What is alkyl halide 1

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Synthesis of haloalkanes

Haloalkanes can be produced by addition or substitution reactions:

  • By adding the hydrogen halidesHCl, HBr and HI (HX) to alkenes, monohalogenated alkanes are easily accessible (Markovnikov addition). The addition of bromine and chlorine gives the corresponding 1,2-dihaloalkanes.
  • Another possibility for the synthesis of haloalkanes is the radical substitution of an alkane with chlorine or bromine. This method can only be used to a limited extent synthetically, because the reaction product consists of a mixture of mono-, di-, tri- and tetrahalogenated alkanes:
  • In the allyl position, on the other hand, you can use N-Bromo-succinimide (NBS) achieve a targeted single substitution. This reaction was first described by Karl Ziegler in 1942.
  • The production of haloalkanes from alcohols is the "usual" method. The simplest method is the direct reaction of an alcohol with HCl, HBr or HI with OH-. However, this reaction only proceeds quickly and at moderate temperatures with tertiary alcohols. In the case of primary and secondary alcohols, the reaction rate is significantly lower, and higher temperatures have to be used. The reaction rate when converting HX with tertiary alkanols, on the other hand, is often so fast that pure HCl or HBr simply has to be passed through the alcohol. After a few minutes, the starting materials have already been converted with good yields. Tertiary alcohols occupy an exceptional position with these direct substituents because the poor leaving group properties of the hydroxyl group are compensated for by the tertiary carbenium ion formed as an intermediate or water is formed as a good leaving group.

Primary and secondary alcohols, which are normally substituted via a mechanism, must be converted into the alkyl halides via the detour of an improved leaving group. Halogenating agents such as thionyl chloride (SOCl2) or phosphorus tribromide (PBr3). These reactions usually take place under mild conditions and the risk of acid-catalyzed rearrangements occurring is low.

Exercise: Synthesis of haloalkanes