Question

$\left(a\right)$ Define saponification equivalent $(S.E.)$ of an ester.
$\left(b\right)$ When is $S.E.$ equal to molecular weight of an ester?
$\left(c\right)$ What is the $S.E.$ of diethyl fumarate?
$\left(d\right)$ Identify $\left(A\right)$ as methyl or ethyl cinnamate from the following data :
$0.75gm$ of $\left(D\right)$ is refluxed with $100.0ml$ of $0.125MKOH$, and the excess $KOH$ is neutralised by back titrating with $27.5ml$ of $0.3MHCl$.

Answer 1

(a) S.E is the weight of an ester that reacts with $1mol$ of $KOH$

(b) If the ester is derived from monocarboxylic acid, its $Mw$ and $S.E$ are identical

(c) If it is diester, then
S.E$=\frac{Mw}{2}=\frac{172.0}{2}=86gm.$ ${eq}^{-1}$

Diethyl fumarate

$C_8{H}_{12}{O}_4=96+12+64\underset{CHCOO{C}_2{H}_5}{\stackrel{CHCOO{C}_2{H}_5}{||}}$

$=172$

(d) Here $m$ moles of $KOH$ used $=100\times 0.125=12.5$
Excess of $KOH=27.5\times 0.3=8.25$
$KOH$ used $=12.5-8.25=4.25m$ mol

$=\frac{4.25}{1000}=0.00425mol.$

Equivalent weight of ester $=0.00425mol$

Since it is monocarboxylic ester $=0.00425\times 0.75=176gm.{mol}^{-1}$

So (A) is ethyl cinnamate $(PhCH=CHCOOEt)$ (since its $Mw=176$) rather than methyl cinnamate $PhCH=CHOOMe$ whose $Mw=162$