Radicals and Selectivity Flashcards
What is a radical and how are they formed?
Any atom or molecule that bears an unpaired electron that is formed through the homolytic breakage of a bond
What calculation can identify whether a radical is charged or not?
Valence electrons - (No. of bonds to atom + lone pairs + radial) = charge
What increases radical stability?
More substitution = greater stability (hyperconjugation of the SOMO) lower energy required for homolytic fission
What is the difference between overlapping a carbocation and a radical?
Radical is slightly less energetically favourable because there is an electron in a higher energy orbital than before thus stabilization energy is lowered
Which is stronger, hyperconjugation or resonance/delocalization?
Resonance (allyl, benzyl) due to delocalization of charge
Under what conditions can radical formation occur?
High temperatures or light (UV) irradiation
What are the 3 steps in radical reactions and what do they involve? draw out mechanism if possible
- Initiation-> formation of radical
- Propagation-> radial + molecule and forms another radical
- Termination (after thousands of propagation)-> radical + radical to form neutral molecule
What are the 2 types of propagation steps are what is the difference?
Productive -> radical + molecule forms a DIFFERENT radical
Non-productive -> radical + molecule forms the same radical
Why do flourine and iodine radical reactions rarely form?
Flourine radical is dangerous (too explosive)
Iodine has low energy requirement for homolytic fission but iodine radicals are much less reactive and the process of iodine extracting a hydrogen from alkane is highly endothermic (thus requires energy input)
What are chemical reactions or processes that involve random or probabilistic behavior.
Stochastic reactions
What has greater selectivity, chlorine radical reactions or bromine?
Bromine because the TS is much closer to the products after propagation thus hyper conjugation would have a bigger effect on reaching the products than with chlorine (which can more easily form the products)
the Ea for formation of chlorination intermediate is lower than that of bromination due to the stability that arises from its products
What is a transition state?
A theoretical structure for a molecule that represents the highest energy in a reaction coordinate (Eg. pentavalent carbon for SN2 reaction)
Why is bromine less exothermic than chlorine for propagation?
Chlorine TS occurs early as it is better at extracting the hydrogen, thus the TS resembles the reactants so lower Ea is required.
Bromine TS occurs later, thus the TS resembled is closer to the products which has a higher activation energy requirement -> less favoured
The position of the TS is determined by the radicals ability to extract H from the C (Stronger radical = faster extraction)
how can products formed through radical reactions be separated?
difference in boiling point can be used
which intermediate is higher in energy than reactants for bromination and chlorination
bromination has intermediate that is higher in energy while chlorination has an intermediate that is lower in energy
