Objectives After completing this section, you should be able to 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. explain what is meant by a primary, secondary, tertiary or quaternary carbon atom. 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. 0 1 2 3 4 5 6 7 8 9 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 C n H 2n +2 . An alkyl group is formed by removing one hydrogen from the alkane chain and is described by the formula C n H 2n +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 \(\PageIndex{2}\). 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 (1 o ) attached to one other C atom Secondary carbons (2 o ) are attached to two other C’s Tertiary carbons (3 o ) are attached to three other C’s Quaternary carbons (4 o ) 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 (1 o ) are attached to carbons bonded to one other C atom Secondary hydrogens (2 o ) are attached to carbons bonded to two other C’s Tertiary hydrogens (3 o ) are attached to carbons bonded to three other C’s Example \(\PageIndex{2}\) Exercises Contributors Dr. Dietmar Kennepohl FCIC (Professor of Chemistry, Athabasca University ) Prof. Steven Farmer ( Sonoma State University ) William Reusch, Professor Emeritus ( Michigan State U. ), Virtual Textbook of Organic Chemistry Organic Chemistry With a Biological Emphasis by Tim Soderberg (University of Minnesota, Morris)