Chemistry · Ch 15 — Hydrocarbons
Isomerism in alkanes
Isomerism in alkanes
Alkanes containing more than three carbon atoms generally show structural isomerism, and specifically chain isomerism, in which the same molecular formula can be assembled either as one continuous (straight) carbon chain or as a chain carrying one or more branches. Methane, ethane and propane have only one possible structure each and so show no chain isomerism, but starting from butane (C4H10) branching becomes geometrically possible: n-butane (a straight 4-carbon chain) and 2-methylpropane/isobutane (a central carbon bonded to three methyl groups) are both valid structures for C4H10. Pentane (C5H12) allows three such arrangements -- n-pentane, 2-methylbutane (isopentane), and 2,2-dimethylpropane (neopentane) -- and the count of possible chain isomers keeps climbing sharply as carbon number increases: Table 15.1 shows butane has 2 isomers, pentane has 3, and hexane already has 5. This rapid growth happens because each additional carbon multiplies the number of distinct positions at which a branch point can be placed, and because different branch patterns on …
Number of carbon atoms, alkane name, number of structural (chain) isomers: 1 carbon, Methane, no structural isomer. 2 carbons, Ethane, no structural isomer. 3 carbons, Propane, no structural isomer. 4 carbons, Butane, two isomers (n-butane and 2-methylpropane/isobutane). 5 carbons, Pentane, three isomers (n-pentane, 2-methylbutane, 2,2-dimethylpropane). 6 carbons, Hexane, five isomers. The table shows that the count of possible chain isomers grows rapidly once branching becomes possible, even thou …
What this figure shows. Shows the full structural (dash) formulas and names of the chain isomers of C4H10 and C5H12. For C4H10: (i) n-butane, drawn as an unbranched chain H3C-CH2-CH2-CH3 with every C-H and C-C bond shown explicitly; (ii) 2-methylpropane (isobutane), a central CH carbon bonded to three CH3 groups. For C5H12: (i) n-pentane, the unbranched five-carbon chain H3C-CH2-CH2-CH2-CH3; (ii) 2-methylbutane (isopentane), CH3-CH(CH3)-CH2-CH3, a four-carbon chain with one methyl branch on C2; (iii) 2,2-dimethylpropane (neopentane), C(CH3)4, a central carbon bonded to four methyl groups. The figure makes explicit that isomers sharing one molecular formula can differ …