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.