The alicyclic hydrocarbons include the cycloalkanes, also called
cycloparaffins
,
cyclanes
, and
naphthenes
, and the cycloalkenes, also known as
cycloolefins
and
cyclenes
, and their alkyl and alkenyl derivatives. Some of these derivatives occur naturally in plants and are commonly known as
terpenes
. Other compounds can be isolated from crude petroleum refinery distillates or catalytically cracked petroleum products. Cycloalkanes are extensively used to produce reformed aromatics and as solvents or synthesis intermediates. Some physical properties of the cycloalkanes are given.
Cyclopropane and cyclobutane are gases and have been used as anesthetics. Cyclopentane and higher members are liquids with low acute and chronic toxicity overall, but still able to induce many toxic responses characteristic of lipophilic compounds, including central nervous system (CNS) depression, dizziness, and headache. For cyclohexane and higher analogs, the margin of safety between CNS depression and death is very narrow and symptomatically barely recognizable. Following ingestion or inhalation, cycloalkanes are exhaled in unchanged form or rapidly metabolized into water‐soluble metabolites, usually glucuronides. Exposure of humans and laboratory animals to high concentrations may cause excitement, loss of equilibrium, stupor, and coma, but rarely death. Oral administration of high doses to animals has additionally resulted in severe diarrhea, vascular collapse, and heart, lung, liver, and brain degeneration. Cycloparaffins and cycloalkenes are also ocular and dermal irritants since, as lipophilic agents, they defat the skin. In many cases, they are also skin sensitizers; their sensitizing activity increases with the number of double bonds. The liquid members up to cyclooctane, and to a lesser extent cyclododecane, are aspiration hazards.
The cyclic olefins are more highly reactive than their paraffin counterparts. They contribute to photochemical smog by reacting with ozone and other small molecular or ionic species. Selected physical properties are given. In C
4
–C
7
cycloalkenes, toxicity increases with molecular weight. Cycladienes and polyenes appear to possess increasingly irritant, toxic, and sensitizing properties, peaking somewhat higher for cycladienes than for monocyclic alkenes. The aspiration hazard also appears higher for the cycloalkenes than the cycloalkanes.