Alcohols, Phenols and ethers 2

Question 1

why are alcohols said to be acidic? what is the order of acidity?

Answer 1

The acidic character of alcohols is due to
the polar nature of O–H bond. An electron-releasing group
(–CH3
, –C2H5
) increases electron density on oxygen tending to
decrease the polarity of O-H bond. This decreases the acid
strength. For this reason, the acid strength of alcohols decreases
1>2>3

Question 2

is water or alcohol a better acid?

Answer 2

Alcohols are, however, weaker acids than water. This can be
illustrated by the reaction of water with an alkoxide.
R-O + H2O —-> R-OH + OH-

This reaction shows that water is a better proton donor (i.e.,
stronger acid) than alcohol. Also, in the above reaction, we note
that an alkoxide ion is a better proton acceptor than hydroxide
ion, which suggests that alkoxides are stronger bases (sodium
ethoxide is a stronger base than sodium hydroxide).

Question 3

alcohols are also bronsted bases why

Answer 3

Alcohols act as Bronsted bases as well. It is due to the
presence of unshared electron pairs on oxygen, which makes
them proton acceptors.

Question 4

describe the acidity of phenols

Answer 4

The reactions of phenol with metals (e.g.,
sodium, aluminium) and sodium hydroxide indicate its acidic
nature. The hydroxyl group, in phenol is directly attached to
the sp2
hybridised carbon of benzene ring which acts as an
electron withdrawing group.

The reaction of phenol with aqueous sodium hydroxide
indicates that phenols are stronger acids than alcohols and water.

Question 5

why are phenols stronger acids than wtaer, alcohol

Answer 5

Due to the higher electronegativity of sp2
hybridised carbon
of phenol to which –OH is attached, electron density decreases
on oxygen. This increases the polarity of O–H bond and results
in an increase in ionisation of phenols than that of alcohols.

In alkoxide ion, the negative charge is localised on oxygen
while in phenoxide ion, the charge is delocalised.
The delocalisation of negative charge (structures I-V) makes
phenoxide ion more stable and favours the ionisation of phenol.
Although there is also charge delocalisation in phenol, its
resonance structures have charge separation due to which the
phenol molecule is less stable than phenoxide ion.

Question 6

explain the effect of substitutents on phenol ring

Answer 6

In substituted phenols, the presence of electron withdrawing
groups such as nitro group, enhances the acidic strength of
phenol. This effect is more pronounced when such a group is
present at ortho and para positions. It is due to the effective
delocalisation of negative charge in phenoxide ion. On the other
hand, electron releasing groups, such as alkyl groups, in
general, do not favour the formation of phenoxide ion resulting
in decrease in acid strength. Cresols, for example, are less acidic
than phenol.

Question 7

pKa and Ka relation

Answer 7

pKa = - logKa

so more the Ka, lesser the pKa and stronger the acid.