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12.8: Reactions of Alkanes

  • Page ID
    401283
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    Learning Objectives
    • Understand the reactions of alkanes: combustion and halogenation.

    Alkanes are relatively stable, nonpolar molecules, that will not react with acids, bases, or oxidizing or reducing reagents. Alkanes undergo so few reactions that they are sometimes called paraffins, from the Latin parum affinis, meaning “little affinity.”

    However, heat or light can initiate the breaking of C–H or C–C single bonds in reactions called combustion and halogenation.

    Combustion

    Nothing happens when alkanes are merely mixed with oxygen (\(O_2\)) at room temperature, but when a flame or spark provides the activation energy, a highly exothermic combustion reaction proceeds vigorously. For methane (CH4), the combustion reaction is as follows:

    \[CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat} \label{1} \]

    As a consequence, alkanes are excellent fuels. For example, methane, CH4, is the principal component of natural gas. Butane, C4H10, used in camping stoves and lighters is an alkane. Gasoline is a liquid mixture of straight- and branched-chain alkanes, each containing from five to nine carbon atoms, plus various additives to improve its performance as a fuel. Kerosene, diesel oil, and fuel oil are primarily mixtures of alkanes with higher molecular masses. The main source of these liquid alkane fuels is crude oil, a complex mixture that is separated by fractional distillation. Fractional distillation takes advantage of differences in the boiling points of the components of the mixture (Figure \(\PageIndex{1}\)). You may recall that boiling point is a function of intermolecular interactions, which was discussed in an earlier chapter.

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    Figure \(\PageIndex{1}\):In a column for the fractional distillation of crude oil, oil heated to about 425 °C in the furnace vaporizes when it enters the base of the tower. The vapors rise through bubble caps in a series of trays in the tower. As the vapors gradually cool, fractions of higher, then of lower, boiling points condense to liquids and are drawn off. (credit left: modification of work by Luigi Chiesa)

    If the reactants of combustion reactions are adequately mixed, and there is sufficient oxygen, the only products are carbon dioxide (\(CO_2\)), water (\(H_2O\)), and energy—heat for cooking foods, heating homes, and drying clothes. Because conditions are rarely ideal, other unwanted by-products are frequently formed. When the oxygen supply is limited, carbon monoxide (\(CO\)) is a by-product:

    \[2CH_4 + 3O_2 \rightarrow​ 2CO + 4H_2O\label{2} \]

    This reaction is responsible for dozens of deaths each year from unventilated or improperly adjusted gas heaters. (Similar reactions with similar results occur with kerosene heaters.)

    Halogenation

    In halogenation reactions, alkanes react with the halogens chlorine (\(Cl_2\)) and bromine (\(Br_2\)) in the presence of ultraviolet light or at high temperatures to yield chlorinated and brominated alkanes. For example, chlorine reacts with excess methane (\(CH_4\)) to give methyl chloride (\(CH_3Cl\)).

    \[CH_4 + Cl_2 \rightarrow​ CH_3Cl + HCl\label{12.7.3} \]

    With more chlorine, a mixture of products is obtained: CH3Cl, CH2Cl2, CHCl3, and CCl4. Fluorine (\(F_2\)), the lightest halogen, combines explosively with most hydrocarbons. Iodine (\(I_2\)) is relatively unreactive. Fluorinated and iodinated alkanes are produced by indirect methods.

    A wide variety of interesting and often useful compounds have one or more halogen atoms per molecule. For example, methane (CH4) can react with chlorine (Cl2), replacing one, two, three, or all four hydrogen atoms with Cl atoms. Several halogenated products derived from methane and ethane (CH3CH3) are listed in Table \(\PageIndex{1}\), along with some of their uses.

    Table \(\PageIndex{1}\): Some Halogenated Hydrocarbons
    Formula Common Name IUPAC Name Some Important Uses
    Derived from CH4
    CH3Cl methyl chloride chloromethane refrigerant; the manufacture of silicones, methyl cellulose, and synthetic rubber
    CH2Cl2 methylene chloride dichloromethane laboratory and industrial solvent
    CHCl3 chloroform trichloromethane industrial solvent
    CCl4 carbon tetrachloride tetrachloromethane dry-cleaning solvent and fire extinguishers (but no longer recommended for use)
    CBrF3 halon-1301 bromotrifluoromethane fire extinguisher systems
    CCl3F chlorofluorocarbon-11 (CFC-11) trichlorofluoromethane foaming plastics
    CCl2F2 chlorofluorocarbon-12 (CFC-12) dichlorodifluoromethane refrigerant
    Derived from CH3CH3
    CH3CH2Cl ethyl chloride chloroethane local anesthetic
    ClCH2CH2Cl ethylene dichloride 1,2-dichloroethane solvent for rubber
    CCl3CH3 methylchloroform 1,1,1-trichloroethane solvent for cleaning computer chips and molds for shaping plastics
    Note To Your Health: Halogenated Hydrocarbons

    Once widely used in consumer products, many chlorinated hydrocarbons are suspected carcinogens (cancer-causing substances) and also are known to cause severe liver damage. An example is carbon tetrachloride (CCl4), once used as a dry-cleaning solvent and in fire extinguishers but no longer recommended for either use. Even in small amounts, its vapor can cause serious illness if exposure is prolonged. Moreover, it reacts with water at high temperatures to form deadly phosgene (COCl2) gas, which makes the use of CCl4 in fire extinguishers particularly dangerous.

    Ethyl chloride, in contrast, is used as an external local anesthetic. When sprayed on the skin, it evaporates quickly, cooling the area enough to make it insensitive to pain. It can also be used as an emergency general anesthetic.

    Bromine-containing compounds are widely used in fire extinguishers and as fire retardants on clothing and other materials. Because they too are toxic and have adverse effects on the environment, scientists are engaged in designing safer substitutes for them, as for many other halogenated compounds.

    Note To Your Health: Chlorofluorocarbons and The Ozone Layer

    Alkanes substituted with both fluorine (F) and chlorine (Cl) atoms have been used as the dispersing gases in aerosol cans, as foaming agents for plastics, and as refrigerants. Two of the best known of these chlorofluorocarbons (CFCs) are listed in Table \(\PageIndex{2}\).

    Chlorofluorocarbons contribute to the greenhouse effect in the lower atmosphere. They also diffuse into the stratosphere, where they are broken down by ultraviolet (UV) radiation to release Cl atoms. These in turn break down the ozone (O3) molecules that protect Earth from harmful UV radiation. Worldwide action has reduced the use of CFCs and related compounds. The CFCs and other Cl- or bromine (Br)-containing ozone-destroying compounds are being replaced with more benign substances. Hydrofluorocarbons (HFCs), such as CH2FCF3, which have no Cl or Br to form radicals, are one alternative. Another is hydrochlorofluorocarbons (HCFCs), such as CHCl2CF3. HCFC molecules break down more readily in the troposphere, and fewer ozone-destroying molecules reach the stratosphere.

    ozone.jpg

    Figure \(\PageIndex{2}\): Ozone in the upper atmosphere shields Earth’s surface from UV radiation from the sun, which can cause skin cancer in humans and is also harmful to other animals and to some plants. Ozone “holes” in the upper atmosphere (the gray, pink, and purple areas at the center) are large areas of substantial ozone depletion. They occur mainly over Antarctica from late August through early October and fill in about mid-November. Ozone depletion has also been noted over the Arctic regions. The largest ozone hole ever observed occurred on 24 September 2006. Source: Image courtesy of NASA, http://ozonewatch.gsfc.nasa.gov/daily.php?date=2006-09-24.


    12.8: Reactions of Alkanes is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.

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