SN1, SN2, E1, E2: choosing the mechanism

A concerted mechanism: the nucleophile attacks opposite the leaving group. Rate law $v = k[\ce{RX}][\ce{Nu-}]$, second order. Stereochemistry: Walden inversion, e.g. (R)-2-bromobutane $+\ \ce{HO-}$ gives (S)-butan-2-ol. Reactivity: methyl > primary > secondary; a tertiary substrate does not react this way, its rear face is hindered.