Hydroxyl Group Substitution

2. Nucleophilic Substitution of the Hydroxyl Group

Using the chemical behavior of alkyl halides as a reference, we are encouraged to look for analogous substitution and elimination reactions of alcohols. The chief difference, of course, is a change in the leaving anion from halide to hydroxide. Since oxygen is slightly more electronegative than chlorine (3.5 vs. 2.8 on the Pauling scale), we expect the C-O bond to be more polar than a C-Cl bond. Furthermore, an independent measure of the electrophilic character of carbon atoms from their nmr chemical shifts (both ¹³C & alpha protons), indicates that oxygen and chlorine substituents exert a similar electron-withdrawing influence when bonded to sp³ hybridized carbon atoms. Despite this promising background evidence, alcohols do not undergo the same S_N2 reactions commonly observed with alkyl halides. For example, the rapid S_N2 reaction of 1-bromobutane with sodium cyanide, shown below, has no parallel when 1-butanol is treated with sodium cyanide. In fact ethyl alcohol is often used as a solvent for alkyl halide substitution reactions such as this.

The key factor here is the stability of the leaving anion (bromide vs. hydroxide). We know that HBr is a much stronger acid than water (by more than 18 powers of ten), and this difference will be reflected in reactions that generate their conjugate bases. The weaker base, bromide anion, is more stable and its release in a substitution or elimination reaction will be much more favorable than that of hydroxide ion, a stronger and less stable base.
Clearly, an obvious step toward improving the reactivity of alcohols in S_N2 reactions would be to modify the –OH functional group in a way that improves its stability as a leaving anion. One such modification is to conduct the substitution reaction in strong acid so that –OH is converted to –OH₂⁽⁺⁾. Since the hydronium ion (H₃O⁽⁺⁾) is a much stronger acid than water, its conjugate base (H₂O) is a better leaving group than hydroxide ion. The only problem with this strategy is that many nucleophiles, including cyanide, are deactivated by protonation in strong acid, effectively removing the nucleophilic co-reactant needed for the substitution. The strong acids HCl, HBr and HI are not subject to this difficulty because their conjugate bases are good nucleophiles and are even weaker bases than alcohols. The following equations illustrate some substitution reactions of alcohols that may be effected by these acids. As was true for alkyl halides, nucleophilic substitution of 1º-alcohols proceeds by an S_N2 mechanism, whereas 3º-alcohols react by an S_N1 mechanism. Reactions of 2º-alcohols may occur by both mechanisms and often produce some rearranged products. The numbers in parentheses next to the mineral acid formulas represent the weight percentage of a concentrated aqueous solution, the form in which these acids are normally used.

Although these reactions are sometimes referred to as "acid-catalyzed" this is not strictly correct. In the overall transformation a strong HX acid is converted to water, a very weak acid, so at least a stoichiometric quantity of HX is required for a complete conversion of alcohol to alkyl halide. The necessity of using equivalent quantities of very strong acids in this reaction limits its usefulness to simple alcohols of the kind shown above. Alcohols having acid sensitive groups would, of course, not tolerate such treatment. Nevertheless, the idea of modifying the -OH functional group to improve its stability as a leaving anion can be pursued in other directions. The following diagram shows some modifications that have proven effective. In each case the hydroxyl group is converted to an ester of a strong acid. The first two examples show the sulfonate esters described earlier. The third and fourth examples show the formation of a phosphite ester (X represents remaining bromines or additional alcohol substituents) and a chlorosulfite ester respectively. All of these leaving groups (colored blue) have conjugate acids that are much stronger than water (by 13 to 16 powers of ten) so the leaving anion is correspondingly more stable than hydroxide ion. The mesylate and tosylate compounds are particularly useful in that they may be used in substitution reactions with a wide variety of nucleophiles. The intermediates produced in reactions of alcohols with phosphorus tribromide and thionyl chloride (last two examples) are seldom isolated, and these reactions continue on to alkyl bromide and chloride products.

The importance of sulfonate ester intermediates in general nucleophilic substitution reactions of alcohols may be illustrated by the following conversion of 1-butanol to pentanenitrile (butyl cyanide), a reaction that does not occur with the alcohol alone (see above). The phosphorus and thionyl halides, on the other hand, only act to convert alcohols to the corresponding alkyl halides.

CH3CH2CH2CH2–OH + CH3SO2Cl

pyridine

CH3CH2CH2CH2–OSO2CH3

Na(+) CN(–)

CH3CH2CH2CH2–CN + CH3SO2O(–) Na(+)

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 S_N2 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 S_N1 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 S_N2 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 S_N1 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 S_N2 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.

For a more complete discussion of hydroxyl substitution reactions, and a description of other selective methods for this transformation Click Here.

Elimination Reactions

3. Elimination Reactions of Alcohols

In the discussion of alkyl halide reactions we noted that 2º and 3º-alkyl halides experienced rapid E2 elimination when treated with strong bases, such as hydroxide and alkoxides. Alcohols do not undergo such base-induced elimination reactions and are, in fact, often used as solvents for such reactions. This is yet another example of how leaving group stability often influences the rate of a reaction.
When an alcohol is treated with sodium hydroxide, the following acid-base equilibrium occurs. Most alcohols are slightly weaker acids than water so the left side is favored.

The elimination of water from an alcohol is called dehydration. Recalling that water is a much better leaving group than hydroxide ion, it is sensible to use acid-catalysis rather than base-catalysis to achieve such reactions. Four examples of this useful technique are shown below. Note that hydrohalic acids (HX) are not normally used as catalysts because their conjugate bases are good nucleophiles and may give substitution products. The conjugate bases of sulfuric and phosphoric acids are not good nucleophiles and do not give substitution under the usual conditions of their use.

The first two examples (top row) are typical, and the more facile elimination of the 3º-alcohol suggests predominant E1 character for the reaction. This agrees with the tendency of branched 1º and 2º-alcohols to give rearrangement products, as shown in the last example. The last two reactions also demonstrate that the Zaitsev Rule applies to alcohol dehydrations as well as alkyl halide eliminations. Thus the more highly-substituted double bond isomer is favored among the products.
It should be noted that the acid-catalyzed dehydrations discussed here are the reverse of the acid-catalyzed hydration reactions of alkenes. Indeed, for reversible reactions such as this the laws of thermodynamics require that the mechanism in both directions proceed by the same reaction path. This is known as the principle of microscopic reversibility. To illustrate, the following diagram lists the three steps in each transformation. The dehydration reaction is shown by the blue arrows; the hydration reaction by magenta arrows. The intermediates in these reactions are common to both, and common transition states are involved. This can be seen clearly in the energy diagrams depicted by clicking the button beneath the equations.

Base induced E2 eliminations of alcohols may be achieved if their sulfonate ester derivatives are used. This has the advantage of avoiding strong acids, which may cause molecular rearrangement and / or double bond migration in some cases. Since 3º-sulfonate derivatives are sometimes unstable, this procedure is best used with 1º and 2º-mesylates or tosylates. Application of this reaction sequence is shown here for 2-butanol. The Zaitsev Rule favors formation of 2-butene (cis + trans) over 1-butene.

CH3CH2CH(CH3)–OH

CH3SO2Cl

CH3CH2CH(CH3)–OSO2CH3

C2H5O(–)Na(+)

CH3CH=CHCH3 + CH3CH2CH=CH2 + CH3SO2O(–) Na(+) + C2H5OH

The E2 elimination of 3º-alcohols under relatively non-acidic conditions may be accomplished by treatment with phosphorous oxychloride (POCl₃) in pyridine. This procedure is also effective with hindered 2º-alcohols, but for unhindered and 1º-alcohols an S_N2 chloride ion substitution of the chlorophosphate intermediate competes with elimination. Some examples of these and related reactions are given in the following figure. The first equation shows the dehydration of a 3º-alcohol. The predominance of the non-Zaitsev product (less substituted double bond) is presumed due to steric hindrance of the methylene group hydrogens, which interferes with the approach of base at that site. The second example shows two elimination procedures applied to the same 2º-alcohol. The first uses the single step POCl₃ method, which works well in this case because S_N2 substitution is retarded by steric hindrance. The second method is another example in which an intermediate sulfonate ester confers halogen-like reactivity on an alcohol. In every case the anionic leaving group is the conjugate base of a strong acid.

Pyrolytic syn-Eliminations Ester derivatives of alcohols may undergo unimolecular syn-elimination on heating. To see examples of these Click Here

This page is the property of William Reusch.   Comments, questions and errors should be sent to whreusch@msu.edu.
These pages are provided to the IOCD to assist in capacity building in chemical education. 05/05/2013