Organic Chemistry Unit 7 Review: Alkenes - Structure and Reactivity

Structure and Properties

• Alkenes contain a carbon-carbon double bond consisting of one sigma (σ) bond and one pi (π) bond
  • The σ bond is formed by the overlap of two sp2 hybridized orbitals
  • The π bond is formed by the sideways overlap of two unhybridized p orbitals
• The carbon-carbon double bond is shorter (1.34 Å) and stronger (611 kJ/mol) compared to a carbon-carbon single bond (1.54 Å, 347 kJ/mol)
• Alkenes are planar molecules with bond angles of approximately 120° around the double bond due to the trigonal planar geometry of the sp2 hybridized carbons
• The presence of the π bond makes alkenes more reactive than alkanes
  • The electron density in the π bond is exposed and can be easily attacked by electrophiles
• Alkenes are nonpolar and immiscible with water due to their hydrophobic nature
• The boiling points of alkenes increase with increasing molecular weight and branching
  • Branching increases the surface area and van der Waals interactions between molecules

Nomenclature

• Alkenes are named by identifying the longest carbon chain containing the double bond and replacing the "-ane" suffix with "-ene"
  • The position of the double bond is indicated by the lower-numbered carbon involved in the double bond (1-butene, 2-pentene)
• When the double bond is equidistant from both ends of the chain, the lower number is assigned to the end with the greater number of substituents (4-methyl-2-pentene)
• Substituents are named and numbered according to their position on the carbon chain (2,3-dimethyl-2-butene)
• The prefixes "cis-" and "trans-" are used to indicate the relative orientation of substituents on the double bond
  • In a cis-alkene, the substituents are on the same side of the double bond
  • In a trans-alkene, the substituents are on opposite sides of the double bond
• When the double bond is in a ring, the prefix "cyclo-" is used, and the position of any substituents is indicated by numbers (1-methylcyclopentene)

Isomerism in Alkenes

• Alkenes exhibit both structural isomerism and stereoisomerism
• Structural isomers have the same molecular formula but different bonding arrangements
  • Chain isomers have different carbon chain lengths (1-butene and 2-methylpropene)
  • Position isomers have the double bond in different positions (1-butene and 2-butene)
• Stereoisomers have the same bonding arrangement but different spatial orientation of atoms
  • Geometric isomers (cis-trans isomers) differ in the relative orientation of substituents across the double bond (cis-2-butene and trans-2-butene)
• Alkenes with more than two different substituents on the double bond exhibit optical isomerism (chirality)
  • These molecules are non-superimposable mirror images called enantiomers
• The stability of alkene isomers depends on the degree of substitution and the orientation of the substituents
  • Trans-alkenes are generally more stable than their cis-isomers due to reduced steric hindrance

Synthesis of Alkenes

• Alkenes can be synthesized through various methods, including elimination reactions, dehydration of alcohols, and the Wittig reaction
• Dehydrohalogenation is an elimination reaction that involves the removal of a hydrogen halide (HX) from an alkyl halide
  • The reaction is typically carried out using a strong base, such as sodium ethoxide or potassium tert-butoxide
  • The regioselectivity of the reaction is governed by Zaitsev's rule, which states that the more stable (more substituted) alkene is the major product
• Dehydration of alcohols involves the removal of water (H2O) from an alcohol using an acid catalyst, such as sulfuric acid or phosphoric acid
  • The reaction proceeds through an E1 mechanism, and the regioselectivity follows Zaitsev's rule
• The Wittig reaction is a powerful method for synthesizing alkenes from aldehydes or ketones
  • The reaction involves the use of a phosphonium ylide (Wittig reagent), which is prepared from a phosphonium salt and a strong base
  • The ylide reacts with the carbonyl compound to form a betaine intermediate, which then undergoes elimination to yield the alkene and a phosphine oxide byproduct

Reactions of Alkenes

• Alkenes undergo various reactions due to the reactivity of the carbon-carbon double bond
• Addition reactions involve the addition of electrophiles or other reagents across the double bond
  • Hydrohalogenation is the addition of a hydrogen halide (HX) to an alkene, forming an alkyl halide
  • Halogenation involves the addition of halogens (Cl2, Br2) to an alkene, forming a vicinal dihalide
  • Hydration is the addition of water (H2O) to an alkene in the presence of an acid catalyst, forming an alcohol
  • Hydrogenation is the addition of hydrogen (H2) to an alkene, forming an alkane, usually in the presence of a metal catalyst
• Oxidation reactions convert alkenes into other functional groups
  • Epoxidation involves the addition of an oxygen atom to an alkene, forming an epoxide (oxirane) using a peroxyacid, such as m-chloroperoxybenzoic acid (mCPBA)
  • Dihydroxylation is the addition of two hydroxyl groups to an alkene, forming a vicinal diol, typically using osmium tetroxide (OsO4) and a co-oxidant
  • Ozonolysis cleaves the carbon-carbon double bond using ozone (O3), forming carbonyl compounds (aldehydes or ketones) upon workup
• Alkenes can also participate in polymerization reactions, forming long-chain molecules called polymers
  • Addition polymerization involves the repeated addition of alkene monomers, such as ethylene or propylene, to form polyethylene or polypropylene

Mechanisms and Stereochemistry

• The reactions of alkenes proceed through various mechanisms, which can be classified as ionic or radical
• Ionic mechanisms involve the formation of carbocations or carbanions as intermediates
  • Markovnikov's rule predicts the regioselectivity of electrophilic addition reactions, stating that the more stable carbocation intermediate leads to the major product
  • Anti-Markovnikov addition occurs when the less stable carbocation intermediate is formed, usually in the presence of peroxides or under free-radical conditions
• Radical mechanisms involve the formation of free radicals (species with an unpaired electron) as intermediates
  • Anti-Markovnikov addition often proceeds through a radical mechanism, such as in the hydroboration-oxidation of alkenes
• The stereochemistry of alkene reactions is determined by the approach of the reagents and the spatial orientation of the substituents
  • Syn addition occurs when the reagents add to the same face of the double bond, leading to a cis-product
  • Anti addition occurs when the reagents add to opposite faces of the double bond, leading to a trans-product
• Concerted reactions, such as the Diels-Alder cycloaddition, proceed in a single step without the formation of intermediates
  • The stereochemistry of concerted reactions is determined by the orientation of the reactants in the transition state

Industrial Applications

• Alkenes are important raw materials for the production of various industrial chemicals and consumer products
• Ethylene, the simplest alkene, is used in the production of polyethylene (plastic bags, containers), ethylene oxide (antifreeze, polyester), and ethylene glycol (coolants, solvents)
• Propylene is used to produce polypropylene (packaging materials, textiles), propylene oxide (polyurethane foams), and acrylic acid (plastics, adhesives)
• Butadiene, a conjugated diene, is a key component in the production of synthetic rubber (styrene-butadiene rubber, nitrile rubber)
• Isoprene, another conjugated diene, is used in the synthesis of natural rubber and terpenes
• Fatty acid derivatives, such as oleic acid and linoleic acid, are used in the production of soaps, detergents, and personal care products
• Terpenes, which are unsaturated hydrocarbons found in essential oils, are used as fragrances, flavors, and pharmaceuticals
  • Examples include limonene (citrus scent), pinene (pine scent), and myrcene (hops, lemongrass)

Key Takeaways and Practice Problems

• Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond, which consists of one σ bond and one π bond
• The presence of the π bond makes alkenes more reactive than alkanes, allowing them to participate in various addition, oxidation, and polymerization reactions
• Alkenes exhibit structural isomerism (chain and position) and stereoisomerism (geometric and optical)
• Alkenes can be synthesized through elimination reactions (dehydrohalogenation, dehydration) and the Wittig reaction
• The reactions of alkenes proceed through ionic or radical mechanisms, and the stereochemistry of the products depends on the approach of the reagents and the orientation of the substituents
• Alkenes are important raw materials for the production of industrial chemicals, plastics, rubbers, and consumer products

Practice Problems:

1. Draw the structural formula and provide the IUPAC name for the following alkenes: a) The alkene with four carbon atoms and a double bond between C2 and C3 b) The alkene with five carbon atoms, a double bond between C2 and C3, and a methyl group on C4
2. Predict the major product of the following reactions: a) The addition of HBr to 2-methylpropene b) The dehydration of 2-butanol using sulfuric acid
3. Propose a synthesis of 2-pentene starting from an alcohol.
4. Explain the difference between Markovnikov and anti-Markovnikov addition, and provide an example of each.
5. Draw the cis and trans isomers of 2-pentene, and explain which isomer is more stable.