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CHEM333 (Bower) > Radical Chemistry > Flashcards

Radical Chemistry Flashcards

1
Q

How do heterolysis and homolysis of bonds differ?

A

Heterolysis is the movement of 2 electrons in 1 direction, forming 2 species w formal charges

Homolysis is the movement of 1 electron in either direction
- producing radicals

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2
Q

What is the purpose of dibenzoyl peroxide?

A

It is a RADICAL INITIATOR

Also only requires heating to between 60-80 degrees to homolysis O-O bond

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3
Q

Why is the primary Br favoured in the radical mechanism?

A

Anything that stabilises an anion OR cation stabilises a radical

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4
Q

Why are tertiary carbon radicals more stable than less substituted radical species?

A
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5
Q

Why’re these not stable radicals?

A

Because there is no resonance stabilisation available

This is because the e- is in an orbital that does NOT overlap with the pi-system
- hence is not in conjugation

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6
Q

What is a SOMO and how do EWGs stabilise radicals?

A

SOMO - Singularly Occupied Molecular Orbital

A SOMO is formed from the stabilization of a radical by an EWG, lowering the single e- in energy

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7
Q

How do EDGs stabilise radicals using SOMOs?

A

The energy of unpaired e- is raised but energy of lone pair drops
- so overall energy is decreased

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8
Q

Give the mechanism for the following reaction in a protic solvent

A
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9
Q

What effect does adding an e- to a carbonyl compound have?

A

The additional e- means the C=O bond is partially broken
- as e- resides in pi* orbital

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10
Q

Give the mechanism - and name - of this reaction
(In aprotic solvent)

A

Pinacol Coupling

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11
Q

Give the mechanism - and name - of this reaction

(Combination of reagents gives Ti(0))

A

McMurray Coupling - Ti(0)

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12
Q

What is this species called?

A

A ketyl radical anion

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13
Q

What is the mechanism, and name, of this reaction?

A

Acyloin condensation

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14
Q

How can the acyloin condensation reaction be improved?

A

By using Me3SiCl
- this traps all alkoxides to prevent base-mediated reactions

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15
Q

What is the mechanism, and name, of this reaction?

A

Allylic bromination - a radical chain reaction

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16
Q

Why is it important to limit Br radical generation in this reaction?

A

Because in the termination step, side products can form

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17
Q

What is the driving force in alkane chlorination?

A

The driving force is H-Cl bond formation

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18
Q
A
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19
Q

Why is 2˚ chlorination favoured?

A

Energetic > statistics

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20
Q

Why is 3˚ chlorination less favourable here?

A

9:1 ratio of 1˚ H atoms to 3˚ H atoms.

Statistics wins

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21
Q

Why is this reaction so highly selective?

A

The reaction is likely reversible

Both processes are disfavoured but 3˚ proton has a lower abstraction energy, and so follows this path

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22
Q

How can halogenation of alkanes be made more selective?

A

Can be made more selective due to H-X bond strengths

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23
Q

Is the second step of halogenation always endothermic or exothermic?

A

2nd step is ALWAYS EXOTHERMIC

24
Q

Which of these is the most and least stable?

25
What side reaction can occur within this reaction? How is it mitigated?
NBS provides a slow release of Br2 in situ - traces of HBr often found in NBS Br2 is only formed as allylic radical forms, keeping [Br2] low
26
What compound is used to dehalogenate alkanes?
Bu3Sn-H It has a high affinity for halogens
27
Why does Bu3Sn-H work for dehalogenating alkanes?
Because it is energetically favourable for both the Sn and C atoms to gain / lose Br / H respectively
28
What is the reactivity trend down group 7?
R-I > R-Br > R-Cl > R-F (inert)
29
What is the reactivity of 3˚ to 1˚ alkanes?
3˚ > 2˚ > 1˚ > aryl/vinyl
30
What is the mechanism for alkyl halide dehalogenation?
31
Give the mechanism of the C-C bond formation reaction
32
Give the mechanism of this reaction
Slow addition of Bu3SnH is important to maintain ~10x concentration of acrylynitrile
33
What’s the role of acetone here?
To act as a protecting group
34
What is the role of NaBH4 in this reaction?
It is used to reduce Bu3Sn-I to Bu3Sn-H in situ This makes Bu3SnH catalytic - ensuring that a low concentration of Bu3SnH is maintained (NaBH3CN can be used instead as it doesn’t reduce aldehydes or ketones)
35
What’s a major limitation of using alkenes for C-C radical bond formation?
That Z MUST be an EWG to give an electrophilic alkene - with alkyl radicals
36
Why do these radicals not react like this?
Because both the radical and alkene are electrophilic, and so they don’t like to react with each other
37
Do electrophilic radicals reside in high or low energy SOMOs?
Low energy SOMO, next to EWG
38
Do nucleophilic radicals reside in high or low energy SOMOs?
High energy SOMO, next to EDGs
39
What orbital do electrophilic radicals have the best overlap with?
Best overlap with HOMO of nucleophilic alkene More willing to accept an electron than give one up
40
What orbital do nucleophilic radicals have the best overlap with?
Best overlap with LUMO of electrophilic alkene More willing to give up an electron than accept one
41
42
How do polar nucleophiles and radicals differ in reactions with alpha-beta unsaturated ketones?
43
How do polar nucleophile and radicals differ in reactions with X-H bonds?
44
How do polar nucleophile and radicals differ in reactions with alkyl halides?
45
Why are intramolecular radical reactions so good?
Radical and radical trap are part of the same molecule - held close together No danger of Bu3Sn radical addition - hence [Bu3SnH] can be higher Stronger C-X bonds can be used as radical precursors
46
Why are -exo cyclisation products preferred over -endo products?
Due to the orientation of the pi* orbital, it is much easier to form 5/6-exo instead of 6/7-endo
47
How are Grignard reagents and organosamarium reagents similar in the formation of organometallic reagents?
They are both the source of electrons
48
What is the role of SmI2 in this reaction?
Electron donation, forming radicals Reducing the ester and alkene FGs
49
Give the mechanism of this reaction
5-membered ring formed - 5-exo is favoured due to orbital overlap Methyl radical avoids bulky OSmI2 Acetone traps the organometallic species
50
51
Give the mechanism, and name, of this reaction
The Birch Reduction Final deprotonation determines structure Kinetically controlled reactions of hexadieneyl anion take place at the central carbon
52
What positions are deprotonated, in order, in the birch reduction shown?
Para first, Ipso next This is because EWGs stabilise e- density at ip[so and para positions on ring
53
What positions are deprotonated, in order, in the birch reduction shown?
Ortho first, then Meta As EDGs stabilise e- density at ortho and meta positions
54
Give the mechanism of the reaction shown
55
Give the mechanism of the reaction shown
Vinyl anion is geometrically unstable - interconverting between E and Z conformers) E conformation is most stable and hence this is the Mpro product - high stereoselectivity
56
What’s the typical sequence for Birch reductions?
57
What’s the typical sequence for Birch reductions?

CHEM333 (Bower) (6 decks)

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