Carbonyl Compounds (Chapter 35). Carbonyl compounds C O Carbonyl group sp 2 hybridized carbon Coplanar bonds, 120 o bond angle p-p overlap bond Two types.

Carbonyl Compounds

(Chapter 35)

Carbonyl compounds

C O Carbonyl group

sp2 hybridized carbonCoplanar bonds, 120o bond anglep-p overlap bond

Two types of compounds

Aldehyde Ketone C O

R’

RC O

H

R

Polarity of carbonyl bondDipole moment

propan-1-ol 1.69Dpropanone 2.88ethanal 2.72

C+ O-

-bond -ve inductive effect

-bond readily polarizable (mesomeric effect)

C+ O- C+ O:-

Chemical reaction

No electrophilic additions (why?)

C O

H-Br

C+ OH

:Br-

C OH

Br

Chemical reaction

No electrophilic additions (why?)

C OH

Br

H

Big +ve,unstableproduct

Energetic Factor

(Unstable)

Chemical reaction

No electrophilic additions (why?)

Ea

Big +ve Ea ,unstableTransitionstate

Kinetic Factor

(Unstable)

C+ OH

Chemical reaction

No nucleophilic substitution

C ONu-:

C O-

Nu

Nucleophilic Addition

C ONu -:

H/R’

R

Reactivity:1. Electrophilicity of C atom of the C=O group.2. Strength of Nu:-

3. Steric effect at the carbonyl group

C O-

Nu

H/R’

R

E+

C E

CN

H/R’

R

Addition of HCN

C O

H/R’

R

+ H-CN C OH

CN

H/R’

R

Reactivity: HCHO > CH3CHO > ArCHO > CH3COCH3

> CH3COR > RCOR > ArCOAr

Nucleophilic Addition

Addition of HCN (using KCN+H+)

C OCN-:

C O-

CN

H/R’

R

H/R’

HCN

R

C OH

CN

H/R’

RCyanohydrin(2-hydroxynitrile)

Note:HCN is NOTused.Because1. Toxic2. Weak acid little CN-

slow

fast

Usefulness in organic synthesis

C OH

CN

H/R’

R H2O,H+

refluxC OH

COOH

H/R’

R

c.H2SO4,heat

-C=C-COOH(,-unsaturated acid)

•+1 carbon (longer carbon chain)•2 functional group

Addition of sodium hydrogensulphate(IV)

C O-

SO3H

R’

RRoom temp.

Na+

C OH

SO3- Na+

R’

RSodium hydrogensulphate(IV) adduct, isolated as colourless crystals

C O

R’

RNa+ HSO3-

..

C-S bond is formed as S is more nucleophilic than O

Addition of sodium hydrogensulphate(IV)

•Limit to aliphatic aldehydes and sterically unhindered ketones (steric effect)

% product from 1mol NaHSO3 in 1 hour:

C=OCH3

H89%

C=O

CH3

CH3

56%

C=OCH3

C2H5

36%

=O

35%

C=O

(CH3)2CH

CH312%

C=O

(CH3)3C

CH3 6%

C=O

C2H5

C2H5 2%

C=OPh

CH3 1%

Addition of sodium hydrogensulphate(IV)

•Reversible (can be reversed by aq. Alkali or acid by shifting eqm. position to LHS by HSO3

- + H+=> SO2 , HSO3- + OH- => SO3

2-)

•Use to purify liquid or gaseous carbonyl compounds which are difficult to purify by direct recrystallization.

Addition-elimination (condensation)

C O

R’

RN:HO

H

H(Hydroxylamine)

HO N+ C O-

H

H

R’

R

HO N C OH

H R’

R

-H2OHO N C

R’

R(Oxime)

Addition-elimination

C O

R’

RNH-NH2

....

NO2

NO2

2,4-dinitrophenylhydrazine(Brady’s reagent)

NH-N=C....

NO2

NO2

2,4-dinitrophenylhydrazone(yellow or light orange crystals)

R’

R

Phenylhydrazone

•Products have sharp and characteristic melting point.•Used as the identification of the original aldehyde and ketone

Note:1. NH3 does not react2. Predict the product obtained by adding H2N-NH2 to propanal.

Oxidation

1. KMnO4/H+ , K2Cr2O7/H+ (Strong oxidizing agent)

RCHO => RCOOH RCH2COCH2R’ => RCOOH + R’ CH2COOH + RCH2COOH + R’COOH C6H5CHO => C6H5COOH requiring reflux for hours

Oxidation

2. Tollen’s reagent (silver mirror test)

Reagent: 2Ag+ + 2OH- => Ag2O + H2O Ag2O + 4NH3 + H2O => 2Ag(NH3)2OH

2[Ag(NH3)2]+ + RCHO + 3OH-

=> RCOO- +2H2O + 4NH3 + 2Ag (mirror)

No reaction with ketone(Tollen’s reagent is a mild O.A.)

Oxidation

3. Fehling’s reagent

Reagent: alkaline solution of copper(II) tartrate

RCHO + 2Cu2+ + 5OH- => RCOO- + 3H2O + Cu2O (Fehling) (brick-red)

Note: No reaction with Ketones and Aromatic Aldehydes

Reduction

Reducing agent: LiAlH4 Lithium Tetrahydridoaluminate NaBH4 Sodium Tetrahydridoborate

Both equivalent to a source of hydride ion, H-.

RC O

H/RH-

R O-

CH/R H

R OHC

H/R H

H+

Reduction

LiAlH4 must be kept dryi.e. in solution of dry ether

LiBH4 is less powerful,can be used in aqueous solution.

Reducing agent: H2/Ni, similar to alkene

RC O

H/R

H2/NiRCH2OH

Triiodomethane reaction

(RCOCH3 + I2/OH- => RCOO- + CHI3 ,yellow ppt.)

CH3

C OH/R

OH-

-H2O

-CH2

C OH/R

X2CH2X

C OH/R

-X-

CX3

C OH/R

CX3

HO C O-

H/R

CX3- + R/HCOOH CHX3 + RCOO-

X2

-X-

X2

-X-

OH-