alkyl chains

Meaning: An alkyl group is an alkane with one hydrogen removed, bonded to the rest of the molecule through a carbon (written R in structures). It has only sp³ carbons with C–C and C–H single bonds, so it is nonpolar and fairly unreactive. Chains can be straight or branched. Alkyl groups are also weakly electron-donating, which helps stabilize carbocations.

Equation: alkyl group = CₙH₂ₙ₊₁, where n is the number of carbons (unitless). The parent alkane is CₙH₂ₙ₊₂.

On the MCAT: Expect trends in physical properties. In a series of alcohols or carboxylic acids, a longer alkyl chain lowers water solubility because the nonpolar part outweighs the polar group, and raises boiling point because of more London dispersion forces. Branching lowers boiling point relative to a straight-chain isomer because surface contact is smaller. Alkyl chains also appear as fatty acid tails (soaps, micelles, membranes) and as side chains of alanine, valine, leucine and isoleucine, which tend to sit in a protein's hydrophobic core.

Common confusion: Students assume a polar group such as –OH guarantees water solubility. Only short-chain compounds are well solubilized; as the alkyl chain lengthens, the molecule behaves more like a hydrocarbon. Also, an alkyl group is a substituent (CₙH₂ₙ₊₁), not a complete alkane (CₙH₂ₙ₊₂).