Organic Syntheses, Coll. Vol. 5, p.998 (1973); Vol. 47, p.103 (1967).
To a 250-ml. round-bottomed flask
fitted with a glass-blade stirrer
, a pressure-equalizing dropping funnel
, a thermometer
and a reflux condenser
equipped with a nitrogen bubbler (Note 1)
are added 6.0 g. (0.25 mole) of magnesium powder (Note 2) 50 ml. of decahydronaphthalene (Note 3)
, and a crystal of iodine
. The flask is swept with nitrogen
, and a nitrogen
atmosphere is maintained throughout the reaction. The mixture is heated to reflux without stirring, and from the dropping funnel there is added slowly one-fifth of a solution of 11.3 g. (0.1 mole) of chlorobenzene (Note 4)
and 9.0 g. (0.15 mole) of dry 2-propanol
. Reaction is almost immediately apparent in the region of the iodine
crystal, and as the reaction becomes progressively more vigorous (ca.
15 minutes) the stirrer is started and the external heating is reduced (Note 5)
. The remainder of the chlorobenzene
solution is added over a 30-minute period; this rate of addition causes the mixture to reflux gently without external heating. An additional 25 ml. of decahydronaphthalene
is added to facilitate the stirring, and the mixture is heated under reflux for one additional hour.
To minimize loss of volatile products such as benzene
, it is advisable to employ a dry ice condenser
on top of the conventional condenser.
Magnesium powder (Grade 4) from Magnesium Elektron, Inc., 610 Fifth Avenue, New York 20, New York, or from Magnesium Elektron Ltd., Manchester, England
, was employed within six months of the date of its grinding by the manufacturer. The use of older or coarser material may lead to lengthened induction periods, particularly when chlorides are used.
Freshly distilled decahydronaphthalene
was used. With the more easily reduced halides, and where the boiling point of the neutral reduction product was close to that of decahydronapthalene
, an excess of 2-propanol
was used as the reaction medium. Other hydrocarbons and secondary or tertiary alcohols may be employed for convenience in particular reductions. Diethyl ether
were not found to be generally suitable media.
The checkers found it necessary to distil the chlorobenzene
just before use.
When there is no sustained reaction after 10 minutes, initiation can often be accomplished by the addition of another crystal of iodine
(no stirring) and/or a small amount of an easily reduced halide such as 1-bromobutane
These washings remove the bulk of the 2-propanol
This drying also removes the last traces of 2-propanol
The checkers used a Nester-Faust 44-cm. spinning-band column
Certain halides, notably fluorides, are comparatively inert under these reaction conditions. In such cases the entrainment method
can be used, and reduction can be accomplished in the presence of a reactive halide such as 1-bromonaphthalene
. Also with certain halides, such as chlorocyclohexane
, the tendency for dehydrohalogenation is diminished by the use of such entraining agents.
The present procedures are based on those briefly described by the submitters in conjunction with E. T. Blues,2
and are based on the observation that magnesium
does not, under normal conditions, readily react with secondary and tertiary alcohols in the absence of a halogen or an organic halide; little or no hydrogen
is evolved during the reduction. Magnesium
reacts readily with primary alcohols, evolving hydrogen
, and the system is much less active in the reduction of organic halides. 2-Propanol
is recommended as a general-purpose alcoholic component, but other secondary and tertiary alcohols can also be employed.
4. Merits of the Preparation
Reduction with magnesium
provides a simple and effective procedure for the reduction of alkyl and aryl chlorides, bromides, and iodides; with an entraining agent some alkyl fluorides are attacked. Groups such as amino, phenolic hydroxyl, ester carbonyl, and ethylenic linkages have not interfered. Nitro compounds must be absent as they inhibit the reaction with magnesium
. Many carbonyl compounds, for example, p-bromobenzophenone
, undergo much simultaneous reduction of the carbonyl groups, but acetone
was obtained in fair yield from chloroacetone
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