Ch. 13 – Radical Chemistry Flashcards
What is a radical?
a reactive intermediate with single unpaired electron
How are radicals formed?
homolysis of covalent bond by adding heat or light
What is geometry and hybridization of carbon radical?
sp2, trig planar
Cleavage of weaker bonds form _______ stable radicals (more or less).
more
More stable radicals have ________ (more or less) subs.
more
Why are more alkyl groups helpful for making more stable radicals?
they are more polarizable and donate electron density
Where is the unpaired electron of a radical located?
in the unhybridized p orbital
What is bond dissociation energy?
energy required to break a bon homolytically
What is radical initiation?
formation of two radicals from sigma bond cleavage (sigma bond must be relatively weak)
What is radical propogation?
one radical is consumed and another is formed
What is radical propogation (hydrogen abstraction)?
pulling out elemental hydrogen and making new H-X bond
What is radical propogation (addition to pi bond)?
addition of radical X dot to pi bond to from new C-X bond
What is radical termination?
two radicals combine to form a sigma bond
What is a radical initiator?
something with a weak bond that serves a source of radical and kick starts the reaction (RO-OR)
Why is light or heat needed to make radicals?
energy is needed to break bonds
How is an alkyl halide made from an alkane?
radical chemistry! by adding X2 and light or heat
What is the selectivity of alkane radical reactions with Cl2 and Br2?
Br2 is selective
Cl2 is unselective
Why is Cl2 so unselective?
H-abstraction is exothermic with Cl so poor selectivity is caused by hammond’s postulate… anything that would stabilize a radical is meaningless (you cant see what is on the other side of the hill)
Why is light/heat needed for radical reactions?
provides energy for homolytic bond cleavage
Do radicals rearrange?
NO
What is the basis of propogation?
a radical reacts with another reagent to form a new sigma bond and another radical
What is the basis of termination?
two radicals combine without generating more
What is the trend for ease of radical formation?
3º-H will form radical quickest and most readily
In chlorination, what substitution will form in greater abundance?
more sub’d carbon that Cl attaches to will form in greatest abundance but not by that much
Why is bromination highly selective and what is it highly selective for?
highly selective for weakest (most sub’d) C-H bond because it will give most stable radical
bromination is exothermic overall but has endothermic transition state so it resembles products most. There is a greater energy barrier so it will form only the more stable radical
What is the RDS of alkane halogenation?
hydrogen abstraction
