Chemistry Reference
In-Depth Information
Before it has been supposed that AcOH is formed from acetoxy-radical AcO* as a result
of its splitting off valency-saturated CA molecule under the light action (O). Data show that
this is not so. Besides, it is stated [170] that AcO* radicals do not have end life-time and
instantly decay with carbon dioxide splitting off according to the reaction
АсО * → СН 3 + СО 2
(4)
In connection with this, acetic acid is not formed from acetoxy-radicals and this allows to
exclude them from the reaction (3).
Another possible photoreaction of acetoxyalkyl radical - is acetyl radical Ac* splitting
off.
С * -ОАс + hν → С-О + Ас *
(5)
Transition of EPR triplet spectrum with splitting of 2,8 mTl into singlet spectrum 0,9 mTl
wide at polyvinylacetate [171] is explained by photoreaction (5). However, acetyl radical in
vacuum does not lead to formation of acetic acid. That is why the reaction (5) does not
explain large yields of AcOH at photolysis of polyvinylacetate and CA and, probably, it does
not present the main way of phototransformation of radicals R*.
At the same time photoreaction of acetyl alkyl radicals (reaction 3 in scheme 3), leading
to chain process of acetic acid formation, indicates the proceeding of Norrish reaction of the
2 nd type,
(6)
Norrish reaction of the 2 nd type is well known for ketones and esters. Acetyl group in
radical R has greater degree of freedom than in valency-saturated molecule RH, and this
facilitates the formation of intermediate six-member complex, through which Norrish reaction
proceeds.
Vinyl radical R vin *, being formed during reaction (6) simultaneously with AcOH, is very
active even at lowered temperatures. For example, they easily break chlorine atom from CCl 4
[160]. That is why vinyl radical, being formed during reaction (6), easily breaks hydrogen
atom from polymer, giving again R according to the reaction
(7)
Thus, sequence of reactions (6) and (7) provides chain process of acetic acid formation
(see reaction 3 in Scheme 3).
Search WWH ::




Custom Search