Engineering Chemistry S Datta

PHOTOCHEMISTRY 555
This dissociation is favoured if a relatively stable alkyl radical is formed and usually
favoured in vapor phase rather than in solvent.
The other examples are cleavage of Cl 2
to Cl atoms, cleavage of O—O bonds in peroxy
compounds, and C—N bonds in aliphatic azo compounds (since stable N 2
is the product).
(2) Decomposition into molecules. Aldehydes can also cleave to form CO and alkane.
R—C—H ⎯⎯→
hν
⏐⏐
O
R—H + CO
Carbonyl compounds possessing γ-hydrogen atoms proceed by a shift of γ–H to oxygen
with subsequent cleavage to an olefine and an enol. This is Norrish type II reaction.
R 2
CH—CR 2
—C—R′
⏐⏐
O
⎯⎯→
hν
R 2
C = CR 2
+ R 2
CH—C—R′ ⎯⎯→ R 2
C = CR
⏐⏐
⏐
O
OH
The reaction can occur from both the excited singlet and the triplet states. Sometimes
the intermediate can also cyclize to a cyclobutanol as side product. Carboxylic esters, anhydrides
can also give this type of reaction.
O
H
RC 2 C—R
h
O
H
RC 2 C—R
Singlet/triplet
.
OH
.
RC 2 C—R
. *
diradical
CR 2 = CR 2 + HO—C—R
CR 2
O = C—R
R OH
CHR 2
R
R
R
R
3. Intramolecular rearrangement. Trimesityl compound to enol ether
R
R
Me
Me
Ar—CH—C—Ar
h
Ar—CH = C—Ar
Where Ar is
Ar O
OAr
Me
4. Photoisomerization. The common example is the cis-trans isomerization of stilbene.
Ph
C=C
H
cis-isomer
Ph
H
h
Ph H
C=C
H Ph
trans-isomer