ch 11

Chapter 11: The Unsaturated Hydrocarbons Alkenes, Alkynes, and Aromatics

11.1 Alkenes and Alkynes: Structure and Physical Properties

• Both alkenes and alkynes are categorized as unsaturated hydrocarbons.

• Alkene Functional Group: Characterized by a carbon-carbon double bond.

• Alkyne Functional Group: Characterized by a carbon-carbon triple bond.

• Simplest alkene: Ethene (ethylene) with molecular formula $C<em>2H</em>4$.

• Simplest alkyne: Ethyne (acetylene) with molecular formula $C<em>2H</em>2$.

Aliphatic Hydrocarbon Structure Comparison

• Comparison of Alkanes, Alkenes, and Alkynes:

  • General Formulas:

  • Alkane: $C<em>nH</em>{2n+2}$

  • Alkene: $C<em>nH</em>{2n}$

  • Alkyne: $C<em>nH</em>{2n-2}$

  • Example Structure:

  • Alkane (Ethane): $C<em>2H</em>6$

  • $H-C-C-H$

  • Alkene (Ethene): $C<em>2H</em>4$

  • $H-C=C-H$

  • Alkyne (Ethyne): $C<em>2H</em>2$

  • $H-C extless br extit{eq} C-H$

Structural Comparison of Five-Carbon Molecules

• Structure details comparing different hydrocarbons based on chain length:

  • Basic tetrahedral zig-zag shape for long-chain alkanes (e.g., Pentane).

  • Planar around the double bond (e.g., Pent-1-ene).

  • Linear at the triple bond (e.g., Pent-1-yne).

Physical Properties

• Alkenes and alkynes share physical properties similar to alkanes:

  • Nonpolar nature.

  • Insoluble in water.

  • Highly soluble in nonpolar solvents.

  • Boiling points increase with molar mass.

• Table 11.1: Molecular and Condensed Formulas of Selected Alkenes and Alkynes:

  • Ethene: $C<em>2H</em>4$, $CH<em>2=CH</em>2$, Melting Point: $-169.1 \, ^\circ C$, Boiling Point: $-103.7 \, ^\circ C$

  • Propene: $C<em>3H</em>6$, $CH<em>2=CHCH</em>3$, Melting Point: $-185.0 \, ^\circ C$, Boiling Point: $-47.6 \, ^\circ C$

  • But-1-ene: $C<em>4H</em>8$, $CH<em>2=CHCH</em>2CH_3$, Melting Point: $-185.0 \, ^\circ C$, Boiling Point: $-6.1 \, ^\circ C$

  • Methylpropene: $C<em>4H</em>8$, and so on…

11.2 Alkenes and Alkynes: Nomenclature

• Naming Rules:

  • Base name comes from the longest chain containing the multiple bond.

  • Change the suffix from –ane to –ene (for alkenes) or –yne (for alkynes).

  • Number the chain from the end that gives the first carbon of the multiple bond the lower number.

  • Indicate the position of the first carbon of the multiple bond just before the suffix.

  • Prefixes for branches/substituents follow the same conventions as alkanes.

Comparison of Names

• Examples:

  • Ethane: $CH<em>3-CH</em>3$

  • Ethene: $CH<em>2=CH</em>2$

  • Ethyne: $CH extless br extit{eq} C$

  • Prop-1-ene: $CH<em>2=CH-CH</em>3$

  • Prop-1-yne: $CH extless br extit{eq} C-CH_3$