Chapter 8

Alkenes and Alkynes IIAddition Reactions

Introduction: Additions to AlkenesAn addition to a C-C double bond

C C

H X

H OSO3H

H OH

HA (cat)

X X

H C C X

H C C OSO3H

H C C OH

X C X

H+

carbocation

X H X+

HX

Addition Products

HX

H+

carbocation

+ X

alkene

X HC

H

C

Electrophile Nucleophile

+ X

C

H

C

electrophile

+ X

nucleophile

C C

H X

Electrophiles include postive reagents such as proton (H+), neutral reagent such as bromine and the Lewis acids such as BH3, BF3 and AlCl3Metal ions that contain vacant orbitals: Ag+, Hg2+ or Pt2+

CH2 CHCH

H X

H2C CHCH3

H X

Markovnikov addition product

carbon atomwith greater number of hydrogen atoms

+ HBr

Br

Br+

littlemajor

+ HBrBr

Br

major minor

The Rate Determining stepStep 1 is the rate determined step

Formation of carbocation

CH3CH CH2

HBr

slow

CH3CH2CH2

1o

Br-CH3CH2CH2 Br

CH3CHCH3Br-

2o

CH3CHCH3

Br

1-bromopropanelittle formed

2-bromopropanemain product

A mechanism for the reactionPredict the products and show the full

mechanism

+ HCl

Modern Statement of markovnikov’s RuleIn the ionic addition of an unsymmetrical

reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon of the double bond so as to yield the more stable carbocation as intermediate

H3C

C

HC

CH2 + I Cl CH2

I

ClCl I

H

H

2-methylpropene 2-chloro-1-iodo-2-methylpropane

Regioselective ReactionsWhen a reaction that can potentially yield

two or more constutional isomers actually produces only one (or a predominance of one), the reaction is said to be regioselectiveHX + unsymetrical alkene more than

product

An exception of Markovnikov’s RuleWhen alkenes are treated with HBr in

presence of peroxide, an anti-Markovnikov addition

CH3CH CH2 + HBr

ROORCH3CH2CH2Br

8.3 Stereochemistry of the Ionic Addition to an AlkeneThe carbocation is formed in the first step of

the addition is trigonal planarachiral

C2H5 CH

CH2

H X

C2H5

H

CH2

H

+ X

a

b

C2H5

X

HCH3

(S)-2-halobutane 50%

b

a

C2H5

HCH

X(R)-2-halobutane50%

8.4 Addition of Sulfuric Acid AlkenesWhen alkenes are treated with cold

concentrated sulfuric acid, they dissolve because they react with electrophilic addition to form alkyl hydrogen sulfate

H O

O

O

O

H

Alkene sulfuric acid

H

O

O

O

O

H

OSO3H

H

carbocation hydrogensulfate ion

alkyl hydrogen sulfate

Alcohols from Alkyl Hydrogen SulfateAlkyl hydrogen sulfates can be easily

hydrolyzed to alcohols by heating them with waterMarkovnikov addition of H- and -OH

CH3CH CH2cold

H2SO4

CH3CHCH3

OSO3H

CH3CHCH3

OH

H2O, heat

exampleProvide mechanistic explanation for the

following observationThe addition of hydrogen chloride to 3-methyl-

1-butene produces two products: 2-chloro-3-methylbutane and 2-chloro-2-methylbutane

Give the structure and name of the product that would be obatained from ionic addition of IBr to propene

exampleIn one industrial synthesis of ethanol, ethene

is first dissolved in 95% sulfuric acid. In a second step of water is added and the mixture is heated. Out the reaction involved

8.5 Addition of Water to Alkene:Acid-Catalyzed Hydration Method for preparation of low molecular

weight alcohols Usually regioselective

+ HOHH3O

H OH

2-methylpropene

+ HOHH3O

25oH

HO

2-methyl-2-propanol

+ H O

H

H

slow

step 1

H

O

H

H

alkene donates an electron pair to a proton to form the more stable 3o carbocation

step 2

O

H

H

fast

O

H

H

The carbocation reacts with a molecule of water to form a protonated alcohol

step 3

O

H

H O

H

Hfast

O H H O

H

H+

A transfer of a proton to a molecule of water leads to the product

Rate determining step

However, formation of 1o carbocation does not take place

+ H O

H

H

slow

step 1

H

O

H

H

alkene donates an electron pair to a proton to form the more stable 3o carbocation

+ H O

H

H

slow

step 1

O

H

H

alkene donates an electron pair to a proton to form the more stable 1o carbocation

H

HH

RearrangementThe carbocation formed initially invariable

rearranges to a more stable one if possibleProvide the full mechanism

H2SO4

H2OHO

3,3-dimethyl-1-butene 2,3-dimethyl-2-butanolmajor