Preparation of acetanilide - PowerPoint Presentation

Slide1

Preparation of acetanilide

This

is by acetylation of aniline.

Acetanilide

is the acyl derivative of the primary aromatic amine, aniline.

It

is prepared when aniline in its

HCl

salt form is treated with acetic anhydride and anhydrous sodium acetate, stirring, then upon cooling the product (acetanilide) will solidify as glistening plates.

We

use acetic anhydride for acetylation instead of other nucleophiles donating because acetic anhydride is cheap, readily available, easily handles and do not form corrosive

HCl

gas although the reaction is moderate.

Acetylation

is done before

chlorosulfonation

because aniline can’t be

monosubstituted

at p-position only because it is very reactive, e.g. aniline nitration give

trinitroaniline

, so we can’t get

monosubstituted

sulfonamide.

In aniline Electrophilic

substitution occurs on o- and

p-position

but in

Acetanilide ,

In part of time, electrons are shared with the ring and sometimes, they are shared with C of carbonyl which

is cause

electron deficient due to

polarization.

Also, in order to decrease activity of aromatic ring (deactivation of the aromatic ring) by decreasing electronic density on the ring particularly on the 2-ortho position. This is done by acetylation of aniline where the acetyl group will exert inductive effect and steric hindrance effect, then

monosubstitution

on the p-position will be obtained.

Mechanism of the reaction (role of

HCl

):

HCl acid is added to solubilize aniline at the beginning since aniline is water-insoluble, so we add HCl to get anilinium

chloride which is water-soluble.HCl is used to activate acetic anhydride since there is no reaction between aniline and acetic anhydride.

In

this case we need aniline in the free form (i.e., the 2 electrons of N are free), and in order to obtain free aniline to attach to acetic anhydride to give active acetanilide, we must liberate it by the addition of weak acid salt (anhydrous sodium acetate) which liberates aniline, then the 2 electrons of N are free to attach.

Properties of acetanilide:

It crystallizes in colorless glistening plates.

It is sparingly soluble in water, easily soluble in alcohol to give neutral solution.

Hot dilute acids or alkalis slowly hydrolyze acetanilide to aniline and acetic acid or their salts.

Melting

point is 114



C.gives yellowish to white precipitate of p-bromine acetanilide solution when react with bromin

. This test is given to distinguish between acetanilide and phenacetin

and

paracetamol

.

Procedure:

Add 4.9 ml aniline to 135 ml H

2

O + 4.5 ml conc. HCl.In another small beaker dissolve 0.5 g of anh

. sod. acetate with 30 ml H2O, add this to the aniline mixture.

Add 6.5 ml acetic anhydride gradually.

Mix and cool for 5 min. until white glistening plates of acetanilide appear.

Filtrate.If we want recrystallization of acetanilide use hot water or mixture of water/alcohol.

Calculate % of yield.