Conjugation

overlapping of p- orbitals across intervening sigma bonds.

1. MM UNIVERSITY SADOPUR, AMBALA TOPIC : PRESENTATION ON conjugation,cross -conjugation and hyper-conjugation MADE BY: Arun Sharma MSC CHEM.1 st year

2. CONJUGATION Conjugation is the overlap of one p-orbital with another across an intervening sigma bond

3. A system of connected p- orbitals with delocalized electrons with alternating single and multiple bonds. The compound may be cyclic, linear or mixed. Conjugated system

4. Types Conjugated ; C=C-C=C Cumulated ; C=C=C Isolated / non conjugated; C=C-C-C=C

5. Some examples Cyclic conjugated system; Linear conjugated system; Mixed conjugated system;

6. Stability of conjugated system systems containing conjugated double bonds are more stable than those containing non conjugated double bonds.

7. Cross-conjugation Special type of conjugation. Three pi-bonds are present, out of which two are not conjugated with each other although each is conjugated with the third i.e Interruption of the strict alteration of single bond and double bond.

8. Hyper-conjugation The interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital to give an extended molecular orbital that increases the stability of the system It is also known as sigma-pi conjugation no bond resonance. It is a permanent effect.

9. Structural requirements of hyper-conjugation Compound should have sp2 hybrid carbons of alkene , arene , carbocation or free radical. Carbon adjacent to sp2 hybrid carbon should have atleast one alpha-hydrogen. Alpha-carbon should have sp3 hybridization.

10. Types σ (C-H), ϖ conjugation: occurs in alkenes and alkyl substituted aromatic compounds- σ (C-H),+ ive charge conjugation: occurs in alkyl carbocations -

11. σ (C-H),odd electron conjugation: occurs in alkyl free radicals- Hyper-conjugating structures-

12. Number of hyper-conjugating structures =number of α -hydrogens+1 Greater the no. of hyper-conjugating structures, more is the stability of the system .

13. Stability of alkenes: tetrasubstituted>trisubstituted>disubstituted>monosubstituted > Stability of alkyl free radicals: Stability of carbocations: > > >