Alkanes and Alkenes Study Notes

Chapter 2: Alkanes and Alkenes

Chapter 2A: Alkanes & Cycloalkanes

### Chapter 2B: Alkenes & Cycloalkenes

Table of Contents

  1. Alkanes

    • Definition and General Formula

    • Physical Properties of Alkanes & Cycloalkanes

    • Reaction of Alkanes

    • IUPAC Nomenclature for Alkanes

  2. Cycloalkanes

    • Nomenclature Rules for Cycloalkanes

  3. Alkenes

    • Definition and General Formula

    • Physical Properties of Alkenes

    • Naming of Alkenes

    • Preparation and Reactions of Alkenes

- Hydrocarbon Characterization Test

1. Alkanes

Definition and General Formula

  • Alkanes: Saturated hydrocarbons having the general formula ext{CnH}_{2n+2}

  • Cycloalkanes: Saturated hydrocarbons with carbon atoms arranged in a ring, having the general formula ext{CnH}_{2n}

Physical Properties of Alkanes & Cycloalkanes

  1. Physical State (Phase)

    • At room temperature (25°C) and atmospheric pressure (1 atm):

      • Unbranched alkanes with:

      • $C1$ to $C4$: Gases

      • $C5$ to $C{17}$: Liquids

      • $C_{18}$ and larger: Solids

  2. Boiling Point

    • As the length of the carbon chain increases (without branches):

      • The size of the alkane increases

      • The strength of Van der Waals forces between molecules increases

      • More energy is required to break the bonds, leading to an increase in boiling point.

    • Graph: "Boiling Point vs C numbers" indicating:

      • Gases (low boiling point) to Liquids to Solids (high boiling points).

    • Effect of Branching: Branching of the alkane chain lowers the boiling point. Example data:

      • Molecular Structure A: Boiling Point 37°C.

      • Molecular Structure B: Boiling Point 28.5°C.

      • Molecular Structure C: Boiling Point 9°C.

  3. Solubility

    • Alkanes are less dense than water and are almost totally insoluble in water.

    • Liquid alkanes and cycloalkanes are soluble in one another and in non-polar solvents.

    • Good non-polar solvents for alkanes include:

      • Benzene ($ ext{C}6 ext{H}6$)

      • Carbon tetrachloride ($ ext{CCl}_{4}$)

      • Chloroform ($ ext{CHCl}_{3}$)

Reaction of Alkanes

  1. Combustion: Alkanes burn in excess oxygen producing carbon dioxide (CO₂), water (H₂O), and heat.

    • General Reaction: ext{C}n ext{H}{2n+2} + (n + rac{3}{2})O2 → nCO2 + (n + 1)H_2O + ext{Heat}

  2. Halogenation: This is a substitution reaction where chlorine or bromine substitutes for hydrogen in alkanes.

    • Example: ext{CH}4 + ext{Cl}2 → ext{CH}_3 ext{Cl} + ext{HCl} (requires UV light).

2. Cycloalkanes

Nomenclature Rules for Cycloalkanes

  1. If the number of carbons in the ring is equal to or greater than the number of carbons in the side chain, the ring becomes the parent compound (suffix).

  2. If the number of carbons in the side chain is greater than that in the ring, the ring becomes a substituent (prefix).

    • Example: isopropylcyclohexane and 2-cyclopropylhexane.

  3. Identical substituents on the ring should be given the lowest consecutive positions.

  4. For different substituents, number them according to alphabetical priority.

3. Alkenes

Definition and General Formula

  • Alkenes: Unsaturated hydrocarbons with double bonds ( ext{C=C}) and the general formula ext{C}n ext{H}{2n} with n ext{ ≥ 2}.

  • Exhibit cis-trans isomerism due to restricted rotation around the carbon-carbon double bond.

Physical Properties of Alkenes

  • Similar to alkanes but generally have lower boiling points due to lesser Van der Waals forces as result of unsaturation.

Naming of Alkenes

  1. The parent name is derived by identifying the longest chain containing the double bond and ending it with the suffix "-ene".

    • Example: ext{CH}3 ext{CH}= ext{CH} ext{CH}2 ext{CH}_3 is named 2-pentene.

  2. The position of the double bond is indicated by the lower number given to its carbon.

  3. When multiple substituents are present, they are named in alphabetical order with the position of the double bond taking priority.

Preparation of Alkenes

  1. Dehydrogenation of Haloalkanes:

    • Uses KOH as base to eliminate H and halide to form an alkene.

    • Saytzeff’s Rule: The more substituted alkene will be the major product.

  2. Dehydration of Alcohols:

    • Removal of water to produce alkenes using acidic catalysts like H₂SO₄ or H₃PO₄.

Reaction of Alkenes

  1. Halogenation, Hydrogen Halide Addition, and Hydration:

    • React with halogens or acids to form haloalkanes or alcohols, respectively. The addition follows Markonikov's rule in unsymmetrical alkenes.

    • Example: Hydrohydration with ext{H}2 ext{SO}4.

  2. Oxidation Reactions:

    • Reactions with potassium permanganate (KMnO₄) can yield glycols (diols) or carbonyl-containing products depending on reaction conditions (basic vs. acidic).

  3. Ozonolysis:

    • Cleavage of alkenes using ozone, forming aldehydes and ketones as products in a highly energetic reaction.

Hydrocarbon Characterization Test

  • Alkenes are characterized by their capacity to decolorize bromine water, indicating unsaturation via their ability to undergo halogenation reactions.

  • Alkanes, being saturated, do not react under the same conditions, maintaining their color as they cannot engage in such addition reactions.

NB: Ensure to refer to additional exercises and examples provided in the transcript for ample practice and better understanding of alkanes and alkenes for examination preparation and practical applications in organic chemistry.