2. Q. Explain the relative magnitudes of the following pairs of β-deuterium isotope effects (per deuterium) for solvolysis (CSE 2006).
❖ A secondary kinetic isotope effect was measured.
❖ Generation of vinyl cation
❖ Hybridization of vinyl cation
❖ Hyperconjugation in vinyl cation
❖ Role of α substituent in vinyl cation stabilization
Insight
Vinyl Cation
Electron-deficient carbon is an
integral part of a π unsaturation
(disubstituted unsaturated analogs
of trisubstituted carbocations)
2
3. Generation of Vinyl cations
1) Electrophilic addition to alkynes and allenes
2) Heterolysis of the (C=)C-X bond: 2 condition
❖ super leaving groups
❖ Intermediate vinyl cations be stabilized by α substituents with
electron-donating ability or ß substituents that are capable of
positive charge dispersal
Fluorosulfonates
Trifluoromethanesulfonates
(triflates)
Nonafluorobutanesulfonates
(nonaflates)
Increased strength of the carbon-halogen bond was attributed to
❑ change in hybridization of carbon from sp3 in alkyl halides to sp2
in vinyl halides.
❑ partial double-bond character
3
4. Hyperconjugation in sp-hybridized Vinyl cations
➢ The carbon Cα with the formal positive charge is sp hybridized and the Vacant p-orbital is orthogonal to a C+=Cβ bond
➢ For vinyl cations there are two different positions for hyper conjugating substituents, β-substituent can be attached at the
single bonded adjacent carbon as in trigonal carbocations. The β -substituent can also be attached at the doubly bonded β
carbon of the vinyl cation.
4
5. Evidence suggests that vinyl cations are linear
Solvolysis Rates 1 > 2 > 3 > 4
Solvolysis rates of the simple cycloalkenyl triflates
❖ As ring size decreases, the rate of hydrolysis also diminishes. Implying that the formation of the linear vinyl cation is
disfavored due to increasing ring strain
The X-ray Structure of a Vinyl Cation
Phenyl cations?
5
6. Cyclic dienyl triflates solvolysis rates
Solvolysis Rates
➢ Cycloheptadienyl triflate solvolyzes at almost the same rate as 1-cycloheptenyl triflate, but cyclooctadienyl triflate reacts about
103 times faster than 1-cyclooctenyl triflate
➢ Second double bond should lead to an increased strain if a linear vinyl cation is formed, so reaction rate lower should be
expected. High rates therefore indicate that the intermediate vinyl cation has been stabilized by the second double bond in the
allylic position.
➢ Stabilizing interaction 8 member ring of the participating orbitals can be achieved, owing to the flexibility
➢ In 7 member ring the stabilizing effect between the interacting orbitals is less suitable. allowing only a partial overlap of the
participating orbitals.
<
~
6
7. Vinyl cations be stabilized by α-substituents
❖ Positive charge in vinyl cations is stabilized by electron donating substituents. Aryl or vinyl groups at the α-carbon delocalize
the positive charge by π-conjugation.
7
8. Stablization in α-arylvinyl system
➢ The ground state of an unhindered α-arylvinyl system is stabilized by a π(α -Ar)- π (C=C) conjugation (1).
➢ The incipient cationic orbital is developed in a perpendicular plane. Stabilization of the ion and of the transition state leading
to it is achieved by overlap of the vacant p orbital and the π system of α-aryl group (2).
➢ However, this stabilizing of π(α -Ar)- p(C+) conjugation can be achieved only at the expense of a simultaneous, complete π(α
-Ar)- π (C=C) deconjugation.
8
9. Q. Explain the relative magnitudes of the following pairs of β-deuterium isotope effects (per deuterium) for solvolysis (CSE 2006).
❖ A secondary kinetic isotope effect was measured indicating strong hyperconjugation.
❖ The more destabilizing substituents in increased isotope effects indicating a greater need for hyperconjugative stabilization
of the developing empty p-orbital with increasing electron withdrawal by the substituents.
❖ β-Deuterium isotope effects in solvolytic reactions have been ascribed to differential hyperconjugative overlap of the C-H(D)
bond with the developing vacant p-orbital at the reaction site.
❖ β-isotope effects are strongly dependent upon the dihedral angle between the empty p-orbital and the β-C-H bond with
maximum effects observed with a dihedral angle of 0°.
❖ Owing to their linear geometry vinyl cations satisfy this requirement perfectly
9
