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Alkenes contain a C=C double bond (also occasionally
called olefins). Alkenes are very common in natural and
synthetic organic compounds.
Ethylene and propylene are the starting materials for
hundreds of synthetic organic compounds and plastics.
Small alkenes are produced by the thermal breakdown of
2C-8C hydrocarbons from petroleum ("cracking")
Chapter 6: Alkenes: structure and reactivity
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Alkenes are unsaturated because they have fewer than
the maximum number of hydrogens in a hydrocarbon
(CnH2n + 2).
Each π bond or ring takes the place of two hydrogens.
Two double bonds�
One triple bond�
If a formula is C6H10, it is 4 H's short of the maximum
(the saturated compound would be C6H14),
so its DOU (degree of unsaturation) is 2.
Halogens are the same as H when calculating DOU�
Oxygens have no effect�
If you're given a formula, try drawing it in a
straight chain with all single bonds. The number of
empty spots is twice the DOU.
�
Two rings•
One ring and one
double bond
•
6.2 Degree of unsaturation
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Alkenes use the suffix -ene.
Name the parent hydrocarbon (which must contain
the double bond - even if there's a longer chain
somewhere else!) If there's a tie, use the chain with
the double bond and the most branches.
1.
Number from the end nearest the double bond. If it's
a tie, number from the end nearest the first branch
point, then second, etc. If that's a tie, differentiate
the branch points alphabetically (ignoring the prefixes
t- and sec-)
2.
Use the double-bond carbon with the lower number.
With multiple double bonds, use diene, triene, etc as
the suffix. Add the prefix cyclo- if the double bond is
in a ring.
3.
6.3 Naming alkenes
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Recall when C is double-bonded, it is sp2-hybridized
The unhybridized p orbitals on the adjacent carbons
combine to make the π bond.
While rotation occurs along σ bonds, the π bond's shape
(above and below the σ) does not allow for rotation.
cis: the two groups point the same direction�
trans: the two groups point opposite directions�
In a 1,2-disubstituted alkene (has one non-hydrogen
group attached to each double-bonded carbon), the
stereochemical descriptors cis and trans can be used:
6.4 Cis-trans isomerism in alkenes
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For trisubstituted or tetrasubstituted alkenes, the
cis/trans designation does not work.
The E/Z desgination uses a series of sequence rules to
assign priorities to groups on each of the double-bonded
carbons.
On one of the double-bonded carbons, rank the
atoms attached to it by their atomic number. Then
rank the atoms on the other double-bonded carbon.
1.
E (Entgegen, apart) - the
high priority groups are on
opposite sides, like trans
�
Z (Zusammen, together) -
the high priority groups are
on ze zame zide, like cis
�
If the 2 atoms attached to the C are tied, look at the
next atoms down the line. Keep going until there's a
difference.
2.
Double bonds count twice. A C=O bond is like two C-O
bonds.
3.
6.5 The E/Z designation
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In general, tetrasubstituted alkenes are the most stable,
due to an effect called hyperconjugation which involves
more overlap of the π electrons with other bonds. The
fewer groups attached, the less stable.
In disubstituted alkenes, cis are less stable than trans -
this can be rationalized by imagining steric strain
between the two groups on the same side of the double
bond in the cis stereoisomer.
Any reaction that can interconvert the stereochemistry
of a double bond will favor the trans isomer because of
its stability.
6.6 Stability of alkenes
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Electrophilic addition to an alkene
usually follows a two-step mechanism,
as we saw in chapter 5.
π bond (weak nucleophile), attacks
the H of HBr (strong electrophile)
which makes a new C-H bond and
breaks the H-Br bond. The other
alkene carbon is now a carbocation.
1.
The bromide ion (nucleophile)
attacks the carbocation (strong
electrophile) to form a new Br-C
bond.
2.
The product is lower in energy than the reactant, so it is
spontaneous overall. The reaction has two steps, each
with a transition state and activation energy:
6.7 Electrophilic addition reactions of alkenes
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Markovnikov's rule: in addition of HX to an
unsymmetrical alkene, like the previous reaction, the
halogen is added to the more substituted carbon.
This is a regiospecific reaction - addition to one atom in
the molecule is favored over another.
This regioselectivity originates from production of the
more substituted carbocation intermediate.
6.8 Orientation of electrophilic addition: Markovnikov's rule
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Carbocation: C with 3 bonds and a + charge
Alkyl substituents stabilize carbocation by an inductive
effect - the polarizable alkyl groups are able to shift
electron density toward the + charge.
Planar structure�
sp2 hybridized (120o
bond angle)
�
One vacant p orbital
perpendicular to the
three hybrid orbitals
�
6.9 Carbocation structure and stability
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In electrophilic addition to an unsymmetrical
alkene, the more highly substituted carbocation
intermediate forms faster
�
More substituted carbocations are more stable
than less substituted. 3o > 2o > 1o > CH3+
�
We know these two facts experimentally:
But, we learned in chapter 5 that rates are related to
activation energy (ΔG‡), and stability is related to Gibbs
free energy change (ΔGo) between reactants and
products.
In endergonic processes, the transition state
resembles the product
a.
In exergonic processes, the transition state
resembles the reactant
b.
The Hammond Postulate: the transition state resembles
the nearest stable species in energy and structure.
6.10 The Hammond postulate
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More stable products (or intermediates) tend to form
faster!
Because the first step is the rate-limiting step in this
reaction (higher ΔG‡ than the second step), it determines
which product will be formed.
Analysis of electrophilic addition
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Some reactions that involve carbocation intermediates
give an unexpected mix of products that can't be
accounted for just by using Markovnikov's rule
(sometimes the nucleophile ends up on a carbon that
didn't have the double bond!)
The discovery of carbocation rearrangements was
conclusive evidence towards the existence of
carbocations themselves.
Hydride shift: a hydrogen atom and its pair of electrons
slides over to an adjacent carbocation, in order to form a
more stable carbocation.
Methyl or alkyl shift: an alkyl group will shift with an
electron pair to an adjacent carbocation.
6.11 Carbocation rearrangements
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