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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