Benzyl bromide
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benzene_200.svg)
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| General | ||||||||||||||||
| Surname | Benzyl bromide | |||||||||||||||
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| Molecular formula | C 7 H 7 Br | |||||||||||||||
| Brief description |
colorless liquid with a pungent odor |
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| properties | ||||||||||||||||
| Molar mass | 171.04 g mol −1 | |||||||||||||||
| Physical state |
liquid |
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| density |
1.44 g cm −3 (20 ° C ) |
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| Melting point |
−4 ° C |
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| boiling point |
198 ° C |
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| Vapor pressure |
0.5 h Pa (20 ° C) |
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| solubility |
slow decomposition in water |
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| Refractive index |
1.575 (20 ° C) |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C | ||||||||||||||||
Benzyl bromide is an organic chemical compound from the group of alkyl halides with the empirical formula C 7 H 7 Br. It is a constitutional isomer to bromotoluenes .
properties
Pure benzyl bromide is a colorless, water-clear liquid at room temperature, which solidifies in crystalline form at around −4 ° C. At 25 ° C it has a density of 1.438 g / cm 3 . It is miscible in all proportions with most common organic solvents , but not with water, which slowly hydrolyzes it to benzyl alcohol and hydrogen bromide .
Benzyl bromide vapors are highly irritating to tears (" lacrimogenic "), which is why it was used in the past as a chemical weapon (" tear gas "). As little as 0.004 mg per liter of air cause strong tear irritation. It attacks the mucous membranes and causes severe irritation. Particularly in the case of intensive contact, for example when swallowing benzyl bromide, the hydrolysis reaction mentioned causes burns from the hydrobromic acid formed . When handling benzyl bromide, it is essential to either work in a fume cupboard or to wear effective respiratory protection. Eye protection is required in any case.
Benzyl bromide is flammable, but not highly flammable.
Extraction and presentation
Benzyl bromide can be produced by the radical bromination of toluene . This reaction is usually carried out at the boiling point and under the action of light ( SSS rule ). Care must be taken that the bromine is added slowly to the toluene and only in the stoichiometrically required amount (one equivalent of Br 2 per equivalent of toluene) so that the benzyl bromide formed cannot react further with an excess of bromine to form benzylidene dibromide . The rate for a second bromination on the side chain is, however, much slower. Benzotribromide is no longer formed.
Usage and reaction examples
As an electrophilic alkylating agent, benzyl bromide is used for the alkylation (benzylation) of nucleophilic substances.
In synthetic organic chemistry, it is often used as a reagent for introducing the benzyl protective group (abbreviation: Bn or Bzl) in alcohols , phenols , carboxylic acids or primary or secondary amines . Typically, the benzyl bromide reacts with an alcoholate (i.e. a previously deprotonated alcohol) in the sense of a nucleophilic substitution ( S N 2 mechanism ) to form a benzyl ether. With carboxylic acids - their salts, such as R – COONa, for example, are usually used here - corresponding benzyl esters are formed; with amines the N -benzylamines. The benzyl protecting group is used quite widely because it is compatible with many other protecting groups.
Benzyl bromide reacts very easily with magnesium under anhydrous conditions to form the Grignard compound benzyl magnesium bromide. This can be implemented in a Grignard reaction with many different reaction partners. The benzyl group in benzylmagnesium bromide can be regarded as a nucleophilic alkylating agent.
The reactions of benzyl bromide with triphenylphosphine or with triethylphosphite offer further, synthetically valuable implementation possibilities . Triphenylphosphine and benzyl bromide lead to benzyltriphenylphosphonium bromide, which can be further processed in a Wittig reaction with ketones or aldehydes to give phenyl- substituted alkenes . In a Michaelis-Arbuzov reaction with benzyl bromide, triethyl phosphite yields the benzylphosphonic acid diethyl ester, which in turn serves as a starting material for the synthesis of phenyl-substituted alkenes from ketones or aldehydes according to the Horner-Wadsworth-Emmons reaction .
Benzyl bromide is the benzyl halide most commonly used for these purposes in the chemical laboratory because it is
- is significantly more reactive than benzyl fluoride , which is also considered to be very toxic,
- is less toxic and in many cases a little more reactive than benzyl chloride and
- is more stable than benzyl iodide, which is more reactive but has a tendency to oxidative decomposition, especially in air .
In syntheses in which the reactivity of the benzyl bromide is insufficient, iodides which are soluble in the reaction medium , e.g. tetrabutylammonium iodide , are added in catalytic (i.e. substoichiometric) amounts , which form the more reactive benzyl iodide with benzyl bromide in situ ( Finkelstein reaction ). After its further reaction, this releases the iodide ion again.
disposal
Residues of benzyl bromide can be destroyed by reaction with aqueous bases (for example dilute sodium hydroxide solution or ammonia solution). The resulting waste must be properly disposed of as halogenated, organic-chemical contaminated hazardous waste.
Individual evidence
- ↑ a b c d e f g h i entry to alpha-bromotoluene in the GESTIS database of IFA , accessed on January 10, 2017(JavaScript required) .
- ↑ Benzyl bromide data sheet from Sigma-Aldrich , accessed on January 24, 2011 ( PDF ).
- ↑ Entry on α-bromotoluene in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
- ^ Fieser and Fieser: Organic Chemistry. 2nd Edition. Verlag Chemie, Weinheim 1982, ISBN 3-527-25075-1 .
- ^ Association of authors: Organikum. 23rd edition. Wiley-VCH, Weinheim 2009, ISBN 978-3-527-32292-3 , p. 206.
literature
- Author community: Organikum . 19th edition. Johann Ambrosius Barth, Leipzig / Berlin / Heidelberg 1993, ISBN 3-335-00343-8 , pp. 175-176.
- MA Tilak: A facile method for the synthesis of benzyl esters using benzyl bromide or iodide and its application to solid phase and conventional peptide synthesis; attempted sterical selection in solid phase synthesis. In: Tetrahedron Lett. 1968, 9 (60), pp. 6323-6326, doi : 10.1016 / S0040-4039 (00) 75465-7 , PMID 5701078 .
- NI Sadova, NI Popik, LV Vilkov: Electron diffraction study on the molecular structure of benzyl chloride and benzyl bromide in the vapor phase. In: J. Mol. Struct. 1976, 31 (1), pp. 131-142, doi : 10.1016 / 0022-2860 (76) 80124-X .