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HYPERCONJUGATION PowerPoint slide useful for first prof
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- 2. OTHER NAME: • Bakerand Nathan Effect • Sigma Bond Effect • No Bond Resonance
- 3. DEFINITION: • The conjugationoccur due to sigma bond. • Conjugation occur due to attachment and detachment of hydrogen to adjacent carbon attach to electron deficient group. • Resonance occur due to repeated separation of hydrogen to any alkyl group is called hyper conjugation.
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- 5. Hyperconjugation: No bondresonance • The electrons of the sigma bond between C and H are involved in delocalization. • In structure to the right: No bond between C and H due to migration of the sigma bond. Hence Hyperconjugation is also called as ‘NO BOND RESONANCE’.
- 6. EXPLANATION: • In orderto explain the hyper conjugation lets us consider a molecule in which three hydrogen atom are attached with carbon 1 and two hydrogen atom are attached with carbon 2. • The c1 are electron sufficient and have high electron density while c2 are electron deficient and low electron density. • The c1 are called α-carbon and hydrogen attached are called α-hydrogen. • C1 have sp3 and c2 have sp2 hybridization.
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- 8. Salient Features ofHyper conjugation: • Hyper conjugation is a permanent effect. This concept was introduced by Baker and Nathan. • It is a “mild or partial sort of conjugation” and it explains how saturated σ-bonded segments interact with π-bonded segments i.e. a double or triple bond. It is also called as delocalization of sigma electrons. • The sigma bond involved in hyper conjugation is usually a C-C or C-H bond.
- 9. Continue… • Greater thenumber of alkyl groups attached to doubly bonded C atoms, greater is the number of contributing structures and greater is the stability. • It is also known as no bond resonance or bond sacrificial resonance or “Baker-Nathan effect” or “sigma-pi (σ-π) conjugation”.’ • Phenomena of hyperconjugation occurs in alkene, alkynes, free radicals (saturated type), carbonium ions (saturated type) and nitro compounds with α-hydrogen.
- 10. CONDITIONS: • Sigma bondshould be weak enough so that electron waves can come out and enter into the adjacent orbitals. • It operates only from alpha carbon. • Bond should be parallel. • More conjugated hydrogen(s) more is the stability of compound.
- 11. TYPES OF HYPERCONJUGATION: •Sacrificial hyper conjugation • Isovalant hyper conjugation • Reverse hyper conjugation
- 12. Sacrificial hyper conjugation: •DEFINITION: • Hyper conjugation occur in neutral or ground state is called sacrificial hyper conjugation. • Sacrificial hyper conjugation, also known as heterovalent hyper conjugation, is a type of hyper conjugation where a contributing structure has one fewer bond than the main Lewis structure, often involving the donation of electrons from a sigma (σ) bond into a p-orbital or π-system. • EXAMPLE:
- 13. Isovalant hyper conjugation: •Isovalent hyper conjugation is a stabilizing interaction in molecules, particularly carbocations and free radicals, where sigma (σ) bonds, typically C-H bonds, interact with an adjacent empty or filled p-orbital or π-system. • The hyper conjugation which is occur due to carbonium ions is called isovalent hyper conjugation.
- 14. Reverse hyper conjugation: •A type of hyper conjugation in which delocalization of sigma bond toward reverse direction is called reverse hyper conjugation. • Reverse hyper conjugation is a situation in which an electron contact is directed from the pi bond to the sigma bond rather than from the sigma bond to the pi bond. • Reverse hyper conjugation, also known as negative hyper conjugation. • In negative hyper conjugation, the electron density flows in the opposite direction (from π- or p-orbital to empty σ*-orbital) than it does in the more common hyperconjugation (from σ-orbital to empty p-orbital). In other words, there is a movement of electron density towards the sigma bond.
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- 16. PHARMACEUTICAL APPLICATION: • 1:Alkenesand their stability: • Stability of alkenes will increase with increase in number of Hydrogen α to unsaturated system. • Increasing order of stability of alkenes because of Hyper conjugation:
- 17. 2:Stability of carbocations(Carbonium ions): • Carbocations have an electron deficient (positively charged) carbon. • The empty p orbital of this sp2 carbon can overlap with σ orbital of C – H bond of adjacent alkyl group (α C – H bond). • This overlap permits individual electrons to help bind together three nucleus: 2 carbon and 1 hydrogen. • The positive charge thus gets dispersed over large volume of space and is stabilized.
- 18. Relative stability ofcarbocations: • In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilization of the cation. • Thus, we have the following relative stability of carbocations :
- 19. Continue… Note: 1. Number ofhyper-conjugating structures = number of α-hydrogens. 2. Greater the no. of hyper-conjugating structures, more is the stability of the system.
- 20. 3:Stability of FreeRadicals: • The stability of free radicals is influenced by hyper conjugation in the same fashion as in case of carbonium ions. • The σ-electrons of the α-C-H bond can be delocalized into the p-orbital of carbon containing an odd electron. • Due to hyper conjugation, the stability of free radicals also follows the same order as that of carbonium ions. i.e. methyl < primary < secondary < tertiary
- 21. Continue… 30 alkyl radical >20 alkyl radical > 10 alkyl radical > methyl radical
- 22. 4: Dipole moment: •Hyper conjugation can increase electron density around atoms, affecting the overall dipole moment of the molecule. • As hyper conjugation affects the development of charges, it as well affects the dipole moment in the molecule. The increase in dipole moment, while hydrogen of formaldehyde (μ = 2.27 D) is replaced by methyl group, that is acetaldehyde (μ = 2.72 D) can be tends to hyper conjugation that tends to development of charges.