Polar addition hydrogen halides

In many addition reactions the attacking reagent, unlike H2, is a polar- molecule. Hydrogen halides are anong the simplest exanples of polar- substances that add to alkenes. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

Addition Reactions. In general, polar molecules such as hydrogen halides add across the B—N bonds, the more electronegative group bonding to boron (91). The adducts are cydotriborazanes such as the product formed by reaction of B-trichloroborazine and hydrogen chloride (eq. 35). X-ray crystal analysis shows the structure exists in a chair conformation (124). 

We use the term substitution with scheme (138) in the sense that it is used for aromatic compounds. Addition is reserved for processes in which a saturated intermediate is formed. To observe retention, we require only that k 2) > k(3) in (138). By analogy with the SB2 reactions at a saturated carbon (Rreevoy et al., 1967), it is probable that some demetalations with acid in a polar solvent proceed in this way. Certainly, the intermediates are wholly analogous to those proposed for the isomerization, hydration, or hydrogen halide addition to alkenes. 

Cabaleiro and Johnson (1967) report that the addition of chlorine to -methyl cinnamate in chloroform or acetic acid is syn-selective, SS 0-75, in chloroform and acetic acid acetoxychloro derivatives are produced as well. Again, Dewar and Fahey (1964) argue that the normal course of addition of hydrogen halides onto olefins is a polar electrophilic process involving classical carbonium ions as intermediates and leading mostly but not exclusively to cis-adducts. A syn-preference was found in the additions of deuterium bromide to acenaphthylene, indene, and cis- and fraws-phenylpropene. In the case of indene, phenylpropene and methyl cinnamate, which are styrene analogs, concerted syn addition is symmetry-allowed (see bottom of p. 273). 

In these compounds the carbon atom is mostly negative and the phosphorus is positive polarized, thus in the case of an addition of proton-active reagents such as hydrogen halides, alcohols, or amines the proton moves to the carbon and the anionic part moves to the phosphorus atom [Eq. (12)] (8,14). 

The interaction of HC1 with an alkene in a non-polar medium results in a simple addition of the hydrogen halide. The centre generated from the proton and monomer is immediately deactivated by the strongly basic and mobile Cl-. In a solvating medium, e.g. nitromethane, styrene oligomerizes in the presence of HC1 [91 ]. Thus the combination of the solvated ions is relatively slow so that propagation can compete with it. 

The interaction of HCl and HBr with indene in methylene chloride and with acenaphthylene in pentane, methylene chloride and acetic acid as well as the reaction of HI with the latter monomer in methylene chloride only yielded the addition products, no polymerisation being detected. However, according to older reports the dimers of both these monomers can be obtained with HCI . Whatever the reason for this discrepancy, it is obvious that indene and acenaphthylene fall in the same category as styrene in that they do not offer the best conditions for the formation of active species in reacting with hydrogen halides. It would be interesting to try these interactions in more polar solvents. 

The crystals of such inorganic substances as carbon dioxide andjthe hydrogen halides and of the majority of organic substances are composed of molecules bound together by van der Waal s forces. If the molecule has a relatively simple structure and only a small polarity, the heat of sublimation is found to be small and for such molecules, London s has calculated the heats of sublimation assuming that only dispersion forces are responsible for the inter-molecular attraction. The attraction energy is considered to be given by =. — (7/r where r is the distance between the molecules and C = J aH (see equation 12.3). In addition to non-polar or weakly polar molecules,... 

The reaction is frequently carried out by passing the dry gaseous hydrogen halide directly into the alkene. Sometimes the moderately polar solvent, acetic acid, which will dissolve both the polar hydrogen halide and the non-polar alkene, is used. The familiar aqueous solutions of the hydrogen halides are not generally used in part, this is to avoid the addition of water to the alkene (Sec. 6.9). 

We have already suggested two reasons why the Br radical adds to the alkene with this characteristic regioselectivity, giving a primary alkyl bromide when the polar addition of HBr to an alkene would give a tertiary alkyl bromide (1) attack at the unsubstituted end of the alkene is less sterically hindered and (2) the tertiary radical thus formed is more stable than a primary radical. In fact, of all the hydrogen halides, only HBr will add to alkenes in this fashion HCI and HI will undergo only polar addition to give the tertiary alkyl halide. Why We need to be able to answer this type of question too. 

We ve already seen several methods for preparing alkyl halides, including the reactions of HX and X2 with alkenes in electrophilic addition reactions (Sections 6.8 and 7.2). The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition. Bromine and chlorine yield trans 3,2 dihalogenated addition products. 

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