B1 Reactions of benzene

Question 1

what reagents are required for the bromination of benzene

Answer 1

Br2
and a lewis acid (AlCl3 or FeBr3)

Question 2

what is a lewis acid

Answer 2

an electron acceptor

Question 3

draw the mechanism for the bromination of benzene

Question 4

draw the complete mechanism for nitration of benzene

Answer 4

(formation of nitronium ion and reaction with benzene)

Question 5

draw the mechanism for the the formation of a nitronium ion

Question 6

draw the mechanism for the sulfonation of benzene

Answer 6

protonation of sulfur trioxide & reaction with benzene

Question 7

draw the mechanism for freidel-crafts acylation

Answer 7

formation of acylium ion and reaction with benzene

Question 8

draw the mechanism for the formation of acylium ion

Question 9

draw what forms in a Friedel-crafts alkylation with a tertiary alkyl chloride

Question 10

draw what forms in a Friedel crafts alkylation with a primary chloride

Question 11

draw the ortho position

Question 12

draw the meta position

Question 13

draw the para position

Question 14

what happens when deuterium is added to a sample of benzene in nmr

Answer 14

if you add deuterium the protons can exchange and will disappear from the spectrum, usually the protons attached to heteroatoms are exchanged

Question 15

draw the mechanism for the exchange of deuterium on benzophenone

Question 16

if X is electron donating will the reaction be faster or slower than with benzene and what are the preferred positions in an SEAr reaction

Answer 16

faster
ortho and para position preferred

Question 17

if X is an electron withdrawing group will the reaction be faster or slower than with benzene and what are the preferred positions in an SEAr reaction

Answer 17

slower
meta position preferred

Question 18

give the general mechanism for electron donating groups on a benzene

Question 19

give the general mechanism for electron withdrawing groups on a benzene

Question 20

which position is an electrophilic attack most likely to occur at and why

Answer 20

ortho and para position because they have a lower chemical shift which means more shielding, more electron density therefor more nucleophilic so the ring is activated by resonance

Question 21

which positions is a nucleophilic attack most likely to occur at and why

Answer 21

meta because it has lower chemical shift so is the least deactivated in comparison to ortho and para

Question 22

what happens to activation and direction during electron donating induction

Answer 22

weak activation and ortho/para directing

Question 23

what happens to activation and direction during electron withdrawing induction

Answer 23

strongly deactivating and meta directing

Question 24

when can the positive charge end up on the ipso carbon

Answer 24

when they are para/ortho directing as they stabilise the positive charge on the ipso carbon whereas meta directing destabilises the positive charge on the ipso carbon

Question 26

what are the consequences to reactions which are meta directing and dont end up with the positive charge on the ipso carbon

Answer 26

X has less of an impact on the reaction as at no point does the positive charge end up on the ipso carbon

Question 27

draw the mechanism to show the movement onto the ipso carbon for ortho position

Question 28

draw the mechanism to show the movement onto the ipso carbon for para position

Question 29

draw the mechanism to show the movement onto the ipso carbon for meta position

Question 30

why is it hard to classify the halogens into following these general trends

Answer 30

because they all have lone pairs so can have a mesomeric effect but they are also electron withdrawing as they are very electronegative so they have an inductive effect

Question 31

what is chemoselectivity

Answer 31

which functional group in a molecule reacts

Question 32

what occurs when a benzimde is acetylated

Answer 32

it reduces the amines electron donation and creates steric hinderance

Question 33

draw the resonance structure of 1-aminoacetophenone

Question 34

why does 1-aminoacetophenone direct to the para position over the ortho position

Answer 34

because the molecule must all be in the same plane so the methyl group will be too close to go to the ortho position causing steric hinderance

Question 35

what can be done to attach a bromine group to the meta position on a phenol

Answer 35

the bromine wont want to attach to the meta position as phenol is strongly ortho/para directing therefor nitrobenzene is used to add the bromine and then substituted to an OH using diazonim salt

Question 36

draw the mechanism for the formation of a diazonium salt

Question 37

how do diazonium salts react

Answer 37

with nucleophiles at the ipso position

Question 38

what is an SEAr reaction

Answer 38

electrophillic aromatic substitution

Question 39

draw the mechanism for an SN1 reaction of a diazonium salt

Question 40

draw the mechanism for a SNAr reaction with florobenzene

Question 41

why can SN2 reactions not occur on benzene

Answer 41

because the s* orbital is inside the benzene ring and it is impossible to have inversion of configuration in a benzene ring

Question 42

what is a SNAr reaction

Answer 42

two step reaction involving discrete additon and elimination where the electron withdrawing group is required to stabilise the anionic intermediate

Question 43

whats the mechanism for the reaction of a nucleophile with ortho EWG-florobenzene

Question 44

what is the mechanism for the reaction of a nucleophile with meta EWG-florobenzene

Question 45

draw the mechanism for the reaction of a nucleophile with para EWG-florobenzene

Question 46

why is flourine the best leaving group for SNAr

Answer 46

because it is the most stable anion as it is the strongest acid therefor has the lowest pka and its electronegativity speeds up the RDS

Question 47

what is the order of best leaving groups for SNAr reactions

Answer 47

F->>Cl-=Br->I

Question 48

what is the RDS in SNAr reactions

Answer 48

the addition step whuch breaks the aromaticity

Question 49

draw the mechanism for the formation of benzyne

Question 50

why is sodiumamide needed for the formation of benzyne

Answer 50

it is a very strong base which reacts with p-chlorotolunene to give a mixture of anilines which is a key intermediate

Question 51

how is benzyne different to normal alkynes

Answer 51

does not require linear geometry at allow orbital overlap as the 6-membered ring allows the extra pi bond to come from electrons in sp2 hybridised orbitals

Question 52

how is benzyne reactive

Answer 52

has a low energy LUMO making it reactive to nucleophiles

Question 53

draw the mechanism for the formation of benzyne from o-aminobenzoic acid

Question 54

draw and name the 3 electron donating groups and the 5 electron withdrawing groups