Understanding Hydrocarbons and Alkanes

• Hydrocarbons consist of only carbon and hydrogen

Hydrocarbon Importance

• Sources of energy
• Used in the manufacture of polymers like polythene.
• Used as solvents for paints.
• Starting materials for dyes and drugs.

Hydrocarbon Classification

• Saturated hydrocarbons contain carbon-carbon and carbon-hydrogen single bonds
• Unsaturated hydrocarbons contain carbon-carbon multiple bonds (double or triple).
• Aromatic hydrocarbons are cyclic compounds with alternating single and double bonds

Alkanes

• Saturated open chain hydrocarbons with carbon-carbon single bonds.
• Methane (CH4) is the first member.
• Alkanes are inert under normal conditions.
• General formula is CnH2n+2

Alkane Structure

• Methane (CH4) has a tetrahedral structure.
• Carbon atom lies at the center.
• Four hydrogen atoms lie at the corners of a regular tetrahedron.
• H-C-H bond angles are 109.5°.
• C-C bonds are 154 pm.
• C-H bonds are 112 pm.
• C-C and C-H bonds are σ bonds

Alkane Isomers

• First three alkanes (methane, ethane, propane) have only one structure.
• Higher alkanes can have more than one structure
• Structural isomers differ in their properties due to differences in their structures.
• Chain isomers differ in the carbon chain arrangement like butane and isobutane

Alkyl Groups

• Alkyl groups are substituents derived from alkanes by removing one hydrogen atom (CH3, C2H5, etc.).
• General formula for alkyl groups is CnH2n+1.

Alkane Preparation

• From Unsaturated Hydrocarbons (Hydrogenation).
• Dihydrogen gas (H2) is added to alkenes and alkynes.
• Finely divided catalysts like platinum, palladium, or nickel are needed.
• From Alkyl Halides: Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.
• From Carboxylic Acids.
• Sodium salts of carboxylic acids are heated with soda lime (NaOH + CaO).
• Kolbe’s Electrolytic Method: Aqueous solutions of sodium or potassium salts of carboxylic acids on electrolysis yield alkanes

Alkane Physical Properties

• Almost non-polar due to covalent C-C and C-H bonds.
• Possess weak van der Waals forces.
• First four members (C1 to C4) are gases, C5 to C17 liquids, and those with 18+ carbons are solids (at 298 K).
• Colorless and odorless.
• Insoluble in water but soluble in non-polar solvents.
• Boiling point increases with molecular mass due to increasing van der Waals forces.
• Branched chains reduce boiling points

Alkane Chemical Properties

• Alkanes are generally inert to acids, bases, oxidizing, and reducing agents.
• Substitution Reactions: Hydrogen atoms can be replaced by halogens, nitro groups, or sulfonic acid groups.
• Halogenation occurs at high temperatures (573-773 K) or in diffused sunlight/UV light.
• Lower alkanes do not undergo nitration or sulphonation.

Halogenation Mechanism

• Proceeds via a free radical chain mechanism.
• Initiation: Homolysis of chlorine molecule by light or heat.
• Propagation: Chlorine free radical attacks methane, forming methyl free radical and HCl. The methyl radical reacts with chlorine to form chloromethane and another chlorine free radical.
• Termination: Consumption of reactants or combination of free radicals.
• Combustion: Alkanes burn completely in air/dioxygen to form carbon dioxide and water, releasing large amounts of heat.

Alkanes - Important Reactions

• Controlled Oxidation: Alkanes react with regulated oxygen/air, with catalysts, to produce oxidation products.
  • Example: Methanol from methane using copper catalyst.
• Isomerization: n-Alkanes isomerize to branched-chain alkanes with anhydrous aluminum chloride and HCl.
• Aromatization:
  • n-Alkanes with six or more carbon atoms are dehydrogenated and cyclized to form benzene and its homologues with oxides of vanadium, molybdenum, or chromium, supported over alumina at 773 K.
• Reaction with Steam: Methane reacts with steam at 1273 K with a nickel catalyst to form carbon monoxide and dihydrogen.
• Used for industrial preparation of dihydrogen gas.
• Pyrolysis: Higher alkanes decompose into lower alkanes, alkenes, etc., on heating.
  • The application of heat to cause decomposition into smaller fragments is called pyrolysis or cracking.

Alkane Conformations

• Rotation around C-C bonds results in spatial arrangements - conformations, conformers, or rotamers.
• Rotation is not completely free, it is hindered by a small energy barrier (1-20 kJ/mol) due to weak repulsive interaction between adjacent bonds (torsional strain).

Ethane Conformations

• Ethane contains a carbon-carbon single bond, with each carbon atom being attached to three hydrogen atoms.
• Eclipsed Conformation: Hydrogens on two carbons are as close as possible.
• Staggered Conformation: Hydrogens are as far apart as possible.
• Skew Conformation: Any intermediate conformation.
• Bond angles and lengths remain the same in all conformations.

Conformational Representations

• Sawhorse Projections: Viewed along the molecular axis, C-C bond drawn diagonally, with substituents attached.
• Newman Projections: Viewed head-on along the C-C bond, front carbon as a point, rear carbon as a circle with substituents attached.
• Relative Stability: Staggered form is more stable due to minimized repulsive forces and energy, while the eclipsed form has higher energy and less stability due to greater electron cloud repulsions.

Alkenes

• Unsaturated hydrocarbons containing at least one double bond. General formula is CnH2n. Also known as olefins.
• Carbon-carbon double bond consists of one strong sigma (σ) bond. Bond enthalpy is about 397 kJ/mol obtained. and one weak pi (π) bond (bond enthalpy about 284 kJ/mol).
• Alkenes have shorter bond length (134 pm) than single bonds (154 pm). Easily attacked by electrophilic reagents. Strength of double bond is significant (bond enthalpy, 681 kJ/mol).

Alkene Nomenclature

• Replace 'ane' with 'ene' in the corresponding alkane name -Number the longest chain containing the double bond with the double bond getting preference for lowest number
• Isomerism: Show structural and geometrical isomerism.

Structural Isomerism

• Ethene (C2H4) and propene (C3H6) have only one structure, but higher alkenes have different arrangements.

Geometrical Isomerism (cis-trans)

• Requires restricted rotation around a double bond.
• Results in different spatial arrangements of atoms or groups.
• cis isomer: Identical atoms or groups are on the same side of the double bond. -trans isomer: Identical atoms or groups are on opposite sides of the double bond.
  • Cis form is generally more polar than the trans form due to dipole moments.

Alkene Preparation

• From Alkynes. Alkynes are partially reduced with dihydrogen using palladised charcoal partially deactivated with poisons to yield cis-alkenes. -Sodium in liquid ammonia yields trans alkenes.
• From Alkyl Halides: Alkyl halides undergo dehydrohalogenation with alcoholic potash to yield alkenes.
• From Vicinal Dihalides: Vicinal dihalides undergo dehalogenation with zinc metal to form alkenes.
• From Alcohols by Acidic Dehydration: Alcohols heated with concentrated sulphuric acid form alkenes by eliminating water.

Alkene Physical Properties

• Resemble alkanes in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen liquids and the higher ones solids. Ethene is a colourless gas with faint sweet smell. Insoluble in water, but fairly soluble in non-polar solvents Boiling point increases with the size of alkenes. Straight chain alkenes have higher boiling points than branched chain isomers

Alkene Chemical Properties

• Due to loosely held π electrons, alkenes show addition reactions. -Addition of dihydrogen forms alkanes -Addition of halogens forms vicinal dihalides and acts as a test for unsaturation.
• Addition of Hydrogen Halides.
• Hydrogen halides (HCl, HBr, HI) add to alkenes to form alkyl halides.
• Follows Markovnikov’s Rule (negative part goes to carbon with fewer hydrogens).
• Anti-Markovnikov Addition or Peroxide Effect. -In presence of peroxide HBr ads contrary to markovnikov rule and happens only with HBr
• Addition of Sulphuric Acid adds to alkenes forming alkyl hydrogen sulfates. -Follows Markovnikov’s Rule. Addition of Water adds with alkene in present of sulphuric acid to form alcohol Ozonolysis cleaves the double bond to form ozonide & later smaller molecules for detecting the position of the bond
• Polymerization results in polythene with large number of ethene at high temperature, pressure, presence of catalyst

Alkynes

• Unsaturated hydrocarbons containing at least one triple bond.
• General formula is CnH2n-2.
• In common system, alkynes are named as derivatives of acetylene. In IUPAC system, they are named as derivatives of the corresponding alkanes replacing 'ane' by the suffix 'yne'.
• Alkynes with more than three carbons have position isomers due to the different location of the triple bond.
• Alkyne are linear molecules with two sp hybridized carbon. Structure shows σ (sigma)/π overlap

Alkyne Preparation

• From calcium carbide and water on industrial scale
• On an industrial scale, ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with coke. Quick lime can be obtained by heating limestone as shown in the following reactions:
• From vicinal dihalides: Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation

Physical Properties of Alkynes:

Follow the same trend of alkenes and alkanes. First three members are gases, the next eight are liquids and the higher ones are solids. All alkynes are colourless. Ethyne has characteristic odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene. Their melting point, boiling point and density increase with increase in molar mass.

Reactivity of Alkynes

• Acidic character
  • Alkynes add sodium and strong bases to form sodium
• Addition Reactions
  • Dihydrogen
  • Halogen
  • Hydrogen Halide depends on the vinylic carbon formation and can be performed by electrophilic or markovnikov's rule.
• The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule.
• Addition of water

Alkyne Polymerisation

• Linear takes place for polyacetylene for electricity
• Cyclic takes place with red hot iron forming benzene

Aromatic Hydrocarbons (Arenes)

• These hydrocarbons are also known as arenes:
• Most of them possess odor and were named aromatic compounds. Some of the compounds were found to contain benzene ring highly unsaturated. Most have benzene ring

Examples of Arenes

• Benzene
• Toluene
• Biphenyl
• Naphthalene

Arenes Nomenclature and Isomerism

• When two hydrogen atoms in benzene molecule become replaced by monovalent group: form as position isomer 1,2 or 1,6 are ortho, 1,3 or 1,5 are meta, 1,4 are para

Arene Formation and Production

• Isomerization occurs to direct isomers from heat and synthesis
• Have specific nomenclature
• Have Kekulé formula for structure which gives the possibility of isomeric 1,2-dibromobenzenes
• Hybridized orbital perpendicular to the plane
• Each carbon in the ring has been determined of equal probability for atoms by their overlapping

Benzene’s Chemical Properties

• Benzene can be isolated by commercial tar, prepared in by cyclic ethyne
• Characterized by electrophilic substation and reactions