# Understanding Mesomeric Effect

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?

• carbonyl group does not show +M effect because in it carbon is highly electron-deficient. In it oxygen cannot donate electron to pi-system from electron pair and donating electron from double bond will leave oxygen with electron sextet, which will not happen under any circumstances, period. – permeakra Mar 20 '14 at 16:22