Bromoethane

Bromoethane, also known as ethyl bromide is a chemical compound of the haloalkanes group. It is abbreviated by chemists as EtBr. This volatile compound has an ether-like odour.

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Synthesis

The preparation of EtBr stands as a model for the synthesis of alkyl bromides in general. It is usually prepared by the addition of HBr to ethylene:

H₂C=CH₂ + HBr → H₃C-CH₂Br

Ethyl bromide is inexpensive and would rarely be prepared in the laboratory. Convenient laboratory syntheses include the action of phosphorus tribromide or thionyl bromide on ethanol. EtBr forms when ethanol is treated with HBr or hydrobromic acid, although this reaction also affords diethyl ether.

Uses

In organic synthesis, EtBr is the synthetic equivalent of the ethyl carbocation (Et⁺) synthon. In reality, such a cation is not actually formed. For example, carboxylates salts are converted to ethyl esters,^[1] carbanions to ethylated derivatives, thiourea into ethylisothiouronium salts,^[2] and amines into ethylamines.^[3]

Being a liquid at room temperature but still very volatile, EtBr is an inexpensive reagent for the preparation of Grignard reagents, which traditionally were used as strong base. Thus EtMgBr deprotonates alkynes:^[4][5][6]

EtBr + Mg → EtMgBr

RC≡CH + EtMgBr → RC≡CMgBr + EtH

This application has been supplanted by the wide availability of organolithium reagents.

Safety

Halocarbons in general are potentially dangerous alkylating agents. Bromides are more superior alkylating agents than chlorides, thus exposure to EtBr should be minimized. EtBr is classified by the State of California as carcinogenic and a reproductive toxin.

References

1. ^ Petit, Y.; Larchevêque, M.. "Ethyl Glycidate from (S)-Serine: Ethyl (R)-(+)-2,3-Epoxypropanoate". Org. Synth. 75: 37; Coll. Vol. 10: 401. 
2. ^ E. Brand, E.; Brand, F. C.. "Guanidodacetic Acid". Org. Synth. 22: 440; Coll. Vol. 3. 
3. ^ Brasen, W. R; Hauser, C. R.. "o-Methylethylbenzyl Alcohol". Org. Synth. 34: 58; Coll. Vol. 4: 582. 
4. ^ Taniguchi, H.; Mathai, I. M.; Miller, S. I.. "1-Phenyl-1,4-Pentadiyne and 1-Phenyl-1,3-Pentadiyne". Org. Synth. 50: 97; Coll. Vol. 6: 925. 
5. ^ A. J. Quillinan, A. J.; Scheinmann, F.. "3-Alkyl-1-alkynes Synthesis: 3-Ethyle-1-hexyne". Org. Synth. 58: 1; Coll. Vol. 6: 595. 
6. ^ Newman, M. S.; Stalick, W. M.. "1-Ethoxy-1-butyne". Org. Synth. 57: 65; Coll. Vol. 6: 564. 

• Makosza, M.; Jonczyk, A.. "Phase-Transfer Alkylation of Nitriles: 2-Phenylbutyronitrile". Org. Synth. 55: 91; Coll. Vol. 6: 897.