vogel_practical_ochem_3.pdf

926 PRACTICAL ORGANIC CHEMISTRY [VI,
VI,26. THE WOHL-ZIEGLER REACTION. APPLICATIONS
OF iV-BROMOSUCCINIMIDE
The direct introduction of a halogen atom (usually bromine) by means of
2V-haloaniine (generally i\ 7 -bromosuccinimide) in the " allyl " position is known
as the Wohl-Ziegler reaction :
s. I / CH2-CH2X
)CH—C=C( + | )NBr
v | / CH2-CH2X
• ;CBr—C=C^ + | X / \ CH2—CH/
NH
/ \ CH2—CH2
Bromination is carried out with anhydrous reagents (to avoid hydrolysis of
the A T -bromoimide), usually in boiling carbon tetrachloride or chloroform solution.
The progress of the reaction can be followed by the fact that at first the
heavy iV-bromosuccinimide is at the bottom of the flask and is gradually replaced
by succinimide, which rises to the surface : the reaction is complete when
all the crystals are floating at the surface (detected by stopping the boiling
momentarily). This can be confirmed (when equimolecular amounts are used)
by transferring a drop of the solution to acidified potassium iodide - starch solution
: iodine should not be liberated. After cooling, the insoluble succinimide
is filtered off, washed with the solvent, and the product isolated, after removal
of the solvent, by distillation or crystallisation.
Two simple applications may be mentioned. With q/cZohexene (I) 3-bromocycZohexene
(II) is obtained in good yield ; the latter upon dehydrobromination
with quinoline affords an 80-90 per cent, yield of 1 : 3-c^cZohexadiene (III) :
Br
A 7 -Bromo- /\ Quinoline y^\
l (I) succinct O 1
' - Q (III >
Methyl crotonate (IV) yields the valuable synthetic reagent methyl y-bromocrotonate
(V) :
iV-Bromo-
CH3CH=CHCOOCH3 (IV) -> BrCH2CH=CHCOOCH3 (V)
succinimide
This compound permits the introduction (in moderate yield) of a four carbon
atom chain at the site of the carbonyl group by the use of the Reformatsky
reaction (compare Section VI,8) :
R'
Zn; |
RCOR'+BrCH2CH=CHCOOCH3 -* RCCH2CH=CHCOOCH3
benzene I
OH
METHYL y-BROMOCROTONATE
JV-Bromosuccinimide. Dissolve, with the aid of rapid mechanical
stirring, 80 g. of pure succinimide (Section V,14) in a mixture of 150 g. of
finely crushed ice and a solution of 32 g. of sodium hydroxide in 200 ml.
of water contained in a litre beaker and cooled externally by ice. Immediately
the imide has dissolved, continue the vigorous stirring and introduce
42 • 5 ml. of bromine in one lot from a separatory funnel supported over
the beaker : it is essential that the bromine be instantly suspended in the
solution. After stirring vigorously for 2 minutes, filter at the pump and