Radical Reactions

heterolytic cleavage

uneven breaking of bond

homolytic cleavage

even breaking of bond

short

Stronger bond length: ____

same

Stronger bond has its overlapping orbitals as the ____ size

different

Weaker bond has its overlapping orbitals as ____ sizes

less

Stronger bond are ___ reactive

stronger

A more stable bond is ___ and less reactive

more

In radical form a stronger bond is ___ reactivity

less

In radical form a weaker bond is ___ reactivity

1°, 2°, 3°, allylic, benzyllic

From least to most stable for radical stability and carbocation stability

benzyllic

First carbon bonded to a benzene or aromatic ring. Carbon next to benze.

tertiary, secondary, primary, methyl

From least to most strong C-H bond strength

Hyperconjugation

Through space orbital interaction. Delocalization of electron density in a sigma bond to an empty or partially empty unhybridized p-orbitals. Stabilizes vacency

Resonance

Delocalization of electron density through unhybridized p-orbitals. Same ____ rules apply for radicals, but use single-headed arrows

allylic and benzyllic

These types of radicals are especially stable

Heat, light, peroxide (chemical, or N=N)

Radial initiators for radical reactions

Initiation

Generates radicals (break weakest bond). 0 to 2 radicals

Propagation

Radical reacts with non-radical. 1 radical to 1 new radical

Termination

2 radicals come together. 2 to 0 radicals

Radical halogenation

Cl-Cl breaks into 2Cl°. A Cl° radical reacts with a tertiary (or secondary) carbon’s hydrogen bond. This forms H-Cl, and the tertiary carbon is left as a radical with an unpaired electron. Another Cl-Cl comes in and reacts with that carbon, making desired non-radical product. A Cl° is left behind, so during the termination step a Cl° will react with Cl° (or another radical) to become non radical Cl-Cl.

Radical halogenation

Cl2 and heat (or hv/peroxide I think)

Transition state

Simultaneous bonds breaking and forming

Hammond postulate

Structure of transition state resembles the species (starting material vs. product/intermediate) that it is closer to in energy

exothermic

early transition state. Short bond is closer to starting material.

endothermic

late transition state. Short bond is closer to product (or intermediate)

F°, Cl°, Br°, I°

Relative reactivity of halogens. The more reactive, the less selective. The order from most to least reactive is ____

I and Br

Selective for tertiary only

Cl

Selective for 2° and 3°

F

Not selective. Will react with 1°/2°/3°

Cl

If you want to selectively halogenate a 2° C-H, use ___

Normal

____ hydrohalogenation: Markovnikov regioselectivity. Carbocation mechanism because strong acid protonates. Also want more stable C+ with H-Br, most basic site=gets protonated

Radical

____ hydrohalogenation: Anti-Markovnikov regioselectivity. H-Br reacts along with peroxide.

Racemic

Stereochemistry with radical reactions

Racemic

Reacting substitute is equally likely to react with either top or bottom of reactant’s orbital.