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2-Bromohexane is the organobromine compound with the formula CH 3 CH(Br)(CH 2) 3 CH 3. It is a colorless liquid. The compound is chiral. Most 2-bromoalkanes are prepared by addition of hydrogen bromide to the 1-alkene. Markovnikov addition proceeds in the absence of free-radicals, i.e. give the 2-bromo derivatives. [2]
This reductive coupling can be viewed as involving two steps. First is the formation of a pinacolate (1,2- diolate ) complex, a step which is equivalent to the pinacol coupling reaction . The second step is the deoxygenation of the pinacolate, which yields the alkene , this second step exploits the oxophilicity of titanium.
Most 1-bromoalkanes are prepared by free-radical addition of hydrogen bromide to the 1-alkene, which is 1-pentene in the case of 1-bromopentane. These conditions lead to anti-Markovnikov addition, giving the 1-bromo derivative. [2] It is also formed by the reaction of 1-pentanol with hydrogen bromide.
2-Phenethyl bromide (C 6 H 5 CH 2 CH 2 Br) is produced via this route from styrene. Hydrogen bromide can also be used to convert alcohols to alkyl bromides. This reaction, that must be done under low temperature conditions, is employed in the industrial synthesis of allyl bromide: HOCH 2 CH=CH 2 + HBr → BrCH 2 CH=CH 2 + H 2 O
For example, in the three-component coupling of aldehydes, amines, and activated alkenes, the aldehyde reacts with the amine to produce an imine prior to forming the aza-MBH adduct, as in the reaction of aryl aldehydes, diphenylphosphinamide, and methyl vinyl ketone, in the presence of TiCl 4, triphenylphosphine, and triethylamine: [19]
In primary (1°) haloalkanes, the carbon that carries the halogen atom is only attached to one other alkyl group. An example is chloroethane (CH 3 CH 2 Cl). In secondary (2°) haloalkanes, the carbon that carries the halogen atom has two C–C bonds. In tertiary (3°) haloalkanes, the carbon that carries the halogen atom has three C–C bonds.
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One attractive feature of the Peterson olefination is that it can be used to prepare either cis- or trans-alkenes from the same β-hydroxysilane. Treatment of the β-hydroxysilane with acid will yield one alkene, while treatment of the same β-hydroxysilane with base will yield the alkene of opposite stereochemistry.