Elecrophilic addition - youtube link
Markovnikov’s rule states that the hydrogen adds to the sp2 carbon of the alkene bonded to the greater number of hydrogens. While all addition reactions add the electrophile to the sp2 carbon bonded to the greater number of hydrogens, they all do not follow Markovnikov’s rule, because hydrogen is not always the electrophile. Hydroboration–oxidation and the addition of HBr in the presence of a peroxide are anti-Markovnikov additions. The addition of hydrogen halides and the acid-catalyzed addition of water and alcohols form carbocation intermediates.
Hyperconjugationcauses tertiary carbocations to be more stable than secondary carbocations, which are more stable than primary carbocations. A carbocation will rearrange if it becomes more stable as a result of the rearrangement. Carbocation rearrangements occur by 1,2-hydride shifts, 1,2-methyl shifts, and ring expansion. HBr in the presence of a peroxide forms a radical intermediate. Radical intermediates do not rearrange. Hydroboration–oxidation is a concerted reaction and does not form an intermediate.
Oxymercuration,alkoxymercuration, and the addition of Br2 and Cl2 form cyclic intermediates.
Oxymercuration and alkoxymercuration are followed by a reduction reaction. Reduction increases the number of C H bonds or decreases the number of C O, C N, or C X bonds (where X denotes a halogen). Hydroboration is followed by an oxidation reaction. Oxidation decreases the number of C H bonds or increases the number of C O, C N, or C X bonds (where, again, X denotes a halogen).
The Hammond postulate states that a transition state is more similar in structure to the species to which it is more similar in energy. Thus, the more stable product will have the more stable transition state and will lead to the major product of the reaction.
Regioselectivity is the preferential formation of one constitutional isomer over another. In heterolytic bond cleavage, a bond breaks such that both electrons in the bond stay with one of the atoms; in homolytic bond cleavage, a bond breaks such that each of the atoms retains one of the bonding electrons.
An alkyl peroxide is a radical initiator because it creates radicals. Radical addition reactions are chain reactions with initiation, propagation, and termination steps. Radicals are stabilized by electron-donating alkyl groups. Thus, a tertiary alkyl radical is more stable than a secondary alkyl radical, which is more stable than a primary alkyl radical.
A peroxide reverses the order of addition of H and Br because it causes instead of to be the electrophile. The peroxide effect is observed only for the addition of HBr. The addition of to a reaction is called hydrogenation.
The heat of hydrogenation is the heat released in a hydrogenation reaction. The greater the stability of a compound, the lower is its energy and the smaller is its heat of hydrogenation. The more alkyl substituents bonded to the sp2 carbons of an alkene, the greater is its stability. Hence, carbocations, alkyl radicals, and alkenes are all stabilized by alkyl substituents. Trans alkenes are more stable than cis alkenes because of steric strain. Electrophilic addition reactions of alkenes lead to the synthesis of alkyl halides, vicinal dihalides, halohydrins, alcohols, ethers, and alkanes.
from - Organic chemistry - Bruice - Chapter 4 summary
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