Carbanions

Pallavi Vyas Semester 1

1. CARBANIONS Submitted by- PALLAVI VYAS 2020-2021 M.Sc chemistry semester 1st S.P.C. GOVT. COLLEGE AJMER

2. Content Introduction of carbanion Formation of carbanion Factors affecting Stability of carbanion Reactions of carbanion

3. Introduction of carbanion A carban carries three bonds and a lone pair of electron, thus making the carbon atom negatively charged. Carbanion is represented as- Carbanion is the conjugate base of a carbon acid.

4. Carbanion has sp hybridization and trigonal pyramidal geometry ( 3b.p. & 1l.p.). It has four sp hybrid orbitals. They have electron rich carbon atom having 8 electrons in its valance shell. They are important chiefly as chemical intermediates 3 3

5. Formation of carbanion The carbanions are obtained by heterolysis in which carban is linked to less electronegative atom/group. The group attached to carban atom leaves behind the bonding electron pair in cleavage; usually departing constituent is a proton. Carbanion are also formed when nucleophile adds to unsaturated compounds.

6. Carbanions are formed from those compounds which contain a functional group capable of weakening a nearby C-H bond.

7. Factors affecting stability of carbanion Inductive effect -: The stability order of carbanion decreases, while moving from primary to tertiary anion as alkyl group has +I effect so due to this increase intensity of negative charge on central carbon of tertiary anion.

8. -: Less the pKa value more is the acidity & thus more is the stability of acid anion. pKa values of some alkanes :-

9. 2. Electronegativity -: When the percentage of s-character increase at the carbanionic carban then stability also increase.

10. 3. Resonance effect. -: Due to resonance,negative charge distribution occur so more the distribution of negative charge more the stability of carbonion. # Relative stability of benzyl & allylic carbanion.

11. #. Relative stability of benzylic carbanion and their substitution.

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13. Reactions of carbanion Carbanions are very reactive & unstable. They undergo chemical reaction as soon as they are formed. Addition reaction:- carbanions combine with another species that has empty orbital in it’s outermost shell. Elimination reaction:- This reaction takes place via formation of carbanion as reaction intermediate.

14. 3. Substitution reaction:- The most important reaction of this category is the alkylation of enolate which is an important synthetic method. 4. Rearrangement reaction:- Rearrangement takes place by 1,2 shift of alkyl or aryl group from carbon to carbon,nitrogen to carbon & sulphur to carbon.

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