Rearrangement hydrogen halides

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES... 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

Unbranched primary alcohols and tertiary alcohols tend to react with hydrogen halides without rearrangement The alkyloxonmm ions from primary alcohols react rap idly with bromide ion for example m an Sn2 process Tertiary alcohols give tertiary alkyl halides because tertiary carbocations are stable and show little tendency to rearrange... 

The reactions of alcohols with hydrogen halides to give alkyl halides (Chapter 4) are nucleophilic substitution reactions of alkyloxonium ions m which water is the leaving group Primary alcohols react by an 8 2 like displacement of water from the alkyloxonium ion by halide Sec ondary and tertiary alcohols give alkyloxonium ions which form carbo cations m an S l like process Rearrangements are possible with secondary alcohols and substitution takes place with predominant but not complete inversion of configuration... 

One of the most important reactions of sulfones in general is the R amberg Backlund rearrangement, which involves the conversion of a-halosulfones to olefins with accompanying loss of hydrogen halide and sulfur dioxide under basic conditions (equation 48)151. 

This relatively ready rearrangement can be a nuisance in the preparative addition of acids, e.g. hydrogen halides (p. 184) to alkenes, or in their acid-catalysed hydration (p. 187) mixed products that are difficult to separate may result or, in unfavourable cases, practically... 

Alcohols react with hydrogen halides (HX) to give alkyl halides. Primary alcohols undergo Sn2 reactions with HX. Primary alcohols with branching on the P-carbon give rearranged products. The temperature must be kept low to avoid the formation of E2 product. 

Allenes are generally prepared by elimination of halogens, hydrogen halides, or water from adjacent carbon atoms, the dehalogenation of gem-dihalocyclo-propanes, rearrangement of acetylenes, and the 1,4-addition to vinylacetylenes. 

In the presence of alkali, hydrogen halide is eliminated producing presumably an electronically-deficient nitrogen fragment, which rearranges to the isocyanate (I) ... 

Quaternisation of triphenylphosphine with an alkyl halide gives a quaternary phosphonium halide (4) which under the influence of a strong base eliminates hydrogen halide to give an alkylidenephosphorane [(5), an ylide]. The latter reacts with an aldehyde or ketone to give first an intermediate betaine (6), which rearranges to the oxaphosphetane (7), which then under the reaction conditions eliminates triphenylphosphine oxide to form an alkene. 

However, the reaction of primary and secondary alcohols with hydrogen halides can generally be a problem since unwanted rearrangement reactions generally occurs. 

---