A2 Substitution and Elimination

Question 1

what makes a good leaving group

Answer 1

ability to readily accept electrons and an organic molecule with a good leaving group, weak C-X bonds and are happy to accept electrons

Question 2

what are the main aspects of a substitution reaction

Answer 2

C-X bond breaking and the X group accepting former bonding electrons to become a lone pair

Question 3

are stronger or weaker acids better leaving groups

Answer 3

stronger (low pka)

Question 4

what is the order of the halides in terms of best to worst leaving group

Answer 4

I > Br > Cl&raquo_space; F
best > worst

Question 5

are alcohols good leaving groups generally

Answer 5

no they cannot act as electrophiles because hydroxide is not a good leaving group because its conjugate acid is water

Question 6

how can alcohols be activated as leaving groups

Answer 6

they can be protonated making the leaving group water not hydroxide as waters conjugate acid is peroxide H30+ which is a strong acid

Question 7

why does protonated alcohol not used as an electrophile often

Answer 7

because it only works in a very small range of non-basic nucleophile (chloride, bromide, iodide) as most nucleophiles are good bases therefor neutralise the reaction

Question 8

why can using alcohol as an electrophile be important

Answer 8

it converts readily available alcohols into useful products

Question 9

draw the mechanism for the chlorination of an alcohol

Question 10

draw the mechanism for the bromination of alcohol

Question 11

why are sulfonate esters useful

Answer 11

because sulfonic acid is a very strong acid and sulfonate ions are excellent leaving groups

Question 12

draw the mechanism for the conversion of an alcohol to a sulfonate ester

Question 13

are alkoxides good leaving groups….explain

Answer 13

no they are not good leaving groups because their conjugate acids are alcohols so ethers cannot be used as SN2 electrophiles

Question 14

what is an epoxide

Answer 14

a three membered ether

Question 15

are epoxides good leaving groups….explain

Answer 15

yes they are good leaving groups because due to them being cyclic the internal bond is 60 not 109.5 adding more strain on the C-O bond providing the extra driving force to overcome the otherwise unfavourable formation

Question 16

how does increasing substitutions around the carbon effect SN2 reactivity

Answer 16

the more substitutions decreases reactivity because there is increased steric hinderance

Question 17

how can a molecule increase rate of SN2 reactions

Answer 17

the presence of neighbouring p-systems (alkenes, aromatics, carbonyl groups) as stabilising overlap of the p-system occurs in the transition state

Question 18

why does inversion occur in SN2 reactions

Answer 18

because the nucleophile must attack from the opposite side of the leaving group resulting in a transition state with 5 bonds to the carbon and when the bond to the leaving group breaks the other groups attached to the carbon are forced to the other side

Question 19

why do SN1 reactions form a racemic mixture

Answer 19

SN1 reactions form a racemic mixture as their is equal likelihood of the nucleophile attacking from either face

Question 20

are SN2 reactions possible on sp2 hybridised carbons

Answer 20

no because their planar structure makes it difficult for the nucleophile to attack the backside of the molecule, their bonds are shorter and stronger so they are more difficult to break

Question 21

when can sp2 hybridisation be seen in SN2 reactions

Answer 21

during the transition state

Question 22

what makes a good nucleophile

Answer 22

high lying HOMO (highest occupied molecular orbital) for effective overlap with the LUMO (lowest unoccupied molecular orbital)
lone pair thats readily available to react with a sigma* orbital

Question 23

are nucleophiles better being more or less basic

Answer 23

more basic

Question 24

put these nucleophile in order from best to worst:
hydroxide, carboxylate, alkoxide. phenoxide

Answer 24

alkoxide > hydroxide > phenoxide > carboxylate
best > worst

Question 25

why are more basic nucleophiles better

Answer 25

because a basicity is a species ability to accept a proton therefor the more basic they are the higher energy lone pairs and stronger electron donors

Question 26

what are the exceptions to more basic nucleophiles being better

Answer 26

does not apply to nucleophiles formed from different elements (cant compare nitrogen and oxygen nucleophiles) it is heavily influenced by structural effects (steric hinderance)

Question 27

is hydroxide a good nucleophile

Answer 27

yes but there if potential competition if there is an acidic proton (competing elimination) therefor to stop this a less basic nucleophile could be used

Question 28

what reactions can occur with hydroxides and what factor effects which reaction dominates

Answer 28

SN2 and E2 reactions can occur, the worse the SN2 electrophile the slower the SN2 reaction therefor the E2 will dominate

Question 29

are carboxylates good nucleophiles....explain

Answer 29

yes they are as they are not basic and do not cause E2 reactions however they are less reactive than hydroxides

Question 30

draw the SN2 and E2 reaction of a hydroxide with chloromethane

Question 31

how can alkyl halides be converted to alcohols

Answer 31

SN2 with carboxylate then ester hydrolysis under basic conditions, draw this mechanism

Question 32

what are phenoxides

Answer 32

anions of phenol

Question 33

are phenoxides good nucleophiles

Answer 33

they are weakly basic and do not cause E2 reactions are fairly reactive

Question 34

how can phenoxides be formed

Answer 34

treating phenol with a base such as hydroxide so that it can deprotonate the phenol

Question 35

draw the williamson ether synthesis

Question 36

are alkoxides good nucleophiles

Answer 36

yes they are very good but the anion must be preformed and they are more reactive than hydroxides so do cause issues in causing E2 reactions (increasing hinderance E2 will dominate)

Question 37

draw the mechanism for the formation of alkoxides from alcohol

Question 38

are thiolates good nucleophiles

Answer 38

yes they are great nucleophiles and they are more acidic than alcohol so can be used when hydroxides are the base

Question 39

are thiols better or worse nucleophiles than their oxygen equivalent

Answer 39

better, although they are less basic because S is lower down the group than O which generally causes nucleophilicity to increase because the molecule is larger therefor more polarisable, they are less likely to form H bonds as you go down the group

Question 40

what is the problem with amines being such good nucleophiles

Answer 40

it can be difficult to stop the reaction after one substitution making it hard to form primary and secondary amines

Question 41

what are quaternary ammonium salts used for

Answer 41

cationic surfactants

Question 42

how can SN2 reactions for amines be controlled

Answer 42

using epoxides stops over-alkylation

Question 43

what is an azide anion

Answer 43

N- = N+ =N-

Question 44

are azide anions good nucleophiles

Answer 44

yes because they are small linear molecules with negative charge on the nitrogen

Question 45

draw the displayed formula for the reaction of an azide reaction with methylbromobenzene

Question 46

how can azide anions be used to make a primary amine

Answer 46

reduced

Question 47

is cyanide a good nucleophile

Answer 47

yes because it is small negatively charged carbon

Question 48

draw the mechanism for an SN2 reaction with cyanide