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of carbon and hydrogen atoms but one molecule might have the carbons connected as a straight chain and the other molecule might have a shorter chain with carbon atoms branching off at intermediate points. These two compounds are called structural isomers. Branching has a stabilizing effect on molecules, and large molecules with several branched chains can have very low cetane numbers.

4.2.1.2 Cyclic hydrocarbons

Some hydrocarbon compounds contain three or more carbon atoms arranged in a ring. They are called cyclic compounds. If the ring contains no double bonds, the structure is saturated and is called a cyclo-paraffin (also known as naphthene). The names of these molecules are derived from the number of carbons in the ring. Most naphthenic hydrocarbons are based on a cyclic structure containing five or six carbons (cyclopentane or cyclohexane) with side chains attached to one of the cyclic carbons.

4.2.1.3 Alkenes and alkynes

Alkenes are compounds that contain a carbon-carbon double bond. Alkenes, also called olefins, are unsaturated. They have fewer hydrogens than an alkane would have for the same number of carbon atoms. An alkyne is a hydrocarbon with one or more carbon-carbon triple bonds.

4.2.1.4 Aromatics

Aromatic compounds contain bonds that cannot be classified as either single or double but are something halfway between. This phenomenon is called resonance. Aromatic compounds are unsaturated and are most commonly found in hexagonal structures with benzene as the most basic example. Aromatics have higher densities than paraffins and, in spite of having slightly lower energy content per unit mass, have higher energy content per unit volume. Aromatics are powerful solvents and tend to cause swelling of elastomers.

4.2.1.5 Oxygenated hydrocarbons

The oxygenated hydrocarbons of primary interest as fuels are alcohols and ethers, although esters are attracting some attention because they can be produced from renewable sources. Aldehydes and ketones are of interest primarily as exhaust pollutants. They are strong irritants and are highly reactive in photochemical smog formation.

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