Properties, Reactions and Some Important Compounds

Physical Properties of Alkanes Boiling point The boiling points of alkanes are generally lower than those of the other compounds with comparable molecular weights As the number of carbon atoms increases, boiling point also increases The boiling point of constitutional isomers of alkanes decreases as branching increases (Ex. Hexane = 68.7, 3-methylpentane = 63.3, 2,3 dimethylbutane = 58.0)

Physical Properties of Alkanes Insoluble in water but soluble in organic solvents such as toluene and diethyl ether Because of the nonpolar nature of alkanes, they are not soluble in (polar) water.

Physical Properties of Alkanes All liquid and solid alkanes are less dense than water; they float on water

Exercise: Arrange the following alkanes in order of decreasing boiling points: 1. butane, decane and hexane 2. 2-methylheptane, octane, 2,2,4-trimethylpentane 3. 2-methylbutane, pentane 2,2 dimethylpropane 4. 3,3-dimethylheptane, nonane, 2,2,4- trimethylhexane

Answer 1. decane, hexane, butane 2. Octane, 2-methylheptane, 2,2,4- trimethylpentane 3. Pentane, 2-methylbutane, 2,2-dimethylpropane 4. Nonane, 3,3-dimethylheptane, 2,2,4- trimethylhexane

Reactions of Alkanes Combustion (Reaction with Oxygen) CH 4 + 2O CO 2 + 2H 2 O kcal/mol

Reactions of Alkanes Halogenation (Reaction with Halogens) – substitution reaction CH 4 + Cl CH 3 Cl + HCl CH 3 Cl + Cl CH 2 Cl 2 + HCl CH 2 Cl 2 + Cl CHCl CCl 4

Important Alkanes Methane – natural gas Propane - component of LPG Butane – fuel in lighter Octane – aviation fuel High MW alkanes are found in waxes of several plants. Example, the wax found in apple skin (C27H56), paraffin wax is used in making candles.

Important Alkanes Haloalkanes 1. Chloroflurocarbons (CFC) – nontoxic, nonflammable, odorless and noncorrosive. Industrial name is freon. Commonly used CFCs are trichlorofluromethane (CCl3F, Freon 11) and dichlorodifluoromethane (CCl2F2, Freon 12). Used as industrial cleaning solvents to prepare surfaces for coatings, to remove cutting oils and waxes from millings and to remove protective coatings. They are also used as propellants for aerosol sprays

Important Alkanes 2. Solvents – several low MW haloalkanes are excellent solvents; used as cleaners and degreasers. Most commonly used are CCl4 (discontinued because it was found to be toxic and carcinogenic), CHCl3 and CH2Cl2.

Where do we get alkanes?

Homework: 1. What are the harmful effects of CFCs? 2. Explain the meaning of octane rating?

End of Lecture