# Homework Help: Which substance is more acidic

1. Jun 18, 2005

### 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

2. Jun 18, 2005

### 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.

Last edited: Jun 18, 2005
3. Jun 19, 2005

### somsom

thanx, but how to compare the singlemuthyl one with
bezene-cooh, which is more acidic ????? help me plz tomorow is i post report , i dunno to do.

4. Jun 19, 2005

### 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

5. Jun 19, 2005

### 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.