8-Alkanes, Alkenes , Alkynes - 2021

Hydrocarbons Overview

  • Hydrocarbons are compounds consisting solely of carbon and hydrogen.

    • Categories of Hydrocarbons:

      • Saturated Hydrocarbons: All carbon atoms are connected with single bonds.

      • Unsaturated Hydrocarbons: Contain one or more double or triple carbon-carbon (C-C) bonds.

Saturated Hydrocarbons

  • Found in two formats:

    • Acyclic Hydrocarbons: Linear or branched chains (e.g., alkanes).

    • Cyclic Hydrocarbons: Ring structures.

Alkanes: Acyclic Saturated Hydrocarbons

  • Definition: Saturated hydrocarbons that do not possess rings.

    • General formula: CnH2n+2 (where n = number of carbon atoms).

    • Examples:

      • Methane (C1H4): Simplest alkane.

      • Ethane (C2H6): Two carbons.

      • Propane (C3H8): Three carbons.

Molecular Structure of Alkanes

  • Simplest alkane (Methane) exhibits a tetrahedral shape due to sp³ hybridization.

    • Bond angles: 109° 28'.

    • Each carbon forms four single bonds (tetrahedral geometry).

Structural Representations of Alkanes

  • Chemical Formula: Indicates type and number of each element.

  • Structural Formulas: Show how atoms are connected.

    • Expanded structural formula, condensed structural formula, abbreviated form for longer chains.

Isomerism in Alkanes

  • Constitutional Isomers: Compounds with the same formula but different atom connections.

  • Example with C4H10:

    • Butane (n-butane): Straight chain.

    • Isobutane: Branched chain.

Chain Isomerism

  • Occurs with increasing carbon count, allowing for branching.

    • Examples: Pentane, Isopentane, Neopentane.

Spatial Isomers (Conformations)

  • Defined by rotation around C-C single bonds:

    • Eclipsed Form: Atoms are aligned.

    • Staggered Form: Atoms are offset, minimizing electron repulsion.

IUPAC Nomenclature for Alkanes

  • Established by the International Union of Pure and Applied Chemistry (IUPAC).

    • Prefix System:

      • Meth- (1 carbon), Eth- (2), Prop- (3), and so on.

      • Alkanes end with “-ane.”

  • Branched-chain alkanes:

    • Alkyl Groups: Formed by removing hydrogen from an alkane.

  • Steps to Name:

    1. Identify the longest chain.

    2. Number the chain for lowest substituent numbering.

    3. Prefix substituent names with their position on the chain.

Multiple Substituents Naming

  • Use di-, tri-, tetra-, etc., for multiple identical substituents.

  • Alphabetical order takes precedence in numbering when different types are involved.

Classification of Carbon Atoms

  • Carbon atoms classified as 1° (primary), 2° (secondary), 3° (tertiary), or 4° (quaternary) based on number of adjacently bound carbon atoms.

Branched-chain Alkyl Groups

  • Branched substituents named according to the alkane they derive from, preferring unbranched structures for naming.

Sources of Alkanes

  • Crude petroleum is purified by exploiting boiling point differences of hydrocarbon mixtures.

    • Boiling points increase with longer carbon chain lengths (approx. 30°C per additional CH2 unit).

Physical Properties of Alkanes

  1. Insoluble in water.

  2. Density: Less than water (0.6 – 0.8 g/mL).

  3. Boiling points rise with carbon count or ring size.

  4. Branched alkanes have lower boiling points due to compactness.

Chemical Properties of Alkanes

  • Characterized by low reactivity due to non-polar C-C and C-H bonds.

Combustion Reactions

  • Alkanes react with O2:

    • Complete combustion forms CO2 and H2O with heat produced.

    • Incomplete combustion can yield CO or other byproducts.

Substitution Reactions

  • Occur when an atom/group in a hydrocarbon is replaced (e.g., halogenation).

Halogenation Reactions

  • Involves substitution of hydrogen atoms in alkanes with halogens (F, Cl, Br, I).

  • Mechanism stages include initiation (formation of free radicals), chain propagation (radical attacks), and interruption (producing stable products).

Unsaturated Hydrocarbons: Alkenes and Alkynes

  • Alkenes: Contain double bonds (e.g., Ethene).

  • Alkynes: Contain triple bonds (e.g., Ethyne).

Structures of Alkenes and Alkynes

  • Alkenes exhibit sp² hybridization, bond angles of 120°.

  • Alkynes exhibit sp hybridization, bond angles of 180°.

Naming Alkenes and Alkynes

  • Change alkane naming conventions to end in “-ene” for alkenes and “-yne” for alkynes.

    • Examples: Ethene (ethylene), Propyne.

Cis-Trans Isomers in Alkenes

  • Occur due to rigidity of the double bond, affecting spatial configurations:

    • Cis: Groups on the same side.

    • Trans: Groups on opposite sides.

  • Identical groups on double-bonded carbons negate cis-trans isomerism.

Addition Reactions of Unsaturated Hydrocarbons

  • Addition Reactions: Instantaneous reactions where reactants add to C=C or C≡C.

  • Common forms:

    • Hydrogenation: Adds H2, using catalysts.

    • Halogenation: Adds halogens (e.g., Br2).

    • Hydrohalogenation: Involves H-X addition.

    • Hydration: Requires acid catalyst for water addition.

Polymerization

  • Process where monomers join to form polymers.

    • Example: Polyethylene from ethylene monomers.

    • Common synthetic polymers include PVC, Teflon, and polystyrene.