Chapter 7 Structure and Synthesis of

Alkenes

A. Alkene Stability

1. Basis of Comparison

The relative stability of alkenes is measured by comparing their heats of hydrogenation

Hydrogenation reactions of simple alkenes are always exothermic so ΔH° is always a negative number

C C C CH H+ H2

ΔH° = heat of hydrogenation

Pt

2. Substitution Effects

2-butene is more stable by 2.7 kcal/mol

+ H2Pt ΔH° = -30.3 kcal/mol

1-butene (terminal, monosubstituted)

+ H2Pt ΔH° = -27.6 kcal/mol

2-butene (internal, disubstituted)

The more energy given off in hydrogenating a double bond, the less stable the alkene

More highly substituted alkenes are more stable

because:

Alkyl groups are electron donating; they contribute electron density via hyperconjugation.

Substitution of alkyl groups on a double bond makes them farther apart (relative to if they were on alkane carbons), reducing steric interactions

CH3 CH3

CH3CH3 H109.5°120°

3. Geometric Isomers

trans-2-butene is more stable by 1.0 kcal/mol In general, trans double bonds are more stable than cis double bonds because steric interactions are reduced in the trans isomer

+ H2Pt ΔH° = -28.6 kcal/mol

cis-2-butene

+ H2Pt ΔH° = -27.6 kcal/mol

trans-2-butene

Cis and trans nomenclature works for disubstituted alkenes but what do we do for tri- and tetrasubstituted alkenes?

An unambiguous system, based on Cahn-Ingold-Prelog priorities, is used to denote alkene stereochemistry as E or Z

cis or trans?

To assign E or Z 1. Use the C-I-P rules to determine which group

attached to each alkene carbon has a higher priority 2.  If the two highest priority groups are on the same side

of the double bond (i.e. cis to each other) the alkene is Z (from zusammen, German for together); if the two highest priority groups are on opposite sides of the double bond the alkene is E (from entgegen, German for opposite)

Br

I

H

Cl

O

CH3O

OH1

2

1

2

2

2

1

1

ZE

3. Cycloalkenes

Double bonds in rings behave like acyclic double bonds (i.e. trans is more stable than cis) except:

a) 3- and 4-membered rings

Alkenes in 3- and 4-membered rings are very

strained (and therefore reactive) because bond angles are limited to 60° and 90°, respectively, when ideal bond angles are 120 °

b) Trans double bonds in 3- to 9-membered rings

Trans double bonds are geometrically

impossible in 3- and 4-membered rings In 5- to 7-membered rings, trans alkenes can

exist but they are very highly strained and are therefore unstable

For 8- and 9-membered rings, trans alkenes are stable but they are less stable than cis alkenes These rules apply to bicyclic rings, as well; if trans substituents are contained within one ring, the ring will be unstable if it contains fewer than eight atoms

trans substituents

stable because the trans alkene substituentsare contained in different rings

not stable because the trans alkene substituentsare contained within a six-membered ring

trans substituents

12

3

45

6

A generalization of this is stated by Bredt’s Rule: A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains eight or more atoms

unstable

1 23

45

6

7

8

1

2

34

561

23

45

6

bridgeheadcarbons

stable

unstable

B. Synthesis of Alkenes 1. Dehydrohalogenation

Reactions may occur by either an E1 or E2 mechanism; for practical purposes E2 is the preferred reaction because E1 is accompanied by SN1 Strong bulky bases are ideal

CH

CX

C C+ Base + X:- + Base-H

CH3 CCH3

CH3O - K+

NN

H

potassium tert-butoxide diisopropylamine triethylamine (Et3N)

E2 reactions generally give Zaitsev orientation, i.e. the most substituted, most stable alkene product is usually the major product Reactions using very bulky bases and very hindered substrates lead primarily to the less substituted product (the Hoffmann product) because the base is sterically hindered from abstracting the proton leading to the Zaitsev product

H

H

Br+ Et3N +

major minor

too sterically hindered

2. Dehydration of alcohols When heated in strong acid, alcohols will dehydrate to form alkenes The reaction is reversible but can be driven to the right by boiling off the lower boiling alkene product

CH3 CCH3

CH3OH H HSO4 + HSO4

-CH3 CCH3

CH3O

H

H

CH3 CCH2

CH3

HOH2

C CH2CH3

CH3H3O+ +

Δ

For a tertiary or secondary alcohol the reaction is E1 Competing SN1 is not a problem; potential nucleophiles are water, which reforms the starting material, and HSO4-, which is too weakly nucleophilic to add reversibly Dehydration is generally not a practical reaction for primary alcohols since the necessary primary carbocation intermediate is too unstable to form in appreciable amounts