The characteristic reaction of alkenes is addition to the double bond.

+ A—BC C A C C B

Reactions of Alkenes

Reactions involving carbocations:

1. Substitutions via Sn1.

2. Eliminations via the E1.

3. Additions to alkenes and alkynes.

Reminder:

Reactions of Alkenes

CH3

+

2° carbocation

CH3

+

3° carbocation

1,2-alkyl shift for stability

Terms: Driven by weaker pi bond to 2, new, stronger sigma bonds.

Overall Transformation: C=C to a new functional group.

Regioselectivity: Which side a specific group is added to.

Stereoselectivity: Whether the addition is cis or trans.

Nucleophile: pi bond has high electron density; reacts with electrophiles.

Electrophile: An electron poor species, which reacts with electron rich

species.

Electrophilic Addition: An electrophile adds to a nucleophile, where two

systems combine to form one product. In actuality, the nucleophile

attacks the electrophile

Alkene Addition Terms

syn addition anti addition

syn Addition versus anti Addition

General Format:

Depending on timing, two different mechanisms are possible:

Two Step: Full Carbocation Mechanism

1: Reaction of the C=C with E+ Carbocation

2: Nucleophilic attack.

Electrophilic Addition

NuE

NuE +

:Nu-Nu E +

E

Nu

+ - E +

One Step: Partial Carbocation Mechanism

Simultaneous formation of the two sigma bonds.

Tips:

1. Find the electrophile.

2. The alkene attacks the electrophile

- Regioselectivity.

3. Figure out if carbocations are formed.

- 2 step reaction is not stereoselective.

Electrophilic Addition

E Nu E Nu

One Step: Partial Carbocation Mechanism

Simultaneous formation of the two sigma bonds.

Tips:

4. Determine if the new sigma bonds are from the same species

- Syn addition.

5. Determine if the two new sigma bonds are from different species

- Anti addition.

Electrophilic Addition

E Nu E Nu

CH3CH2 CH2CH3

H H

CH3CH2CH2CHCH2CH3

Br

(76%)

CHCl3, -30°CC C

HBr

Example

Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate.

Mechanism

Electrons flow fromthe system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.

Mechanism

Free Radical Mechanism:

Free Radical Mechanism

Br Br

H

BrBr

H

Which mechanism is used for the following reactions?

Carbocation

Radical

Learning Check

C C H X C C XHX= I, Br, Cl

+

C C H X C C XHX= I, Br, Cl

+

hv

Electrophilic Addition:

Hydrogenation:

Hydrohalogenation:

Hydration:

Hydroboration:

Alkene Reactions

Nu E

E

Nu

+

C C H H C C

H H

catalyst+

C C H X C C XHX= I, Br, Cl

+

C C H OH C C OHH+H+

C C H OH C C OHH+1. BH3

2. H2O2/NaOH

Halogenation:

Halohydration:

Hydrohalogenation:

Epoxidation:

Ozonolysis:

Alkene Reactions

C C H X C C XHX= I, Br, Cl

+

C C X X C C

X

X

+X= I, Br, Cl

C C X XH2O

C C

OH

X

HX+X= I, Br, Cl

+

C C RCO3H C C

O

RCO2H+ +

C C O3C O CO+

+

work-up

Reaction Type: Electrophilic Addition (Reduction). Used for heats of hydrogenation stability.

Overall Reaction: H2 + Alkene Alkane

Requires: Metal catalyst (Pt, Pd, Ni, Rh).

Regioselectivity: None.

Stereospecificity: Only syn addition products.

Hydrogenation

C C H H C C

H H

catalyst+

H HC C

A

B

X

YH H

Mechanism of Catalytic Hydrogenation

H

H H H

Mechanism of Catalytic Hydrogenation

C CA

B

X

YAbsorption of H

by catalyst

H

H H HCC

AB

XY

Mechanism of Catalytic Hydrogenation

Addition of alkene to catalyst.

H

H H HCC

AB

XY

Mechanism of Catalytic Hydrogenation

H H

H

CC

AB

XY

H

Mechanism of Catalytic Hydrogenation

Addition of H to alkene.

H HH

CC

AB

XY

H

Mechanism of Catalytic Hydrogenation

Alkane leaves catalyst with H in

syn position.

CO2CH3

CO2CH3

(100%)

H2, Pt

Example of syn-Addition

CO2CH3

CO2CH3

H

H

Example 1:

Example 2:

Learning Check

H2

or

When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

Markovnikov's Rule

Occurs due to the formation of the most stable carbocation:

Markovnikov rules extend to other additions which don’t involve halides:

Markovnikov Example

C C

CH3

H X C C

CH3

Br

X= I, Br, Cl+ C C

CH3

Br

+

Major product

CH3 CH3

HOH

H3O+

Br

CH3CH2CHCH3CH2CH3CH2CH

acetic acid

HBr

Mechanistic Basis for Markovnikov's Rule:

Example 1

Br

CH3CH2CHCH3CH2CH3CH2CH

HBr

CH3CH2CH—CH3 + Br – +

CH3CH2CH2—CH2

+

primary carbocation is less stable: not formed

Mechanistic Basis for Markovnikov's Rule:

Example 1

Mechanistic Basis for Markovnikov's Rule:

Example 2

H

CH3

CH3

Cl

HCl

0°C

HCl

H

CH3

CH3

Cl

H H CH3

+

secondary carbocation is less stable: not formed

CH3

H

+

H

Cl–

Reactions that are Anti Markovnikov add the hydrogen to the side with the least number of hydrogens. The ‘group’ is attached to the less substituted carbon.

Reaction does not go through a carbocation intermediate.

Anti Markovnikov

H

H

CH3

H

1. BH3

2. H2O2/ NaOH

H

H

CH3

HH OH

Reaction of Alkenes with HBr (no radical formation).

Reaction Type:

Overall Reaction: HX + Alkenes Alkyl Halide.

Hydrogen Halide Reactivity Order:

HI > HBr > HCl > HF (same as acidity order)

Regioselectivity: Markovnikov Rule.

“For addition of hydrogen halides to alkenes, the H atom adds to the C with the most H atoms already present” meaning it goes through the more stable carbocation intermediate.”

Hydrohalogenation

Electrophilic Addition; Reduction (H) and Oxidation (X)

C C H X C C XHX= I, Br, Cl

+

Reaction of Alkenes with HBr (no radical formation).

Stereoselectivity: None, during mechanism, a planar carbocation is formed.

Requirements: HBr, No light (hv), no peroxides, no radicals created.

Hydrohalogenation

C C H X C C XHX= I, Br, Cl

+

Mechanism:

Hydrohalogenation

H

H

H

CH3

H+

H

C

CH3CH3

+

Br-H

C

CH3CH3

BrProtonate alkene.

Can be viewed as an acid base reaction (Lewis).

Pi electrons act as a Lewis Base.

Bromide ion is nucleophile. Carbocation is electrophile.

Forms most stable carbocation.

Nucleophilic attack.Forms alkyl bromide.