Requirements of mechanisms (froms subs and elims topic)

Question 1

what is a good leaving group?

Answer 1

the conjugate base of a strong acid

Question 2

SN1 will occur when…(intermediate)

Answer 2

relatively stable carbocation

Question 3

SN1 will occur when…(solvent)

Answer 3

polar protic solvents

Question 4

What happens to stereochemistry in SN1?

Answer 4

lost because intermediate is sp2 and planar, forms racemic products

Question 5

regioselectivity of SN1

Answer 5

if intermediate stabilised by resonance, then addition to least steric hindrance

Question 6

rate limiting step of SN1

Answer 6

leaving group leaves

Question 7

how is the intermediate stabilised in SN1?

Answer 7

hyperconjugation (adjacent sigma c-h overlap i.e. tertiary) and/or conjugation with a pi system to spread out +ve charge

Question 8

a quaternary ammonium salt favours…

Answer 8

elimination over substitution

Question 9

angle & overlap requirements for SN2

Answer 9

HOMO of nucleophile must overlap with LUMO of electrophile at 180 degrees

Question 10

what is usually the HOMO of the nucleophile in SN2?

Answer 10

a lone pair

Question 11

what is usually the LUMO of the electrophile in SN2?

Answer 11

the sigma antibonding orbital of C-leaving group

Question 12

SN2 will occur when…(solvent)

Answer 12

polar aprotic solvents because nucleophile not stabilised so “wants” to react

Question 13

SN2 transition state name

Answer 13

trigonal bipyramidal with nucleophile and leaving group in axial positions

Question 14

SN2 will occur when…(electrophile)

Answer 14

least sterically hindered i.e. methyl

Question 15

the SN2 transition state could be stabilised by…

Answer 15

conjugation as pi system overlaps with partial bonds

Question 16

good nucleophiles are…

Answer 16

bases with a weak conjugate acid

Question 17

nucleophiles based on atoms lower down the periodic table are better because…

Answer 17

-ve charge in a higher energy AO so better overlap and lower HOMO-LUMO gap

Question 18

What happens to stereochemistry with SN2?

Answer 18

inverted only at reaction centre

Question 19

A high temperature suggests…

Answer 19

elimination (because positive entropy change)

Question 20

E1 requires a substrate that

Answer 20

forms a stable carbocation and beta hydrogens

Question 21

What type of base is need for E1?

Answer 21

non nucleophilic to attack protons only, avoids competing substitution but doesn’t need to be particularly good because not in rate limiting step

Question 22

stereoselectivity of E1

Answer 22

E/Z conformation leading to least sterically hindered alkene

Question 23

E1 is regioselective to…

Answer 23

most substituted alkene because kinetically & thermodynamically favourable

Question 24

rate limiting step of E1

Answer 24

leaving group leaves

Question 25

E2 requires an substrate that

Answer 25

has an antiperiplanar beta hydrogen, but if multiple then favours least sterically hindered conformation

Question 26

transition state of E2

Answer 26

antiperiplanar, can be shown using a newmann projection

Question 27

opposite of antiperiplanar

Answer 27

gauche (dihedral of 60 degrees vs 180 of antiperiplanar)

Question 28

A small base in E2 favours what type of product?

Answer 28

regioselective to thermodynamic/saytsev product (more substituted)

Question 29

A hindered base in E2 favours what type of product?

Answer 29

regioselective to kinetic/hofmann product (less substituted)

Question 30

When is E2 favoured over E1?

Answer 30

primary substrates because no stable carbocation

Question 31

When is E2 favoured over SN2?

Answer 31

tertiary substrates because beta hydrogens more accessible than alpha carbon

Question 32

what type of base is needed for E2?

Answer 32

non nucleophilic to attack protons only, but a high concentration fo good base speeds up reaction because present in rate determining step

Question 33

what is required in the substrate for E1cb?

Answer 33

an electron withdrawing group (e.g. carbonyl) is adjacent to beta hydrogens but not the reaction centre

Question 34

rate limiting step of E1cb

Answer 34

irreversible elimination of conjugate base (leaving group)

Question 35

intermediate required for E1cb

Answer 35

stabilised anionic intermediate e.g. enolate

Question 36

what type of solvent is dicholoromethane?

Answer 36

non-polar

Question 37

what type of solvent is diethyl ether?

Answer 37

non-polar

Question 38

what type of solvent is hexane?

Answer 38

non-polar

Question 39

what type of solvent is water?

Answer 39

polar protic

Question 40

what type of solvent is acetic acid?

Answer 40

polar protic

Question 41

what type of solvent is methanol?

Answer 41

polar protic

Question 42

what type of solvent is ethanol?

Answer 42

polar protic

Question 43

what type of solvent is isopropanol?

Answer 43

polar protic

Question 44

what type of solvent is DMSO?

Answer 44

polar aprotic

Question 45

what type of solvent is DMF?

Answer 45

polar aprotic

Question 46

what type of solvent is THF?

Answer 46

polar aprotic

Question 47

what type of solvent is acetone?

Answer 47

polar aprotic

Question 48

what type of solvent is ethyl acetate?

Answer 48

polar aprotic

Question 48

what type of solvent is acetonitrile?

Answer 48

polar aprotic

Question 49

mechanism of bromination

Answer 49

cyclic addition, forms bromonium intermediate, then SN2 style step with other bromide