Section 12--Electrophilic Addition of Bromine and Chlorine
to Alkenes
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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
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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