CH6: Radical Reactions Flashcards
What is needed for homolytic bond cleavage
Heat or light
Dialkyl peroxides turn into what with cleavage R-O-O-R
Alkoxyl radicals R-O (-)
What are the most stable radicals?
Allylic and benzylic because of resonance
Alkane halogenation
Is very unstable and leads to a multitude of products
Chlorine radical rxn
Not good, use bromination
How to get monosubstituted alkanes with halogen radical reactions
Use an excess of alkanes
Initiation
The initial step where X2 is split into radicals by heat or light
Chain propagation
A) X* steals an H off of an alkyl group forming an alkyl radical
B) The R* radical attacks X2 forming an alkyl halide and another X* which will go steal another H
A/B repeat
What is radical halogenation?
X2 attacks a symmetrical compound with light or heat adding one X
What is hydrobromination with peroxides?
HX with ROOR leads to the less substituted alkyl halide
Compound symmetry leads to?
A multitude of products
Termination
When 2 radicals link up
Monochlorination of higher alkanes
Leads to multiple products…shit
Monobromination of higher alkanes
Much more selective…gives the predicted product
What happens when an achiral carbon radical is attacked by X*
It may lead to enantiomers because it is trigonal planar, it can fill from either side.
Radical rxns are
SN1
What happens when there is 1 sterogenic center and another one is formed
The enantiomers ex. R,S vs R,R are not made in equal amounts
Steps of hydrobromination in peroxide
1) Peroxide splits to form alkoxy radical R-O*
2) This then takes an H off of HBr forming bromine radical
3) Br* attacks the least substituted side of the alkene yielding a more stable/substituted radical
4) The radical carbon grabs an H off of HBr creating more radical and therefore step 3/4 repeat until termination
Alkene polymerization
Monomeric units use pi electrons to form chains initiated by peroxide
