Question

What chemical starts off the chain reaction in the initiation step of an anti-markovnikov radical addition?

Answer 1

The initiator in a free radical addition reaction is a substance that decomposes into free radicals under mild conditions.

Explanation:

An initiator should have bonds with low dissociation energies (e.g. `"O-O"` bonds) or form stable molecules (e.g. `"N"_2`) on dissociation.

Common initiators are:

Azo compounds

Azo compounds (`"R-N≡N-R"`) decompose into nitrogen and two free radicals on heating or irradiation.

`"R-N≡N-R " stackrel(color(blue)(Δ))stackrelcolor(blue)("or"color(white)(1)hν)(→) "R·" + "N≡N" + "·R"`

AIBN (azo-bis-isobutyronitrile) is a convenient free radical initiator because it decomposes at relatively low temperatures.

upload.wikimedia.org
(from en.wikipedia.org)

Organic peroxides

Two common peroxide initiators are

(a) Di-t-butyl peroxide

chemed.chem.purdue.edu
(from chemed.chem.purdue.edu)

(b) Benzoyl peroxide

(from research.cm.utexas.edu)