Substitution Reactions: SN1 and SN2: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry I Topic 5, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
A reaction class in which an electron-rich species replaces a leaving group at a saturated carbon. The two limiting pathways differ in whether bond breaking precedes bond making.
Nucleophile
An electron-rich species that attacks an electron-poor carbon, forming a new bond with a pair of electrons it supplies. Negative charge and polarizability both increase its strength.
Electrophile
An electron-poor site that accepts a pair of electrons. In substitution it is the carbon made partially positive by an attached electronegative leaving group.
Leaving group
The fragment that departs with the bonding electrons. Good ones are weak bases, so halide ions and sulfonates leave readily while hydroxide and amide essentially never do.
Tosylate
A sulfonate ester made from an alcohol and a sulfonyl chloride. It converts a hydroxyl into an outstanding leaving group without disturbing the configuration at that carbon.
Concerted mechanism
A transformation in which all bond making and breaking occurs in a single step through one transition state, with no intermediate ever formed.
Backside attack
Approach of the nucleophile from the face directly opposite the leaving group. It maximizes overlap with the antibonding orbital of the carbon-leaving group bond.
Walden inversion
The configurational flip that accompanies a concerted substitution, in which the three retained bonds sweep through to the opposite face like an umbrella turning inside out.
Second order rate law
A rate proportional to the concentrations of both substrate and nucleophile. Doubling either one doubles the rate, which is the fingerprint of a one-step pathway.
First order rate law
A rate depending only on substrate concentration, since the slow step is unimolecular ionization. Adding more nucleophile does not speed the reaction up.
Carbocation intermediate
A trivalent, sp2 hybridized, positively charged carbon with an empty p orbital. Its planar geometry allows attack from either face in a later step.
Carbocation stability order
Tertiary beats secondary beats primary beats methyl, because alkyl groups donate electron density through hyperconjugation into the empty p orbital.
Hyperconjugation
Stabilizing overlap between a filled adjacent sigma bonding orbital and an empty or partially filled p orbital. More neighboring alkyl groups means more of this stabilization.
Carbocation rearrangement
Migration of a hydrogen or an alkyl group with its bonding pair from an adjacent carbon to the cationic center, producing a more stable cation and a scrambled product skeleton.
Hydride shift
A rearrangement in which a hydrogen atom moves with its bonding electrons to an adjacent positive carbon. It occurs only when the resulting cation is more stable.
Methyl shift
A rearrangement in which an entire methyl group migrates with its bonding electrons to a neighboring cationic center, changing the carbon skeleton of the product.
Racemization at the reacting center
Loss of stereochemical purity when a planar intermediate is attacked from both faces. The product mixture is usually not exactly even because of ion pairing.
Steric hindrance to substitution
Crowding around the reacting carbon that blocks the trajectory of an incoming nucleophile. It is the reason tertiary halides resist the concerted pathway entirely.
Polar aprotic solvent
A solvent such as acetone, dimethyl sulfoxide, or acetonitrile that dissolves salts without hydrogen bonding to anions, leaving nucleophiles bare and highly reactive.
Polar protic solvent
A solvent with hydroxyl or amino hydrogens that cages anions in a hydrogen-bonded shell. It slows nucleophiles down but stabilizes developing cations and their leaving groups.
Nucleophilicity trend in protic solvent
Larger, more polarizable anions win because solvation penalizes small dense ones most, which reverses the ordering seen in aprotic media.
Allylic and benzylic activation
Positions adjacent to a pi system react quickly by either pathway, since the pi system stabilizes both the cation and the concerted transition state.
Neopentyl substrate
A primary carbon flanked by a fully substituted neighbor. Its bulk blocks backside approach so severely that the substrate is unreactive by the concerted route despite being primary.
Solvolysis
A substitution in which the solvent itself acts as the weak nucleophile. Because solvent concentration is effectively constant, the observed kinetics are first order in substrate.
Vinyl and aryl halide inertness
Halides bonded to sp2 carbon of an alkene or ring resist ordinary substitution, since backside approach is blocked and the resulting cation would be very unstable.