentanone

O H

OCH3

OH

Vanillin

CH3

O

Carvone(spearmint flavour)

O O

H3C CH3

Butadione(butter flavour)

O

H3C CH3

O

CH3

O

O

H3C CH2

CH3

Carbonyls readily undergo Nucleophilic Attack

cis and trans isomers possible

- H2OC O N N

H

H

H

H+ C N N

H

H

Hydrazine Hydrazone

ANHYDROUS

OC

RNH2

OCNHH

R

OCNH R

H

OCNH R

H

CN

R

- H2O

Imine

The reaction is acid-catalysed

H+ + H2O Equilibrium Reactions

Other Condensation ReactionsR

CH

O H3C OH

H3C OH

H+

+R

CH

OCH3

OCH3- H2O

Dimethoxy ACETAL

C O + NNHH

HNO2

O2N

NNH

NO2

O2N

H

H3C

2,4-Dinitrophenylhydrazine

2,4-DinitrophenylhydrazoneAcetaldehyde-

Acid-Catalysed Acetal Formation

Reversed with acid and water

Acetal is stable under basic conditions

O

H R

H+

O

R

H

OHH3C

OCH RO

H

H CH3

OCH RO

H

CH3Hemiacetal

- H2O

- H+

OCH3COCH3

H

R

OCH RO

H

CH3

H

OHH3CH

- H+

OCH3COCH3

H

RAcetal

+ H+

Addition of Hydrogen Cyanide

OOH

CN

NaCN + NH4Cl NaCl + NH3 + HCN

cyclohexanone

Nucleophilic addition reactions

OC

R H+ HCN

HC

R

OH

CN

C O

-CN

CyanohydrinHC

R

O

CNH CN

Professor Victor Grignard (1912 Nobel Prize)Developed this chemistry with Professor P. A. Barbier

CR OH

ProtonationH

H2OAlcohol

CR X

H

H

X = I or Br

CH

HMgXR RCH2

MgX

Grignard Reagent

Mg

Ether

CR X

H

H

X = I or Br

CH

HLiR RCH2

Li

Organolithium Reagent

Li

Ether

C O

R Li

CR O LiADDITION

Examples MgBr

C OH

H

EtherCH

HO MgBr

H3O+

CH

HO H

BenzylalcoholC O MgBr+

Ether

C OH

Triphenylmethanol

Benzyl GroupPhenyl, Ph Group

Ph

2. H3O+

-Hydrogens next to Carbonyl Groups are relatively acidic

OC

R CH

C

H

Hydrogen, pKa = 19-20

Hydrogen, pKa = 40-50

Keto-Enol TautomerismThe negative charge is delocalisedO OH

98.8%1.2%

O

H3C CH3

OH

H3C CH2

>99%

C=O bond stronger than C=C bond so equilibrium lies towards keto-tautomerEnol formation can be acid or base catalysed

OC

R CH

B

OC

R C

OC

R C

- HB

Resonance StabilizedOC

RC

OC

R CH

B

- HBOC

R C

H

Enol