Alkanes and Cycloalkanes: Structure and IUPAC Names

One name, one molecule

A chemist in Sudbury and a chemist in Seoul must be able to read the same name and draw the same molecule. That is the job of the IUPAC system (International Union of Pure and Applied Chemistry). In Grade 11 you used IUPAC rules for ionic and simple molecular compounds. Organic names work like a code: the root tells you how many carbons are in the main chain, the ending tells you the family, and prefixes with numbers tell you what is attached and where.

We start with the simplest family, the alkanes: saturated hydrocarbons, with only single bonds.

Alkyl groups and halogen substituents

When an alkane loses one hydrogen, it can attach to a chain as a branch called an alkyl group. Change -ane to -yl:

• methane → methyl, –CH₃
• ethane → ethyl, –CH₂CH₃
• propane → propyl, –CH₂CH₂CH₃
• butane → butyl, –CH₂CH₂CH₂CH₃

A propyl group attached by its middle carbon, –CH(CH₃)₂, is often called isopropyl (IUPAC: 1-methylethyl).

Halogen atoms can also replace hydrogens. They are named as prefixes: fluoro-, chloro-, bromo-, iodo-. A compound such as CH₃CH₂Cl (chloroethane) is an alkyl halide. Halogens and alkyl groups are both called substituents, and they follow the same naming rules.

IUPAC rules for branched alkanes

1. Find the parent chain: the longest continuous chain of carbons. It does not have to be drawn in a straight line; it can turn corners. If two chains tie for longest, choose the one with more substituents.
2. Number the chain from the end that gives the lowest locant to the first substituent you meet. (If there is a tie, compare the next substituent, and so on.)
3. Name each substituent and give it the number of the carbon it is attached to.
4. If the same substituent appears more than once, use di-, tri-, tetra- and give every locant: 2,2-dimethyl, not 2-dimethyl.
5. List substituents in alphabetical order, ignoring di-, tri- and tetra- (so ethyl comes before dimethyl, because e comes before m).
6. Punctuation: commas between numbers, hyphens between numbers and words, and write the whole name as one word: 2,3-dimethylpentane.

Cycloalkanes

When the two ends of an alkane chain join, a hydrogen is lost from each end and a ring forms. These cycloalkanes have the general formula CₙH₂ₙ. Add cyclo- in front of the alkane name: cyclopropane, cyclobutane, cyclopentane, cyclohexane.

To name a substituted ring:

• One substituent: no number is needed: methylcyclohexane.
• Two or more: number around the ring to give the lowest set of locants. If there is still a choice, the substituent that comes first alphabetically gets carbon 1: 1-ethyl-3-methylcyclohexane.

Cyclohexane is the most common ring in nature and industry. Its carbons are not flat; the ring puckers into a shape called a "chair", which keeps every angle close to 109.5°. Cyclopropane, with 60° angles, is strained and much more reactive.

Physical properties of alkanes

• C–C and C–H bonds are almost non-polar, so alkane molecules attract each other only by weak London dispersion forces.
• Longer chains have more electrons and more surface contact, so the forces grow and the boiling point rises steadily with chain length (see the table). C₁ to C₄ are gases at room temperature, C₅ to about C₁₇ are liquids, and larger alkanes are waxy solids such as paraffin.
• Alkanes are insoluble in water ("like dissolves like"), and liquid alkanes are less dense than water, so gasoline floats on water.
• Alkanes are fairly unreactive, but they burn well. That is why they are our main fuels: natural gas for furnaces, propane for barbecues, gasoline (mainly C₅ to C₁₂) for cars and kerosene for jets. Crude oil is separated into these fractions by fractional distillation, which works because of the different boiling points.