Engineering Chemistry S Datta

MECHANISM OF ORGANIC REACTIONS 171
SHORT QUESTIONS AND ANSWERS
Q. 1. Why is the RX + KOH → ROH + KX called nucleophilic substitution?
Ans. Since the attacking reagent (OH – ) is a nucleophilic reagent, and substitution of X –
takes place by OH – , i.e., substitution of weakly basic group takes place by a strongly basic
group.
Q. 2. How can you ascertain that X – is a weakly basic group and OH – is a strongly
basic group?
Ans. X – is a weak base as its conjugate acid HX is a strong acid; and OH – is a strong base
as its conjugate acid H 2
O is very weak.
Q. 3. What are nucleophilic reagents?
Ans. Nucleophilic reagents can supply an electron pair. Obviously a nucleophilic reagent
attacks a molecule at the point of low electron density. Nucleophilic reagents may be negatively
charged viz. OH – , CN – etc. and neutral molecules like H 2
O, NH 3
with unshared pair of electrons.
Q. 4. Rate equations suggest that there are two different mechanisms for
nucleophilic substitution—explain.
Ans. Hydrolysis of primary halogenoalkane viz. bromobutane is a second order reaction.
The rate equation is,
R = K[C 4
H 9
Br] [OH – ]
The suggested mechanism is:
H
H
CH 3
CH 3
Br
HO C
H:O: :C
:
H
H
+ : Br:
In S N
2
reaction two molecules are involved in the rate determining step.
Hydrolysis of tertiary halogenoalkanes viz. 2-bromo 2-methylpropane is first order. Rate
equation for the reaction is:
Rate = K[C 4
HgBr]
The suggested mechanism is:
:
:
–
CH CH 3 3
Slow
C Br
[Rate determining
step]
CH 3
CH 3
+
C
CH 3
CH 3
+Br –
fast OH –
CH 3
CH 3
CH 3
C
OH