Hydrogen bonding bromide reaction

The bond highlighted m yellow is the peptide bond ) Pencyclic reaction (Section 10 12) A reaction that proceeds through a cyclic transition state Period (Section 1 1) A honzontal row of the penodic table Peroxide (Section 6 8) A compound of the type ROOR Peroxide effect (Section 6 8) Reversal of regioselectivity oh served m the addition of hydrogen bromide to alkenes brought about by the presence of peroxides m the reaction mixture... 

The most significant difference between brominations in protic and non-protic solvents concerns the kinetic law. Whereas in protic media the reaction is first-order in bromine, in halogenated media it is second-order (Bellucci et ai, 1980). CTC ionization is electrophilically assisted by hydrogen bonding by a protic solvent to the leaving bromide and leads to a bromonium-bromide ion pair. In non-protic media, assistance to the bromination step is provided by a second bromine molecule, leading to a bromonium-tribromide ion pair. In other words, in protic media bromination is solvent-assisted (56) while in halogenated media it is bromine-catalysed (57). 

Kelly and coworkers devised a two binding-site host that accelerates an Sn2 reaction between a primary aliphatic amine and an alkyl bromide [17[. Kelly s host (Figure 1.7) acted as template for the two reactants that were able to form three hydrogen bonds to each aminopyridone from the host. The reactants were the aminomethyl- and bromomethylnaphthyridines indicated in Figure 1.7, which bound strongly (K>1O M ) to the aminopyridone moieties from the host. A sixfold acceleration of the Sn2 reaction was observed, but turnover could not be demonstrated (the product precipitated from the CHCI3 solution as the HBr salt). 

Al(01I)4(Il20)2] - The high viscosity of sodium aluminate solutions is explained by hydrogen bonding between these hydrated ions, and between them and water molecules. By reaction of aluminum and its chloride or bromide at high temperature, there is evidence of the existence of monovalent aluminum, Here the aluminum atom is apparently in the sp state, with an electron pair on the side away from the chlorine atom, whereby the single pairs on the two chlorine atoms are shared to form two weak it bonds. 

The products contain a trivalent, positively charged carbon atom surrounded by a solvent shell and a bromide ion which carries a negative charge and is surrounded and hydrogen bonded to a shell of solvent molecules. The distance between the carbon and the bromine atom in the products is much greater than in the reactants. The bond between carbon and bromine is broken as the bromide moves away from the carbon and the pair of bonding electrons ends up in the valence shell of the bromide ion. We can depict this reaction using curved-arrow notation which tracks electron movement. 

Under acidic reaction conditions, however, aliphatic fluoride can be readily displaced (Scheme 4.13), especially in alkyl monofluorides. This is probably because of the strong hydrogen bonds formed by fluoride [75], As illustrated by the second example in Scheme 4.13, for some substrates the fluorides will be even more reactive than the corresponding chlorides or bromides [76]. The first two reactions sketched in Scheme 4.13 can be driven to completion by use of higher temperatures or longer reaction times, to give almost quantitative yields of alkyl iodides. 

Hydrogen bonds, between carbohydrates and hydrophilic groupings of proteins of enzymes, V, 55 Hydrogen bromide, hydrolysis of polysaccharides with, II, 231 Hydrogen cyanide, reaction with osones, II, 80... 

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