Hydrocarbons and IUPAC Nomenclature

Acyclic saturated hydrocarbons, the alkanes, have formula CₙH₂ₙ₊₂. An acyclic hydrocarbon with exactly one C=C has CₙH₂ₙ; one with exactly one C≡C has CₙH₂ₙ₋₂. Each ring or double bond removes two H relative to the acyclic saturated reference, and a triple bond removes four. Thus C₄H₈ could be a monoalkene or a cycloalkane; formula alone does not establish which. Benzene's aromatic bonding needs a separate model, rather than treating it as an ordinary alkane.

For ordinary neutral hydrocarbons, each carbon has a total bond-order sum of four. In CH₃CH=CH₂, the middle carbon has one C–C single, one C=C double and one C–H, totaling four. A double bond counts as two toward valence even though it connects only two atoms.

1) Find the longest continuous carbon chain that contains the multiple bond — that is the parent. 2) Number the chain from the end that gives the multiple bond, or failing that the first substituent, the lowest number. 3) Name each branch as a substituent: methyl, ethyl, propyl, and halogens as fluoro, chloro, bromo, iodo. 4) List substituents alphabetically with their locants, using di-, tri-, tetra- for repeats.

For the simple branched alkanes here, compare the ordered substituent-locant lists from both ends and choose the lower number at the first point of difference, not the smaller sum. List substituent names alphabetically, ignoring multiplicative di-/tri- when alphabetizing. Separate repeated numbers with commas, numbers from words with hyphens, and write the name without spaces. In a simple alkene, choose a suitable parent containing the double bond and give that bond its lower locant before ordinary branch numbering. Full IUPAC parent-selection priorities for complex multi-functional molecules go beyond this introductory method.

Chemists draw lines instead of letters: each vertex and each line end is a carbon, and hydrogens on carbon are not drawn. A zig-zag of four line segments is pentane. Reading skeletal structures quickly is the single most useful organic skill.

To draw 3-methylhexane, first make a six-carbon chain, number it and add a one-carbon branch at C₃. The condensed structure is CH₃CH₂CH(CH₃)CH₂CH₂CH₃, giving seven carbons overall and C₇H₁₆. Recount the longest chain to confirm that the branch did not create a longer parent.

In a skeletal drawing, each unlabeled end and corner is carbon; add enough implicit H for each carbon to reach valence four. Heteroatoms such as O, N and Cl are written explicitly, and their attached H is usually shown. A molecular formula counts all atoms but hides connectivity; a condensed formula groups them while retaining connections. In CH₃CH(CH₃)CH₂CH₃, parentheses mark a branch from the preceding CH, not an extra link at the end of the chain.