Properties, Reactions and Some Important Compounds

Physical Properties of AlkanesBoiling pointThe 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 AlkanesInsoluble 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 AlkanesAll 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 hexane2.2-methylheptane, octane, 2,2,4-

trimethylpentane3.2-methylbutane, pentane 2,2

dimethylpropane4.3,3-dimethylheptane, nonane, 2,2,4-

trimethylhexane

Answer1. decane, hexane, butane2. Octane, 2-methylheptane, 2,2,4-

trimethylpentane3. Pentane, 2-methylbutane, 2,2-

dimethylpropane4. Nonane, 3,3-dimethylheptane, 2,2,4-

trimethylhexane

Reactions of AlkanesCombustion (Reaction with Oxygen)

CH4 + 2O2 ------ CO2 + 2H2O + 212 kcal/mol

Reactions of AlkanesHalogenation (Reaction with Halogens) –

substitution reaction

CH4 + Cl2 ------ CH3Cl + HCl

CH3Cl + Cl2 ----- CH2Cl2 + HCl

CH2Cl2 + Cl2 ----- CHCl3 ---- CCl4

Important AlkanesMethane – natural gasPropane - component of LPGButane – fuel in lighterOctane – aviation fuelHigh 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 AlkanesHaloalkanes1. 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 Alkanes2. 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