The heat of hydrogenation is a measure of stability.
The relative stabilities of related alkenes can be determined by measuring their
heats of combustion.
The thermodynamic stability of the butenes increases in the order: 1-butene < cis2-butene < trans-2-butene.
Highly substituted alkenes are most stable; trans isomers are more
stable than cis.
The relative stability of the alkenes increases with increasing substitution
(hyperconjugation), and trans isomers are usually more stable than cis isomers
(crowding).
An exception to this stability rule is in medium-ring and smaller cycloalkenes. The
trans isomers of cycloalkenes are much more strained than are the corresponding
cis isomers.
The smallest isolated simple trans cycloalkene is trans-cyclooctene which is 9.2
kcal mol-1 less stable than the cis isomer is very twisted.
11-8 Preparation of Alkenes from Haloalkenes and Alkyl
Sulfonates: Bimolecular Elimination Revisited
Two approaches to the synthesis of alkenes are elimination reactions and the
dehydration of alcohols.
Regioselectivity in E2 reactions depends on the base.
Haloalkanes (or alkyl sulfonates) in the presence of strong base can undergo
elimination of HX with the simultaneous formation of a C=C double bond.
In the cases where the hydrogen atom can be removed from more than one
carbon atom in the structure, the regioselectivity of the reaction can be controlled
to a limited extent.
Consider the dehydrobromination of 2-bromo-2-methylbutane.
Elimination of HBr proceeds through attack by the base
on one of the neighboring hydrogens situated anti to the
leaving group.
The transition state leading to 2-methyl-2-butene is
slightly more stabilized than the one leading to 2-methyl1-butene.
The more stable product is formed faster because the
structure of the transition state resembles that of the
products.
Elimination reactions that lead to the more highly substituted alkene are said to
follow the Saytzev rule.
The double bond preferentially forms between the carbon that contained the
leaving group and the most highly substituted adjacent carbon that bears a
hydrogen.
When a more hindered base is used, more of the thermodynamically less favored
terminal alkene is generated.
Removal of a secondary hydrogen (C3 in the starting bromide) is sterically more
difficult than abstracting a more exposed methyl hydrogen when a hindered base
is used.
The transition state leading to the more stable product is increased in energy by
steric interference with the bulky base.
An E2 reaction that generates the thermodynamically less favored isomer is said
to follow the Hofmann rule.
E2 reactions often favor trans over cis.
The E2 reaction can lead to cis/trans alkene mixtures, in some cases with
selectivity.
This and related reactions appear to be controlled by the relative thermodynamic
stabilities of the products. The more stable trans double bond is formed
preferentially.
Complete selectivity is rare in E2 reactions, however.
11-9 Preparation of Alkenes by Dehydration of Alcohols
When alcohols are treated with
mineral acid at elevated
temperatures, dehydration (E1
or E2) occurs, resulting in alkene
formation.
As the hydroxy
bearing carbon
becomes more
substituted, the ease
of elimination of
water increases.
Secondary and tertiary alcohols dehydrate by an E1 mechanism.
The protonated hydroxy forms an alkyloxonium ion providing a good leaving
group: water.
Loss of water forms a secondary or tertiary carbocation.
Deprotonation forms the alkene.
Carbocation side reactions (hydrogen shifts, alkyl shifts, etc.) are possible.
The thermodynamically most stable alkene or alkene mixture usually results from
unimolecular dehydration in the presence of acid.
Whenever possible, the most highly substituted system is generated.
Trans-substituted alkenes predominate if there is a choice.
Treatment of primary alcohols with mineral acids at high temperatures also leads
to alkenes.
The reaction of propanol yields propene.
The reaction proceeds by protonation of the alcohol, followed by attack by HSO4- or
another alcohol molecule (E2 reaction) to remove a proton from one carbon atom and
water from the other.