alkenes report

Depending on the structure of the alkyl halide and alcohol, dehydrohalogenation and dehydration often yield a mixture of isomeric alkenes; however only one isomer generally predominates.

(1841– 1910), was a Russian chemist from Kazan. He worked on organic compounds and proposed Zaitsev's rule, which predicts the product composition of an elimination reaction.

“In dehydrohalogenation and dehydration, the double bond of the alkene is formed

predominantly towards the adjacent carbon atom having fewer hydrogen atoms.”

CH3CH2CHCH3

Br

CH3CH=CHCH3 CH3CH2CH=CH2

H H H HH C C = C C H

H H

Major product(disubstituted alkene)

H H H HH C C C = C H H H

Minor product(monosubstituted alkene)

CH3

|CH3-C-CH2CH3

| OH

CH3

|CH3C=CHCH3

Major product Minor product

CH3

|CH2=C-CH2CH3

Cl | CH3-CH3-C-CH3

| CH3

CH3-CH=C-CH3

| CH3 Major

product

Minor product

CH3-CH2-C-CH3

|| CH2

H2 O

+H3 O Cl

In an addition reaction, carbon-carbon double bonds become single bonds. This means that an unsaturated hydrocarbon becomes a saturated organic compound.

+

Unsaturated hydrocarbon

Saturated organic compound

Why do alkenes undergo addition reactions?

Carbon-carbon double bonds in alkenes are reactive. readily undergoes addition reactions

Hydrogenation

Hydrogenation is the addition of hydrogen to the double bond.

Halogen Addition

This reaction involves the addition of Cl2 or Br2 (I2 is too unreactive) to the alkene, forming a vicinal dihalide.

Hydrohalogenation

Hydrohalogenation is the addition of gaseous hydrogen halide, HX, to an alkene, forming an alkyl halide.

The order of reactivity of HX is HI> HBr > HCl. The order of reactivity of the HX reflects their ability to donate a proton.

Hydrohalogenation

For the type RCH=CHR, the hydrogen of HX may add to any of the doubly-bonded carbon atoms, forming only one product.

Hydrohalogenation

For unsymmetrical alkenes of the type R-CH=CH3, there is the possibility of obtaining two isomeric products during hydrohalogenation.

(1838 –1904), was a Russian chemist. Markovnikov is best known for Markovnikov's rule, elucidated in 1869 to describe addition reactions of H-X to alkenes.

“When a hydrogen halide adds to an

unsymmetrical alkene, the hydrogen adds to the

carbon that already holds the greater

number of hydrogens, and the halogen adds to the carbon having fewer

hydrogens.”

Markovnikov’s Rule

Orientation of Addition of HBrThe addition of HBr was unpredictable in that sometimes addition occurred according to Markovnikov’s rule and at other times, anti-Markovnikov addition was observed. Kharasch and Mayo discovered that anti-Markovnikov addition was promoted when peroxides were present during the reaction.

Orientation of Addition of HBr

Hydration

Hydration is the addition of water to the carbon-carbon double bond to yield an alcohol.

Hydroxylation

Hydroxylation is an oxidation reaction wherein alkenes are converted to 1,2 diols (dihydroxy alcahols containing the two –OH groups on adjacent carbons). The most commonly used oxidizing agent is aqueous KMnO4 solution. The reaction amounts to an addition of two hydroxyl groups to the double bond.

Polymerization

Polymerization is the combining together of alkene molecules (monomers) to form one large molecule, called a polymer.

Alkenes will polymerize in the presence of acid catalysts via a carbonium ion mechanism, or peroxide initiators via a free radical mechanism.

Polymerization

General Equation:

Reaction with ethene

Reaction Conditions

Example

Hydrogenation(Addition of hydrogen)

Pt, Pd, Rh, or Nicatalyst

Halogen AdditionRoom

temperature

Hydrohalogenation(Addition of

hydrogen halide)- C C

H H

H HH

H HC CH H

Cl+ HCl

C CH H

H HH

H HC CH H

H+ H2

C CH H

H HH

H HC CCl Cl

H+ Cl2

Reaction with ethene

Reaction Conditions

Example

Hydration (Addition of steam) H2SO4

Hydroxylation KMnO4

Polymerization peroxide

C CH H

H HH

H HC CH OH

H+ H2O

C C CH H H

H H HCH3

H HC COH OH

HHKMnO4

C C CLH H

H H

H HC CH Cl

nn

Dehydrohalogenation of Alkyl HalidesFor primary alkyl halides, the mechanism is a continuous one-step process.

The basic reagent directly abstracts a hydrogen ion on an adjacent carbon at the same time that the double bond is forming and the halide ion is leaving.

Dehydrohalogenation of Alkyl HalidesFor secondary and tertiary alkyl halides, the mechanism involves two steps.

Step 1: The alkyl halide undergoes slow heterolysis to form a carbonium ion and a halide ion.

Dehydrohalogenation of Alkyl Halides

Step 2: The carbonium ion rapidly loses a proton to the base. The electron pair left behind forms the pi bond between the carbon atoms.

Dehydrohalogenation of Alkyl Halides

Example

MechanismCH3-CH2-CH-CH3

| Cl

CH3-CH=CH-CH3

Step 1 : Loss of Cl produces a 2° carbonium ion

CH3-CH2-CH-CH3

| Cl

CH3-CH=C+H-CH3

Slow, E1

-Cl

2 butene

Mechanism

Step 2 : The carbonium ion rapidly loses a proton to the base. The electron pair left behind forms the pi bond between the carbon atoms.

CH3-CH2-C+H-CH3CH3-CH=CH-CH3

Fast

-H+ (from C2) 2-butene

CH3-CH2-CH=CH2

1-butene

Fast

-H+ (from C2)

Mechanism

Dehydration of alcohols. In dehydration reactions, a molecule of water is eliminated from an alcohol molecule by heating the alcohol in the presence of a strong mineral acid. A double bond forms between the adjacent carbon atoms that lost the hydrogen ion and hydroxide group.

CH3-CH2-OH + H2SO4 CH2=CH2 + H2Oheat

Mechanism

1. Protonation of the alcohol

CH3-CH2-O-H

¨

CH3-CH2-O+-H

|

H

heat

H+ H | |O- O SO O

H |-O O SO O

¨

Mechanism

2. The protonated ethanol loses a water molecule to give a carbonium ion.

CH3-CH2-O+-H

|

H

CH3-C+H2 H2O

Mechanism

3. The carbonium ion loses a proton forming the alkene.

CH2-C+H2

|

H

CH2=CH2

H |-O O SO O

¨

H H | |O O SO O

When propylene polymerizes into polypropylene, the methyl groups appear regularly on alternate carbons of the main chain.

nCH2=CH -CH2CH-CH2-CH-CH2-CH-

CH3 CH3 CH3 CH3

OR

(-CH2-CH-)n

CH3

H+

When a hydrogen halide adds to an unsymmetrical alkene, the hydrogen adds to the carbon that already holds the greater number of hydrogen, and the halogen adds to the carbon having fewer hydrogen.

CH3CH=CH2 + HCl CH3CHCH3

|

Cl

Used in the production of margarine

Hydrogen + vegetable oil Margarine200 °C, nickel catalyst

The greater the amount of hydrogen used, the more ___________ the fat and the more _________ the margarine becomes.

saturated solid

CH3CH=CH2 + HCl CH3CH-CH3 + CH3CH2-CH2Cl

Step 1. The H+ electrophile protonates the pi bond

CH3CH=CH2 + H:Cl CH3C+H-CH3 + :Cl-

CH3CH=CH2 + H:Cl CH3C2H-C+H2 + :Cl-

Step 2. The carbocation reacts with the halide anion to form the product.

CH3C+H-CH3 + :Cl- CH3CHCH3

ClCH3C2H-C+H2 + :Cl- CH3CH2CH2Cl

A. Chain Initiating Steps

CH3

|BrCH2CHCH3

HOOH CH3

CH2=CHCH3 + HBr2-Methylpropene

B. Chain Propagating Steps

C. Chain Terminating Step

Br· + Br· Br:Br

alkenes report

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