Question

Arrange the following carbanions in increasing order of their stability.

Answer 1

Hint: Stability of carbanions decreases as we are moving from primary to tertiary carbanion. Because of the +I effect, the methyl groups increase the intensity of the negative charge on central carbon in tertiary carbanion which makes tertiary carbanion more unstable.

Complete step by step answer:
- The stability of carbanions is as follows.
Primary carbanion > secondary carbanion > tertiary carbanion.
- As per the above order we can say that primary carbanion is more stable then secondary and tertiary carbanion.
- Due to +I effect tertiary carbanion is less stable than secondary and primary carbanions.
- Due to the presence of – I affect the stability of the carbanions increases.
- If molecules (carbanions) contain electron withdrawing groups, they withdraw the electrons from the carbanions and make the carbanions more stable. This effect is called –I effect.
- In the question there are three molecules having carbanions in their structures. They are as follows.

- All the given molecules contain primary carbanions.
- The molecule-I contains primary carbanion adjacent to a double bond.
- The molecule-I shows resonance as follows.
 

- But still molecule-I is unstable because a large amount of electrons creates high repulsions.
- Coming to the molecule-II, it contains only primary carbanion. There are no – I groups and no double bond in it. Then it is more stable than molecule-I.
- In molecule-III there is a presence of fluorine (an electron withdrawing group) makes molecule-III more stable due to – I effect.
- Therefore the increasing order of the stability of the given carbanions is as follows.

Note: If there is a presence of any electron donating groups (+I effect) in carbanions then the carbanions are more unstable than the normal carbanions. The stability of the carbanions will increase by the presence of electron withdrawing groups.