General Methods of Preparation of Aliphatic Aldehydes & Ketones

INTRODUCTION

Both aldehydes & ketones contain carbonyl group as their functional group.

Structure of carbonyl group

Both aldehydes & ketones have carbonyl group as the functional group. The carbonyl carbon is sp² hybridised & it uses sp² hybrid orbitals to form 3$\sigma$ bonds, one with oxygen atom & remaining 2 with two other atoms or groups (R or H). All these 3$\sigma$ bonds lie in same plane at the angle of 120º.

The unhybridized p – orbital of carbonyl carbon form $\pi$ - bond with oxygen atom by sidewise overlapping with half filled

p – orbital of oxygen atom.

Since carbon & oxygen have different values of electronegativity, the bond between carbon & oxygen is polar. Infact electron density around the oxygen atom is increased which causes the development of partial positive charge ($\delta$+) on carbon & partial negative charge ($\delta$-) on oxygen.

Thus the carbonyl carbon is an electrophilic & carbonyl oxygen is nucleophilic centre.

Orbital picture of carbonyl group

Illustration 1. Give the IUPAC name for

(i) CH₃CHO

Solution: (i) Ethanal

(ii) 2, 2 – diphenyl ethanal

GENERAL METHODS OF PREPARATION OF ALIPHATIC ALDEHYDES & KETONES

1. From Alcohols

(a) By Direct oxidation: Aldehydes are prepared by oxidation of 1º alcohols. Refer to Alcohols

(b) By catalytic dehydrogenation

When vapours of 1º or 2º alcohols are passed over copper gauze, they get dehydrogenated to form aldehydes or ketones.

$\underset{n-propylalcohol}{C{H}_{3}C{H}_{2}C{H}_{2}OH}\stackrel{Cu/573K}{\to }\underset{\mathrm{Pr}opionaldehyde}{C{H}_{3}C{H}_{2}CHO}+H_2$

Dehydrogenation reaction is a better method of preparation because there is no risk of further oxidation of aldehyde.

(c) By using PCC

PCC stands for pyridinium chlorochromate. It is an equimolar mixture of CrO₃, HCl and pyridine. It is used to oxidise 1° alcohol to aldehyde and 2° alcohol to ketones without affecting double or triple bond.

Illustration 2. Complete the following reaction by writing down the major product:

Solution:

2. From Acid chlorides

Aldehydes are prepared from acid chlorides by reaction with H₂ in the presence of palladium catalyst supported on BaSO₄.

Ketones are obtained by reacting acid chlorides with dialkyl cadmium.

$2RCOCl+CdR{{}^{\prime }}_2\to 2RCO{R}^{\prime }+CdC{l}_2$

Illustration 3. Prepare ethanal by reduction method.

Solution:

3. From fatty acids

(a) By heating calcium salt of fatty acid

Aldehydes are obtained by heating calcium salt of fatty acids with calcium formate.

Ketones are formed by distilling calcium salt of fatty acids alone.

Similarly mixed ketones, can also be obtained by similar reactions:

(b) By passing vapours of fatty acids over manganese oxide

In this method, formic acid alone gives formaldehyde. Acetic acid gives acetone & the mixture of two acids gives acetaldehyde.

$2HCOOH\underset{575K}{\overset{MnO}{\to }}HCHO+H_{2}O+C{O}_2$

$2C{H}_{3}COOH\underset{575K}{\overset{MnO}{\to }}C{H}_3-\stackrel{\stackrel{O}{||}}{C}-C{H}_3+H_{2}O+C{O}_2$

$C{H}_{3}COOH+HCOOH\underset{575K}{\overset{MnO}{\to }}C{H}_{3}CHO+H_{2}O+C{O}_2$

4. From Alkynes: (Refer to hydrocarbons)

5. By reductive ozonolysis of alkenes: (Refer to hydrocarbons)

Illustration 4. What happens when (give equation only)?

(i) Ethyne is treated with dilute H₂SO₄ in the presence of HgSO₄.

(ii) propan-2-ol is treated with Cu at 573 K.

Solution: (i) $CH\equiv CH\underset{HgS{O}_4}{\overset{dil.H_{2}S{O}_4}{\to }}C{H}_{3}CHO$