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Material Type: Exam; Class: Organic Chemistry; Subject: Chemistry and Biochemistry; University: University of Southern Mississippi; Term: Unknown 1989;
Typology: Exams
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Addition of Bromine and Chlorine to AlkenesAlkanes do not react with Br
dark 25
o^ C
X
X
Examples of the Halogenation Reaction
2
2
1,2-dichlorobutane
cyclopentene
A Mechanism for the Bromination Reaction
Step 1 Electrophilic Attack
:
: : (^) ::
δ+ δ− : Br 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
Br
H
H
Nucleophilic attack by bromideion in an S
2 reaction yields theN
trans
-1,2-dibromide.
Br
(A)
(B)
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
Example: Reaction of Cyclopentene
2
OH
Br
Br
HO
X^
OH
X^
X
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
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:
(i) OsO
, pyridine 4
(ii) Na
SO 2
/H 3
O or NaHSO 2
/H 3
O 2
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
stereochemistry is set
O^ - Mn O^
cis-
1,2-cyclopentanediol
OsO
4
pyridine, 25
o^ C
Os O^
stereochemistry is set
NaHSO
3
cis-
1,2-cyclopentanediol
Examples of Oxidative-Cleavage with Hot Potassium Permanganate
adipic acid
HO
OH
O
O
cyclohexene
KMnO
O, heat
The dicarboxylate salt ,
is produced initially which is neutralized by acid in a workup step.
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
2
.
KMnO
O, heat
(^5) 3-methyl-2-pentene
5
4
3
O^
HO
2
1 O
KMnO
O, heat
1-methylcyclohexene
a