Substitution vs Elimination

Mechanism	Substrate	Nucleophile/Base	Leaving Group	Solvent
$S_{N}1$	Tertiary and/or resonance stabilised.	Good Nucleophile, Weak Base	Good leaving group.	Polar Protic.
$S_{N}2$	Primary or 0$°$	Good Nucleophile, Weak Base	Good leaving group.	Polar Aprotic.
$E1$	Tertiary or resonance stabilised.	Weaker Base	$L_1$ should be a good leaving group.	Polar.
$E2$	Tertiary or resonance stabilised.	Stronger Base	Good leaving group.	Polar Aprotic.