aliphatic

Meaning: Aliphatic carbon compounds lack an aromatic ring. Aromatic rings are cyclic, planar, and fully conjugated with 4n + 2 π electrons (Hückel's rule). Aliphatic includes alkanes, alkenes, alkynes, conjugated dienes, cycloalkanes such as cyclohexane, and their derivatives, for example alcohols and carboxylic acids built on non-aromatic carbon chains. Aliphatic carbons may be sp³, sp², or sp hybridized.

On the MCAT: Expect questions that make you classify a structure as aliphatic or aromatic, then predict a consequence. In IR spectra, aliphatic C–H stretches appear just below 3000 cm⁻¹, while aromatic and vinylic C–H stretches appear just above. In ¹H NMR, aliphatic hydrogens appear far upfield (roughly 0.9 to 2 ppm), while aromatic hydrogens appear near 6.5 to 8 ppm. In biochemistry, alanine, valine, leucine, and isoleucine have aliphatic side chains, which are nonpolar and hydrophobic, whereas phenylalanine, tyrosine, and tryptophan have aromatic side chains.

Common confusion: Students equate aliphatic with open-chain or saturated. Neither is required. Cyclohexane and cyclohexene are aliphatic rings, and ethene and ethyne are aliphatic despite their π bonds. The deciding feature is the absence of aromaticity, not the absence of rings or double bonds. A ring with double bonds is aromatic only if it meets the Hückel criteria.