Alkenes, Alkynes, and Addition Reactions: Structure, Properties, and Mechanisms

Alkenes: Structure and Nomenclature

Structure of Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their structure and bonding influence their chemical reactivity and physical properties.

• General formula: CnH2n

• Bonding: Double bond consists of one sigma (σ) and one pi (π) bond.

• Hybridization: Each carbon in the double bond is sp2 hybridized.

• Geometry: Trigonal planar around double-bonded carbons; bond angles ≈ 120°.

Example: Ethene (ethylene), C2H4, is the simplest alkene.

Nomenclature of Alkenes

• Parent chain: Longest chain containing the double bond.

• Numbering: Number the chain to give the double bond the lowest possible number.

• Suffix: Replace '-ane' with '-ene' (e.g., ethene, propene).

• Cis/trans (E/Z) isomerism: If each carbon of the double bond has two different groups, geometric isomerism is possible.

Example: 2-butene can exist as cis-2-butene and trans-2-butene.

Alkyne Structure and Nomenclature

Structure of Alkynes

Alkynes are hydrocarbons with at least one carbon-carbon triple bond.

• General formula: CnH2n-2

• Bonding: Triple bond consists of one sigma (σ) and two pi (π) bonds.

• Hybridization: Each carbon in the triple bond is sp hybridized.

• Geometry: Linear around triple-bonded carbons; bond angles = 180°.

Example: Ethyne (acetylene), C2H2.

Nomenclature of Alkynes

• Parent chain: Longest chain containing the triple bond.

• Numbering: Number the chain to give the triple bond the lowest possible number.

• Suffix: Replace '-ane' with '-yne' (e.g., ethyne, propyne).

Physical Properties of Alkenes and Alkynes

• Nonpolar molecules; insoluble in water, soluble in organic solvents.

• Boiling points increase with molecular weight.

• cis isomers often have higher boiling points than trans isomers due to dipole moments.

Degree of Unsaturation

Definition and Calculation

The degree of unsaturation (index of hydrogen deficiency) indicates the number of rings and/or multiple bonds in a molecule.

• Formula: $\text{Degree of Unsaturation} = \frac{2C + 2 + N - H - X}{2}$

• C = number of carbons, N = number of nitrogens, H = number of hydrogens, X = number of halogens.

• Each ring or double bond increases the degree by 1; each triple bond increases it by 2.

Example: C4H6 has a degree of unsaturation of 2 (could be two double bonds, a triple bond, or a ring and a double bond).

Conformations and Stereochemistry of Alkenes

• Cis/trans (E/Z) isomerism: Arises due to restricted rotation around the double bond.

• Cahn-Ingold-Prelog rules: Used to assign E/Z configuration based on priority of substituents.

Reactivity of Alkenes: Addition Reactions

General Mechanism

Addition reactions involve the addition of atoms or groups across the double bond, converting it to a single bond.

• Common types: Hydrohalogenation, hydration, halogenation, hydroboration-oxidation, hydrogenation.

• Regioselectivity and stereoselectivity are important considerations.

Electrophilic Addition Mechanism

1. Electrophile attacks the π bond, forming a carbocation intermediate.

2. Nucleophile attacks the carbocation, yielding the addition product.

Example: Addition of HBr to propene yields 2-bromopropane (Markovnikov product).

Markovnikov's Rule

• In the addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens (less substituted), and the halide attaches to the more substituted carbon.

• Rationale: More stable carbocation intermediate is formed.

Anti-Markovnikov Addition

• Occurs in the presence of peroxides (e.g., HBr with ROOR), where the halide attaches to the less substituted carbon.

Hydration of Alkenes

• Addition of water (H2O) in the presence of acid catalyst (H2SO4).

• Follows Markovnikov's rule; forms alcohols.

Hydroboration-Oxidation

• Two-step reaction: 1) Addition of BH3 (borane) to alkene, 2) Oxidation with H2O2, NaOH.

• Anti-Markovnikov addition; syn addition (both H and OH add to the same side).

Halogenation

• Addition of X2 (Cl2, Br2) to alkenes forms vicinal dihalides.

• Proceeds via a halonium ion intermediate; anti addition (opposite sides).

Halohydrin Formation

• Addition of X2 in the presence of H2O; forms halohydrins (X and OH on adjacent carbons).

• OH attaches to the more substituted carbon.

Hydrogenation

• Addition of H2 across the double bond using a metal catalyst (e.g., Pd/C, Pt, Ni).

• Syn addition; reduces alkene to alkane.

Alkynes: Structure, Properties, and Reactions

Physical Properties

• Similar to alkenes; linear geometry, higher boiling points than alkenes of similar mass.

• Terminal alkynes are slightly acidic (can form acetylide ions with strong bases).

Addition Reactions of Alkynes

• Undergo similar addition reactions as alkenes but can add two equivalents of reagent (e.g., halogenation, hydrohalogenation).

• Hydration yields ketones (Markovnikov) or aldehydes (anti-Markovnikov, via hydroboration-oxidation).

Conjugated Dienes

Structure and Stability

• Conjugated dienes have alternating double and single bonds (e.g., 1,3-butadiene).

• More stable than isolated dienes due to delocalization of π electrons.

Reactivity

• Undergo 1,2- and 1,4-addition reactions with electrophiles.

• Diels-Alder reaction: [4+2] cycloaddition with dienophiles to form six-membered rings.

Summary Table: Major Alkene Addition Reactions

Reaction	Reagents	Regioselectivity	Stereochemistry	Product
Hydrohalogenation	HX	Markovnikov	Mixed	Alkyl halide
Hydration	H2O, H2SO4	Markovnikov	Mixed	Alcohol
Hydroboration-Oxidation	1) BH3, 2) H2O2, NaOH	Anti-Markovnikov	Syn	Alcohol
Halogenation	Br2, Cl2	None	Anti	Vicinal dihalide
Halohydrin Formation	Br2/Cl2 + H2O	OH to more substituted C	Anti	Halohydrin
Hydrogenation	H2, Pd/C	None	Syn	Alkane

Key Study Tips

• Know the basic types of reactions for each functional group.

• Learn reagents and their roles (nucleophile, electrophile, oxidizing/reducing agent).

• Understand mechanisms and be able to predict major/minor products.

Additional info: Some mechanistic details, such as the formation of carbocation intermediates and stereochemical outcomes, were expanded for clarity and completeness.