Lecture 1

Question 1

Self Perpetuating Reaction

Answer 1

After formation of initial radical using light, no further photochemical generation needed as the reaction proceeds by itself

Question 2

Radical Inhibitors Examples

Answer 2

Phenols, Quinines, Diphenlamines, iodine etc.

Question 3

Acetone and HBr?

Answer 3

• with no O2/HOOH present, 2-methyl,bormo ethane formed
• with O2/HOOH - generation of tertiary radical and generation of 1-bromo2-methylpropane - why?

Question 4

Draw two radical initiators

Answer 4

-dibenzoyl peroxide
- AIBN (azoisobutyronitrile)

Question 5

Why do radical reactions mainly occur in the gas phase?

Answer 5

Less energy demanding - more energy demanding in liquid phase as for heterolytic fission, chargers are stabilised by solvent - homolytic fission doesn’t have an energy stabilisation from this.
- Radical chem therefore often occurs in non polar solvents if its in a solution

Question 6

Which organometalllic compounds have weak carbon metal bonds so that they can undergo homolytic fission easily?

Answer 6

R-HG-R —-> R* + *HG-R or organocolbalts

Question 7

Photolysis - which functional groups can absorb light?

Answer 7

1. Ketones (near UV region)
2. Alkyl hypochlorides (RO-Cl)
3. Nitriles (RO-NO)
4. Initiators (AIBN) and Dibenzoyl peroxide)

Question 8

Advantages of Photolysis as initiator reaction

Answer 8

• low temps,
• strong bonds are broken (e.g. azo)

Question 9

Thermolysis - disadvantages
1. in gas phase - which are main reactions?
2. In solution - which are main functional groups reacting? Side reaction?

Answer 9

Disadvantages: High temps, need weak ish bonds
1. Decomposition of metal alkyls
2. Peroxides or azo compounds (think two initiators)
- need long reaction times and high temps
Side reaction - abstraction of H from solvent to form dimers (in absence of other species, conc. dependant reaction)

Question 10

Why do radical reactions have so many side products?

Answer 10

Not as selective as heterolytic reactions

Question 11

C-C bond fission - how is it accomplished?

Answer 11

Redox reactions using Fe2+/Fe3+ and Cu+/Cu2+

Question 12

What reactions does Copper facilitate?

Answer 12

1. Decomposition of acyl peroxides (think initiator)
2. Sandmeyer reaction - diazonium salts to aromatic halides

Question 13

Stable phenoxy radical generation

Answer 13

1 electron oxidation of K3Fe(CN)6

Question 14

How to form radical at an electrophilic carbon?

Answer 14

If molecule contains C=O group, alpha carbon is radical is use Mn(III)(OAc)3, if alpha position is blocked, gamma position turns radical

Question 15

Why do many radical detection methods involve colours?

Answer 15

Inolve unpaired electrons between low energy energy levels

Question 16

Radical Detection Methods

Answer 16

1. Colour
2. Use radical to initiate polymerisation
3. ESR (electron spin resonance spec) or EPR
4. Spin trapping - if lifetime of radical is too short, react it with substrate that is diamagnetic to increase its lifetime
5. Make radical in frozen matrix e.g. argon

Question 17

Shape of radical orbitals?

Answer 17

Most are sp2, but sometimes between sp2 and sp3

Question 18

Carbon radical shape

Answer 18

Usually say that carbon radicals are planar and that the radical (in a SOMO) is sp2 hybridised with the unpaired electron in the p orbital. SER can measure the sigma character in the orbital of the unpaired electron

Question 19

Why do conjugated systems make more stable radicals?

Answer 19

Electron can be delocalised.

Question 20

What substituents stabilise radicals?

Answer 20

• Electron withdrawing and donating
• tertiary carbon radical more stable than primary
• Bulky groups (think initiators)
• conjugation ( delocalises radical)

Question 21

What changes when attacking a carboxylic acid if using an electron withdrawing radical or electron donating?

Answer 21

If using electron withdrawing radical (e.g. Cl) then beta radical is formed as beta radical is electron rich
if using electron donating radical (e.g. Me) then alpha radical is formed and alpha carbon was electron poor
This is explained by thinking of energy of SOMOs

Question 22

Three ways radicals react

Answer 22

1. Radical + Radical give spin paired molecule
2. Radical + spin paired molecule give new radical plus spin paired molecule.
3. Radical gives new radical + spin paired molecule

Question 23

Do radicals prefer to react with each other or with spin paired molecules?

Answer 23

Spin paired - radicals are usually too reactive and preferentially react with solvent or other substrates. Think about this in terms of radical inhibition - if O2 present and two radicals, instead of two radicals reaction they may react with O2 to form O-O (inhibition of polymerisation/dimerisation)

Question 24

What does NBS selectively attack?

Answer 24

Weak C-H Bonds