Engineering Chemistry S Datta

176 ENGINEERING CHEMISTRY
As the planar carbocation is formed in rate determining step and the process of formation
is slow, it is quite likely that attack by OH – would take place with equal readiness from either
side of planar carbocation leading to 50/50 mixture of the (+)– and (–)–forms. That is, always
racemisation takes place during S
1
N
reaction.
In S N
2 reaction:
R
R
R
HO – +
R
C
R
Br
– – HO C
HO
R
The back side attack by the nucleophile always leads to inversion of configuration in
S
2
N
reaction.
Q. 23. Discuss the effect of solvent on S
1
N
and S
2
N
reactions.
Ans. The change of solvent (I) affects the rate of the reaction (II) which may change the
mechanistic pathway.
S
1
N
reaction becomes faster if the solvent polarity increases.
The solvolysis of (CH 3
) 3
Br becomes 3 × 10 4 times faster in 50 % ethanol than in ethanol
alone. Developing ion pair gets solvated easily and helps the process ionisation.
Increase in solvent polarity has little effect in S
2
N
reaction; only a slight decrease in
reaction rate is observed. Change of solvent may have marked effect on S
2
N
reaction. The
reaction rate becomes 4.5 × 10 4 fold faster than the reaction CH 3
I with N – 3
if we change the
solvent from CH 3
OH to HCON(CH 3
) 2
(DMF). Because in CH 3
OH due to hydrogen bonding N – 3 is
solvated and its nucleophilic activity is reduced. Whereas in DMF the hydrogen loading is
absent and N
–
3
remains unsolvated and powerful nucleophile can attack.
Increase in solvent polarity and ion solvating ability may change reaction pathway from
S
2
N
→ S 1 N
. Transfer from protic polar solvent to polar nonprotic solvent may change the reaction
pathway from S
1
N
→ S
2
N
by increasing the effectiveness of the nucleophilic.
Q. 24. The order of reactivity of halogen acids towards an alcohol is—
HI > HBr > HCl.
Explain.
Ans. The alcohol is first protonated, there is attack by X – to give RX. So, the reaction is
nucleophilic substitution and the leaving group is H 2
O.
HX
ROH ⎯⎯→ R – OH + 2 ⎯⎯→
X – [ − δ
+
X ---R---O H ] 2 ⎯⎯→ RX + H 2
O.
Nucleophilicity of X – decreases in order: I – > Br – > Cl – .
Hence the order of reactivity of HX is
HI > HBr > HCl.
Q. 25. For a given HX, the order or reactivity of the alcohols follows the order
3° > 2° > 1°—explain.
C
T.S.
Br
R
R
R