Carbonyl Compounds

Carbonyl Compounds (Chapter 35)

Carbonyl compounds C

O

Carbonyl group sp2 hybridized carbon Coplanar σ bonds, 120o bond angle p-p overlap π bond

Two types of compounds

H

Aldehyde

C R

O

R’ Ketone

C R

O

Polarity of carbonyl bond Dipole moment propan-1-ol propanone ethanal

Cσ+

Oσ-

1.69D 2.88 2.72

σ-bond

-ve inductive effect

π-bond

readily polarizable (mesomeric effect)

Cσ+

Oσ-

C+

O:-

Chemical reaction H C

O H-Br

C+ O :Br-

No electrophilic additions (why?)

C OH Br

Chemical reaction No electrophilic additions (why?)

Energetic Factor

C OH Br (Unstable)

∆H

Big +ve, unstable product

Chemical reaction No electrophilic additions (why?) Big +ve Ea , unstable Transition state

Kinetic Factor

C+ OH (Unstable)

Ea

Chemical reaction No nucleophilic substitution

Nu-:

C

O

C ONu

Nucleophilic Addition H/R’

H/R’ Nu -:

C

R C

O

R

O-

Nu H/R’ R C CN

E

E+

Reactivity: 2. Electrophilicity of C atom of the C=O group. • Strength of Nu:• Steric effect at the carbonyl group

Addition of HCN H/R’

H/R’ C R

O

+ H-CN

R C

OH

CN

Reactivity: HCHO > CH3CHO > ArCHO > CH3COCH3 > CH3COR > RCOR > ArCOAr

Nucleophilic Addition Addition of HCN (using KCN+H+) H/R’ CN : -

C

slow

O

R H/R’ fast

Cyanohydrin (2-hydroxynitrile)

R C CN

OH

H/R’ R C CN HCN

O-

Note: HCN is NOT used. Because 5. Toxic 6. Weak acid little CN-

Usefulness in organic synthesis H/R’

H/R’ R C

OH

H2O,H+ reflux

CN c.H2SO4,heat

-C=C-COOH (α,β-unsaturated acid)

R C

OH

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

Addition of sodium hydrogensulphate(IV) C-S bond is formed as S is more nucleophilic than O

R’ C

..

Na+ HSO3-

Room temp.

O

R R’

Sodium hydrogensulphate(IV R C OH ) adduct, isolated as SO3- Na+ colourless crystals

R’ R C

+ Na O

SO3H

-

Addition of sodium hydrogensulphate(IV) •Limit to aliphatic aldehydes and sterically unhindered ketones (steric effect) % product from 1mol NaHSO3 in 1 hour: CH3 H

C=O

C2H5

CH3 C=O

CH3 89% 56% (CH3)2CH (CH3)3C C=O C=O CH3 CH3 12% 6%

=O

C=O CH3 36% C2H5 C=O C2H5 2%

35% Ph C=O 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- => SO32-) •Use to purify liquid or gaseous carbonyl compounds which are difficult to purify by direct recrystallization.

Addition-elimination (condensation) H HO N:

R’ C

O

R H

H

HO N+ C H

(Hydroxylamine)

H HO N

R’ C R

OH

-H2O

R’ R

R’ HO N (Oxime)

C

R

O-

Addition-elimination R’ NO2

..

..

NH-NH2

C

O

NO2

R

NO2 2,4-dinitrophenylhydrazine (Brady’s reagent)

..

..

NH-N=C

R’ R

NO2 2,4-dinitrophenylhydrazone (yellow or light orange crystals)

Phenylhydrazone •Products have sharp and characteristic melting point. •Used as the identification of the original aldehyde and ketone Note: • NH3 does not react • Predict the product obtained by adding H2N-NH2 to propanal.

Oxidation • KMnO4/H+ , K2Cr2O7/H+ (Strong oxidizing agent) RCHO => RCOOH RCH2COCH2R’ => RCOOH + R’ CH2COOH + RCH2COOH + R’COOH C6H5CHO => C6H5COOH requiring reflux for hours

Oxidation 1. 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 1. 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-.

R

R

C O H/R H-

C H/R

OH

H

+

R

OH

C H/R

H

Reduction LiAlH4 must be kept dry i.e. in solution of dry ether LiBH4 is less powerful, can be used in aqueous solution. Reducing agent: H2/Ni, similar to alkene R C H/R

O

H2/Ni

RCH2OH

Triiodomethane reaction CH3 C H/R

O

X2

X2

-X

-

OH

-

-X-

-H2O

CX3 C H/R

CH2 C H/R -

OH-

O

CX3- + R/HCOOH

O

CH2X C O H/R

X2 -X-

CX3 HO C

O-

H/R CHX3 + RCOO-

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