lecture 14
TRANSCRIPT
ELECTROPHILIC ADDITION REACTIONS
Types of alkene addition reactions
ELECTROPHILIC ADDITION
What does the term "electrophilic addition" imply ?
An electrophile, E+, is an electron poor species that will react with an electron rich species such as C=C.
An addition implies that two systems combine to give a single entity.
An electrophilic addition reaction is an
addition reaction which happens because
what we think of as the "important"
molecule is attacked by an electrophile.
The "important" molecule has a region of
high electron density which is attacked by
something carrying some degree of
positive charge.
Generalized mechanism for electrophilic addition
Addition of HX to Alkenes
+ H X H X
Mechanism for HX addition to alkenes
CH3
|
CH2 = C – CH3 > CH3CH = CHCH3 ,
CH3CH2–CH=CH2 , CH3CH=CH2 >
CH2=CH2 > CH2=CHCl
Reactivity of Alkenes towards Acidic Reagents
Evidence for the MechanismEvidence for the Mechanism
Orientation of addition as well as reactivity of
alkenes are consistent with the mechanism i.e.
formation of carbocations as an intermediate.
Rate of reaction depends upon the concentration
of both the alkene and the reagent.
Reaction requires an acidic reagent.
Products obtained are unexpected ones,
wherever structure permits rearrangements.
REGIOSELECTIVITY
WHY THIS REGIOSELECTIVITY?
MARKOVNIKOV RULE
The addition of HX and other acidic reagents to
carbon-carbon double bond is found to form
major products in which the electrophile (proton)
is found to add to that doubly bonded carbon to
which more hydrogens are bonded.
These observations were first made by Markovnikov.
Addition of HBr to Isobutene and Markovnikov Rule
Protonation of double bond yields the most stable carbocation
?CH
CH3
H3C
CH3
CH2
HCl
CH
CH3
H3C
CH3
CH2
H+CH3
H3C
CH3
Cl-
H
CH3
CH3
H3C
CH3
H
CH3
Cl
1,2-methyl shiftCH
H3C
H3C
CH3
CH3
Cl-
CH
H3C
H3C
CH3
CH3
Cl
CH
H3C
H3C
CH3
CH3
Cl
CH3
H3C
CH3
H
CH3
Cl +
3-Chloro-2,2-dimethylbutane 2-Chloro-2,3-dimethylbutane
STEREOCHEMISTRY OF THE IONIC ADDITION TO AN ALKENE
Achiral reactants produce achiral products.
Orientation of addition :Stereochemistry of Orientation of addition :Stereochemistry of HH--Br addition.Br addition.
H3C C CHCH3
H
H
+ Br
Br H
Br H
++H Br
H Br
No Control ofNo Control ofStereochemistryStereochemistryBrBr-- adds to adds to either side either side of of carbocationcarbocation..
Regiochemistry is perfect.Only the product from secondarycarbocation is formed.
But stereochemistry is not controlled
H-Br
Addition of HBr in Presence of peroxide
Anti-Markovnikov product
Free - Radical addition to alkenes Mechanism
Essence of the mechanism is that hydrogen
and bromine add to the double bond
homolytically rather than heterolytically.
The intermediate is a free radical rather than
a carbocation.
Like halogenation of alkanes, this is a chain
reaction, this time involving addition rather
than substitution.
Free - Radical addition to alkenes Mechanism
FACTS SUPPORTING THE MECHANISM
• A few molecules of peroxide can change
the orientation of addition of many
molecules of HBr.
• This strongly indicates a chain reaction.
• So, too, does the fact that a few molecules
of inhibitor (such as hydroquinone) are
able to prevent this change in orientation.
• The anti - Markovnikov addition is caused
not only by the presence of peroxides but
also by irradiation
• This is done with irradiation using light of
a wavelength known to dissociate HBr
into hydrogen and bromine free radicals.
You can now predict the product of the reactions given below:
Br
+?HBr
Br
+ HBrPeroxide
?Br
Br
Br
Br
HX addition to alkenes
The order of reactivity of the HX is
HI > HBr > HCl >HF, and unless the alkene is highly
substituted, HCl reacts so slowly that the reaction is
not useful as a preparative method.
The addition of HX to an unsymmetrical alkene can
occur in two ways.
In practice, however, one product only predominates
When a reaction, potentially can yield two or more
constitutional isomers, but yields only one of them as
the major product, then such a reaction is known as
regioselective reaction.
Markovnikov vs Anti- MarkovnikovAddition of HBr to Alkenes
In the absence of peroxides, the reagent that
attacks the double bond first is a proton.
Because proton is small, steric effects are
unimportant.
It attacks carbon atom by ionic mechanism
so as to form the more stable carbocation.
The result is Markovnikov addition.
In the presence of peroxides, the reagent that
attacks the double bond first is the larger
bromine atom.
It attaches itself to the less hindered carbon
atom by a radical mechanism, so as to form
the more stable radical intermediate.
The result is anti-Markovnikov addition.
Addition of Addition of IClICl to alkenesto alkenes