Understanding Mesomeric Effect

Question

I am getting confused between the type of mesomeric effect (+M or -M) shown by compounds containing $\ce{-COX}$ where $\ce{X}$ is an electronegative element.

Examples: $\ce{-CONHCH3}$, $\ce{-COOCH3}$, $\ce{-COOH}$.

According to me, all these compounds must show both a +M as well as a -M effect. Since the group (e.g. $\ce{-COX}$) can accept as well as donate $\pi$ electrons (as in $\ce{CH_2=CH-COOH}$).

However, the book says these compounds show only -M effect. Please explain me the reason behind it.

In the above example, isn't the group showing +M effect, since it's donating the electrons?

Answer 1

Following up on Permeakra's comment, the relative importance of resonance structures is typically determined by:

1) Maximize atoms that fulfill the octet rule. 2) Electronegative atoms have negative formal charges and avoid positive charges. 3) Minimize charge separation.

Carboxylic acid derivatives (esters, amides, etc.) have resonance structures that break #2, but since all atoms fulfill the octet rule, they are important contributors to the overall electronic picture of the compounds. Hopefully the image below is legible.

In general, resonance (mesomeric) structures that contain an electronegative atom that both breaks the octet rule AND bears a formal positive charge are not reasonable resonance structures and do not contribute to the overall electronic picture of the compound. The resonance structure of the carboxylic acid drawn in the original question falls into this category as the 'top' oxygen both breaks the octet rule and contains a positive charge.