8b Flashcards
when benzene has a substituent,, where can an electrophile be added
it can be added in the ortho or para position bc this is where there is a cationic + charge when drawing resonance structures.
the + is never on the meta
2 types of nucleophilic attacks on aromatics are
SNAR (looks like sn2)
diazonium salts (looks like sn1)
describe sn2 briefly
sn2 = 1 step
nucleophile attacks C of cyclohexane and kicks off LG.
this can occur bc the substituent isnt in the same plane as the ring
why doesnt sn2 occur in benzene or other aromatics
bc the substituent is in the same plane as the ring,,, the nuc therefore cannot overlap its homo with the lumo (C-X antibonding orbital) as this is in the centre of the pi system
the nuc cannot enter here as it will be repelled by the high electron density
preventing it from achieving orbital overlap and breaking the C-X bond.
sn2 also occurs with inversion of stereochem,, benzene // aromatic compounds cannot invert
sn2 occurs with
inversion of stereochemistry
aka r –> s
axial to equitorial etc
okay describe how SNAR works,, nucleophilic substitution of an aromatic
u have ur aromatic with a substituent that u want to subs for a diff one,, aka u want to swap this one for another one.
u take ur nuc and u do conjugate addition,, to the double bond close to the susbtituent and u conjugate this only an ewg. u then use this negative charge on the substituent to kick the original group off,, leaving behind the nuc.
when do we include the substitent pi electrons as pat of our ‘finding the homo and lumo’
when theyre part of a carbonyl
aka alkene and carbonyl (aliphatic structure)
we count the carbonyl electrons
we dont include the pi electrons on the nitro group when its bonded on a benzene!!!
when is SNAR faster,, with Ar-F or Ar-I
Ar-F is faster bc its a better ewg,, making C more positive ,, and electrophilic
what isnt the rate determining step in an SNAR reaction
breaking the Ar-X (halogen) bond isnt in the rate determining step
SNAR is what order
its second order
meaning its rate law includes both the nucleophile and the aryl halide
so theyre both included in the rds
rate law for SNAR
rate = k [aryl halide] [ nucleophile]
FOCl NBr
ISCH
electronegativity scale
fossile nbr isch
what halogen makes the C of the aromatic more delta positive
F makes the C more delta positive,, compared to Ar- I
the reacton of SNAR has how many parts,, and what does this mean
SNAR has 2 parts and has an intermediate (when the negative charge was conjugated onto the ewg after the conjugate addition part)
meaning its energy diagram has 2 peaks and a trough (where the intermediate is)
for conjugate addition to occur in SNAR,, the original substituent and the ewg must be
it must be ortho or para to one another,, not meta,, as the resonance forms show the negative charge cannot reach the ewg
if u draw the bonds,, it means u break the Ar- EWG if u have the original substituent on the meta position.
list the EWG,, that need to be ortho or para to the original subsituent for it to collect the negative chargw and successfully kick off the original subs,, when the new subs/nuc attacks the Ar.
NO2 - nitro
CN triple bond (nitrile)
carbonyl (ketone)
SO2 (sulfoxide)
