Alkenes, Alkynes, and Dienes Help
Nomenclature and Structure
Alkenes (olefins) contain a carbon-carbon double bond and have the general formula CnH2n. These unsaturated hydrocarbons are isomeric with the saturated cycloalkanes.
In the IUPAC (International Union of Pure and Applied Chemistry) nomenclature system, the longest continuous chain of C's containing the double bond is assigned the parent, with the suffix changed from -ane to -ene. The chain is numbered so that the position of the double bond is designated by assigning the lower possible number to the first doubly bonded C.
Geometric (cis/trans) Isomerism
The carbon-carbon double bond consists of a σ bond and a π bond. The π bond has electron density above and below the plane containing the carbon atoms. The π bond is weaker and more reactive than the σ bond because these electrons are more exposed and more weakly bound. Alkenes readily undergo addition reactions. The π bond prevents free rotation about the carbon-carbon double bond and therefore an alkene having two different substituents on each doubly bonded C has geometric isomers. For example, there are two 2-butenes:
Geometric (cis/trans) isomers are stereoisomers because they differ only in the spatial arrangement of the groups. They are diastereomers and have different physical properties (m.p., b.p., etc.). For IUPAC rules, in place of cis or trans, the letter Z is used if the higher-priority substituents on each carbon are on the same side of the double bond. The letter E is used if they are on opposite sides.
Preparations of Alkenes
Elimination. Also called β-eliminations, these reactions constitute the principal laboratory method to make double bonds.
Dehydrohalogenation. Alkenes are typically prepared by treating an alkyl halide with a strong base, often using KOH in ethanol. This reaction proceeds in an antiperiplanar manner (see Chapter 6).
Dehydration. Alcohols are heated with acid to prepare alkenes. Since carbocations can be formed in these reactions, a number of side reactions can interfere.
The more groups on the C=C group (i.e., the more substituted the alkene), the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is:
R2C=CR2 > R2C=CRH > R2C=CH2 ,RCH=CHR > RCH=CH2
Partial Reduction of Alkynes
Addition of one equivalent of hydrogen to an alkyne produces an alkene. Either E or Z alkenes can be produced, depending on the conditions. Different products are possible because the reactions proceed by different mechanisms.
Reactions of Alkenes
Addition of H2. Alkenes undergo addition reactions at the double bond. The most fundamental of such reactions is the addition of hydrogen. This reaction only occurs in the presence of a catalyst, typically palladium metal supported on activated carbon (Pd/C). Since this reaction involves hydrogen atoms on the surface of the catalyst, both of the hydrogens are added to the same face of the alkene.
Electrophilic Addition Reactions. The π electrons of alkenes are a nucleophilic site that reacts with electrophiles. The addition of HBr proceeds by the following mechanism. First, the nucleophilic π electrons attack the electrophile, producing a new carbon-hydrogen bond and a carbon cation (carbocation):
The carbocation (a strong electrophile) is then attacked by bromide ion (a nucleophile), producing the final product.
The secondary carbocation shown here is more stable than the primary cation that would result from protonation at the central carbon. Alkyl groups, such as the CH3 group in the cation above, are electron-donating groups. These electron-donating groups help stabilize the electron-deficient carbocation. As a result, there is a strong preference for the formation of highly substituted cations instead of less-substituted cations.
Other addition reactions occur in an analogous fashion. With many electrophiles, there is a very specific stereochemical outcome that is dictated by the reaction mechanism. For example, addition of Br2 to cyclohexene produces the trans isomer of 1,2-dibromocyclohexane:
The reaction proceeds by attack of the nucleophilic π-electrons on Br2, which produces a bridged bromonium ion.
Bromide ion then attacks the bromonium ion from the opposite side of the molecule from the first bromine (in an Sn2 fashion, see Chapter 6) opening the bromonium ion and producing the trans product.
Alkenes also react with carbenes, highly reactive 6-electron species. The most common method is to treat the alkene with CH2I2 and zinc-copper alloy. This reaction, known as the Simmons-Smith reaction, produces cyclopropanes. The stereochemistry of the alkene is preserved in this reaction. Trans alkenes yield trans cyclopropanes.
One of the most important reactions of alkenes is hydration. Hydration reactions add water to the double bond, producing alcohols.
Hydration in an anti-Markovnikov manner can be accomplished by a 2-step procedure known as hydroboration. In this case, borane (BH3) adds across the double bond to place the boron on the least substituted carbon. Oxidation with hydrogen peroxide (H2O2) results in formation of the alcohol.
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