Addition Reactions of Alkenes
The most characteristic reaction of alkenes is addition to
the double bond.
Hydrogenation of Alkenes
Exothermic reaction with heat given off (heat of
hydrogenation).
The reaction is slow due to a high activation energy and
is normally catalyzed by finely divided transition metals.
Possible Stereochemistry of Addition
The alkene is planar so addition of A and B could be
either syn (same side) or anti (opposite sides).
Practice Question
Which of the alkenes below will produce 2-
methylbutane on catalytic hydrogenation?
A) 1 and 3
B) 1, 2 and 3
C) 2 and 4
D) 2, 3 and 4
Heats of Hydrogenation and Stability
Since more substituted alkenes are more stable the heat of
hydrogenation of substituted alkenes should be lower.
Heats of Hydrogenation and Stability
less stable
more stable
Addition of Hydrogen Halides to Alkenes
Addition of H-X to an alkene yields an alkyl halide according
to the equation:
Addition of Hydrogen Halides to Alkenes
More substituted double bonds are more electron rich so
they react faster.
HI is the strongest acid so it reacts fastest in the series.
Selectivity of Addition of HX to Alkenes
Addition of HX is highly selective:
Markovnikov’s Rule for Addition of HX
Markovnikov’s rule: when an unsymmetrically substituted
alkene reacts with a hydrogen halide, the hydrogen adds
to the carbon that has the more hydrogens, and the halogen
adds to the carbon that has fewer hydrogens.
Practice Question
The reaction of 3-methyl-1-butene with HBr
produces 2-bromo-3-methylbutane and which
other alkyl halide?
A) B)
C) D)
Acid-Catalyzed Hydration of Alkenes
Electrophilic addition of water to alkenes is an acid
catalyzed reaction.
Markovnikov’s rule is followed.
Rate of the Hydration Reaction
The indication that more substitued alkenes react fastest
suggests carbocation intermediates
Hydration and Dehydration
The hydration of alkenes is the reverse of the dehydration
of alcohols.
These are equilibrium reactions and the real question is:
How can you ensure the reaction goes in the desired
direction?
Hydroboration–Oxidation of Alkenes
Anti-Markovnikov Addition of Water
An indirect method to add water It involves sequential
hydroboration and oxidation. The water is added in an
anti-Markovnikov way.
Hydroboration-Oxidation of Alkenes
Hydroboration is the addition of boron hydride to the alkene:
Oxidation replaces the boron substituent with hydroxyl.
Hydroboration-Oxidation of Alkenes
Boron adds to the least substituted C of the alkene:
The oxidation then forms the alcohol:
Hydroboration-Oxidation of Alkenes
The reaction equation is normally written as:
Hydroboration-Oxidation of Alkenes
No carbocation intermediates so no rearrangements.
Several different forms of BH3 can be used.
Stereochemistry of Hydroboration-Oxidation
The H and OH are added in a syn orientation.
Addition of Halogens to Alkenes
Electrophilic addition of halogens to alkenes yields vicinal
dihalides (X = Br, Cl)
Stereochemistry of Addition of Halogens
Trans-dihalides are exclusively formed by anti addition.
Rates of Addition of Halogens to Alkenes
Rate of reaction increases with substitution suggesting that
flow of electrons from the alkene to bromine is rate
determining.
Halohydrin Formation
Reaction of bromine with alkenes in the presence of water
yields vicinal halohydrins (halogen and a hydroxyl on
adjacent carbons).
Halohydrin Formation
The reaction is regioselective and the hydroxyl group
always attaches to the most substituted end of the alkene.
Epoxidation of Alkenes
Reaction of alkenes with peracids yields epoxides.
For example:
Rate of Epooxidation
Rate of reaction increases with increasing substitution of
the alkene.
Ozonolysis of Alkenes
Ozone is the triatomic allotrope of oxygen.
Ozone reacts with alkenes to form ozonides which undergo
hydrolysis in water to carbonyl compounds.
Ozonolysis of Alkenes
Typical reactions are:
Problem Solving with Ozonolysis
Determine the structure of the alkene given the products:
Since the carbonyl carbons were the two ends of an alkene
we can figure out what the alkene was:
Addition of HBr to Alkenes
Addition of HBr to alkenes can be directed to either
of two regioisomers:
Light Initiation of Addition of HBr
The free radical reaction can be initiated by light with, or
without, peroxides.
Introduction to Organic Chemical Synthesis:
Retrosynthetic Analysis
One approach to the synthesis of a given a target is to
think backwards to the starting material and the
reactions needed.
For example: How do you synthesize 1,2-epoxycyclo-
hexane from cyclohexanol?
The double arrow is used in retrosynthetic analysis.
Retrosynthetic Analysis
Determine which compound the epoxidcan be synthesized
from and then how that compound can be synthesized
from cyclohexanol.
Then work forward and fill in the reagents required:
