Carbanion

This Powerpoint presentation helps us to know the some basic of carbanion

1. Presented by, D.sujitha M.Pharm Ist sem department of pharmaceutical chemistry CARBANION

2. CONTENT: Introduction Structure of carbanion Stability of carbanion Formation of carbanion Application Reference

3. INTRODUCTION Carbanion is a unit that contains a negative charge on a carbon atom. Base/nucleophile depending upon the reaction conditions. Negative charge gives good nucleophilic properties to the unit that can be used in the formation of new carbon–carbon bonds.

4. CHARACTERISTIC OF CARBANION : Pyramidal,sp3 hybridised. Bond angle 109.28. Geometry-tetrahedral. It has 8 electrons. Act as good base or nucleophile.

5. Carbanion generally formed by heterolytic cleavage

6. Carbanions undergoes rapid interconversion between 2 pyramidal forms. Tetrahedron can undergo inversion or retain its stereochemistry depending upon the attached substituents. Methyl carbanion has a barrier to inversion of about 2 kcal mol–1, whereas the trifluoromethyl carbanion has a barrier of 120 kcal mol–1. A fluorine atom is thus more stabilizing than a hydrogen atom because of its high electronegativity

7. Stability of carbanion Inductive effect Resonance effect or delocalisation Aromaticity Electronegativity of carbanionic carbon Stabilisation by carbanion and enolate ion n Stabilisation by carbanion and enolate ion Stability of carbanion

8. Carbanion works as a nucleophile when attacks any electron-deficient center except a proton. Stability and reactivity of a carbanion is determined by several factors. • The inductive effect: electronegative atoms adjacent to the charge will stabilize the charge.

9. Hybridization of the charge-bearing atom: the greater the s-character of the charge-bearing atom the more stable the anion. • Extent of conjugation of the anion: Resonance effects can stabilize the anion. This is especially true when the anion is stabilized as a result of aromaticity.

10. Inductive effect: Is an electronic effect due to polarisation of bond within a molecule or ion. Electronegative atoms adjacent to the charge will stabilize the charge Inductive effect decrease with distance 2 type: +I effect and –I effect.

11. Alkyl substitution at the carbanionic site results in an intensification of the carbanionic character because of the electron-donating character of the alkyl groups. Order of stability in carbanions is the reverse of that of carbocations that is: vinyl > phenyl > cyclopropyl > primary > secondary > tertiary

12. Resonance effect If negatively charged carbon is in conjugation with a double bond the resonance effects will stabilize the anion by spreading out the charge by rearranging the electron pairs. Number of resonating structure is increase the stability of carbanion is increase. Delocation of negative charge increase the stability of carbanion

13. AROMATICITY In some carbanions the lone pair of electron of the negative charge is involved in delocalization to add on to the aromatic character of the molecule which gives them extra stability. cyclopentadienyl anion

14. Electronegativity of carbanionic carbon Stability directly propotional to electronegativity of carbanionic carbon which is directly propotional to % S- character of carbanionic carbon. The S-character is increase, then electronegativity also increase.

15. Stabilisation by carbanion & enolate ion Stabilised by >C=o, -NO2, and CN groups present on carbanionic carbon These groups stabilise carbanion by resonance effect. Stabilization of carbanion through formation of enolate. Contribution of structure (II) will be more than (I)because in (II) negative charge is present on electronegative oxygen

16. Formation of carbanion

17. Deprotonation from a C–H Bond: When proton is abstracted from a carbon center then the resulting anion is called as carbanion. The acidic hydrogen of an organic substrate can be abstracted by an appropriate base for example carbanions generated from carbonyl compound.

18. DECARBOXYLATION Decarboxylation of carboxylate leads to formation of carbanions intermediate.

19. Reduction of C–X Bond with Metal Metals which are less electro negative than carbon such as Mg, Na, Hg, etc react with alkyl halide at appropriate condition to form the carbon metallic bond where carbon contain negative charge and the metal contain positive charge  

20. A Negative Ion Adds to a Carbon–Carbon Double or Triple Bond The addition of a negative ion to a carbon–carbon double or triple bonds leads to a carbanion. The addition of a negative ion to a carbon–oxygen double bond does not give a carbanion, since the negative charge resides on the oxygen. Carbanions are also formed when a nucleophile adds to an α,β-unsaturated compounds

21. APPLICATION Aldol condensation Cannizzaro reaction Dieckmann reaction Michael reaction Perkin reaction Clasein condensation Benzoin condensation

22. Aldol condensation. The reaction of an aldehyde or ketone with dilute base or acid to form a beta - hydroxycarbonyl product.

23. Michael addition reaction: Reagents : commonly bases such as NaOH or KOH. The first step is the formation of the enolate . Enolates tend to react with α,β -unsaturated ketones via  conjugate addition . A conjugate addition with a carbanion nucleophile is known as the  Michael reaction  or  Michael addition.

24. Grignard reagents The strongest bases are obtained from the reaction of metal with organohalogen compounds to give reagents known as Grignard reagents Grignard reagents are examples of organo metallic carbanions.

25. Perkin reaction The condensation of aromatic aldehyde with anhydrides is called as Perkin reaction. In this reaction formation of α,ß unsaturated acid.

26. REFERENCE: Reactive intermediates in organic chemistry structure, mechanism and reactions by Maya Shankar Singh, wileyvch publication.Pg:no : 65-157. Advanced organic chemistry, reaction, mechanisms, and structure by Jerry March. Pg: no: 175-185.