AlkenesAlkene 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
Cistrans 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 Zaitsevs rule summarizes the results of numerous experiments in which alkene mixtures were produced by -elimination. In its original form, Zaitsevs rule stated that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the -carbon having the fewest hydrogens.
Zaitsevs rule as applied to the acid-catalyzed dehydration of alcohols is now more often expressed in a different way: -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 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
Brnsted acid, transferring a proton to a Brnsted 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 dont 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-dimethyl1-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 alkenes 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 catalysts 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
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 Markovnikovs 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)
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)
Stereochemistry of reaction (syn-addition of water)
Addition of halogen to alkenes (anti-addition)
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