Properties of Hydrocarbons
The properties of hydrocarbons are critical for understanding their behavior and applications in various fields. This section particularly emphasizes alkenes, which are unsaturated hydrocarbons, containing at least one carbon-carbon double bond.
Physical Properties
Alkenes resemble alkanes in their basic physical properties, but they differ in their chemical behavior. The first three alkenes are gases, while the following fourteen are liquids, and higher members solidify. For instance, ethene (C2H4) is a colorless gas with a faint sweet odor. Alkenes are generally insoluble in water but mix well with non-polar solvents such as benzene and petroleum ether. The boiling point increases with molecular size—approximately 20-30 K for each -CH2- addition to the chain. As a general rule, straight-chain alkenes have higher boiling points than branched-chain varieties due to the stronger van der Waals forces among molecules.
Chemical Properties
Alkenes exhibit rich chemical reactivity due to the presence of the pi bond, allowing for various addition reactions where electrophiles add across the double bond. These reactions often follow Markovnikov's rule, directing the attachment of substituents based on the stability of possible intermediates. Reaction types involve:
1. Addition of hydrogen (
H2) to form alkanes.
2. Halogen addition forming vicinal dihalides.
3. Addition of hydrogen halides (e.g., HBr) forming haloalkanes.
4. Free radical reactions where the product distribution can reflect the conditions (principally influenced by peroxide during HBr addition).
5. Reactions with water to produce alcohols in the presence of acid, also following Markovnikov’s rule.
Overall, understanding the properties of hydrocarbons, specifically alkenes, is vital for their applications in the chemical industry, including polymer production and organic synthesis.