Alkenes

Alkene Nomenclature

The longest continuous chain that includes the double bond forms the base name of

the alkene, and the chain is numbered in the direction that gives the doubly bonded

carbons their lower numbers. The locant (or numerical position) of only one of the

doubly bonded carbons is specified in the name.

Isomerism in alkenes

there are four isomeric alkenes of molecular formula C4H8

The pair of isomers designated cis- and trans-2-butene have the same constitution;

both have an unbranched carbon chain with a double bond connecting C-2 and C-3.

They differ from each other, however, in that the cis isomer has both of its methyl

groups on the same side of the double bond, but the methyl groups in the trans isomer

are on opposite sides of the double bond.

Isomers that have the same constitution but differ in the arrangement of their atoms in

space are classified as stereoisomers. cis-2-Butene and trans-2-butene are

stereoisomers, and the terms “cis” and “trans” specify the configuration of the double

bond. Stereoisomeric alkenes are sometimes referred to as geometric isomers

Cis–trans stereoisomerism in alkenes is not possible when one of the doubly bonded

carbons bears two identical substituents.

Relative stabilities of alkenes

we saw how to use heats of combustion to compare the stabilities of isomeric alkanes.

We can do the same thing with isomeric alkenes. Consider the heats of combustion of

the four isomeric alkenes of molecular formula C4H8. All undergo combustion

according to the equation

We see that the isomer of highest energy (the least stable one) is 1-butene. The isomer

of lowest energy (most stable) is 2-methylpropene. In general, alkenes with more

highly substituted double bonds are more stable than isomers with less substituted

double bonds.

Like the sp2-hybridized carbons of carbocations and free radicals, the sp2-hybridized

carbons of double bonds are electron attracting, and alkenes are stabilized by

substituents that release electrons to these carbons (alkyl groups).

Analogous data for a host of alkenes tell us that the most important factors governing

alkene stability are:

1. Degree of substitution (alkyl substituents stabilize a double bond)

2. Van der Waals strain (destabilizing when alkyl groups are cis to each other)

Degree of substitution. We classify double bonds as monosubstituted,

disubstituted, trisubstituted, or tetrasubstituted according to the number of carbon

atoms that are directly attached to the C=C structural unit.

Preparation of alkenes

Elimination reactions

Dehydration of alcohols

In the dehydration of alcohols,the student should observe that the H and OH are lost

from two adjacent carbons. An acid catalyst is necessary.

Regioselectivity in alcohols dehydration

Zaitzef rule

Zaitsev’s rule summarizes the results of numerous experiments in which alkene

mixtures were produced by β-elimination. In its original form, Zaitsev’s rule stated

that the alkene formed in greatest amount is the one that corresponds to removal of

the hydrogen from the β-carbon having the fewest hydrogens.

Zaitsev’s rule as applied to the acid-catalyzed dehydration of alcohols is now more

often expressed in a different way: β-elimination reactions of alcohols yield the most

highly substituted alkene (more stable) as the major product

In addition to being regioselective, alcohol dehydrations are stereoselective

Mechanism

The carbocations are key intermediates in alcohol dehydration.

a three-step mechanism for the sulfuric acid-catalyzed dehydration of tert-

butyl alcohol.

Steps 1 and 2 describe the generation of tert-butyl cation

Step 3 is the step in which the double bond is formed.

Step 3 is an acid-base reaction in which the carbocation acts as a Brønsted

acid, transferring a proton to a Brønsted base (water). This is the property of

carbocations that is of the most significance to elimination reactions.

General features for the carbocation stability

Because alkyl groups stabilize carbocations, we conclude that they release electrons to

the positively charged carbon, dispersing the positive charge. They do this through a

combination of effects. One involves polarization of the - bonds to the positively

charged carbon. The other is hyperconjufation: Carbocation is stabilized by

delocalization of the electrons in the neighbouring C-H bonds of the methyl group

into the vacant 2p orbital of the positively charged carbon.

primary carbocations are too high in energy to be intermediates in most chemical

reactions. If primary alcohols don’t form primary carbocation then how do they

undergo elimination? For primary alcohols it is believed that a proton is lost from the

alkyloxonium ion in the same step in which carbon-oxygen bond cleavage takes

place.

Rearrangement in alcohol dehydration

Some alcohols undergo dehydration to yield alkenes having carbon skeletons different

from the starting alcohols. Not only has elimination taken place, but the arrangement

of atoms in the alkene is different from that in the alcohol

The two alkenes present in greates amount, 2,3-dimethyl-2-butene and 2,3-dimethyl-

1-butene, both have carbon skeletons different from that of the starting alcohol.

carbocation could either lose a proton to give an alkene having the same carbon

skeleton or rearrange to a different carbocation, as shown in mechanism. The

rearranged alkenes arise by loss of a proton from the rearranged carbocation. Why do

carbocations rearrange? The answer is straight-forward once we recall that tertiary

carbocations are more stable than secondary carbocations. Thus, rearrangement of a

secondary to a tertiary carbocation is energetically favorable. The carbocation that is

formed first in the dehydration of 3,3-dimethyl-2-butanol is secondary; the rearranged

carbocation is tertiary. Rearrangement occurs, and almost all of the alkene products

come from the tertiary carbocation. Rearrangement occur due methyl group shifts

from C-3 to the positively charged carbon at C2 to finally afford the most stable 3o

carbocation.

Mechanism

Hydride shift often occur in hydration of primary alcohols

Write the mechanism of this reaction

Addition reactions of alkenes

Hydrogenation is the addition of H2 to a multiple bond.

The bonds in the product are stronger than the bonds in the reactants; two C-H _

bonds of an alkane are formed at the expense of the H-H - bond and the -

component of the alkene’s double bond. The overall reaction is exothermic. Heat of

hydrogenation is a positive quantity equal to -H° for the reaction. Heat of

hydrogenation could be used to estimates the stabilities of alkene isomers

Stereochemistry of alkene hydrogenation

hydrogen atoms are transferred from the catalyst’s surface to the alkene. Although the

two hydrogens are not transferred simultaneously, it happens that both add to the

same face of the double bond, Syn addition.

The term syn addition describes the stereochemistry of reactions such as catalytic

hydrogenation in which two atoms or groups add to the same face of a double bond.

When atoms or groups add to opposite faces of the double bond, the process is called

anti addition.

Electrophilic addition of hydrogen halide to alkene

Mechanism

Both steps in this general mechanism are based on precedent. It is called electrophilic

addition because the reaction is triggered by the attack of an electrophile (an acid) on

the -electrons of the double bond. Using the two -electrons to form a bond to an

electrophile generates a carbocation as a reactive intermediate; normally this is the

rate-determining step.

Regioselectivity in hydrogen halide addition

Markonikov rule

Markovnikov’s rule states that when an

unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds

to the carbon that has the greater number of hydrogen substituents, and the halogen

adds to the carbon having fewer hydrogen substituents.

Mechanism basis of Markonikov rule

Rearrangement in HX addition

As all carbocation reaction mechanisms, rearrangement could be expected in some

cases

Free radical addition of HBr (anti Markonikov)

Mechanism

Addition of sulfuric acid (Markonikove rule)

Acid catalyzed hydration of alkene

Predict the mechanisms of the following reactions

Hydroboration –oxidation of alkenes (anti markonikov addition of water)

Mechanism

Stereochemistry of reaction (syn-addition of water)

Addition of halogen to alkenes (anti-addition)

Mechanism

Ozonolysis of alkenes

Oxidation of alkenes

Reaction with KMnO4

Oxidative cleavage of alkene

this reaction give as well as ozonolysis good information about the structure feature

of alkene

Polymerization of alkenes

Free radical polymerization of ethylene

The following table represent some alkenes used to form polymers and their

applications in industry