Alkenes
Mechanism for electrophilic addition of alkenes
- two steps:
(1) The close approach of the electrophile brings about polarization of C=C, which subsequently
undergoes a heterolytic fission to form a carbocation / carbonium ion.
(2) The carbocations attains stability by accepting an electron pair from another nucleophile to form
addition product.
- equation for bromination (97/9(b)(iii))
- equation for hydrobromination (95/12(a)(ii), 98/6(a)(ii))
Inductive effect
- electron-withdrawing (-F, -Cl, -Br) causes negative I effect
- electron-donating (alkyl groups) causes positive I effective
Relative stability of carbocations
- As the number of alkyl groups on positive C atom increases, positive I effect increases. Thus positive
charge on C atom is dispersed more (vice versa, intensified), that is, charge density decreases. This
leads to an increase in the stability of carbocation and thus the major product.
Other examples of addition reaction
- Reaction with [MnO4]-/H+: CH2CH2 + [O] + H2O ------> CH2OHCH2OH
- Hydration: CH2CH2 + H2O (g) ------(H3PO4, 300*C, 65 atm)----> CH3CH3OH
- Hydrogenation: CH2CH2 + H2 ------(Ni, 150-200*C)-----> CH3CH3
- Catalytic hydrogenation used in hardening of edible oils (triesters) to form margarine
(1) Conditions: H2, Ni, 150-200*C (99/13(d)(i))
(2) The equation
(3) Comparison of edible oils and margarine
| edible oils (triesters) | margarine |
| R1, R2 and R3 contains more C=C bonds | R1', R2' and R3' contains less C=C bonds |
| molecular shapes are less regular | molecular shapes are more regular |
| molecules packed less closely | molecules packed more closely |
| lower m.p. | higher m.p. |
| liquid | solid |
(4) Medical evidence:
(a) animal fat is saturated and have few C=C bonds, thus dangerous to health
(b) vegetable oil is unsaturated and have more C=C bonds, thus safer
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