Intro to radicals

Question 1

What is a radical?

Answer 1

Molecular entitiy with an unpaired electron

C-based ones are generally very reactive

Question 2

What is the structure of a C-centred radical?

Answer 2

Slightly pyamidal with unpaired e- in sp³ orbital

However the methyl radical is flat

Question 3

What is the stability of radicals?

Answer 3

Generally follows the C-H or O-H bond dissociation energy

Aromatics stabilise them through delocalisation

7e- most stable with 3º>2º>1º

Question 4

What are the main factors which stabilise radicals?

Answer 4

Overlap with filled or/and empty adjacent orbitals

Question 5

What is a persistent radical?

Answer 5

Long-lived radicals whihc are kinetically stable (usually for steric reasons)

Question 6

What are some general features of radical reactions?

Answer 6

Usually exothermic - favours more stable products as breaks weak ones and forms stable ones

Radicals add to π-bonds or abstract atoms

Attack univalent and sometimes divalent atoms

Question 7

What do radicals attack?

Answer 7

1. halides
2. olefins / alkynes
3. 1,5-HAT
4. carbonyls

Question 8

What are the general steps in a radical reaction?

Answer 8

Homolysis/recombination

Redox (initiation & termination)

Addition/β-scission to π bonds

Homolysis/recombination

Question 9

What is the Kolbe electrolysis?

Answer 9

Oxidative decarboxlyation of a carboxylic acid salts

Leads to radicals which dimerise

Question 10

What is 1,5-HAT?

Answer 10

Intramolecular 1,5-hydrogen atom transfer

Common for alkyoxy radicals with correct orientation of C-H bond

6-membered TS

Question 11

What is disproportionation in a radical reaction?

Answer 11

Form of terminaton

H-atom abstracted which is β to the radical centre

Question 12

What is the wohl-ziegler bromination?

Answer 12

Allylic/benzylic bromination of hydrocarbons using an NBS and trace (PhCO₂)₂ & HBr

Goes via the “goldfinger mechanism”

Question 13

What does NBS provide a reaction?

Answer 13

Low steady state conc of Br₂ by reaction with HBr formed in H-atom abstraction step

Low [Br₂] and [HBr] prevent unwanted radical and polar reactions

Question 14

What type of process is it when a Br radical adds to an alkene?

Answer 14

Reversible as there is nothing to trap the adduct radical

When unsymm, Br radical abstracts H to give the most stable radical

Question 15

What is the Hofmann-Loffler-Freytag reaction?

Answer 15

N-halogenated amine → Cyclic amine (e.g.pyrollodine)

Reagents: H₂SO₄ and heat/hv followed by base

Question 16

What does the Hofmann-Loffler-Freytag mechanism rely upon?

Answer 16

Weakness of N-Cl or N-Br bond

1,5-HAT

Question 17

Why are 6-membered TS favoured?

Answer 17

Thermodynamically favoured

Question 18

Why is a solvent cage sometimes required?