Docsity
Docsity

Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity


Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan


Guidelines and tips
Guidelines and tips

Electrophilic Addition of Bromine and Chlorine to Alkenes | CHE 255, Exams of Organic Chemistry

Material Type: Exam; Class: Organic Chemistry; Subject: Chemistry and Biochemistry; University: University of Southern Mississippi; Term: Unknown 1989;

Typology: Exams

Pre 2010

Uploaded on 08/19/2009

koofers-user-sut
koofers-user-sut 🇺🇸

10 documents

1 / 30

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
Section 12--Electrophilic Addition of Bromine and Chlorine
to Alkenes
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16
pf17
pf18
pf19
pf1a
pf1b
pf1c
pf1d
pf1e

Partial preview of the text

Download Electrophilic Addition of Bromine and Chlorine to Alkenes | CHE 255 and more Exams Organic Chemistry in PDF only on Docsity!

Section 12--Electrophilic Addition of Bromine and Chlorine

to Alkenes

Addition of Bromine and Chlorine to AlkenesAlkanes do not react with Br

or Cl 2

in the absence of 2

ultraviolet or visible light at room temperature.Alkenes rapidly undergo additions reactions with Br

and Cl 2

in the 2

dark (no radiation) at room temperature:

  • X

dark 25

o^ C

X

X

Examples of the Halogenation Reaction

1-butene

+^

Cl

2

  • 9

o^ C

ChlorinationCH

CH 3

CH=CH 2

2

1,2-dichlorobutane

CH

CH 3

CHClCH 2

Cl 2

cyclopentene

+^

Br

  • 5

o^ C

CCl

/ethanol 4

Bromination

Note: Only the

trans

-1,2-dibromocyclopentane is formed. This result

is understandable when the mechanism of this reaction is examined

trans-

1,2-dibromocyclopentane(Racemic Form)

Br

Br

Br

Br

A Mechanism for the Bromination Reaction

Step 1 Electrophilic Attack

:

: : (^) ::

δ+ δ− : Br Br

As the Br

approaches 2

the alkene, theexposed

π^

electrons

induce a polarizationof the nonbondingelectrons in the Br

: : + bromonium ion

: : (^) : :

  • +^

Br

Br

.

The closer electropositive bromine atomattaches to the two carbons of the alkenesimultaneously to form a bridgingbromonium ion

.

The proposed mechanism for bromination of an alkene follows thegeneral two-step mechanism of addition. The first step is electrophilicattack by the Br

on the p bond to give a cation intermediate, a bridging 2

bromonium ion that controls the stereochemistry of the addition reaction

Because of the structuralsymmetry of the cyclopentylbromonium ion, reaction bypaths (A) and (B) occur withequal probability

bromonium ion

Br

H

H

Nucleophilic attack by bromideion in an S

2 reaction yields theN

trans

-1,2-dibromide.

Br

(A)

(B)

(A)

Br

Br

(B)

Br

Br

racemic form

Stereospecific ReactionsA reaction where one particular stereoisomeric form of the startingmaterial reacts to give a particular stereoisomeric form of the productis a stereospecific reaction. In such reactions, the reaction mechanismintroduces stereospecificity. The brominations of cyclopentene andcyclohexene are examples of stereospecific reactions.

Halohydrin FormationWhen the halogenation reaction is carried out in thepresence of other nucleophiles that compete with halide ion,other addition products are formed.

In water, H

O is a competing nucleophile: 2 X 2 H^2

O

an alkene

Example: Reaction of Cyclopentene

Br

2

H^2

O

cyclopentene

OH

Br

Br

HO

trans

-2-bromocyclopentanol

a halohydrin (as a racemic form)

vicinal dihalide

X^

OH

X^

X

A Mechanism for Bromohydrin Formation

Br

2 H^2

O

Br Br

δ+ δ−

bromonium ion

O Br+ H^

H

A^

B

A H^2

O^

Br

B Br

OH^ +

.

Bromohydrin formation is a stereospecific reaction. Thisreaction specificity is explained by the formation and reaction ofa bromonium ion intermediate

OH

Br

(-H

  • )

Br

HO

(-H

  • ) Water attacks the bromonium ionin an S

2 reaction by paths A and B.N

Oxidations of Alkenes--Syn HydroxylationThe stereospecific formation of 1,2-diols (or glycols) fromalkenes may be carried out in two ways:

KMnO

, HO 4

cold H

O 2

HO

OH

(i) OsO

, pyridine 4

(ii) Na

SO 2

/H 3

O or NaHSO 2

/H 3

O 2

HO

OH

The Mechanism of the Hydroxylation ReactionThe mechanisms of the two hydroxylation reactions arevery similar. The initial step is oxidative addition to thealkene to produce cyclic ester-type intermediates

two electron reduction

Mn O O

O O

permanganate ion

O manganate ester

O Mn O^

O

two electron reduction

Os O O

O O

osmium tetroxide

O osmate ester

O Os O^

O

Examples of Syn-Hydroxylation

MnO

  • 4 cold
O

stereochemistry is set

O^ - Mn O^

O
H^2
O
OH

cis-

1,2-cyclopentanediol

OH
HO

OsO

4

pyridine, 25

o^ C

O^
+^
O

Os O^

O

stereochemistry is set

H^2
O

NaHSO

3

cis-

1,2-cyclopentanediol

OH
HO

Section 17--Oxidative Cleavage of Alkenes

Examples of Oxidative-Cleavage with Hot Potassium Permanganate

adipic acid

HO

OH

O

O

cyclohexene

KMnO

, HO 4
H^2

O, heat

+H

The dicarboxylate salt ,

is produced initially which is neutralized by acid in a workup step.

  • O

O 6

O

O

+K

+K

Terminal alkenes yield a carboxylic acid and CO

: 2

KMnO

, HO 4

H^2

O, heat

+H

1-hexene

6

5

4

3

2

1

+ CO

2

6 pentanoic acid

2 O

OH

carbon dioxide

Predicting the Products of the Oxidative-Cleavage Reaction

.

In these oxidations, the carbons of the alkene are oxidized to thehighest level possible. Products may be predicted by replacing theC=C with C=O + O=C and allowing for the further oxidation ofaldehyde products to carboxylic acidsExamples

KMnO

, HO^4
H^2

O, heat

+H

(^5) 3-methyl-2-pentene

5

4

3

O^

HO

2

1 O

KMnO

, HO 4
H^2

O, heat

+H

1-methylcyclohexene

a

a

O

HO

O