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2.3: Alkyl Groups

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  • Objectives

    After completing this section, you should be able to

    1. recognize and name any alkyl group that can be considered to have been formed by the removal of a terminal hydrogen atom from a straight-chain alkane containing ten or fewer carbon atoms.
    2. explain what is meant by a primary, secondary, tertiary or quaternary carbon atom.
    3. represent the various types of organic compounds using the symbol “R” to represent any alkyl group.

    Key Terms

    Make certain that you can define, and use in context, the key terms below.

    • alkyl group
    • methyl group
    • isopropyl group
    • sec-butyl group
    • isobutyl group
    • tert-butyl group
    • primary carbon
    • secondary carbon
    • tertiary carbon
    • quaternary carbon

    Study Notes

    The differences among primary, secondary, tertiary and quaternary carbon atoms are explained in the following discussion. A convenient way of memorizing this classification scheme is to remember that a primary carbon atom is attached directly to only one other carbon atom, a secondary carbon atom is attached directly to two carbon atoms, and so on.

    The IUPAC system requires first that we have names for simple unbranched chains, as noted above, and second that we have names for simple alkyl groups that may be attached to the chains. Examples of some common alkyl groups are given in the following table. Note that the "ane" suffix is replaced by "yl" in naming groups. The symbol R is used to designate a generic (unspecified) alkyl group.

    Table \(\PageIndex{1}\): Alkyl Group names
    Group CH3 C2H5 CH3CH2CH2 (CH3)2CH– CH3CH2CH2CH2 (CH3)2CHCH2 CH3CH2CH(CH3)– (CH3)3C– R–
    Name Methyl Ethyl Propyl Isopropyl Butyl Isobutyl sec-Butyl tert-Butyl Alkyl

    Alkyl Groups

    Alkanes can be described by the general formula CnH2n+2. An alkyl group is formed by removing one hydrogen from the alkane chain and is described by the formula CnH2n+1. The removal of this hydrogen results in a stem change from -ane to -yl. Take a look at the following examples.


    The same concept can be applied to any of the straight chain alkane names provided in Table \(\PageIndex{2}\).

    Table \(\PageIndex{2}\): straight chain alkane
    Name Molecular Formula Condensed Structural Formula
    Methane CH4 CH4
    Ethane C2H6 CH3CH3
    Propane C3H8 CH3CH2CH3
    Butane C4H10 CH3(CH2)2CH3
    Pentane C5H12 CH3(CH2)3CH3
    Hexane C6H14 CH3(CH2)4CH3
    Heptane C7H16 CH3(CH2)5CH3
    Octane C8H18 CH3(CH2)6CH3
    Nonane C9H20 CH3(CH2)7CH3
    Decane C10H22 CH3(CH2)8CH3
    Undecane C11H24 CH3(CH2)9CH3
    Dodecane C12H26 CH3(CH2)10CH3
    Tridecane C13H28 CH3(CH2)11CH3
    Tetradecane C14H30 CH3(CH2)12CH3
    Pentadecane C15H32 CH3(CH2)13CH3
    Hexadecane C16H34 CH3(CH2)14CH3
    Heptadecane C17H36 CH3(CH2)15CH3
    Octadecane C18H38 CH3(CH2)16CH3
    Nonadecane C19H40 CH3(CH2)17CH3
    Eicosane C20H42 CH3(CH2)18CH3

    Classification of carbon atoms

    Carbons have a special terminology to describe how many other carbons they are attached to.

    • Primary carbons (1o) attached to one other C atom
    • Secondary carbons (2o) are attached to two other C’s
    • Tertiary carbons (3o) are attached to three other C’s
    • Quaternary carbons (4o) are attached to four C's

    Example \(\PageIndex{1}\)

    You will find that hydrogen atoms are also classified in this manner. A hydrogen atom attached to a primary carbon atom is called a primary hydrogen; thus, isobutane, has nine primary hydrogens and one tertiary hydrogen.

    • Primary hydrogens (1o) are attached to carbons bonded to one other C atom
    • Secondary hydrogens (2o) are attached to carbons bonded to two other C’s
    • Tertiary hydrogens (3o) are attached to carbons bonded to three other C’s

    Example \(\PageIndex{2}\)




    Consider the following molecule. How many carbons are in the longest chain? Find a primary and quaternary carbon, and label an ethyl group.





    A = 4° Carbon

    B = Ethyl Group

    C = 1° Carbon

    The longest chain is 10 carbons long


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