Organic Chemistry: Alkenes and Reactions

Hydrocarbons

• Hydrocarbons are compounds composed of carbon and hydrogen.

Classification of Hydrocarbons

• Hydrocarbons are classified into three main categories based on the types of carbon-carbon bonds:
  • Saturated hydrocarbons
  • Unsaturated hydrocarbons
  • Aromatic hydrocarbons

Alkanes

• Alkanes are saturated open-chain hydrocarbons containing only carbon-carbon single bonds.
• General formula: CnH2n+2
  • n stands for the number of carbon atoms
  • 2n+2 stands for the number of hydrogen atoms
• All H-C-H bond angles are 109.5°.
• Types of carbon atoms:
  • Primary (1°): Attached to one other carbon atom
  • Secondary (2°): Attached to two other carbon atoms
  • Tertiary (3°): Attached to three other carbon atoms
  • Quaternary (4°): Attached to four other carbon atoms

Preparation of Alkanes

• From unsaturated hydrocarbons: Dihydrogen gas adds to alkenes and alkynes in the presence of platinum, palladium or nickel catalysts which creates alkanes. This process is called hydrogenation.
• From alkyl halides: Alkyl halides (except fluorides) with zinc and dilute hydrochloric acid yields alkanes.
• Wurtz reaction: Alkyl halides with sodium metal in dry ether produce higher alkanes.
• From carboxylic acids: Sodium salts of carboxylic acids undergo decarboxylation (eliminating carbon dioxide) by heating with soda lime (mixture of sodium hydroxide and calcium oxide) leading to a reduction of the alkane chain by one carbon.
• Kolbe's electrolytic method: An aqueous solution of the sodium or potassium salts of a carboxylic acid produces an alkane via electrolysis.

Physical Properties of Alkanes

• Alkanes are almost nonpolar molecules due to covalent C-C and C-H bonds and the small difference in electronegativity between carbon and hydrogen atoms.
• First four members (C1 to C4) are gases.
• C5 to C17 are liquids.
• Those with 18 or more carbons are solids at room temperature (298 K).
• Alkanes are colorless and odorless.

Chemical Properties of Alkanes

• Substitution reactions: One or more hydrogen atoms can be replaced by halogens, nitro group, or sulfonic acid group.
• Halogenation: Halogenation occurs at high temperatures (573-773 K) or in the presence of diffused sunlight or ultraviolet light.
• Reactions:
  • CH₄ + Cl₂ → CH₃Cl + HCl (UV light)
  • CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (UV light)
  • CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (UV light)
  • CHCl₃ + Cl₂ → CCl₄ + HCl (UV light)
• Mechanism
  • Initiation: The reaction starts with the homolysis of chlorine molecules in presence of heat or light.
  • Propagation: Chlorine free radical attacks methane generating a methyl free radical with the formation of HCl
  • Termination: This reaction stops due reactant consumption and/or other side reactions.
• Combustion: Alkanes, when heated in the presence of air or oxygen, are completely oxidized into carbon dioxide and water with the release of heat (used as fuel). CH4 + 2O2 → CO2 + 2H2O; ΔH° = -840 kJ/mol
• Controlled oxidation: Alkanes react with oxygen/air at a controlled/regulated rate, and high temperature/pressure, and catalysts to yield a variety of oxidation products.
• Isomerization: n-alkanes transformed to branched-chain alkanes upon heating in the presence of anhydrous aluminum chloride and hydrogen chloride.
• Aromatization: n-alkanes with six or more carbon atoms on heating to 773K at 10-20 atmospheric pressure with vanadium, molybdenum or chromium oxides on alumina undergo dehydrogenation and cyclization to produce benzene and its homologues.
• Reaction with steam: Methane reacts with steam at 1273 K in the presence of nickel catalysts to form carbon monoxide and dihydrogen. (Used for industrial preparation of dihydrogen).
• Pyrolysis (Cracking): Higher alkanes decompose into lower alkanes/alkenes, etc. when heated to high temperatures (decomposition into smaller molecules using heat).

Conformations

• The rotation of C-C bonds results in different spatial arrangements called conformations, conformers, or rotamers.
  • Eclipsed: Hydrogen atoms are closest to each other.
  • Staggered: Hydrogen atoms are farthest apart for minimal repulsion.
  • Skew: Intermediate between eclipsed and staggered.

Representations of Conformations

• Line-wedge, Sawhorse, and Newman projections show different ways to depict the same conformation.

Alkenes

• Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond.
• General formula: CnH2n
• Nomenclature: The suffix "ene" is used in alkenes

Isomerism in Alkenes

• Structural isomerism (position isomerism and chain isomerism)
• Geometrical isomerism:
  • Cis: Identical groups on the same side of the double bond.
  • Trans: Identical groups on opposite sides of the double bond.

Preparation of Alkenes

• From alkynes: Partial reduction of alkynes (with a calculated amount of dihydrogen) in the presence of Lindlar's catalyst (partially deactivated palladium-on-charcoal catalyst) yields alkenes.
• From alkyl halides (dehydrohalogenation): Alkyl halides with alcoholic potassium hydroxide, eliminate hydrogen halides, to produce alkenes. This is a β-elimination reaction.
• From vicinal dihalides (dehalogenation): Vicinal dihalides react with zinc metal to form an alkene.

Physical Properties of Alkenes

• The first three members are gases.
• The next fourteen are liquids.
• Higher alkenes are solids.

Chemical Properties of Alkenes

• Addition reactions: Electrophiles add to the carbon-carbon double bond to form addition products.
  • Addition of dihydrogen (hydrogenation): Alkenes react with dihydrogen gas in the presence of nickel, palladium, or platinum to form alkanes.
  • Addition of halogens: Halogens like bromine or chlorine react to form vicinal dihalides.
  • Addition of hydrogen halides (Markovnikov and Anti-Markovnikov addition): Hydrogen halides add to unsymmetrical alkenes either by Markovnikov (the negative part of the addendum attacks carbon with fewest hydrogens) or Anti-Markovnikov (peroxide effect) mechanisms.
    • Addition of water (acidic conditions): Alkenes react with acidified water to form alcohols, in accordance with Markovnikov's rule.
  • Oxidation: Alkenes react with cold, dilute, aqueous potassium permanganate to produce vicinal glycols. Acidic potassium permanganate or potassium dichromate oxidize alkenes to ketones or acids, depending on the alkene structure.
  • Ozonolysis: Ozone adds to alkenes to form ozonides, which are then cleaved by Zn-H2O to yield smaller molecules.
• Polymerization: Large numbers of ethene molecules combine at high temperatures, pressure, and in the presence of a catalyst (forming polythene).

Alkynes

• Alkynes contain at least one carbon-carbon triple bond.
• General formula: CnH2n-2
• Nomenclature: The suffix "yne" is used.
• Position isomerism exists.
• Preparation:
• From calcium carbide by reaction with water.
• From vicinal dihalides by reaction with alcoholic potassium hydroxide undergoing dehydrohalogenation.

Physical Properties of Alkynes

• First three are gases.
• Next 8 are liquids. Higher are solids.
• All are colourless.

Chemical Properties of Alkynes

• Acidic character: Sodium and sodamide react with alkynes to liberate dihydrogen gas, forming sodium acetylides.
• Addition reactions:
  • Addition of dihydrogen (hydrogenation): React with hydrogen gas with a catalyst (Pt or Pd/C) to form alkenes.
  • Addition of halogens: React with halogens—forming di-halides.
  • Addition of hydrogen halides: React with hydrogen halides, producing alkyl halides. The addition follows Markovnikov's rule in normal conditions.
  • Addition of water: React with water under certain conditions to produce a carbonyl compound.
• Polymerization: Can polymerize.

Aromatic Hydrocarbons

• Commonly known as arenes.
• Structure of benzene: Kekule suggested an oscillating nature of double bonds. Experimentally, the bond lengths are equal, indicating delocalised electrons.
• Benzene is a planar molecule.
• Aromatic compounds have sp² hybridized carbon atoms.
• Aromaticity: A compound is aromatic if it meets Huckel's rule: Planar, complete delocalization of π electrons, has (4n+2) π electrons in the ring where n is an integer (0,1,2...).
• Preparation of benzene: From cyclic polymerisation of ethyne, decarboxylation of aromatic acids (such as reacting sodium salt of benzoic acid with soda lime).
• Reactions of benzene:
  • Reaction with zinc dust: Convert phenol to benzene.
  • Electrophilic substitution reactions (nitration, halogenation, sulphonation, Friedel-Crafts alkylation, acylation).

Physical Properties of Aromatic Hydrocarbons

• Usually colorless liquids or solids.
• Characteristic aroma.
• Immiscible with water, but miscible with organic solvents.

Chemical Properties of Aromatic Hydrocarbons

• Electrophilic substitution reactions.
  • Nitration: Introduction of a nitro group (using a mixture of concentrated nitric acid and concentrated sulfuric acid).
  • Sulphonation: Replacement of a hydrogen atom with a sulfonic acid group (using fuming sulfuric acid).
  • Halogenation: Reaction with halogens (e.g., chlorine) use a Lewis acid catalyst.
  • Friedel-Crafts alkylation: Reaction of benzene with alkyl halides using aluminum chloride as a catalyst.
  • Friedel-Crafts acylation: Reaction of benzene with acyl halides or acid anhydrides using aluminum chloride as a catalyst.
• Addition reactions: Rare in normal conditions but can happen under very vigorous conditions (e.g. hydrogenation under Pt catalyst at high temperature and pressure to produce cyclohexane) or reactions with chlorine in the presence of UV light.

Other relevant information

• Various reactions of these compounds are possible including combustion (burning in air to give carbon dioxide and water) and numerous other specific reactions that are not explicitly detailed in the provided summaries.