Which substance is more acidic

[somsom]

H3C-bezene-cooh

H3Cp-bezene-cooh
----H3Cm|
second one has 2 ch3 attaching ti benezene at para and meta

can u tell me how to know which one is more acidic ?thanx

-somsom

 

[Curious3141]

It's been a heck of a long time since I last did this (13 years), but IIRC, the methyl group is a weak electron donor. That means the dimethylphenylcarboxylate ion will be destabilised by the two methyl groups, leading to a shift to the left in the equilibrium :

$$HA \rightarrow H^+ + A^-$$

where $A^-$ denotes the dimethylphenylcarboxylate ion.

This will of course, lead to a reduction in [itex][H^+][/tex], and a reduction in acidity of the dimethyl compound.

The single methyl group would have the same effect, but less so, so it should be the more acidic of the two.

 

[somsom]

thanx, but how to compare the singlemuthyl one with
bezene-cooh, which is more acidic ? help me please tomorow is i post report , i don't know to do.

 

[somsom]

sorry forgot to tell you a thin that i am 31 yrs old, and now had to toolk a course in chemistry for a job, still looking

 

[Curious3141]

Same reasoning that I posted. Every methyl group makes the compound a less strong acid.

So in terms of decreasing acidity (in order of increasing pH in solution),

benzoic acid > methylbenzoic acid > dimethylbenzoic acid > trimethylbenzoic acid...

and so on.