Alcohol

Introduction 

Nomenclature

In the IUPAC system of nomenclature, functional groups are normally designated in one of two ways. The presence of the function may be indicated by a characteristic suffix and a location number. This is common for the carbon-carbon double and triple bonds which have the respective suffixes ene and yne. Halogens, on the other hand, do not have a suffix and are named as substituents, for example: (CH₃)₂C=CHCHClCH₃ is 4-chloro-2-methyl-2-pentene. If you are uncertain about the IUPAC rules for nomenclature you should review them now.
Alcohols are usually named by the first procedure and are designated by an ol suffix, as in ethanol, CH₃CH₂OH (note that a locator number is not needed on a two-carbon chain). On longer chains the location of the hydroxyl group determines chain numbering. For example: (CH₃)₂C=CHCH(OH)CH₃ is 4-methyl-3-penten-2-ol. Other examples of IUPAC nomenclature are shown below, together with the common names often used for some of the simpler compounds. For the monofunctional alcohols, this common system consists of naming the alkyl group followed by the word alcohol. Alcohols may also be classified as primary, 1º, secondary, 2º & tertiary, 3º, in the same manner as alkyl halides. This terminology refers to alkyl substitution of the carbon atom bearing the hydroxyl group (colored blue in the illustration).

Many functional groups have a characteristic suffix designator, and only one such suffix (other than "ene" and "yne") may be used in a name. When the hydroxyl functional group is present together with a function of higher nomenclature priority, it must be cited and located by the prefix hydroxy and an appropriate number. For example, lactic acid has the IUPAC name 2-hydroxypropanoic acid.

Compounds incorporating a C–S–H functional group are named thiols or mercaptans. The IUPAC name of (CH₃)₃C–SH is 2-methyl-2-propanethiol, commonly called tert-butyl mercaptan. The chemistry of thiols will not be described here, other than to note that they are stronger acids and more powerful nucleophiles than alcohols.

Back to the to

Preparation of Alcohols

can be prepared in a number of ways including the following: 1)Grignard reaction. 2) Reduction of an aldehyde, ketone, or carboxylic acid with the appropriate reducing agent.

3) Substitution reaction of hydroxide or water on the appropriate alkyl halide. 4) Addition reactions to alkenes (covered in the alkene section).

An alcohol preparation process is disclosed by catalytic hydrogenation of the corresponding carbonyl compounds at high temperature and pressure in liquid phase. A catalyst is used that contains copper on a SiO2-containing substrate in the presence or absence of one or several elements selected among magnesium, barium, zinc or chromium.

Back to the to

Reactions of Alcohols

1. Substitution of the Hydroxyl Hydrogen

Alcohols can behave as weak acids, undergoing deprotonation. The deprotonation reaction to produce an alkoxide salt is either performed with a strong base such as sodium hydride or n-butyllithium, or with sodium or potassium metal.

2 R-OH + 2 NaH → 2 R-O^-Na⁺ + 2H₂↑

2 R-OH + 2Na → 2R-O⁻Na + H₂

E.g. 2 CH₃CH₂-OH + 2 Na → 2 CH₃-CH₂-O⁻Na + H₂

Water is similar in pK_a to many alcohols, so with sodium hydroxide there is an equilibrium set up which usually lies to the left:

R-OH + NaOH <=> R-O^-Na⁺ + H₂O (equilibrium to the left)

It should be noted, though, that the bases used to deprotonate alcohols are strong themselves. The bases used and the alkoxides created are both highly moisture sensitive chemical reagents.

The acidity of alcohols is also affected by the overall stability of the alkoxide ion. Electron-withdrawing groups attached to the carbon containing the hydroxyl group will serve to stabilize the alkoxide when formed, thus resulting in greater acidity. On the other hand, the presence of electron-donating group will result in a less stable alkoxide ion formed. This will result in a scenario whereby the unstable alkoxide ion formed will tend to accept a proton to reform the original alcohol.

With alkyl halides alkoxides give rise to ethers in the Williamson ether synthesis.

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.

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 S_N2 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.

Esterification

To form an ester from an alcohol and a carboxylic acid the reaction, known as Fischer esterification, is usually performed at reflux with a catalyst of concentrated sulfuric acid:

R-OH + R'-COOH → R'-COOR + H₂O

In order to drive the equilibrium to the right and produce a good yield of ester, water is usually removed, either by an excess of H₂SO₄ or by using a Dean-Stark apparatus. Esters may also be prepared by reaction of the alcohol with an acid chloride in the presence of a base such as pyridine.

Other types of ester are prepared similarly- for example tosyl (tosylate) esters are made by reaction of the alcohol with p-toluenesulfonyl chloride in pyridine.

2. Nucleophilic Substitution of the Hydroxyl Group

The OH group is not a good leaving group in nucleophilic substitution reactions, so neutral alcohols do not react in such reactions. However, if the oxygen is first protonated to give R−OH₂⁺, the leaving group (water) is much more stable, and the nucleophilic substitution can take place. For instance, tertiary alcohols react with hydrochloric acid to produce tertiary alkyl halides, where the hydroxyl group is replaced by a chlorine atom by unimolecular nucleophilic substitution. If primary or secondary alcohols are to be reacted with hydrochloric acid, an activator such as zinc chloride is needed. Alternatively the conversion may be performed directly using thionyl chloride.

Alcohols may likewise be converted to alkyl bromides using hydrobromic acid or phosphorus tribromide, for example:

3 R-OH + PBr₃ → 3 RBr + H₃PO₃

In the Barton-McCombie deoxygenation an alcohol is deoxygenated to an alkane with tributyltin hydride or a trimethylborane-water complex in a radical substitution reaction.

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 a S_N2 mechanism, whereas 3º-alcohols react by a 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.

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 bromine, 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.

 Back to the to

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.

R–O–H   +   Na⁽⁺⁾ OH^(–)     R–O^(–) Na⁽⁺⁾   +   H–OH

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 reversable 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.

More useful is the E1 elimination reaction of alcohols to produce alkenes. The reaction generally obeys Zaitsev's Rule, which states that the most stable (usually the most substituted) alkene is formed. Tertiary alcohols eliminate easily at just above room temperature, but primary alcohols require a higher temperature.

This is a diagram of acid catalysed dehydration of ethanol to produce ethene:

A more controlled elimination reaction is the Chugaev elimination with carbon disulfide and iodomethane.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.

The E2 elimination of 3º-alcohols under relatvely 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.

Back to the to

---

© M.EL-Fellah ,Chemistry Department, Garyounis University