Lecture 1 Flashcards
Self Perpetuating Reaction
After formation of initial radical using light, no further photochemical generation needed as the reaction proceeds by itself
Radical Inhibitors Examples
Phenols, Quinines, Diphenlamines, iodine etc.
Acetone and HBr?
- with no O2/HOOH present, 2-methyl,bormo ethane formed
- with O2/HOOH - generation of tertiary radical and generation of 1-bromo2-methylpropane - why?
Draw two radical initiators
-dibenzoyl peroxide
- AIBN (azoisobutyronitrile)
Why do radical reactions mainly occur in the gas phase?
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
Which organometalllic compounds have weak carbon metal bonds so that they can undergo homolytic fission easily?
R-HG-R —-> R* + *HG-R or organocolbalts
Photolysis - which functional groups can absorb light?
- Ketones (near UV region)
- Alkyl hypochlorides (RO-Cl)
- Nitriles (RO-NO)
- Initiators (AIBN) and Dibenzoyl peroxide)
Advantages of Photolysis as initiator reaction
- low temps,
- strong bonds are broken (e.g. azo)
Thermolysis - disadvantages
1. in gas phase - which are main reactions?
2. In solution - which are main functional groups reacting? Side reaction?
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)
Why do radical reactions have so many side products?
Not as selective as heterolytic reactions
C-C bond fission - how is it accomplished?
Redox reactions using Fe2+/Fe3+ and Cu+/Cu2+
What reactions does Copper facilitate?
- Decomposition of acyl peroxides (think initiator)
- Sandmeyer reaction - diazonium salts to aromatic halides
Stable phenoxy radical generation
1 electron oxidation of K3Fe(CN)6
How to form radical at an electrophilic carbon?
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
Why do many radical detection methods involve colours?
Inolve unpaired electrons between low energy energy levels
Radical Detection Methods
- Colour
- Use radical to initiate polymerisation
- ESR (electron spin resonance spec) or EPR
- Spin trapping - if lifetime of radical is too short, react it with substrate that is diamagnetic to increase its lifetime
- Make radical in frozen matrix e.g. argon
Shape of radical orbitals?
Most are sp2, but sometimes between sp2 and sp3
Carbon radical shape
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
Why do conjugated systems make more stable radicals?
Electron can be delocalised.
What substituents stabilise radicals?
- Electron withdrawing and donating
- tertiary carbon radical more stable than primary
- Bulky groups (think initiators)
- conjugation ( delocalises radical)
What changes when attacking a carboxylic acid if using an electron withdrawing radical or electron donating?
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
Three ways radicals react
- Radical + Radical give spin paired molecule
- Radical + spin paired molecule give new radical plus spin paired molecule.
- Radical gives new radical + spin paired molecule
Do radicals prefer to react with each other or with spin paired molecules?
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)
What does NBS selectively attack?
Weak C-H Bonds
