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Bond energies to bromine tend to be lower than those to chlorine but higher than those to iodine, and bromine is a weaker oxidising agent than chlorine but a stronger one than iodine. This can be seen from the standard electrode potentials of the X 2 /X − couples (F, +2.866 V; Cl, +1.395 V; Br, +1.087 V; I, +0.615 V; At, approximately +0.3 V).
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The reaction mechanism for an alkene bromination can be described as follows. In the first step of the reaction, a bromine molecule approaches the electron-rich alkene carbon–carbon double bond. The bromine atom closer to the bond takes on a partial positive charge as its electrons are repelled by the electrons of the double bond.
Illustrative of the bromination of an alkene is the route to the anesthetic halothane from trichloroethylene: [6] Iodination and bromination can be effected by the addition of iodine and bromine to alkenes. The reaction, which conveniently proceeds with the discharge of the color of I 2 and Br 2, is the basis of the analytical method.
In organic chemistry, an addition reaction is an organic reaction in which two or more molecules combine to form a larger molecule called the adduct. [1] [2] An addition reaction is limited to chemical compounds that have multiple bonds. Examples include a molecule with a carbon–carbon double bond (an alkene) or a triple bond (an alkyne).
A typical example is shown below; note that if possible, the H is anti to the leaving group, even though this leads to the less stable Z-isomer. [27] An example of an E2 Elimination. Alkenes can be synthesized from alcohols via dehydration, in which case water is lost via the E1 mechanism. For example, the dehydration of ethanol produces ethylene:
A few other images are of low quality but it might be hard to find versions of higher quality: Ba (oxidises too easily), Pm, Ra, Pa (radioactives). A few images are only available as fair-use: Po, Np, Cm. It may be possible to find a free alternative (although it would be quite difficult).
Reaction is slower with alkynes than alkenes. [3]: 750 In the paradigmatic example, hydrogen bromide radicalyzes to monatomic bromine. These bromine atoms add to an alkene at the most accessible site, to give a bromoalkyl radical, with the radical on the more substituted carbon.