Benzyl bromide

• Phenylmethylbromid
• (Bromomethyl) -benzene
• α -bromotoluene

Colorless liquid with a pungent odor

Liquid

1.44 g · cm -3 ( 22 ° C)

4 ° C

198 ° C.

0.6 hPa ( 20 ° C)

In water slow decomposition

1.575 (20 ° C)

Attention

Template: Infobox chemical / molecular formula search available

Benzyl bromide is an organic chemical compound from the substance group of alkyl halides with the formula C7H7Br. It is a constitutional isomer of the Bromtoluolen.

Properties

Pure benzyl bromide is a colorless, water-clear liquid crystalline solidifies at about -4 ° C at room temperature. It has a density of 1.438 g/cm3 at 25 ° C. The most common organic solvents, it is miscible in all proportions, but not with water, by which it is slowly hydrolyzed to benzyl alcohol and hydrogen bromide.

The vapors of benzyl bromide are strong lachrymatory ( " lacrimogen " ), which is why in the past as a chemical warfare agent (" tear gas " ) was used. Already 0,004 mg per liter of air evoke strong irritant tears. It attacks the mucous membranes and lead to severe irritation. Especially in close contact, such as when swallowing of benzyl bromide, it is mentioned by the hydrolysis reaction to burns by the resulting hydrobromic acid. When dealing with benzyl bromide is therefore essential to operate in either an extraction system or wear an effective respiratory protection. Eye protection is required in every case.

Benzyl bromide, although combustible, but not flammable.

Production and representation

Benzyl can be prepared by free radical bromination of toluene. This reaction is usually carried out at the boiling point and under the action of light (SSS - rule). It is important to make sure that the bromine is slowly and only in the stoichiometrically required amount (an equivalent of Br2 per equivalent of toluene ) was added to the toluene so that the benzyl bromide formed can not react with a bromine excess to Benzylidendibromid on. The speed for a second bromination at the side chain is, however, considerably lower. Benzyltribromid is no longer formed.

Use and response examples

As an electrophilic alkylating agent benzyl bromide for the alkylation ( benzylation ) used by nucleophilic substances.

In the organic synthetic chemistry, it is often used as a reagent for introduction of the benzyl protecting group (abbreviation: Bn or Bzl) with alcohols, phenols, carboxylic acids, or primary or secondary amines. Typically, the benzyl bromide reacts with an alkoxide (i.e., a previously deprotonated alcohol) in terms of a nucleophilic substitution ( SN2 mechanism ) to form a benzyl ether. With carboxylic acids - here are generally salts thereof, such as R- COONa, used - arise corresponding benzyl ester; with amines, the N- benzyl amines. The benzyl protecting group is used quite frequently, because it is compatible with many other protecting groups.

With magnesium benzyl bromide reacted under anhydrous conditions very easy to the Grignard reagent benzyl magnesium bromide. This can be reacted in a Grignard reaction with a variety of reactants. The benzyl group in benzylmagnesium bromide can be considered as a nucleophilic alkylating agent.

Further, synthesis of chemically valuable implementation options offered by the reactions of benzyl bromide with triphenylphosphine or triethyl phosphite with. Triphenylphosphine with benzyl bromide leading to benzyltriphenylphosphonium bromide, which can be further processed in a Wittig reaction with ketones or aldehydes to form phenyl- substituted alkenes. Triphenylphosphite provides in a Michaelis- Arbuzov reaction with benzyl bromide to Benzylphosphonsäurediethylester, which in turn serves as the starting material for the synthesis of phenyl- substituted alkenes from ketones or aldehydes by the Horner-Wadsworth -Emmons reaction.

Benzyl bromide is used in chemical laboratories the benzyl halide used for this purpose most frequently because it

• Is significantly more reactive than Benzylfluorid, which is also regarded as very toxic,
• Less toxic and in many cases is somewhat more reactive than benzyl and
• Although more stable than the more reactive, and particularly to the air for oxidative decomposition tendency benzyliodide.

In syntheses, in which the reactivity of the benzyl bromide is not sufficient to be used in catalytic (i.e. sub-stoichiometric ) amounts in the reaction medium -soluble iodides, such as tetrabutylammonium iodide are added, which in situ with benzyl bromide, the reactive form benzyl iodide ( Finkelstein reaction). This is in accordance with its further reaction, the iodide ion again.

Disposal

Of benzyl radicals can be destroyed by reaction with aqueous base ( for example, dilute sodium hydroxide or ammonia solution). The resulting waste must be properly disposed of as halogen-containing organic- chemically contaminated hazardous waste.