Conjugation & hyperconjugation (1).pptx

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1. CONJUGATION & HYPERCONJUGATION By: Dr. Farhat Saghir.

2. CONJUGATION Conjugation is the overlap of one p-orbital with another across an intervening sigma bond Conjugated molecules have alternating single , double or triple bonds For example:

3. The molecule is non conjugated when it do not have alternating single double or triple bond For example : Conjugated and non-conjugated dienes : Conjugated dienes are two double bond separated by a single bond For example:

4. Non Conjugated Dienes Non-conjugated dienes having two double bonds are separated by more than one single bond For example: Cyclic conjugated system Linear conjugated system

5. Stability of conjugated dienes : Conjugated dienes are more stable than non-conjugated dienes due to factors such as De localization of charge through resonance and heat of hydrogenation , It is shown that as alkene become more stable , They contain less energy so release less heat during hydrogenation conjugated dienes have a lower heat of hydrogenation that is -226 kJ/mol Than non conjugated dienes have a heat of hydrogenation that is -252 kJ/mol

6. The stability of conjugated dienes is due to the positioning of the p-orbitals and ability to overlap which give strength to single bond between two double bonds Types of conjugation Hyper conjugation CROSS conjugation Hyper conjugation It is the stabilizing interaction that result from interaction of the electrons (usually c-h c-c) with an adjacent empty or partially filled orbital or pi orbital to give an extended molecular orbital that increases the stability of system. example

7. Cross conjugation: Cross-conjugation is a special type of conjugation in a molecule, when in a set of three pi bonds only two pi bonds interact with each other by conjugation, while the third one is excluded from interaction . Example:

8. Hyperconjugation Hyper-conjugation is a special case of resonance in which sigma electrons of a carbon-hydrogen or carbon-carbon bond are in conjugation with pi-electrons of the delocalized system or with p-orbitals.

9. Explanation Hyper-conjugation may involve the interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to give extended molecular orbitals. Hyper-conjugation is directly proportional to stability. According to classical resonance theory, electron delocalization could occur only via parallel overlap of p orbitals . According to hyper-conjugation, a variant of resonance theory, electron delocalization could also occur via parallel overlap of p orbitals with hybridized orbitals involved in forming the structures

10. Applications Stability of carbocations Stability of free redical Stability of Alkenes

11. 1. Relative stability of alkyl carbocations consider the ethyl carbocation (CH3CH2 + ), a primary carbocation. The empty p orbital on C1 and the sp3 -hybridized orbital on C2 participating in C2— H1 σ-bond are more or less parallel, allowing parallel overlap, which lowers the electron deficiency at C1 but makes the H1 electron deficient. In same way, C2—H2 σ-bond and C2—H3 σ-bond could also share electrons with the empty p orbital on C1.

12. Molecular orbital approach :

13. Resonance approach :

14. Cont.… Based on the above resonance contributing structures, the resonance hybrid of the ethyl carbocation can be shown roughly as follows. Due to hyper-conjugation, in the ethyl carbocation, the net charge of +1 is shared by a total of four atoms, one carbon atom and three hydrogen atoms. No. of hyper-conjugated structures is directly proportional to stability.

15. Stability of Carbocations More the number of CH3 groups bonded to positively charged C of carbocation, the greater is the interaction and so is stability. tertiary> secondary> primary

16. 2.Relative stability of free radicals Hyper-conjugation in alkyl free radicals disperses unpaired electron over large volume of space thus providing stability as shown for ethyl free radical (CH3CH2˙). The order of stability depends on the extent of delocalization. The greater the delocalization, the more is the stability. The order of alkyl free radical stability is, 3° free radical > 2° free radical > 1° free radical > free radical

17. Cont.… No. of sigma hydrogens is directly proportion to resonating structures

18. 3. Relative stability of alkenes Overlap of σ-orbital of C–H bond with π-orbital of adjacent C–C double bond gives rise to canonical structures. Delocalization of electrons occurs over three nuclei and thus stabilizes the alkene.

19. More the substituents, more is the opportunity for hyper-conjugation and more stable is the alkene. Stability of alkenes will increase with increase in number of Hydrogens α to unsaturated system.

20. conclusions More Substituted alkene is more stable than less substituted alkene.

21. Negative hyper-conjugation It is also called “reverse hyper-conjugation” It operates in opposite direction to that of hyper-conjugation. The interaction between filled π or p orbitals and adjacent antibonding σ* orbitals is referred to as "negative hyper conjugation", as for example in the fluoro-ethyl anion:

22. it is exerted by sigma bonded electrons alpha-halo alkyl group. alpha-c-x bonds when interact with nearby sigma or pi-bonding orbital. Negative hyper conjugation stabilizes the molecule by dispersion of negative charge.

23. Effect on bond length Hyper-conjugation leads to shortening of C–C sigma (σ) bond adjacent to multiple bond. For example, C–C σ-bond in methyl acetylene is 1.46 A° in length, much less as compared to 1.54 A° found in saturated hydrocarbons. For methyl acetylene, this can be explained by hyper conjugation between alkyl and alkynyl parts.