Engineering Chemistry S Datta

MECHANISM OF ORGANIC REACTIONS 175
Ans.
CH 3
I > CH 3
Cl > CH 3
Br > CH 3
F
as bond dissociation energies (D) is of the order C—F > C—Cl > C—Br > C—I.
Thus C—I bond is readily broken and the reactivity of CH 3
I is greatest.
Q. 18. Aqueous KOH with RX gives alcohol whereas alcoholic KOH alkene—
explain.
Ans. In aqueous medium, S N
takes place,
RX + KOH ⎯⎯→ ROH + KX
whereas in alcoholic medium
C 2
H 5
OH + KOH C 2
H 5
O – K + + H 2
O.
The – OC 2
H 5
is strongly basic and preferentially help elimination.
–
OC2H5
RCH 2
CH 2
X ⎯⎯⎯→ RCH = CH 2
Q. 19. Why do haloalkanes give cyanides with KCN – and isocyanides with AgCN?
Ans. Both the reagents act as nucleophilic and release CN – ; CN – being ambident
nucleophile, there are two active sites in the CN –
–
..
–
C
..
≡N ←⎯→ C
..
= N
..
Ag + promotes R + formation from RX by precipitating AgX and reaction takes S N
1
path.
R – X + Ag + [CN – slow
] ⎯⎯→ Ag X↓ + R + + [CN – fast
] ⎯⎯→ R – N + ≡ C !
In the absence of such promotion by Ag + e.g. with Na + [CN – ] the S
2
N
reaction is found
N ≡ C – + R – X ⎯→ [N ≡ C---R---X] → N ≡ C ≡ R + X –
T.S.
Q. 20. What do you mean by leaving group?
Ans. The charged species (group) that is replaced during a substitution reaction is a
leaving group.
Q. 21. What is the characteristic of a better leaving group?
Ans. The stronger or harder as a base, worse as a leaving group. – OH, OR – , NH
–
2
are
harder bases and are the worst as a leaving group. The best leaving groups are anions of
strong acids e.g., halide ions (X – ).
Q. 22. Discuss the stereochemistry of S
1
N
and S
2
N
reactions.
Ans. In S
1
N
reaction racemisation takes place that means (+)– and (–)– isomers form in
equal amounts if we take an optically active RX.
R
R
C
Br
Slow
R
C
+
OH –
Fast
HO C 50%
R
R
R
( )
R
R
(+)–
R
+
Br –
R
R
C
OH 50%
R