Alkenes

Definition and General Characteristics

What Are Alkenes?

Alkenes: hydrocarbons containing at least one carbon-carbon double bond (C=C). Classified as unsaturated hydrocarbons. General formula: CnH2n for acyclic, mono-substituted alkenes.

Functional Group

Functional group: alkene double bond, reactive site. Confers distinct chemical reactivity from alkanes. Enables addition reactions.

Importance

Key intermediates in organic synthesis, polymer production, petrochemical industry. Basis for production of plastics, alcohols, and other chemicals.

"The double bond in alkenes is a versatile functional group enabling diverse synthetic transformations." -- Clayden, Greeves, Warren, Wothers

Nomenclature of Alkenes

IUPAC Rules

Longest chain containing C=C selected as parent. Numbering starts from end nearest double bond. Position of double bond indicated by lowest possible number.

Common Names

Some alkenes have trivial names (e.g., ethylene, propylene). Used mainly in industrial contexts.

Substituents and Multiples

Substituents named as alkyl groups. Multiple double bonds indicated by suffixes -diene, -triene, with locants.

Example: 3-methyl-1-buteneLongest chain: 4 carbonsDouble bond at C1Methyl substituent at C3

Structure and Bonding

Hybridization

Each alkene carbon: sp2 hybridized. Three sp2 orbitals form sigma bonds; unhybridized p orbital forms pi bond.

Double Bond Composition

Double bond = 1 sigma + 1 pi bond. Sigma: head-on overlap; pi: side-on overlap of p orbitals. Pi bond restricts rotation.

Molecular Geometry

Trigonal planar geometry around alkene carbons. Bond angles approx. 120°. Planarity essential for pi bond formation.

Orbital diagram:sp2 orbitals form C-C sigma bond and C-H sigma bondsp orbitals on each carbon overlap side-on → pi bond

Physical Properties

Boiling and Melting Points

Boiling points higher than alkanes of similar molar mass due to increased polarity of double bond. Melting points depend on symmetry and chain length.

Solubility

Nonpolar overall. Insoluble in water. Soluble in organic solvents like hexane, ether.

Density and State

Low density liquids or gases at room temperature for lower alkenes. Higher alkenes are liquids or solids.

Chemical Properties

Reactivity Overview

Alkenes undergo electrophilic addition, oxidation, polymerization. Double bond is nucleophilic site.

Electrophilic Addition

Common reactions: addition of HX, X2, H2O. Markovnikov's rule governs regioselectivity.

Oxidation and Polymerization

Oxidation: epoxidation, dihydroxylation. Polymerization: radical or coordination mechanisms to produce polymers.

General electrophilic addition:Alkene + Electrophile (E+) → Carbocation intermediate → Addition of nucleophile (Nu-)

Synthesis of Alkenes

Dehydration of Alcohols

Acid-catalyzed elimination of water from alcohols. Follows Zaitsev's rule for major product.

Dehydrohalogenation

Base-induced elimination of HX from alkyl halides. Strong bases favor E2 mechanism.

Other Methods

Wittig reaction: carbonyl + phosphonium ylide → alkene. Catalytic cracking of alkanes.

Isomerism in Alkenes

Structural Isomerism

Position isomers: double bond at different carbons. Chain isomers: different carbon skeletons.

Geometric (Cis-Trans) Isomerism

Restricted rotation about C=C leads to cis (same side) and trans (opposite side) isomers. Different physical and chemical properties.

Optical Isomerism

Occurs if alkene contains chiral centers. Not common for simple alkenes but relevant in substituted derivatives.

Cis-2-butene vs Trans-2-butene:Cis: methyl groups on same side, higher boiling pointTrans: methyl groups on opposite sides, more stable

Reaction Mechanisms

Electrophilic Addition Mechanism

Step 1: Electrophile attacks pi bond, forms carbocation intermediate. Step 2: Nucleophile attacks carbocation. Stereochemistry: anti addition common.

Radical Addition

Initiated by radicals, e.g. halogenation under UV light. Radical chain mechanism with propagation and termination.

Polymerization Mechanisms

Radical polymerization: initiator generates radical, adds to alkene, propagates chain. Coordination polymerization: transition metal catalysts control stereochemistry.

Industrial Applications

Polymer Production

Alkenes polymerize to form polyethylene, polypropylene, polystyrene. Basis of plastics industry.

Petrochemical Industry

Feedstock for synthesis of alcohols, detergents, antifreeze, synthetic rubber. Produced via cracking.

Organic Synthesis

Precursors to alcohols, diols, epoxides, and other functionalized molecules. Used in pharmaceuticals and agrochemicals.

Spectroscopic Identification

Infrared (IR) Spectroscopy

C=C stretch absorption at 1620-1680 cm⁻¹. =C-H stretch near 3020-3100 cm⁻¹.

Proton Nuclear Magnetic Resonance (¹H NMR)

Alkenic protons resonate downfield (4.5-6.5 ppm). Splitting patterns indicate substitution.

Carbon-13 NMR

Alkenic carbons appear between 100-150 ppm. Chemical shifts depend on substitution pattern.

IR spectrum:C=C stretch: 1650 cm⁻¹ (medium intensity)=C-H stretch: 3080 cm⁻¹ (weak)NMR proton shifts:Alkenic H: 5.0-6.5 ppm

Environmental and Safety Considerations

Toxicity and Exposure

Low molecular weight alkenes can be irritants and asphyxiants. Flammable gases pose explosion risk.

Environmental Fate

Alkenes degrade via atmospheric oxidation. Some contribute to photochemical smog formation.

Handling and Storage

Stored under pressure or as liquids at low temperature. Require inert atmosphere to prevent polymerization.

Summary

Key Points

Alkenes: unsaturated hydrocarbons with C=C double bond. Exhibit unique reactivity due to double bond. Undergo electrophilic addition, polymerization. Important in industry and synthesis.

Applications

Used in producing polymers, chemicals, intermediates. Spectroscopic techniques confirm structure. Safety critical due to flammability and reactivity.

Future Directions

Development of selective catalysts, green synthesis methods, and sustainable polymer materials continues to advance alkene chemistry.

References

• Clayden, J., Greeves, N., Warren, S., Wothers, P. Organic Chemistry. Oxford University Press, 2012, pp. 250-300.
• Smith, M. B., March, J. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley, 2007, vol. 1, pp. 430-500.
• Morrison, R. T., Boyd, R. N. Organic Chemistry. Prentice Hall, 1992, vol. 2, pp. 620-670.
• Anslyn, E. V., Dougherty, D. A. Modern Physical Organic Chemistry. University Science Books, 2006, pp. 350-390.
• Carey, F. A., Sundberg, R. J. Advanced Organic Chemistry: Part A: Structure and Mechanisms. Springer, 2007, 5th ed., pp. 280-320.