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Reactions of Alkenes…the details

• Electrophilic addition reactions, general

– General mechanism (ex: hydrogen halides)
– Carbocations & transition states
– Regioselectivity
– Rearrangement of carbocations

• Specific reactions

– Addition of water and alcohol
– Addition of halogens
– Oxymercuration-reduction
– Hydroboration-oxidation
– Addition of radicals
– Addition of hydrogen

• Relative stabilities of alkenes

Electrophilic Addition of Alkenes

Every reaction:

1. Starts with addition of electrophile to sp

2

C

2. Ends with addition of nucleophile to second sp

2

C

Example: Addition of Hydrogen Halides

2

What is the product?

We must examine the

mechanism…

a more stable
carbocation

Carbocation formation is the rate-limiting step

Carbocation Stabilities

3

Delocalization of Electrons

Alkyl groups decrease the concentration of positive
charge in the carbocation

Hammond postulate: the transition state will be more
similar to the species that it is closer to energetically

Exergonic reaction: early transition state

Endergonic reaction: late transition state

4

I: early transition state
II: mid-transition state

III: later transition state

A more stable transition state gives a faster reaction

In a regioselective reaction, one constitutional isomer is the
major or the only product

Regioselectivity: Markovnikov’s Rule

The electrophile adds to the sp

2

carbon that is bonded to

the greater number of hydrogens

5

Carbocation Rearrangement

1,2-hydride shift

a more stable
carbocation

a more stable
carbocation

Carbocation Rearrangement

1,2-methyl shift

Carbocation Rearrangement

a more stable

carbocation

Ring Expansion

6

Carbocation does not always rearrange …

Addition of Water to Alkene

Remember steps 1 & 2

Addition of water is also called hydration

Hydration of Propene

7

Acid-Catalyzed Addition of Alcohol

1. Addition of

electrophile?

2. Addition of

nucleophile?

Addition of Halogens to Alkene

1. Addition of

electrophile?

2. Addition of

nucleophile?

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Addition of Halogens

in the Presence of Water

1. Addition of

electrophile?

2. Addition of

nucleophile?

Why is H

2

O the

nucleophile instead

of Br

-

?

Consider the transition states …

Why a secondary alcohol instead of a primary alcohol?

Oxymercuration and Mercuration

of Alkene lead to hydration

1. Addition of

electrophile?

2. Addition of

nucleophile?

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Demercuration by Reduction

Addition of Borane

Hydroboration–Oxidation

Anti-Markovnikov’s rule in product formation

Anti-Markovnikov

addition

Markovnikov

addition

(a pericyclic reaction)

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Formation of Alkyl Boranes

OH Replacement of Boron

Anti-Markovnikov Addition

of an OH Group

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Synthesis of Bromobutane
2 ways: (2 different isomers)

Generation of Radicals

Addition of Radicals to Alkene

Formation of bromine radical

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Addition of radicals to alkene, cont.

Formation of most stable alkyl radical and

reformation of Br radical

Addition of radicals to alkene, the end.

Three different ways that 2 radicals can combine to

make an electron pair

Relative Stabilities of Alkyl Radicals

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Calculating the Change in H

°

Note: only in Br case are both propagation steps

exothermic

Addition of Hydrogen to Alkenes

Catalytic Hydrogenation of an Alkene

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Potential Energies of Pentene Isomers

Relative Stabilities of Alkenes

Steric Strain in Alkenes

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Relative Stabilities of Dialkyl-

Substituted Alkenes