methyl and primary
Summary
A methyl carbon has no other carbons attached; a primary (1°) carbon has one. Methyl and primary alkyl halides are usually the best SN2 substrates because backside attack is unhindered, and they generally avoid SN1 because their carbocations are unstable.
Detail
Meaning: Carbons are classified by how many other carbons they bond to. Methyl (CH₃–X) has none; primary (RCH₂–X) has one; secondary has two; tertiary has three. For substitution, this sets the crowding around the carbon bearing the leaving group and the stability of any carbocation. Methyl and primary centers are open to backside attack, so SN2 is fast. Their carbocations get little hyperconjugation or inductive stabilization from alkyl groups, so SN1 and E1 are generally slow unless resonance helps (allylic or benzylic).
Equation: SN2 rate = k[Nu][RX], where k is the rate constant (m³ mol⁻¹ s⁻¹) and [Nu] and [RX] are nucleophile and substrate concentrations (mol/m³). SN2 reactivity: methyl > 1° > 2° >> 3°.
On the MCAT: A passage lists alkyl halides and asks which reacts fastest with a strong nucleophile, especially in a polar aprotic solvent. Choose methyl or primary. For SN1 the order reverses: 3° > 2° >> 1° > methyl. Primary alcohols also appear in oxidations: PCC gives an aldehyde, while stronger oxidants give a carboxylic acid.
Common confusion: Primary is not "least reactive" overall; it is slowest for SN1 but fastest for SN2. Primary does not guarantee fast SN2, either: a neopentyl halide is primary but hindered by the adjacent quaternary carbon. For amines and amides, primary counts groups on nitrogen instead.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.