Alkenes and Alkynes: Properties and Reactions

Alkenes and alkynes are the two major classes of unsaturated hydrocarbons, distinguished by the presence of carbon-carbon multiple bonds. Their chemical reactivity — centered on the pi bonds that constitute the multiple bond — is significantly greater than that of alkanes, and this reactivity forms the basis of a vast number of reactions that are clinically and pharmacologically relevant. For HAAD candidates, understanding the reactions of alkenes and alkynes is essential not only for pure chemistry questions but also for understanding polymerization reactions (which produce many biomedical polymers), fatty acid chemistry (which underpins lipid biochemistry), prostaglandin synthesis (which involves alkenes), and the metabolism of unsaturated fats. This chapter covers the structure, preparation, properties, and key reactions of alkenes and alkynes.

Electronic Structure of Multiple Bonds

The Carbon-Carbon Double Bond (C=C)

The double bond in an alkene consists of two distinct components:

1. A sigma (σ) bond: Formed by the head-on overlap of sp² hybrid orbitals on each carbon; this is the stronger bond (347 kJ/mol)
2. A pi (π) bond: Formed by the sideways overlap of unhybridized p orbitals perpendicular to the molecular plane; this is the weaker bond (268 kJ/mol)

The carbon atoms in a double bond are sp² hybridized — each carbon has three sp² hybrid orbitals arranged at 120° to each other in a trigonal planar geometry, with one unhybridized p orbital perpendicular to the plane. This explains the planar geometry of ethene (H₂C=CH₂) — all six atoms lie in the same plane, with H-C-H and H-C=C angles of approximately 120°.

The Carbon-Carbon Triple Bond (C≡C)

The triple bond in an alkyne consists of three components:

1. One sigma (σ) bond: sp hybrid orbital overlap along the internuclear axis
2. Two pi (π) bonds: Sideways overlap of two sets of unhybridized p orbitals, perpendicular to each other

The carbon atoms in a triple bond are sp hybridized — each carbon has two sp hybrid orbitals at 180° to each other (linear geometry), with two unhybridized p orbitals perpendicular to each other and to the molecular axis. This gives linear geometry to alkynes — the four atoms bonded to the two triple-bonded carbons are collinear (e.g., HC≡CH is linear at 180°).

Physical Properties of Alkenes and Alkynes

The physical properties of alkenes and alkynes are similar to those of alkanes of comparable molecular weight:

• State: Alkenes and alkynes with 2–4 carbon atoms are gases at room temperature (e.g., ethene, propene, but-1-ene, ethyne); those with 5 or more carbons are liquids or solids
• Solubility: Practically insoluble in water; soluble in non-polar organic solvents (hexane, benzene, chloroform)
• Density: Less than water (all alkenes and alkynes float on water)
• Boiling point: Increases with increasing molecular weight; like alkanes, straight-chain isomers have higher boiling points than branched isomers; alkynes have slightly higher boiling points than alkanes of similar molecular weight due to stronger London dispersion forces (more linear shape → larger surface area → stronger instantaneous dipole-induced dipole interactions)

Preparation of Alkenes and Alkynes

Industrially, alkenes are produced primarily by the cracking (pyrolysis) of petroleum fractions — heating large alkanes to high temperatures (500–900°C) to break them into smaller, more valuable alkenes and alkanes.

Laboratory/preparative methods for alkenes:

1. Dehydrohalogenation of alkyl halides: Elimination of HX (where X = Cl, Br, I) from an alkyl halide using a strong base (e.g., NaOH, KOH, alcoholic KOH): CH₃–CH₂–Br + KOH(alc) → CH₂=CH₂ + KBr + H₂O

   The Zaitsev’s Rule (Saytzeff’s Rule) states that the major product is the more substituted alkene (the alkene with the most alkyl substituents on the double bond).

2. Dehydration of alcohols: Elimination of water from an alcohol using a strong acid catalyst (e.g., H₂SO₄ at 170°C): CH₃–CH₂–OH + H₂SO₄(170°C) → CH₂=CH₂ + H₂O

3. Catalytic hydrogenation of alkynes (partial reduction): Controlled hydrogenation using a poisoned catalyst (e.g., Lindlar’s catalyst — Pd/CaCO₃ poisoned with lead acetate and quinoline) gives cis-alkenes: CH≡CH + H₂ →(Lindlar) cis-CH₂=CH₂

Reactions of Alkenes

Alkenes undergo electrophilic addition reactions — the electron-rich pi bond acts as a nucleophile and attacks an electrophile. The pi electrons are relatively loosely held and are the first to react.

Addition of Halogens (Halogenation)

Alkenes react with halogens (Cl₂, Br₂, I₂) at room temperature to form vicinal dihalides (two halogens on adjacent carbons): CH₂=CH₂ + Br₂ → BrCH₂–CH₂Br (1,2-dibromoethane)

The bromine test: The decolorization of reddish-brown bromine solution is a classic test for unsaturation (C=C double bonds). This test is particularly important in organic chemistry practical work and in identifying unsaturated compounds in pharmaceutical analysis.

Mechanism: The pi electrons of the alkene attack the electrophilic end of the halogen molecule (Br–Br), forming a cyclic bromonium ion intermediate. The bromide ion then attacks from the backside (anti addition), resulting in anti addition of the two bromine atoms to the double bond.

Addition of Hydrogen Halides (Hydrohalogenation)

Alkenes react with HBr, HCl, and HI to form alkyl halides: CH₂=CH₂ + HBr → CH₃–CH₂Br (bromoethane)

Markovnikov’s Rule: When adding HX to a unsymmetrical alkene (one with different substituents on each carbon of the double bond), the hydrogen atom adds to the carbon with more hydrogen atoms (the less substituted carbon), and the halogen adds to the more substituted carbon: CH₃–CH=CH₂ + HBr → CH₃–CH(Br)–CH₃ (2-bromopropane, not 1-bromopropane)

Anti-Markovnikov addition: In the presence of peroxides (RCOO–), the addition of HBr to alkenes occurs in the opposite (anti-Markovnikov) manner — this is the Kharasch effect or peroxide effect: CH₃–CH=CH₂ + HBr(peroxides) → CH₃–CH₂–CH₂Br (1-bromopropane)

Addition of Water (Hydration)

Acid-catalyzed hydration: Water adds to alkenes in the presence of dilute H₂SO₄ to form alcohols: CH₂=CH₂ + H₂O →(H⁺) CH₃–CH₂OH (ethanol)

This is the industrial method for producing ethanol. Like hydrohalogenation, this follows Markovnikov’s rule — water adds so that the –OH group goes to the more substituted carbon: CH₃–CH=CH₂ + H₂O →(H⁺) CH₃–CH(OH)–CH₃ (propan-2-ol)

Hydrogenation

Alkenes can be hydrogenated (reduced) to alkanes by adding H₂ in the presence of a metal catalyst (Ni, Pd, or Pt) at moderate temperatures: CH₂=CH₂ + H₂ →(Ni, heat) CH₃–CH₃

This reaction is exothermic (hydrogenation is always exothermic). Catalysts speed up the reaction but are not consumed. Hydrogenation of unsaturated vegetable oils (converting liquid unsaturated fats to saturated solid fats) is the basis of margarine production.

Oxidation Reactions

With cold, dilute KMnO₄ (Baeyer’s test): Alkenes react with cold dilute KMnO₄ solution to form diols (vicinal diols — two –OH groups on adjacent carbons). The purple KMnO₄ solution is decolorized: 3CH₂=CH₂ + 2KMnO₄ + 4H₂O → 3HO–CH₂–CH₂–OH + 2MnO₂ + 2KOH

With hot, concentrated KMnO₄: Hot concentrated KMnO₄ cleaves the double bond, producing carboxylic acids or ketones (depending on substitution pattern):

• Unsubstituted carbon (CH₂=) → CO₂
• Disubstituted carbon (=CH–) → carboxylic acid (R–COOH)
• Trisubstituted carbon (=C<) → ketone (R–CO–R’)

Ozonolysis: Reaction of alkenes with O₃ followed by reductive workup (Zn/H₂O or DMS) cleaves the double bond to give carbonyl compounds (aldehydes or ketones). This is a powerful structural analysis tool — the products of ozonolysis reveal where the double bond was located in the original molecule.

Reactions of Alkynes

Addition of Halogens and Hydrogen Halides

Alkynes undergo addition reactions similar to alkenes but can add two molecules of reagent (one across each pi bond): CH≡CH + Br₂ →(1st addition) BrCH=CHBr →(2nd addition) Br₂CH–CHBr₂

Hydrogen halides add according to Markovnikov’s rule (for the first addition): CH≡CH + HBr → CH₂=CHBr (bromoethene, vinyl bromide)

Hydration (Kucherov Reaction)

Alkynes can be hydrated (water added across the triple bond) in the presence of HgSO₄ and dilute H₂SO₄ to form ketones (not aldehydes, except for acetylene which gives acetaldehyde): CH≡CH + H₂O →(Hg²⁺, H₂SO₄) [CH₂=CH–OH] → tautomerizes to CH₃–CHO (acetaldehyde)

For internal alkynes: CH₃–C≡CH + H₂O →(Hg²⁺) CH₃–CO–CH₃ (propanone/acetone)

This follows Markovnikov’s addition of water to the triple bond.

Acid-Catalyzed Dehydration

Like alcohols, alkynes can be formed from dihalides by elimination of two molecules of HX. Conversely, vicinal dihalides (two halogens on adjacent carbons) undergo double dehydrohalogenation to form alkynes: R–CHX–CHX–R’ + 2KOH(alc) → R–C≡C–R’ + 2KCl + 2H₂O

Conversion to Acetylides

Alkynes with a terminal –C≡CH group (terminal alkynes) are weakly acidic and can be deprotonated by strong bases (Na, NaNH₂, Grignard reagents) to form acetylide anions: HC≡CH + Na →(liquid NH₃) H–C≡C⁻Na⁺ + ½H₂ HC≡CH + NaNH₂ → H–C≡C⁻Na⁺ + NH₃

These acetylide anions are useful nucleophiles and can undergo SN2 reactions with alkyl halides to form longer-chain alkynes (this is a method for carbon-carbon bond formation in organic synthesis).

⚡ Exam tip: Remember Markovnikov’s rule — hydrogen adds to the carbon with more hydrogens. Anti-Markovnikov addition only occurs with HBr in the presence of peroxides. Alkenes decolorize bromine water (the bromine test for unsaturation). Hot KMnO₄ cleaves double bonds to carboxylic acids. Remember the tautomerization of enols to carbonyl compounds (keto-enol tautomerism).

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