Hückel's rule
In organic chemistry, hydrocarbons (compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. "aleiphar", fat, oil), also known as non-aromatic hydrocarbons. Aliphatics can be cyclic; however, hydrocarbons with conjugated pi-systems that obey Hückel's rule are instead considered to demonstrate aromaticity. Aliphatic compounds can be saturated, like hexane, or unsaturated, like hexene and hexyne. Open-chain compounds (whether straight or branched) contain no rings of any type, and are thus aliphatic.