Organic Free-Radical Substitutions and Additions

Radicals are electron

deficient

What is the charge of a radical

neutral (no formal charge)

What is not required for radical reactions

solvent

Identify the mechanism

free-radical substitution

How can the addition of HBr to alkenes, and the polymerization of alkenes/dienes be initiated

heat, peroxides, or UV

All radical reactions are

chain reactions

What is the initiation step of radical reactions

radical generation

What is the propagation step of radical reactions?

radical reacts with a stable molecule to produce a new radical (this step can be repeated many times)

What is the termination step of radical reactions?

radicals are destroyed and reaction is stopped.

On average how many propagation steps does a radical undergo before termination?

100’s or 1000’s

The energy required for the generation of organic radicals reflects their

relative stability

Stable radicals require the BLANK energy to form and vice versa, and are generated most rapidly

least

The stability of radicals generally increases with increasing

substitution on a carbon atom that carries the unpaired electron

Why are allylic and benzylic radical easy to generate

they are stabilized through resonance

identify the form

allylic radical resonance form

What is the order of stability for radicals?

A reaction usually involves radicals if any of the following reactants or conditions are present

1. AIBN (radical initiator)

2. peroxides

3. UV light

4. high temp

Why can electrophilic substitution not produce a radical

must happen in a solvent (cannot be gas phase)

To determine which hydrogen atom is most easily abstracted to generate a radical, what must be determined

which radical intermediate is the most stable

Typical radicals (including benzylic) are what geometry, and why

planar, the carbon is sp² hybridized

in free radical substitution, the radical can be attacked from either face with equal probability, which implies what of the stereochemistry

it will be a 50/50 mixture (racemic) of R and S regardless of if it begins as and R or and S molecule

In free radical bromination, this step is exothermic or endothermic?

endothermic

What is Hammond postulate?

a transition state resembles the structure of the nearest stable species (reactant, intermediate or product)

For an exergonic reaction, the transition state is most similar in energy to

the reactants

For an endergonic reaction, the transitions state is most similar in energy to

the products

what is the “reactivity-selectivity principle”

highly reactive species are nonselective in their reactions, relatively non-reactive species are very selective in their reactions

Which energy diagram signifies a reaction with high regioselectivity

Top

what is the heating of molecules to extremely high temperatures called

pyrolysis

what is a typically pyrolysis reaction

cleavage of sigma-bonds to give radicals

In free radical bromination, which radical will attach the carbon

Br

the polymer must have BLANK bonded to each carbon of the monomer regardless of the type of bond in the monomer

the same number of hydrogens

Addition polymers give a polymer with a BLANK that has the same BLANK as the monomer

repeating unit, chemical formula

Addition polymerization reactions typically proceed by way of BLANK

highly reactive species (free radical or carbocation)

Condensation polymers are formed by BLANK, accompanied by formation of BLANK

reaction of two functional groups to link monomer units, small molecule (like water or HCl)

Condensation polymers do NOT involved what kind of intermediate

free radical

Free radical polymerization reactions produce only

addition polymers

When determining the best reactant for a desired product, it is most important to keep in mind the likelihood of what

desired regioselectivity

In free-radical polymerization of vinyl monomers, addition occurs almost exclusively in

heat to tail fashion

What is head to tail addition

propagating radical attacks less hindered, less substituted carbon