organic acid

This type of question always appears in AL Exam.(about2-3M)

Q1)

．The acidity of a compound depends on the relative stability of the acid molecule and the conjugated base.

．The higher stability of the conjugated base is, the higher acidity of the molecule is.

．C6H5SO3- is more stable than CH3COO-

．Because the mesomeric effect can disperse the negative charge to a greater extent. (OR show resonance structure)

．While methyl group is electron donating group, it intensities the negative charge and hence it is more unstable.

∴C6H5SO3H is more acidic than CH3COOH.

Q2)

．The acidity of a compound depends on the relative stability of the acid molecule and the conjugated base.

．The higher stability of the conjugated base is, the higher acidity of the molecule is.

．C6H5SO3- is more stable than C6H5O-

．Because the mesomeric effect in C6H5SO3- can disperse the negative charge more than that in C6H5O-. (OR show resonance structure)

∴C6H5SO3H is more acidic than C6H5OH

2008-09-16 19:03:15 補充：
Q1) If more the no of resonance structures can be drawn, then the mesomeric effect is stronger.

If we ignore the benzene ring and the methyl group, C6H5SO3- has one more X=O double bond than CH3COO-. (X is the central atom)

Hence the former one has more resonance structure.

2008-09-16 19:03:32 補充：
(I know than the -ve charge cannot be "drawn" into the benzene, but even we assume that benzene ring does not have -M effect, it still have -I effect. However methyl group has +I effect. Benzene can still disperse the -ve charge)

2008-09-16 19:03:43 補充：
If you are AL student and studying PM, it may be difficult to accept this kind of argument.

But if you keep this attitude in learning AL chemistry, you will feel very upset. (Because inorg have more these kinds of questions.)