Question

What is the effect of substituent -ch3 on acidity of phenol 2puc?

Answer 1

The alkyl substituents have a negligible effect on acidity while halogens increase the acidity as substituents in straight chain carboxylic acids.In aromatic carboxylic acids the effect is different.

Explanation:

the acidity value is given by the Ka and pKa values i.e. Ka is a measure of acid strength and -log of Ka value gives pKa for very strong acids pKa values are negative .When an alkyl substituent is attached to the carboxylic acid e.g.
X- CH2COOH the value of Ka and pKa are as follows
X is the substituent on the carboxylic acid.

X Ka pKa
H 1.8x10−5 4.7
CH3 1.3x10−5 4.9
CH3(CH2)5 1.3x10−5 4.9
as already stated if the substituents are alkyl the acidity has negligible effect as compared to H only .2 value increases.

Now we consider halogens as substituents and observe the Ka and pKa values accordingly

X Ka pKa
F 2.5x 10−32.6Cl1.4x10−3 2.9

Electronegative atoms tend to pull the electrons towards themselves by inductive force so the ionization of H+ becomes easy and as a result the acidity increases.
this effect decreases as the substituents have long alkyl chains halogen attached for example

X Ka pKa
Cl-CH2 1.0x10−44.0Cl−CH2−CH23.0x10−5 4.5
this decreases acidity because halogen withdraws electrons more when they are directly attached to the carboxylic acid as the alkyl chain increases it becomes difficult for halogen to withdraw electrons.

For sp2 hybridized substituents ,as they tend to have a pi-bond they have a tendency to withdraw electrons as compared to sp3 and sp is more electron withdrawing than sp2. i.e.

X Ka pKa
benzoic acid 6.3x10−54.2CH2=CH−COOH5.5x10−5 4.3
CH=C=COOH 1.4x10−2 1.8

Answer 2

Because of the moving of electrons across hydrogen, an increase in negative ions and a consequent decrease in acidity will result.

Reason:

Alkyl substituents little affect the acidity, whereas halogens make straight-chain carboxylic acids more acidic when they are substituted. The outcome differs in aromatic carboxylic acids.

The group that donates electrons is -CH3.

because of the moving of electrons across hydrogen, an increase in negative ions and a consequent decrease in acidity will result.

The methyl group (-CH3) is an electron donor (sometimes referred to as an electron release), which might reduce the stability of the phenoxide ion because the inductive effect causes the negative charge on the phenoxide ion to become more concentrated. This reduces the rightward tilt of the equilibrium and lowers the phenols' acidity.

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