11th Chemistry NCERT notes

Resonance Structure

A number of organic compounds cannot be accurately represented by one structure. For example, benzene is ordinarily represented as

Resonance Structure of Benzene

This structure has three C-C bonds and three C=C bonds.

Carbon-carbon double bond length = 1.34 A
Carbon-carbon single bond length = 1.54A.

But it has been determined experimentally that all carbon-carbon bonds in benzene are identical and have same bond length (1.39A).

Thus, the structure of benzene cannot be represented by single structure. It can be represented equally well by ‘he energetically similar structures I and II. The two structures are called resonance structures.

Resonance Structure

Actual structure of benzene is resonance hybrid of structures I and II.
Another example of resonance is provided by nitromethane (CH3N02) which can be represented by two Lewis structures.

Resonance Structure

The actual structure of nitromethane is a resonance hybrid of the two canonical forms I and II.

Resonance energy:

The difference in the energy between the most stable contributing structure for a compound and its resonance hybrid is called as resonance energy or resonance stabilization energy.

Resonance Effect

The polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and a lone pair of electrons present on an adjacent atom. There are two types of resonance or mesomeric effects designated as R or M effect.

Positive Resonance Effect (+R effect)

Those atoms which lose electrons towards a carbon atom are said to have a +M effect or +R effect. For example:
-Cl, -Br, -I, -NH2, -NR2, -OH, -OCH3

Positive Resonance Effect (+R effect)

Negative Resonance Effect (-R effect)

Those atoms or groups which draw electrons away from a carbon atom are said to have a -M effect or -R effect.
For example: -NO2, -C≡N, -CHO

Negative Resonance Effect (-R effect)


How useful was this post?

Click on a star to rate it!

Average rating 3.7 / 5. Vote count: 12

No votes so far! Be the first to rate this post.

As you found this post useful...

Follow us on social media!

We are sorry that this post was not useful for you!

Let us improve this post!

Tell us how we can improve this post? Please mention your Email so that we can contact you for better feedback.


Please enter your comment!
Please enter your name here