addition to carbon carbon multiple bonds

Question 1

what is the intermediate in bromination of alkenes

Answer 1

bromonium ion

Question 2

addition of Br2 to alkenes leads to syn or anti addition?

Answer 2

anti due to sn2 inversion during attack of the bromonium ion

Question 3

explain the regeoselectivity of hydrobromination of alkenes

Answer 3

markovnikov’s rule - bromine on more substituted carbon because carbocation more stabilised (hammond postulate)

Question 4

compare the nucleophilicity of alkenes and dienes

Answer 4

dienes are more nucleophilic than isolated alkenes because they have a higher energy HOMO

Question 5

what is the difference between reactions of enolates with bromine in basic vs acidic conditions

Answer 5

in basic conditions, multiple substitution occurs and then C-C bond cleavage producing CHBr3 and a carboxylate
in acidic conditions, only monosubstitution occurs

see chloroform reaction

Question 6

why are multiple substitutions of enols avoided in acidic conditions

Answer 6

bromine destabilises the positive charge on the transition state

Question 7

in a conjugated 1,4 a,b unsaturated carbonyl, which group is the soft electophile and which the hard

Answer 7

soft electrophile - C=C double bond
hard electrophile - C=O double bond

Question 8

what reagents are needed for addition of lithium cuprates

Answer 8

i) R2CuLi
Me3SiCl
ii) H+ work up

Question 9

what role does Me3SiCl play during addition of organocuprates

Answer 9

Si atom in molecule acts as lewis acid - bonding to the O-
silyl enol ether is hydrolysed to a ketone at the end of the reaction

Question 10

What composes Lindlar’s Catalyst

Answer 10

Pd, CaCO3, Pb(OAc)2

Question 11

What is the purpose of using Lindlar’s catalyst for the hydrogenation of alkynes

Answer 11

so that the alkyne is more susceptible to hydrogenation than the alkene (preventing double hydrogenation)

Question 12

Alkyne -> trans alkene

Answer 12

Na in NH3 (l)
radical mechanism

Question 13

Alkyne -> cis alkene

Answer 13

hydrogen gas and Lindlar’s catalyst

Question 14

which alkenes can be reduced using Na in NH3 (l)

Answer 14

extremely electron defficient alkenes such as those bonded to multiple electron withdrawing groups

Question 15

are more or less substituted radicals more stable?

Answer 15

more substituted radicals are more stable due to stabilisation by hyperconjugation

Question 16

describe the general reaction:
allylic bromination with NBS

Answer 16

• radical chain reaction
• NBS suspended in tetrachloromethane reacts rapidly with HBr to form Br2 -> provides constant low concentration of Br2

Question 17

why is NBS needed for allylic bromination of alkenes

Answer 17

• allylic C-H bonds are weaker and allylic radicals favoured by resonance stabilisation
• NBS provides constant low concentration of Br2 so unlikely for competing reaction to occur where the bromine would add to double bond
• use of CCl4 also discourages formation of bromonium cation

Question 18

how to hydrogenate alkenes - cis selective mechanism

Answer 18

hydrogen gas in the presence of finely divided metal catalyst, i.e. Pd, Pt dispersed over C

Question 19

how to hydrogenate conjugated alkenes

Answer 19

use NADPH as hydride source
enolate intermediate is protonated by adding acid

Question 20

how can you form an epoxide from an alkene

Answer 20

1. peracids
2. O2 at high temperature over silver cat

Question 21

explain the stereospecifity of epoxidation with peracids

Answer 21

cis-alkene gives cis-epoxide and trans-alkene gives trans-epoxide

Question 22

how does the subsitution of alkenes affect the rate of epoxidation with peracids

Answer 22

more substituted alkenes epoxidise faster because electron donation from the substituents increases the energy of the π HOMO so it is more nucleophilic

Question 23

which carbon is attacked during nucleophilic ring opening of epoxides in acidic vs basic conditions

Answer 23

acidic: attack at more substituted end
basic: attack at less substituted end

Question 24

what reagents to make diol via anti addition from alkene

Answer 24

i) m-CPBA
ii) H2O and HClO4

Question 24

what reagents to make diol via syn addition from an alkene

Answer 24

OsO4 in aqueous acetone OR MnO4- (aq) - alkakine permanganate

Question 25

how can dihydroxylation using OsO4 be made more economic

Answer 25

use catalytic amount of Os(VIII) and a stoichiometric amount of reoxidant such as NMO

Question 26

what reagent for the cleavage of 1,2-diols to produce carbonyls

Answer 26

HIO4 (aq) - periodic acid OR Pb(OAc)4 in benzene

Question 27

what can ozonolysis be considered as achieving?

Answer 27

double oxidation

Question 28

what does ozonolysis produce

Answer 28

C-C bond cleavage leading to the production of two aldehydes

Question 29

what are the key features of the diels-alder reaction

Answer 29

1. we need a diene in which the C=C bonds can orient cis to each other 2. dienophiles with EWG work better 3. reaction is concerted and pericyclic

Question 30

why do dienophiles with EWG work better for Diels-Alder reaction

Answer 30

lowers LUMO energy, reducing HOMO-LUMO gap so that reaction is faster

Question 31

how do you hydrate an alkene such that the hydroxyl group is on the less substituted carbon

Answer 31

add borane BH3 -> B ends up on less substituted carbon add NaOH + H2O2 (-> HOO- + Na+ + H2O)

Question 32

describe the steps of alcoxymercuration (and reagent)

Answer 32

Add Hg(OAc)2 alkene attacks Hg forming mercurinium ion water opens mercurinium ion reduction using NaBH4 leading to cleavage of C-Hg bond