Structure: • The alkene functional group consists of two sp2 hybridized C atoms bonded to each ...

20 Pages·2016·0.23 MB·English

Structure:
• The alkene functional group consists of
two sp2 hybridized C atoms bonded to
each other via a σ and a π bond.
• The π bond is produced by the side-to-
side overlap of the π-orbitals not utilized in
the hybrids (see left).
• The substituents are attached to the C=C
unit via σ bonds.
• The 2 C of the C=C and the 4 atoms
attached directly to the C=C are all in the
same plane.

Reactivity:
• A π bond is a region of
high electron density (red)
so alkenes are typically
nucleophiles.
• Alkenes react with
electrophiles (e.g. H+, X+)
• Alkenes typically undergo
addition reactions in which
the π bond is converted to
two new stronger σ bonds.
• Overall reaction
: Electrophilic addition

Electrophilic addition reactions are an important class of reactions that allow
the interconversion of C=C and C≡C into a range of important functional groups.
Conceptually, addition is the reverse of elimination

What does the term "electrophilic addition" imply ?
A electrophile, E+, is an electron poor species that will react with an electron rich
species (the C=C)
An addition implies that two systems combine to a single entity.

Depending on the relative timing of these events, slightly different mechanisms
are possible:
• Reaction of the C=C with E+ to give a carbocation (or another cationic
intermediate) that then reacts with a Nu-
• Simultaneous formation of the two s bonds
The following pointers may aid your understanding of these reactions:
• Recognise the electrophile present in the reagent combination
• The electrophile adds first to the alkene, dictating the regioselectivity.
• If the reaction proceeds via a planar carbocation, the reaction is not
stereoselective
• If the two new s bonds form at the same time from the same species,
then syn addition is observed
• If the two new s bonds form at different times from different species,
then anti addition is observed
Carbocations (review)
Stability:

The general stability order of simple alkyl carbocations is: (most stable) 3o > 2o >
1o > methyl (least stable)

This is because alkyl groups are weakly electron donating due to
hyperconjugation and inductive effects. Resonance effects can further stabilize
carbocations when present.
Structure:

Alkyl carbocations are sp2
hybridized, planar systems at the
cationic C center.
The p-orbital that is not utilised in
the hybrids is empty and is often
shown bearing the positive charge
since it represents the orbital

available to accept electrons.

Reactivity:
As they
have an
incomplete
octet,
carbocation
s are
excellent
electrophile
s and react
readily with
nucleophile
s
(substitutio
n).
Alternativel
y, loss of
H+ can
generate a
p bond

(elimination
).
The
electrostati
c potential
diagrams
clearly
show the
cationic
center in
blue, this is
where the
nucleophile
will attack.

Rearrangements:
Carbocations are prone to rearrangement via 1,2-hyride or 1,2-alkyl shifts
provided it generates a more stable carbocation. For example:

Notice that the "predicted"
product is only formed in 3%
yield, and that products with a
different skeleton dominate.
The reaction proceeds via
protonation to give the better
leaving group which departs to
give the 2o carbocation
shown. A methyl group rapidly
migrates taking its bonding

electrons along, giving a new 2o carbocation to 3o carbocation
skeleton and a more stable 3o
carbocation which can then lose
H+ to give the more stable alkene
as the major product.
This is an example of a 1,2-alkyl shift. The numbers indicate that the alkyl
group moves to an adjacent position.
Similar migrations of H atoms, 1,2-hydride shifts are also known.
Reactions involving carbocations:
1. Substitutions via the S 1
N
2. Eliminations via the E1
3. Additions to alkenes and alkynes (HX, H3O+)
Reaction Type: Electrophilic Addition
Summary
• Alkenes can be reduced to alkanes with H2 in the presence of metal
catalysts such as Pt, Pd, Ni or Rh.
• The two new C-H s bonds are formed simultaneously from H atoms
absorbed into the metal surface.
• The reaction is stereospecific giving only the syn addition product.
• This reaction forms the basis of experimental "heats of hydrogenation"
which can be used to establish the stability of isomeric alkenes.
Related reactions
• Hydrogenation of alkynes

CATALYTIC
HYDROGENATION
Step 1:
Hydrogen gets
absorbed onto the
metal surface.

Step 2:
Alkene approaches
the H atoms
absorbed on the
metal surface.
Step 3:
C=C reacts with the
H atoms on the
surface forming the
two new C-H s
bonds.
Reaction type: Electrophilic Addition
Summary
• When treated with HX alkenes form alkyl halides.
• Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity
order).
• Regioselectivity predicted by Markovnikov's rule :
"For addition of hydrogen halides to alkenes, the H atom adds to the C
with the most H atoms already present"
• Reaction proceeds via protonation to give the more stable carbocation
intermediate.
• Not stereoselective since reaction proceeds via planar carbocation.
• For HBr, care must be taken to avoid the formation of radicals when an
alternate mechanism occurs.
MECHANISM FOR REACTION OF ALKENES
WITH HBr

Step 1:
An acid/base reaction. Protonation of the alkene
to generate the more stable carbocation. The p
electrons act as a Lewis base.
Step 2:
Attack of the nucleophilic bromide ion on the
electrophilic carbocation creates the alkyl
bromide.
Markovnikov's Rule

• Markovnikov's rule (1870s), based on experimental observation, states
that :

"when an unsymmetrical alkene reacts with a hydrogen halide to give an
alkyl halide, the hydrogen adds to the carbon that has the greater number
of hydrogen substituents, and the halogen to the carbon having the fewer
number of hydrogen substituents"

• Modern mechanistic knowledge indicates reaction occurs via protonation
to give the more stable carbocation.

Propene can protonate to give two different carbocations, one 2o and the other
1o. Formation of the more stable 2o carbocation is preferred.
• The carbocation then reacts with the nucleophile to give the alkyl bromide
Reaction of Alkenes with HBr (radical)

Reaction type: Radical Addition
Summary
• When treated with HBr, alkenes form alkyl bromides.
• However, under these conditions, the regioselectivity is anti Markovnikov

• Peroxides facilitate the formation of a bromine radical, RO. + HBr → ROH
+ Br.
• Reaction proceeds via the more stable radical intermediate.
• Contrast with non-radical alternative:

Regular Radical

Conditions HBr (dark, N atmosphere) H Br (peroxides, uv light)
2
Electrophile H+ Br.
Intermediate carbocation radical
Regioselectivity Markovnikov Anti-Markovnikov
• The change in selectivity can be rationalized by the reversal of the order in
which the -H and -Br add