7.6.4      Aldehydes and Ketones

 

General Formula   CnH2nO      n ³ 1

Nomenclature

ALKANALS           Aldehydes    R¾C=O

                                                      ½  

                                                      H

 

Name

Molecular formula

Structural formula

b.pt.  /oC

Density /g cm-3

 

methanal

 

CH2O

 

   

 H¾C=O

       ½

       H

 

-21

 

 

 

ethanal

 

C2H4O

     H 

      ½

H¾C¾C=O

      ½   ½

      H  H

 

20

 

 

 

propanal

 

C3H6O

      H   H 

      ½   ½  H¾C¾C¾C=O

      ½   ½    ½

      H  H    H

 

49

 

 

benzaldehyde

 

 

 

C7H6O

 

 

 

 

H¾C=O

      ½

 

178

 

 

ALKANONES        Ketones        R¾C=O

     ½

                                                     R’

 

Name

Molecular formula

Structural formula

b.pt.  /oC

Density /g cm-3

 

propanone

 

C3H6O

 

   

 CH3¾C=O

          ½

          CH3

 

56

 

 

 

butan-2-one

 

C4H8O

    

      CH3¾CH2¾C=O

                   ½  

                   CH3 

 

80

 

 

phenylethanone

 

C3H6O

 

    CH3¾C=O

             ½

 

 

 

202

 

 

 

Exercise 1

Write structural formula for:

(i)      butanal                  

(ii)     pentan-2-one

(iii)    pentan-3-one            

(iv)    cyclohexanone

(v)     3-methylhexanal           

(vi)    pentan-2,4-dione

(vii)  4-chloro-2-methylhexan-3-one

(viii) 4-hydroxybenzaldehyde

 

Exercise 2

Name the following compounds:

(i)       CH3CH2CH2CHO                 

(ii)      CH3CH2COCH3

(iii)     C2H5COC2H5            

(iv)    (CH3)2CHCHO

(v)     (CH3)2CHCOCH3           

(vi)             CH3CH=CHCHO

(vii)          CH3CH=CHCOCH3

 

Physical properties

Methanal HCHO is a gas, the rest are colourless liquids.  The simpler members are soluble in water.

 

 

Structure of the carbonyl group

 


C= O

 

The C=O is polarised because of the greater electronegativity of oxygen. Because of this, carbonyl compounds can

(i)                            undergo nucleophilic attack on the carbon atom and

(ii)                         electrophilic attack on the oxygen (this is only important when the electrophile is H+ )

 

The carbonyl group resembles the alkene group in that both groups contain a σ bond and a π bond between the bonded atoms. One might expect that the π electrons in the C=O bond, like those in the C=C bond would react with electrophiles like HBr or Br2. This does not happen. The reason is that the oxygen atom in the carbonyl group is able to keep control of the π electrons and does not make them available for bonding to an electrophile.

          δ+  δ-

          C = O                                                  C = C

 

polar bond                                           non polar bond

electron deficient carbon is                 the double bond is attacked

attacked by nucleophiles                       by electrophiles

 

The rate of nucleophilic attack depends on the size of the δ+ charge on the carbon. Therefore aldehydes are more reactive than ketones.

    δ+  δ-                        δ+  δ-                    δ+  δ-

H¾C=O         >      CH3¾C=O    >        CH3¾C=O

      ½                              ½                           ½

      H                              H                           CH3

 

 

Reaction of  simple aldehydes and ketones with hydrogen cyanides and 2,4-dinitrophenylhydrazine.

 

Nucleophilic addition reactions of aldehydes and ketones

 

Addition of hydrogen cyanide H-CN

When hydrogen cyanide is added to a cold solution (10-20oC) of the carbonyl compound addition occurs.  To avoid the danger of working with the poisonous gas HCN, it is generated in the reaction mixture (‘in situ’) by the action of sulphuric acid on potassium cyanide.

                   H2SO4  +  KCN                 HCN(g)  +  KHSO4

 

Hydrogen cyanide is a weak acid

HCN   ¨   H+  +   C≡N- 

     (cyanide ion – a nucleophile)

The reaction is catalysed by a base to increase the concentration of CN- by moving the equilibrium to the right.

(i) Aldehyde

                                                                             CN

                                                                             ½

CH3 ¾C=O    +        HCN                               CH3 ¾C¾OH

          ½                                                                  ½

          H                                                                 H

ethanal                                                         2-hydroxypropanonitrile

 

CH3CHO  +  HCN                       CH3CH(OH)CN

                                                2o alcohol

 

 

 

(ii)  Ketone

                                                                             CN

                                                                             ½

CH3 ¾C=O    +        HCN                               CH3 ¾C¾OH

         ½                                                                   ½

         CH3                                                               CH3

propanone                                           2-hydroxy-2-methylpropanonitrile

 

(CH3)2CO  +  HCN                               (CH3)2C(OH)CN

 

 

Mechanism of the addition reaction of hydrogen cyanide and propanone viewed as nucleophilic addition.

 

Mechanism

HCN  ¨  H+  +  C≡N- 

 

 

                                                          CN                                    CN

    δ+    δ-                                                 ½               H+                               ½

CH3 ¾C=O                                CH3 ¾C¾O-                       CH3 ¾C¾OH

         ½                                               ½                                      ½

         CH3                                           CH3                                   CH3

                                                intermediate

CN-

The reaction is initiated by nucleophilic attack of the cyanide ion on the electron deficient carbon atom of the carbonyl group. The intermediate extracts a proton (H+) from the hydrogen cyanide, regenerating a cyanide ion, CN-

Condensation reactions of aldehydes and ketones

 

Condensation reactions involve

(i)                            addition of a compound followed by

(ii)                         the elimination of a small molecule (such as H2O, HCl)

 

The general reaction is

(i)

CH3                                                    CH3

½                                                                                 ½

C=O       +         H2 ¾X                       C=X    +    H2O

½                                                                                 ½

H                                                       H

 

(ii)

CH3                                                    CH3

½                                                                                 ½

C=O       +         H2 ¾X                       C=X    +    H2O

½                                                                                 ½

CH3                                                    CH3

 

Examples of H2X :

H2 ¾NOH             hydroxylamine

H2 N¾NH2           hydrazine

H2 N¾NH             phenylhydrazine

 ½

 

 

H2N¾NH              2,4-dinitrophenylhydrazine

           ½    NO2               (2,4-DNPH)

 

           ½

NO2

 

 

 

 

Reaction with 2,4-dinitrophenylhydrazine

CH3¾C=O    +        H2 ¾NNH                       CH3 ¾C=N¾NH     +        H2O

         ½                             ½     NO2                                ½     NO