Bondline / Reactions

SN1 substitution

The leaving group departs first to give a carbocation, which the nucleophile then traps. Best for tertiary substrates; gives a racemic mixture at a stereocentre.

Substitution · 3 steps

Conditions

Weak nucleophile (often the solvent, e.g. water or ethanol), polar protic solvent. Rate = k[RX].

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

BrH2O+
Step 1Slow, rate-determining step: the C–Br bond breaks on its own, giving a planar tertiary carbocation.
H2OOH+HH2OH₂O++
Step 2Water attacks the flat carbocation (from either face).
OH+HH2OOHH3O+H₂O++
Step 3A second water molecule removes the proton from the oxonium ion.

About substitution reactions

In a substitution, a nucleophile replaces a leaving group on a saturated carbon. Whether it happens in one step (SN2) or through a carbocation (SN1) depends on the substrate, the nucleophile and the solvent.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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