[thedy]

Hi,I have read many articles here and on web,but I haven t got clear answer.In my chemistry book is  writen,that benzoic acid and acids with conjugated system are more acidic,than without benzene ring or conjugated system,because carboxyl anion is stabilized by involving negative charge to benzene ring or if we have conjugated system.
But it is common fact,that COOH has -M effect,so it attract from system,doesn it?So it cannot be stabilized...

https://docs.google.com/drawings/d/1l8Kl2LfkvT4n1ncvybql3Ocv9m8M_ayJXkw5iguCtgo/edit

This link shows,how is carboxyl anion stabilized.But COOH has -M effect,not +,like in picture.Picture is from my chemistry book.
Thanks a lot....

[Dan]

Ok - the drawings in your book are complete voodoo chemistry, it's incorrect and utterly misleading.

The negative charge in the benzoate, propenoate or even propynoate anions are not conjugated with the unsaturated systems.



The increased acidity is an inductive effect. The more s character in an sp^x hybridised carbon, the less electron donating it it is [so most most donating is sp³ > sp² > sp least donating]. The reason for this is that orbitals with more s character are smaller, and so the electrons are closer to the nucleus and experience a stronger attractive force and are less readily released/donated inductively.

Perhaps another way of seeing it is that, for example, propanoic acid is less acidic than propenoic acid because propyl has a greater +I effect than propenyl due to the sp³ vs sp² orbitals involved in the C-COOH sigma bond.

[thedy]

The negative charge in the benzoate, propenoate or even propynoate anions are not conjugated with the unsaturated systems.

Is it 100% certainity?Because,here: http://www.chembook.co.uk/chap22.htm it is writen in paragraph 22.3.3. Effects of the rest of the molecule on acid strength. ,like it is conjugated...
And second link: http://www.scribd.com/doc/49439658/Acid-Basic-Strength where is writen in paragraph iii):,,Benzoic Acid > Carboxylic Acid
Reason: The benzoate anion enjoys stabilization from both resonance and delocalization of the negativecharge into the benzene ring
.
Hence, the benzoate anion is more stable than the carboxylate anion,relative to their respective undissociated acid molecules"

So,who does have true?I m completelly confused,in deep depression

[thedy]

And thanks for answer....And I forgot,please try explain,why carboxyl group is not in conjugation with benzene.I know,that negative charge on oxygen is separated from ring by two single bonds.

BUT,if I draw resonance,you can notice,that this negative charge is augmented over whole carboxyl group,and that means,in one of several resonance structures carbon has in carboxyl group negative charge(I know,small probability) and this charge is in conjugation,because we have free electron pair on carbon(negative charge,as I wrote above)---single bond---double bond.....Am I right?Please explain me....

[Dan]

Is it 100% certainity?Because,here: http://www.chembook.co.uk/chap22.htm it is writen in paragraph 22.3.3. Effects of the rest of the molecule on acid strength. ,like it is conjugated...
And second link: http://www.scribd.com/doc/49439658/Acid-Basic-Strength where is writen in paragraph iii):,,Benzoic Acid > Carboxylic Acid
Reason: The benzoate anion enjoys stabilization from both resonance and delocalization of the negativecharge into the benzene ring
.
Hence, the benzoate anion is more stable than the carboxylate anion,relative to their respective undissociated acid molecules"

So,who does have true?I m completelly confused,in deep depression

Yes, 100%. I am sure all other qualified chemists will agree with me. Both the sources you posted are wrong. Don't get depressed - it's not your fault.

The negative charge is stabilised by resonance between the two oxygen atoms but it is not (significantly) delocalised onto the benzene ring by resonance.

Here are the resonance structures of benzoate:

[thedy]

Thanks a lot you give me a meaning of life:D. ...