Engineering Chemistry S Datta

:
MECHANISM OF ORGANIC REACTIONS 179
(i) The electronegative atom ‘O’ of phenol gets positively charged due to resonance. So
bonding electrons of O—H bond will be pulled strongly towards O-atom making release
of H + easier. Moreover, the anion formed on the release of H + by phenol molecule is
resonance stabilised. So, phenol can behave as an acid.
(ii) Due to resonance, o- and p- positions of the benzene ring of phenol molecule become
electron rich. Benzene having a delocalised π-cloud is susceptible to electrophilic
substitution. When benzene ring is attached to a group containing a ‘key atom’ (here
‘O’ has lone pair of electrons) the resonance effect gives rise to increased electron
densities in the o- and p- positions. This increased electron density activates the
benzene ring towards electrophilic substitution S 2 E
. The electrophiles can attack at
those positions leading to electrophilic substitution. So phenol is o-, p-orienting*.
Q. 33. pK a
values for NH 3
, CH 3
NH 2
, (CH 3
) 2
NH are 9.25, 10.64 and 10.77
respectively. Justify.
..
Ans. NH 3 is a Lewis base, it can donate a pair of electrons. When this donation capacity
is increased by any effect, the basicity is increased. The (+I)-effect of CH 3
groups increases the
electron density of N atom in CH 3
NH 2
and this effect is more pronounced in (CH 3
) 2
NH due to
two CH 3
groups performing as stronger bases than NH 3
.
NH 3
:
CH 3
NH 2
:
pK = 9.25
a pK a = 10.64
CH 3
CH 3
NH
:
pK a = 10.77
Q. 34. pK a
values of NH 3
, CH 3
NH 2
and PhNH 2
are 9.25, 10.64 and 4.62 respectively.
Justify.
Ans. NH 3
is basic, CH 3
NH 2
is more basic, due to the (+I) effect of the –CH 3
group. The
pK a
values are in accordance with the above fact. But we see that pK a
of aniline is 4.62 i.e., it
is less basic than NH 3
.
The resonating structures of aniline is:
NH 2
:
:
+
NH 2
:
+
NH 2
:
+
NH 2
:
NH 2
Example of resonance
Due to resonance, the electron density over ‘N’ atom becomes less than that over NH 3
.
So aniline (pK a
= 4.62) becomes less basic than NH 3
.
Q. 35. Do you expect that p-nitroaniline is less basic than aniline?
Ans. Yes, the electron withdrawing effect of –NO 2
group in p-nitroaniline makes electron
density of ‘N’ atom less than that in aniline and that is why p-nitroaniline becomes less basic.
.. ..
*This property common to other groups, like NH2, : OR
.. , helps electrophilic substitution and are o-,
p-orienting.