Carbonyl Compounds

Carbonyl compounds C O Two types of compounds H R Carbonyl group sp 2 hybridized carbon Coplanar  bonds, 120 o p-p overlap  bond bond angle R’ R C O

Polarity of carbonyl bond Dipole moment propan-1-ol propanone ethanal C  + O  1.69D

2.88

2.72

 -bond -ve inductive effect  -bond readily polarizable (mesomeric effect) C  + O  C + O: -

Chemical reaction C O H-Br C + O :Br H 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?) Kinetic Factor C + OH (Unstable) E a Big +ve E a unstable , Transition state

Chemical reaction Nu : No nucleophilic substitution C O C O Nu

Nucleophilic Addition H/R’ Nu : R C O H/R’ R C E CN H/R’ R C O Nu E + Reactivity: 1. Electrophilicity of C atom of the C=O group.

2. Strength of Nu: 3. Steric effect at the carbonyl group

Addition of HCN H/R’ R C O + H-CN H/R’ R C OH CN Reactivity: HCHO > CH 3 CHO > ArCHO > CH 3 COCH 3 > CH 3 COR > RCOR > ArCOAr

Nucleophilic Addition Addition of HCN (using KCN+H + ) H/R’ slow H/R’ CN : C O R C O R CN H/R’ fast HCN Cyanohydrin (2-hydroxynitrile) R C CN OH Note: HCN is NOT used.

Because 1. Toxic 2. Weak acid little CN -

Usefulness in organic synthesis R H/R’ C OH H 2 O,H + reflux CN c.H

2 SO 4 ,heat H/R’ R C OH COOH ( -C=C-COOH  ,  -unsaturated acid) •+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’ Room temp.

R’ Na + ..

HSO 3 Sodium R C O R’ R C SO 3 O H hydrogensulphate(IV) R C OH adduct, isolated as colourless crystals SO 3 Na + Na +

Addition of sodium hydrogensulphate(IV) •Limit to aliphatic aldehydes and sterically unhindered ketones (steric effect) % product from 1mol NaHSO 3 in 1 hour: (CH 3 CH 3 ) 2 H C=O 89% CH C=O CH 3 12% CH 3 CH 3 C=O 56% (CH 3 ) 3 C CH 3 C=O 6% C 2 H 5 CH 3 C 2 H 5 C=O C 2 H 5 36% C=O 2% 35% =O Ph CH 3 C=O 1%

Addition of sodium hydrogensulphate(IV) •Reversible (can be reversed by aq. Alkali or acid by shifting eqm. position to LHS by HSO 3 + H + => SO 2 , HSO 3 + OH => SO 3 2 ) •Use to purify liquid or gaseous carbonyl compounds which are difficult to purify by direct recrystallization.

Addition-elimination (condensation) R’ H HO N: H (Hydroxylamine) R C O H HO N + R’ C O H R H HO N R’ C OH -H 2 O R R’ HO N (Oxime) C R

Addition-elimination NO 2 R’ ..

..

NH-NH 2 R C O NO 2 ..

..

NH-N=C R’ R NO 2 2,4-dinitrophenylhydrazine (Brady’s reagent) NO 2 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: 1. NH 3 does not react 2. Predict the product obtained by adding H 2 N-NH 2 to propanal.

Oxidation 1. KMnO 4 /H + , K 2 Cr 2 O 7 /H + (Strong oxidizing agent) RCHO => RCOOH RCH 2 COCH 2 R’ => RCOOH + R ’ CH 2 COOH + RCH 2 COOH + R ’ COOH C 6 H 5 CHO => C 6 H 5 COOH requiring reflux for hours

Oxidation 2. Tollen’s reagent (silver mirror test) Reagent: 2Ag + + 2OH Ag 2 O + 4NH 3 => Ag 2 O + H 2 O + H 2 O => 2Ag(NH 3 ) 2 OH 2[Ag(NH 3 ) 2 ] + => RCOO + RCHO + 3OH +2H 2 O + 4NH 3 + 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 + 2Cu 2+ + 5OH => RCOO + 3H 2 O + Cu 2 O (Fehling) (brick-red) Note: No reaction with Ketones and Aromatic Aldehydes

Reduction Reducing agent: LiAlH 4 Lithium Tetrahydridoaluminate NaBH 4 Sodium Tetrahydridoborate Both equivalent to a source of hydride ion, H .

R C H/R H O R H/R C O H H + R H/R C OH H

Reduction LiAlH 4 must be kept dry i.e. in solution of dry ether LiBH 4 is less powerful, can be used in aqueous solution.

Reducing agent: H 2 /Ni, similar to alkene R C H/R O H 2 /Ni RCH 2 OH

Tri-iodomethane reaction CH 3 C H/R X 2 -X O X 2 -X OH -H 2 O CX H/R CX 3 3 C O CH 2 C H/R OH O HO X 2 -X CH 2 X C H/R O CX 3 C O + R/HCOOH H/R CHX 3 + RCOO (RCOCH 3 + I 2 /OH => RCOO + CHI 3 , yellow ppt.)