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 of compounds Aldehyde Ketone C O R R C O H R

Polarity of carbonyl bond Dipole moment propan-1-ol1.69D propanone2.88 ethanal2.72 C + O - -bond -ve inductive effect -bond readily polarizable (mesomeric effect) C + O - C+C+ O: -

Chemical reaction No electrophilic additions (why?) C O H-Br C+C+ O H :Br - COH Br

Chemical reaction No electrophilic additions (why?) COH Br H Big +ve, unstable product Energetic Factor (Unstable)

Chemical reaction No electrophilic additions (why?) EaEa Big +ve E a, unstable Transition state Kinetic Factor (Unstable) C+C+ OH

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

Nucleophilic Addition C O Nu - : 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 CO-O- Nu H/R R E+E+ CE CN H/R R

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

Nucleophilic Addition Addition of HCN (using KCN+H + ) C O CN - : CO-O- CN H/R R HCN R COH CN H/R R Cyanohydrin (2-hydroxynitrile) Note: HCN is NOT used. Because 1.Toxic 2.Weak acid little CN - slow fast

Usefulness in organic synthesis COH CN H/R R H 2 O,H + reflux COH COOH H/R R c.H 2 SO 4,heat -C=C-COOH (, -unsaturated acid) +1 carbon (longer carbon chain) 2 functional group

Addition of sodium hydrogensulphate(IV) CO-O- SO 3 H R R Room temp. Na + COH SO 3 - Na + R R Sodium hydrogensulphate(IV) adduct, isolated as colourless crystals C O R R Na + HSO 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 NaHSO 3 in 1 hour: C=O CH 3 H 89% C=O CH 3 56% C=O CH 3 C2H5C2H5 36% =O 35% C=O (CH 3 ) 2 CH CH 3 12% C=O (CH 3 ) 3 C CH 3 6% C=O C2H5C2H5 C2H5C2H5 2% C=O Ph CH 3 1%

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

Addition-elimination (condensation) C O R R N:HO H H (Hydroxylamine) HON+N+ CO-O- H H R R NCOH HR R -H 2 O HONC R R (Oxime)

Addition-elimination C O R R NH-NH 2.. NO 2 2,4-dinitrophenylhydrazine (Bradys reagent) NH-N=C.. NO 2 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.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.Tollens reagent (silver mirror test) Reagent: 2Ag + + 2OH - => Ag 2 O + H 2 O Ag 2 O + 4NH 3 + H 2 O => 2Ag(NH 3 ) 2 OH 2[Ag(NH 3 ) 2 ] + + RCHO + 3OH - => RCOO - +2H 2 O + 4NH 3 + 2Ag (mirror) No reaction with ketone (Tollens reagent is a mild O.A.)

Oxidation 3.Fehlings reagent Reagent: alkaline solution of copper(II) tartrate RCHO + 2Cu OH - => 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 CO H/R H-H- RO-O- C H ROH C H/R H H+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 CO H/R H 2 /Ni RCH 2 OH

Triiodomethane reaction (RCOCH 3 + I 2 /OH - => RCOO - + CHI 3, yellow ppt.) CH 3 CO H/R OH - -H 2 O - CH 2 CO H/R X2X2 CH 2 X CO H/R -X - CX 3 CO H/R CX 3 HOCO-O- H/R CX R/HCOOHCHX 3 + RCOO - X2X2 -X - X2X2 OH -