CHEM1032-Lecture 5-Physical and Chemical properties

Physical and Chemical Properties

  • Study of hydrocarbons focuses on:

    • General types of reactions

    • Alkanes and Cycloalkanes (substitution reactions)

    • Alkenes and Cycloalkenes (addition reactions)

    • Alkynes (addition reactions)

    • Uses of alkanes and alkenes

General Types of Reactions

  1. Substitution Reaction:

    • Reaction: AB + CD → AD + CB

  2. Addition Reaction:

    • Reaction: A + B → C

  3. Elimination Reaction:

    • Reaction: A → B + C

Main Types of Reactions of Organic Compounds

  1. Substitution: An atom(s) replaces another.

  2. Addition: Atom(s) are added across double (=) or triple (≡) bonds.

  3. Elimination: Atom(s) are removed from a compound.

  4. Rearrangement: Atom(s) shift resulting in a different molecule.

Combustion Reaction

  • All hydrocarbons undergo combustion (exothermic reaction):

    • General equation: Hydrocarbon + Oxygen → Carbon Dioxide + Water + Energy

  • Example reactions:

    1. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Energy

    2. C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) + Energy

Physical Properties of Alkanes and Cycloalkanes

  • Very weak Van der Waals and dipole-dipole interactions:

    • C1 – C4: gases

    • C5 – C17: liquids

    • C18: solids

  • Branched alkanes have lower boiling points than straight chain isomers; more branching = less interaction.

Chemical Properties of Alkanes and Cycloalkanes

  1. Combustion:

    • Alkanes + Oxygen → Carbon Dioxide + Water + Energy.

  2. Substitution:

    • Alkanes undergo substitution to produce alkyl halides in the presence of UV light (e.g., chlorination, bromination).

    • Example: CH4 + Br2 → CH3Br + HBr

  3. Elimination: (Dehydrogenation)

    • Two σ bonds broken and one π bond formed.

Alkyl Halides

  • Weakly polar; insoluble in water.

  • Boiling/melting points increase with size and number of carbon atoms.

  • Examples:

    • Ethane: b.p. -89 °C; Bromoethane: b.p. 39 °C

Nomenclature of Alkyl Halides

  • Importance of correctly naming alkyl halides using IUPAC rules.

Physical Properties of Alkenes and Cycloalkenes

  • Similar properties to alkanes; unsaturated compounds.

  • Increase in boiling/melting points with an increase in carbon number.

  • Insoluble in water, nonpolar.

Chemical Properties of Alkenes and Cycloalkenes

  1. Preparation:

    • Dehydrohalogenation or dehydration of alcohols.

  2. Reactions:

    • Addition reactions (e.g. hydrohalogenation, halogenation, hydration, halohydrin formation, hydrogenation).

Alkyne Properties

  • Similar to alkanes and alkenes but higher boiling points.

  • Prepared via elimination.

Uses of Hydrocarbons

  • Alkanes: Methane as natural fuel, butane in gasoline.

  • Alkenes: Used in bags, films, and PVC plastics.

  • Alkyl Halides: Utilized in natural defense mechanisms by organisms and produced in various marine environments.