nucleophilic aromatic substitution

why does electrophilic aromatic substitution work with benzene as the nucleophile?

benzene derivatives are generally electron rich

which S_N1 reactions happen for benzene derivatives?

only for diazonium salts as N₂ is the best leaving group

what is the general mechanism for diazonium S_N1?

when can S_NAr happen?

good leaving group

ortho para electron withdrawing group

mechanism for nucleophilic substitution

double bond reformed

why do S_N2 reactions never happen on benzene rings? 3

σ* orbital is inaccessible inside benzene ring

impossible to have inversion of configuration in benzene

S_N2 doesnt happen at sp² hybridised carbons

how many steps do S_NAr reactions have?

what are they?

why is an EWG needed?

two steps

discrete addition and elimination steps

EWG needed to stabilise anionic intermediate

what is the S_NAr mechanism?

how is S_NAr different from S_EAr?

EWG activate the ring

when does the EWG stabilise the anionic intermediate?

ortho or para

how does ortho EWG stabilise anionic intermediate?

how does para EWG stabilise anionic intermediate?

why does meta EWG not stabilise anionic intermediate?

negative charge never ends up on ipso carbon

what is the best leaving group for S_NAr?

why?

how is this different from S_N2?

fluoride

Sn2 - best leaving group is most stable anion (largest ion as charge spread out)

SnAr - favours most electronegative

how does electronegativity of LG affect SnAr?

speeds up rate determining step

how does geometry need to be for alkyne orbital overlap?

needs linear geometry to allow for effective orbital overlap of 2 right angle pairs of p orbitals

how is benzyne different from normal alkynes?

extra pi bond comes from electrons in sp² hybridised orbitals

low energy LUMO

what happens to benzyne when it is not in the presence of a nucleophile?

dimerises

how to make benzyne via diazonium salt?

nucleophilic substitution mechanism with diazonium ion

which end of the triple bond does benzyne react with?

either end