octane (n.)

before vowels oct-, word-forming element meaning "eight," from Greek okta-, okt-, from PIE *okto(u) "eight" (see eight). The variant form octo- often appears in words taken from Latin, but the Greek form is said to be the more common in English.

word-forming element in chemical use, indicating a chain of carbon atoms with no double bonds, proposed 1866 by German chemist August Wilhelm von Hofmann (1818-1892) to go with -ene, -ine (2), -one.