Structure and Reactivity of Alcohols

Structure and Reactivity of Alcohols: Acidity, Reactions, and Activating the Hydroxyl Group for Substitution

Alcohols, characterized by the presence of a hydroxyl (-OH) group, share some striking similarities with water (H-OH), yet their reactivity differs significantly due to the presence of the alkyl (R) group. This section will explore the acidity of alcohols, their reactions, and a crucial concept in organic chemistry: activating the hydroxyl group for substitution reactions, drawing parallels with water where appropriate.

Acidity of Alcohols

Like water, alcohols possess a slightly acidic character due to the polar O-H bond. The hydrogen atom of the hydroxyl group can be donated to a strong base, forming an alkoxide ion (RO⁻) analogous to the hydroxide ion (HO⁻) formed when water acts as an acid. However, alcohols are generally weaker acids than water. The alkyl group (R) attached to the oxygen in an alcohol influences its acidity. Electron-donating groups (like alkyl groups themselves) decrease acidity by destabilizing the negative charge on the resulting alkoxide ion, while electron-withdrawing groups increase acidity.

The acidity of alcohols allows them to react with strong bases to form alkoxides. Alkoxides are important reagents in organic synthesis, serving as strong nucleophiles and bases.

Reactions of Alcohols

Alcohols participate in a variety of reactions, including:

• Reactions with Acids: Alcohols react with strong acids (like sulfuric acid) to form ethers via a dehydration reaction. This reaction involves the protonation of the hydroxyl group, followed by the loss of water and subsequent attack by another alcohol molecule.
• Oxidation: Alcohols can be oxidized to aldehydes, ketones, or carboxylic acids, depending on the type of alcohol (primary, secondary, or tertiary) and the oxidizing agent used.
  
• Esterification: Alcohols react with carboxylic acids (or acid derivatives like acid chlorides or anhydrides) to form esters. This is a condensation reaction catalyzed by acid we will discuss later.

Activating the Hydroxyl Group for Substitution

A crucial point to understand is that the hydroxyl group itself is a poor leaving group in substitution reactions. Direct displacement of the -OH group by a nucleophile is generally unfavorable. This is because the hydroxide ion (HO-) is a strong base and a poor leaving group. Therefore, to carry out substitution reactions at the carbon bearing the hydroxyl group, we must first activate the -OH group by converting it into a better leaving group.  

This activation typically involves transforming the hydroxyl group into a species that is more stable upon departure and therefore facilitates nucleophilic attack. Several methods exist for achieving this, and we will explore some important examples shortly. These methods essentially involve converting the -OH group into a leaving group that is more readily displaced.  

Two common reagents used for this activation, which you will encounter frequently, are thionyl chloride (SOCl₂) and phosphorus tribromide (PBr₃). 1 These reagents react with alcohols to form alkyl chlorides and alkyl bromides, respectively. 2 The mechanisms of these reactions involve converting the hydroxyl group into a much better leaving group, thereby enabling subsequent nucleophilic substitution by the halide ion (chloride or bromide). 3 Understanding these mechanisms is key to mastering alcohol reactivity. We will delve into the detailed mechanisms of these reactions in the following sections.