Preparing Alkyl Halides from Alcohols

The most general method for preparing alkyl halides is to make them from alcohols. A great many alcohols are commercially available, and we ll see later that a great many more can be obtained from carbonyl compoun k. Because of the importance of the reaction, many different reagents have been used for transforming alcohols into alkyl halides. 

The simplest method for converting an alcohol to an alkyl halidi involves treating the alcohol with HCl, HBr, or HI  

For reasons that will be discussed in the next chapter (Section 11.16), tlu reaction works best when applied to tertiary alcohols, R3COH. Primary aiu secondary alcohols also react, but at slower rates and at higher reaction temperatures. Although this is not a problem in simple cases, more com plicated molecules are sometimes acid-sensitive and are destroyed by thi reaction conditions. 

The reaction of HX with a tertiary alcohol is so rapid that it s often cai ried out simply by bubbling the pure HCl or HBr gas into a cold ether solution of the alcohol. Reaction is usually complete within a few minutes. 

Primary and secondary alcohols are best converted into alkyl halides by treatment with such reagents as thionyl chloride (SOCI2) or phosphorus tribromide (PBrs). These reactions, which normally take place readily under mild conditions, are less acidic and less likely to cause acid-catalyzed rearrangements than the HX method. 

Thomson . Click Organic Interactive to use a web-based palette to design a synthesis of alkyl halides, beginning with alcohols. 

LA I Cl L Will L/C Ui LUMCU All OCLtlUII L11C ICdLLlL/ll WL/A1 0 UOl WlLU lUllCliy 

Draw three resonance forms for the cyclohexadienyl radical. 

The major product of the reaction of methylenecyclohexane with JV-bromo-succinimide is l-(hromomethyl)cyclohexene. Explain. 

What products would you expect from reaction of the following alkenes with NBS If more than one product is formed, show the structures of all. 

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A more efficient method of preparing alkyl halides from alcohols involves reactions with thionyl chloride (SOCL). 

As already known, a halogen atom at position 4 in the pyridazine ring is more reactive towards nucleophiles then one at position 3. This has been confirmed also with 3,4-dichloro-5,6-diphenylpyridazine and various nucleophiles (81MI2). The high reactivity of 6-halo-1,3-dimethylpyridaz-inium halides or phosphorodichloridates has been used to prepare alkyl halides from alcohols (80S746). 

Other substitution reactions we ve seen include some of the reactions used for preparing alkyl halides from alcohols. We said in Section 10.7, for example, that alkyl halides can be prepared by treating alcohols with HX—reactions now recognizable as nucleophilic substitutions of halide on the protonated alcohols. Tertiary alcohols react by an S>jl pathway involving unimolecular dissociation of the protonated alcohol to yield a carbo-cation, whereas primary alcohols react by an 8 2 pathway involving direct bimolecular displacement of H2O from the protonated alcohol (Figure 11.23). 

Other Ways to Prepare Alkyl Halides from Alcohols... 

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