Because of its enhanced acidity, the hydrogen atom on the hydroxyl group is rather easily replaced by other substituents. A simple example is the facile reaction of simple alcohols with sodium (and sodium hydride), as described in the first equation below. Another such substitution reaction is the isotopic exchange that occurs on mixing an alcohol with deuterium oxide (heavy water). This exchange, which is catalyzed by acid or base, is very fast under normal conditions, since it is difficult to avoid traces of such catalysts in most experimental systems.
|2 R–O–H + 2 Na 2 R–O(–)Na(+) + H2|
|R–O–H + D2O R–O–D + D–O–H|
The mechanism by which many substitution reactions of this kind take place is straightforward. The oxygen atom of an alcohol is nucleophilic and is therefore prone to attack by electrophiles. The resulting "onium" intermediate then loses a proton to a base, giving the substitution product. If a strong electrophile is not present, the nucleophilicity of the oxygen may be enhanced by conversion to its conjugate base (an alkoxide). This powerful nucleophile then attacks the weak electrophile. These two variations of the substitution mechanism are illustrated in the following diagram.
The preparation of tert-butyl hypochlorite from tert-butyl alcohol is an example of electrophilic halogenation of oxygen, but this reaction is restricted to 3º-alcohols because 1º and 2º-hypochlorites lose HCl to give aldehydes and ketones. In the following equation the electrophile may be regarded as Cl(+).
(CH3)3C–O–H + Cl2 + NaOH (CH3)3C–O–Cl + NaCl + H2O
Alkyl substitution of the hydroxyl group leads to ethers. This reaction provides examples of both strong electrophilic substitution (first equation below), and weak electrophilic substitution (second equation). The latter SN2 reaction is known as the Williamson Ether Synthesis, and is generally used only with 1º-alkyl halide reactants because the strong alkoxide base leads to E2 elimination of 2º and 3º-alkyl halides.
One of the most important substitution reactions at oxygen is ester formation resulting from the reaction of alcohols with electrophilic derivatives of carboxylic and sulfonic acids. The following illustration displays the general formulas of these reagents and their ester products, in which the R'–O– group represents the alcohol moiety. The electrophilic atom in the acid chlorides and anhydrides is colored red. Examples of specific esterification reactions may be selected from the menu below the diagram, and will be displayed in the same space.
Some examples of alcohol substitution reactions using this approach to activating the hydroxyl group are shown in the following diagram. The first two cases serve to reinforce the fact that sulfonate ester derivatives of alcohols may replace alkyl halides in a variety of SN2 reactions. The next two cases demonstrate the use of phosphorus tribromide in converting alcohols to bromides. This reagent may be used without added base (e.g. pyridine), because the phosphorous acid product is a weaker acid than HBr. Phosphorous tribromide is best used with 1º-alcohols, since 2º-alcohols often give rearrangement by-products resulting from competing SN1 reactions. Note that the ether oxygen in reaction 4 is not affected by this reagent; whereas, the alternative synthesis using concentrated HBr cleaves ethers. Phosphorus trichloride (PCl3) converts alcohols to alkyl chlorides in a similar manner, but thionyl chloride is usually preferred for this transformation since the inorganic products are gases (SO2 & HCl). Phosphorus triiodide is not stable, but may be generated in situ from a mixture of red phosphorus and iodine, and acts to convert alcohols to alkyl iodides. The last example shows the reaction of thionyl chloride with a chiral 2º-alcohol. The presence of an organic base such as pyridine is important, because it provides a substantial concentration of chloride ion needed for the final SN2 reaction of the chlorosufite intermediate. In the absence of base chlorosufites decompose on heating to give the expected alkyl chloride with retention of configuration
Tertiary alcohols are not commonly used for substitution reactions of the kind discussed here, because SN1 and E1 reaction paths are dominant and are difficult to control. This aspect of alcohol chemistry will be touched upon in the next section.
The importance of sulfonate esters as intermediates in many substitution reactions cannot be overstated. A rigorous proof of the configurational inversion that occurs at the substitution site in SN2 reactions makes use of such reactions. An example of such a proof will display above when the An Inversion Proof button beneath the diagram is pressed. Abbreviations for the more commonly used sulfonyl derivatives are given in the following table.
|Name & Abbrev.||Mesyl or Ms||Tosyl or Ts||Brosyl or Bs||Trifyl or Tf|
For a more complete discussion of hydroxyl substitution reactions, and a description of other selective methods for this transformation Click Here.