Carbonyl Compounds

Carbonyl compounds C O C O R’ R H Carbonyl group sp2 hybridized carbon Coplanar  bonds, 120o bond angle p-p overlap  bond Two types of compounds Aldehyde Ketone C O R’ R H

Polarity of carbonyl bond Dipole moment propan-1-ol 1.69D propanone 2.88 ethanal 2.72 C+ O- -bond -ve inductive effect -bond readily polarizable (mesomeric effect) C+ O:-

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

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

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

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

Nucleophilic Addition H/R’ R C O Nu -: H/R’ R E+ C E CN H/R’ R Reactivity: Electrophilicity of C atom of the C=O group. Strength of Nu:- Steric effect at the carbonyl group

Addition of HCN C O H/R’ R + H-CN OH CN Reactivity: HCHO > CH3CHO > ArCHO > CH3COCH3 > CH3COR > RCOR > ArCOAr

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

Usefulness in organic synthesis H2O,H+ reflux C OH COOH H/R’ R C OH CN 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-S bond is formed as S is more nucleophilic than O C O- SO3H R’ R Room temp. Na+ C O R’ R Na+ HSO3- .. C OH SO3- Na+ R’ R Sodium hydrogensulphate(IV) adduct, isolated as colourless crystals

Addition of sodium hydrogensulphate(IV) Limit to aliphatic aldehydes and sterically unhindered ketones (steric effect) % product from 1mol NaHSO3 in 1 hour: C=O CH3 H 89% C=O CH3 56% C=O CH3 C2H5 36% =O 35% C=O (CH3)2CH CH3 12% C=O (CH3)3C CH3 6% C=O C2H5 2% C=O Ph 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) R’ R N: HO H (Hydroxylamine) HO N+ C O- H R’ R HO N C OH H R’ R -H2O HO N C R’ R (Oxime)

Addition-elimination C O R’ R NH-N=C .. NO2 2,4-dinitrophenylhydrazone (yellow or light orange crystals) R’ R NH-NH2 .. NO2 2,4-dinitrophenylhydrazine (Brady’s reagent)

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 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 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 C O H/R R O- C H/R H R OH C H/R H H+ 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 O H/R H2/Ni RCH2OH

Tri-iodomethane reaction CH3 C O H/R OH- -H2O -CH2 X2 CH2X -X- CX3 HO O- CX3- + R/HCOOH CHX3 + RCOO- (RCOCH3 + I2/OH- => RCOO- + CHI3 ,yellow ppt.)