REACTIONS OF ALCOHOLS

Reactions of Alcohols

Alcohols are capable of being converted to metal salts, alkyl halides, esters, aldehydes, ketones, and carboxylic acids.

Alcohols are only slightly weaker acids than water, with a K _a value of approximately 1 × 10 ⁻¹⁶. The reaction of ethanol with sodium metal (a base) produces sodium ethoxide and hydrogen gas.

This reaction is identical to the reaction of sodium metal with water.

However, the latter reaction occurs faster because of the increased acidity of water (K _a value of 1 × 10 ⁻¹⁵). Likewise, similar reactions occur with potassium metal.

The acidity of alcohols decreases while going from primary to secondary to tertiary. This decrease in acidity is due to two factors: an increase of electron density on the oxygen atom of the more highly‐substituted alcohol, and steric hindrance (because of the alkyl groups, which inhibit solvation of the resulting alkoxide ion). Both of these situations increase the activation energy for proton removal.

The basicity of alkoxide ions increases while going from primary to tertiary. This increase in basicity occurs because the conjugate base of a weak acid is strong. The weaker the acid, the stronger the conjugate base.

Alkyl halide formation

Alcohols are converted to alkyl halides by S _N1 and S _N2 reactions with halogen acids.

Primary alcohols favor S _N2 substitutions while S _N1 substitutions occur mainly with tertiary alcohols.

A more efficient method of preparing alkyl halides from alcohols involves reactions with thionyl chloride (SOCl ₂).

This reaction is rapid and produces few side reaction products. In addition, the sulfur dioxide and hydrogen chloride formed as byproducts are gasses and therefore easily removed from the reaction. Mechanistically, the alcohol initially reacts to form an inorganic ester.

The chloride ion produced by this reaction, acting as a nucleophile, attacks the ester in an S _N2 fashion to yield molecules of sulfur dioxide, hydrogen chloride, and an alkyl halide.

Because the reaction proceeds mainly by an S _N2 mechanism, the alkyl halide produced from an optically active alcohol will have the opposite relative configuration from the alcohol from which it was formed.

Because thionyl bromide is relatively unstable, alkyl bromides are normally prepared by reacting the alcohol with phosphorous tribromide (PBr ₃).

This reaction proceeds via a two‐step mechanism. In the first step, the alcohol reacts with the phosphorous tribromide.

The second step is an S _N1 or S _N2 substitution in which the bromide ion displaces the dibromophosphorous group.

In a similar manner, alkyl iodides are prepared by reacting an alcohol with phosphorous triiodide.

Ester formation

Esters are compounds that are commonly formed by the reaction of oxygen‐containing acids with alcohols. The ester functional group is the

  

Alcohols can be converted to esters by means of the Fischer Esterification Process. In this method, an alcohol is reacted with a carboxylic acid in the presence of an inorganic acid catalyst.

Because the reaction is an equilibrium reaction, in order to receive a good yield, one of the products must be removed as it forms. Doing this drives the equilibrium to the product side.

The mechanism for this type of reaction takes place in seven steps:

1. The mechanism begins with the protonation of the acetic acid.

2. The π electrons of the carboxyl group, , migrate to pick up the positive charge.

3. The oxygen of the alcohol molecule attacks the carbocation.

4. The oxonium ion that forms loses a proton.

5. One of the hydroxyl groups is protonated to form an oxonium ion.

6. An unshared pair of electrons on another hydroxy group reestablishes the carbonyl group, with the loss of a water molecule.

7. The oxonium ion loses a proton, which leads to the production of the ester.

Alkyl sulfonate formation. Alcohols may be converted to alkyl sulfonates, which are sulfonic acid esters. These esters are formed by reacting an alcohol with an appropriate sulfonic acid. For example, methyl tosylate, a typical sulfonate, is formed by reacting methyl alcohol with tosyl chloride.

Other sulfonyl halides that form alkyl sulfonates include:

These groups are much better leaving groups than the hydroxy group because they are resonance stabilized. Alcohol molecules that are going to be reacted by S _N1 or S _N2 mechanisms are often first converted to their sulfonate esters to improve both the rate and yield of the reactions.

Formation of aldehydes and ketones. The oxidation of alcohols can lead to the formation of aldehydes and ketones. Aldehydes are formed from primary alcohols, while ketones are formed from secondary alcohols.

Because you can easily further oxidize aldehydes to carboxylic acids, you can only employ mild oxidizing agents and conditions in the formation of aldehydes. Typical mild oxidizing agents include manganese dioxide (MnO ₂), Sarett‐Collins reagent (CrO ₃—(C ₅H ₅N) ₂), and pyridinium chlorochromate (PCC),

Following are several examples of the oxidation of primary alcohols:

Because ketones are more resistant to further oxidation than aldehydes, you may employ stronger oxidizing agents and higher temperatures. Secondary alcohols are normally converted to ketones by reaction with potassium dichromate (K ₂Cr ₂O ₇), potassium permanganate (KMnO ₄), or chromium trioxide in acetic acid (CrO ₃/CH ₃COOH). Following are several examples of the oxidation of secondary alcohols:

Carboxylic acid formation. Upon oxidation with strong oxidizing agents and high temperatures, primary alcohols completely oxidize to form carboxylic acids. The common oxidizing agents used for these conversions are concentrated potassium permanganate or concentrated potassium dichromate. Following are several examples of this type of oxidation: