Hydrocarbons Ques 53
53. Base strength of :
(A) $H_3 C- \overline{C}H_2$
(B) $H_2 C= \overline{C}H$ and
(C) $H-C \equiv \overline{C}$
is in the order of :
[2008]
(a) $(B) > (A) > (C)$
(b) $( C ) > (B) > (A)$
(c) $(A) > (C) > (B)$
(d) $(A) > (B) > (C)$
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Answer:
Correct Answer: 53.(d)
Solution:
- (d) The amount of $s$-character in various hybrid orbitals is as follows.
$s p=50 \%, s p^{2}=33 \%$ and $s p^{3}=25 \%$
Therefore $s$-character of the $C-H$ bond in acetylene $(s p)$ is greater than that of the $C-H$ bond in alkene ( $s p^{2}$ hybridized) which in turn has greater $s$-character of the $C-H$ bond in alkanes.
Thus, owing to a high $s$-character of the $C-H$ bond in alkynes, the electrons constituting this bond are more strongly held by the carbon nucleus with the result the hydrogen present on such a carbon atom can be easily removed as proton.
The acidic nature of three types of $C-H$ bonds follows the following order
$ \equiv C-H>=C-H>-C-H $
Further, as we know that conjugate base of a strong acid is a weak base, hence the correct order of basicity is
$ H-C \equiv \stackrel{(-)}{C}<CH_2=\stackrel{(-)}{C} H<\stackrel{(-)}{CH_2}-CH_3 $
BETA
