Thread: Organic Chemistry - Acidity

This is taken from Organic Chemistry 8th Edition of T. W. Graham Solomons and Craig B. Fryhle. This is just what I've understood so far. So, correct me if I'm wrong.

Basically, the following points leads to increased acidity of molecules.

• Charge delocalization, e.g. CH3NH2 is more acid than CH4.
• Greater s orbital character in carbon hybridization, e.g. CH3CH=CH2 is more acid than CH3CH2CH3.
• Lower position within a group of the periodic table, e.g. HCl is more acid than HF.
• Position further to the right within a given row of the periodic table, e.g. HCl is more acid than H2S.

But I do have problems. For example, problem 3.28.B.

Arrange the following compounds in order of decreasing acidity.
CH3CH2CH2OH
CH3CH2CO2H
CH3CHClCO2H

Interestingly, I did that rightly. I thought that CH3CHClCO2H is the most acid because the chlorine atom and the two oxygen atoms provide more charge delocalization than the two oxygen atoms do in CH2CH2CO2H. Based on the same reason, CH2CH2CO2H is more acid than CH3CH2CH2OH.

But how about the following groups?

1. CH3CH2OH2+, CH3CH2OH and CH3OCH3.
2. CH3NH3+, CH3NH2 and CH3NH-.

I'm thinking that how stabile the anion of a compound, how more acid the compound is. But I don't have any courage to use it as an assumption.

Any help would be appreciated greatly.

p.s. How can you do subscripts with BBCode?

But how about the following groups?


1. CH3CH2OH2+, CH3CH2OH and CH3OCH3.
2. CH3NH3+, CH3NH2 and CH3NH-.

Just see that CH3CH2OH2+ and CH3CH2OH are conjugate acid-base pair in Bronstedt terms. CH3CH2OH2+ is then stronger acid than CH3CH2OH because it can easily loose proton (something which is positive-charged easily loose positive-charged particles). CH3CH2OH is weaker acid because it forms CH3CH2O- and H+ ions and it's hard to separate opposite charged ions).

Now, second point will be easy for you.

Originally Posted by Neecze

But how about the following groups?


1. CH3CH2OH2+, CH3CH2OH and CH3OCH3.
2. CH3NH3+, CH3NH2 and CH3NH-.

Just see that CH3CH2OH2+ and CH3CH2OH are conjugate acid-base pair in Bronstedt terms. CH3CH2OH2+ is then stronger acid than CH3CH2OH because it can easily loose proton (something which is positive-charged easily loose positive-charged particles). CH3CH2OH is weaker acid because it forms CH3CH2O- and H+ ions and it's hard to separate opposite charged ions).

Now, second point will be easy for you.

OK. Thanks. I think I've figured it out.