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10.5: Preparing Alkyl Halides from Alcohols

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    After completing this section, you should be able to

    1. write an equation for the conversion of an alcohol to an alkyl halide.
    2. list a given series of alcohols in increasing or decreasing order of reactivity with hydrogen halides.
    3. identify the alkyl halide formed when a given alcohol reacts with thionyl chloride, phosphorus tribromide, or a hydrogen halide.
    4. identify the alcohol which should be used to prepare a given alkyl halide using one of the reagents specified in Objective 3.
    5. select the most appropriate reagent for converting a given alcohol to a given alkyl halide.
    Study Notes

    The use of thionyl chloride for converting alcohols to alkyl chlorides has the added benefit that both of the by-products, sulfur dioxide and hydrogen chloride, are gases. This characteristic simplifies the isolation and purification of the reaction product.

    In the laboratory, one can test for the presence of alcohols with Lucas reagent (a mixture of concentrated hydrochloric acid and zinc chloride). Lucas reagent converts alcohols to alkyl chlorides: tertiary alcohols fastest followed by secondary alcohols; primary alcohols do not react to any significant extent. Thus, Lucas reagent can help distinguish among primary, secondary and tertiary alcohols due to going through a substitution reaction.

    General Reaction

    When alcohols react with an acid halide, a substitution takes place producing an alkyl halide and water:

    reaction equation for alcohol plus HX yields alkyl halide plus water.svg

    • The order of reactivity of alcohols is 3° > 2° > 1° > Methyl.
    • The order of reactivity of the hydrogen halides is HI > HBr > HCl (HF is generally unreactive).

    Tertiary alcohols react reasonably rapidly HCl, HBr, or HI, but for primary or secondary alcohols the reaction rates are too slow for the reaction to be of much importance. For the reactions that do occur, bubbling HX into an alcohol solution yields a haloalkane or alkyl halide. Below the reaction shows, tert-butanol and hydrochloric acid reacting to yield t-butyl chloride and water.


    A tertiary alcohol reacts with hydrochloric acid to produce a tertiary haloalkene and water. .svg


    Secondary, tertiary, allylic, and benzylic alcohols appear to react by a mechanism that involves the formation of a carbocation, in an SN1 reaction with the protonated alcohol acting as a leaving group.

    The SN1 mechanism is illustrated by the reaction of tert-butyl alcohol and aqueous hydrochloric acid (H3O+, Cl-). The first two steps in this SN1 substitution mechanism are protonation of the alcohol to form an oxonium ion. Protonation of the alcohol converts a poor leaving group (OH-) to a good leaving group H2O which makes the dissociation step of the SN1 mechanism more favorable. In step 3, the carbocation reacts with a nucleophile (a halide ion) to complete the substitution.

    mechanism for reaction of tert-butyl alcohol with aqueous HCl to give tert-butyl chloride.svg

    Carbocation rearrangements are extremely common in organic chemistry reactions are are defined as the movement of a carbocation from an unstable state to a more stable state through the use of various structural reorganizational "shifts" within the molecule. Once the carbocation has shifted over to a different carbon, we can say that there is a structural isomer of the initial molecule.

    Not all acid-catalyzed conversions of alcohols to alkyl halides proceed through the formation of carbocations. Primary alcohols and methanol react to form alkyl halides under acidic conditions by an SN2 mechanism. In these reactions the function of the acid is to produce a protonated alcohol. The halide ion then displaces a molecule of water (a good leaving group) from carbon; this produces an alkyl halide:

    halide anion reacts with a protonated primary alcohol OR methanol to give a primary alkyl halide and water (a good leaving group).svg

    Again, acid is required. Although halide ions (particularly iodide and bromide ions) are strong nucleophiles, they are not strong enough to carry out substitution reactions with alcohols that are not activated (converted to a better leaving group). Direct displacement of the hydroxyl group does not occur because the leaving group would have to be a strongly basic hydroxide ion.

    the SN2 reaction of bromide with an unactivated alcohol does not occur to produce an alkyl bromide and hydroxide, because hydroxide is a bad leaving group..svg

    Conversion of Alcohols into Alkyl Halides Using SOCl2 or PBr3

    The most common methods for converting 1º- and 2º-alcohols to the corresponding chloro and bromo alkanes (i.e. replacement of the hydroxyl group) are treatments with thionyl chloride (SOCl2) and phosphorus tribromide (PBr3), respectively. These reagents are generally preferred over the use of concentrated HX due to the harsh acidity of these hydrohalic acids and the carbocation rearrangements associated with their use.

    General Reaction

    a primary or secondary alcohol reacts with thionyl chloride to produce an alkyl chloride, sulfur dioxide, and hydrochloric acid.svg

    a primary or secondary alcohol reacts with phosphorus tribromide to give an alkyl promide and HOPBr2.svg


    cyclohexanol reacts with thionyl chloride to give chlorocyclohexane.svg

    (R)-2-pentanol reacts with phosphorus tribromide to give (S)-2-bromopentane.svg


    Both of these reagents form an alkyl halide through an SN2 mechanism. The mechanism for both reactions starts by converting the hydroxide of the alcohol into a better leaving group through formation of an intermediate. Thionyl chloride creates an intermediate chlorosulfite (-OSOCl2) compound and phosphorus tribromide makes an intermediate dibromophosphite (-OPBr2) compound. These intermediate compounds can subsequently be eliminated as a leaving group during an SN2 reaction with the corresponding nucleophilic halide ion. Since these reactions proceeds through a backside attack, there is inversion of configuration at the carbon

    Thionyl chloride

    Notice that during the reaction with thonyl chloride hydrochloric acid (HCl) and sulfur dioxide (SO2) are produced as byproducts.

    mechanism for reaction of (S)-2-pentanol with thionyl chloride to give (R)-2-chloropentane, sulfur dioxide, and hydrochloric acid.svg

    Phosphorus tribromide

    During this reaction HOPBr2 is made as a biproduct.

    (R)-2-butanol reacts with phosphorus tribromide to give (S)-2-bromobutane and HOPBr2.svg

    Stereochemical Considerations

    The SN2 reaction with the corresponding nucleophilic halide ion contained in the mechanism of both reactions causes an inversion of configuration at the carbon atom.

    (S)-2-pentanol reacts with thionyl chloride to give (R)-2-chloropentane.svg

    (R)-2-pentanol reacts with phosphorus tribromide to give (S)-2-bromopentane_2.svg

    Exercise \(\PageIndex{1}\)

    1) Predict the alcohol required for the synthesis of the following halides:

    a. 2-chloro-3-methylbutane, b. 4-bromo-3,5-dimethylheptane, c. (3-iodobutyl)benzene.svg



    a. 3-methylbutan-2-ol, b. 3,5-dimethylheptan-4-ol, c. 4-phenylbutan-2-ol.svg

    10.5: Preparing Alkyl Halides from Alcohols is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Jim Clark, Steven Farmer, Dietmar Kennepohl, Layne Morsch, Tim Soderberg, & Tim Soderberg.