Alkenes + Alkynes

Question 1

orbital interactions in additions reactions

Answer 1

π-bond (HOMO) of alkene with LUMO of electrophile

Question 2

addition of HX - regioselectivity

Answer 2

tertiary carbon = favoured

Question 3

Markovnikov’s rule

Answer 3

hydrogen adds to least sub. carbon atom

Question 4

anti-markovnikov product

Answer 4

[presence of peroxide]

major product = least sub. alkyl halide

Question 5

what controls radical reactions?

Answer 5

strength of bonds

Question 6

what stabilises alkyl radicals?

Answer 6

hyperconjugation with adjacent σ-bonds (tertiary = most stable)

conjugation with adjacent π-systems -> single e- delocalised + resonance structures can be drawn

Question 7

SOMO

Answer 7

singly occupied molecular orbital

Question 8

syn-addition

Answer 8

addition to one face of alkene

Question 9

addition of bromine - stereochemistry

Answer 9

syn-addition of bromine

Nu then approaches at 180 to leaving group = INVERSION of configuration

Question 10

regioselectivity of ring-opening

Answer 10

Nu adds to most sub. end of bromonium ion

loose sn2 transition state with δ+ better stabilised on more sub. carbon atom

Question 11

NBS

Answer 11

alternative source of electrophilic bromine

Question 12

which is more reactive? more or less sub. alkene

Answer 12

more sub.

Question 13

which is more reactive? Z or E

Answer 13

Z

Question 14

which is more reactive? 1,1-disub. or 1,2-disub.

Answer 14

1,1-disub.

Question 15

why are more sub. alkenes more stable and more reactive?

Answer 15

[stability]

stabilised by hyperconjugation - overlap of σC-H and πC-C

σC-H and πC-C close enough in energy to interact - lowers 𝚿2

overall drop in 𝚿1 is greater than rise in 𝚿2 -> therefore combined energy of all electrons is lower through hyperconjugation = more stable

[reactivity]

HOMO raised by hyperconjugation compared to original πC-C

Question 16

palladium

Answer 16

insoluble heterogeneous catalyst for hydrogenation of alkenes

Question 17

Rh(PPh3)3Cl - Wilkinson’s catalyst

Answer 17

soluble homogeneous catalyst for hydrogenation of alkenes

Question 18

hydrogenation - stereochemistry

Answer 18

syn-addition

Question 19

what does hydroboration followed by oxidation produce?

Answer 19

anti-markovnikov regioselectivity

Question 20

why are borane derivatives electrophilic?

Answer 20

empty p orbital on boron atom

Question 21

hydroboration - electronics

Answer 21

δ+ builds up on more sub. end of alkene

Question 22

hydroboration - sterics

Answer 22

boron adds to least hindered end of alkene

Question 23

hydroboration - stereochemistry

Answer 23

stereospecific - H and boron add to same face (sin-addition)

oxidation doesn’t affect stereochemistry as migration step occurs with retention

Question 24

epoxidation - stereochemistry

Answer 24

stereospecific

O added to 1 face of alkene - syn-addition

Question 25

epoxide ring-opening - basic conditions

Answer 25

steric hinderance = controlling factor

i.e. good Nu @ least hindered position

Question 26

epoxide ring-opening - acidic conditions

Answer 26

mixture of regioisomers

Question 27

dihydroxylation - stereochemistry

Answer 27

stereospecific

both O atoms added to one face of alkene - syn-addition

Question 28

why is osmium tetroxide not widely used? what’s the alternative?

Answer 28

v. toxic and expensive

N-oxide (NMO) used to oxidise Os(VI) back into Os(VIII)

Question 29

why is using sodium periodate beneficial?

Answer 29

avoids use of stoichiometric lead

Question 30

O3 structure

Answer 30

bent

central O = positively charged

2 terminal O share negative charge

Question 31

structure of alkynes

Answer 31

carbon-carbon triple bond

sp on each C make up 2 σ bonds

remaining p orbitals overlap to form 2 π bonds

substituents = 180 degrees from each other

Question 32

why are terminal alkynes easier to deprotonate than alkanes/alkenes?

Answer 32

[lower pKa]

alkynes deprotonate to give sp-hybridised orbital

more s-character = anion closer to nucleus = greater stability

Question 33

what is required to get an aldehyde product during the hydroboration of terminal alkyne reaction?

Answer 33

sterically demanding borane to avoid side reactions