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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: $\ce -CONHCH_3$, $\ce -COOCH_3$, $\ce -COOH$. According to me, all these compounds must show both +M as well as -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?

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?

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: $\ce -CONHCH_3$, $\ce -COOCH_3$, $\ce -COOH$. According to me, all these compounds must show both +M as well as -M effect. Since the group (eg $\ce -COOH$) can accept as well as donate $\pi$ electrons (as in $\ce CH2=CH-COOH$). 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?

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: $\ce -CONHCH_3$, $\ce -COOCH_3$, $\ce -COOH$. According to me, all these compounds must show both +M as well as -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?

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: $\ce -CONHCH_3$, $\ce -COOCH_3$, $\ce -COOH$. According to me, all these compounds must show both +M as well as -M effect. Since the group (eg $\ce -COOH$) can accept as well as donate $\pi$ electrons (as in $\ce CH2=CH-COOH$). However, the book says these compounds show only -M effect. Please explain me the reason behind it.

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: $\ce -CONHCH_3$, $\ce -COOCH_3$, $\ce -COOH$. According to me, all these compounds must show both +M as well as -M effect. Since the group (eg $\ce -COOH$) can accept as well as donate $\pi$ electrons (as in $\ce CH2=CH-COOH$). 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?