Carbocation ppt
- 1. CARBOCATIONS BOBBY SOLANKI M.Sc Chemistry Sharda University Greater Noida
- 2. CARBOCATIONS • A carbocation is an ion with a positively-charged carbon atom. • It has Planar structure and sp2 hybridized • bond angle 120o • Has six electrons C Empty unhybridised p-orbital sp2 Hybridisation of carbon Planar Strucutre of carbnion +
- 3. Generation of carbocation: 2.From alkenes: 1.
- 4. R + N 2 + R N 2 + •By the heterolysis of alkyl diazonium salt R R R R R R + -e -e + . R + . . R + •By the removal of an electron from a neutral molecule or a free radical
- 6. The most stable of all alkyl cations is the tert-butyl cation. : The electron-donating effect of alkyl groups increases the electron density at the charge-bearing carbon, reducing the net charge on the carbon, and in effect spreading the charge over the α carbons. + CH3 H C C 3 CH3 + + + CH3 H3C CH2 H3C CH CH3 +I effect
- 7. : Tertiary carbocations are more stable (and form more readily) than secondary carbocations; primary carbocations are highly unstable because, while ionized higher order carbons are stabilized by hyperconjugation, unsubstituted (primary) carbons are not. CH3 + CH2 H H + + H H H H H CH C + H H H H NONE Three for 3C-H bonds H H Six for 6C-H bonds H H HH H Nine for 9C-H bonds
- 8. 3. +R groups stabilize the carbocations: 2 + H3 C O CH H C 3 O + CH 2 CH2 + + CH CH2 CH + CH2 + HC CH2 CH2 + R CH2 + R C + H CH2 CH2 + H2C + CH2 4. Some carbocations are stabilized due to aromatization. For eg; + + Cyclopropenyl cation is stable due to aromatization
- 9. 1. R+ cation act as an electrophile to react with nucleophiles.For Eg; R+ + Nu- R Nu 2. Some carbocations act as Bronsted acid to lose a proton . For Eg; H H R C+ R R R R R R H + + → Reactions of carbocations:
- 10. O R H + + C OH R R + R H2C CH2 + H + 3 H C CH2 + O R R + H3C F BF3 O + CH 3 R R + BF4 _ R R O CH3 + + By the addition of a cation to a neutral molecule. Addition reaction:
- 11. Wagner-Meerwein rearrangement: • When alcohol containing more than two alkyl or aryl group on β carbon are treated with acid, the product formed is generally a rearranged product, rather than simple substitution or elimination product. This reaction is called Wagner - Meerwein rearrangement. • Newly generated carbocation is stabilized generally by loss of a proton to give olefin.
- 12. The earliest examples of Wagner-Meerwein rearrangement was noticed in bicyclic terpenes. R H R1 OH H R2 R3 R H R1 OH2 R2 R3 R R1 R2 H R3 R2 R1 H R3 R R2 R1 R R3 Note: that in the initial step a proton is consumed and in the last step a proton is released. Mechanism involves rearrangement of the carbocation intermediate.
- 13. The migrating group can be alkyl, aryl, hydrogen or ethoxycarbonyl. In the case of unsymmetrical diols, which one of the hydroxy group gets protonated it is important. As seen earlier in this module, in general, the hydroxyl group that can generate a more stabilized carbocation is the one which gets protonated. CH3 H CH3 CH3 H3C C C OH OH Pinacol CH3 O H3C C C CH3 CH3 Pinacolone Pinacol Rearrangement: When vicinal diol (also known as pinacol) is treated with acids, it rearranges to give aldehyde or ketone. This reaction is called as Pinacol rearrangement.
- 14. Mechanism : R1 C R3 C C A plausible mechanism can be represented as follows; R2 R4 R2 R4 H R1 C R3 OH OH2 -H2O R1 C OH C OH OH R2 R4 R3 R1 C OH C R2 R4 R3 -H O R1 C C R2 R3 R4
- 15. In this reaction, the hydroxyl group on carbon bearing two phenyl groups will be protonated faster to form a more stable benzylic carbocation. OH OH Ph Ph H2SO4 O Ph Ph
- 16. The following reaction represents a special case of Wagner-Meerweinrearrangement. Generally, a mixture of rearranged and non-rearranged products isformed. NH2 HNO2 OH 2 CH OH + HNO2 CH2NH2 NH2 OH HNO2 OH + Demjanov rearrangement:
- 17. Mechanism is as follows: N O HO N O H2O N O O N O O N O H -H2O N O N O + O NH2 N N OH H NO2 N N OH2 N N H -HNO2 H HN N O -H2O -N2 H2O OH