An alcohol is an organic molecule characterized by a hydroxyl group (—OH) attached to a carbon chain (R—OH). Dehydration is a chemical process involving the removal of a water molecule (\(\text{H}_2\text{O}\)) from a reactant. When dehydration is applied to an alcohol, two entirely different types of molecules can result. The specific structure that forms is determined by the conditions of the reaction, particularly the temperature applied during the process.
The Process of Alcohol Dehydration
The chemical removal of water from an alcohol requires a strong acid catalyst, such as concentrated sulfuric acid or phosphoric acid. The first step involves the alcohol’s oxygen atom bonding with a hydrogen ion (\(\text{H}^+\)) from the acid, creating a positively charged intermediate known as an alkyloxonium ion. This protonation is necessary because the original hydroxyl group is a poor leaving group. By turning the hydroxyl group into a water molecule, the molecule transforms the poor leaving group into an excellent one. The subsequent reaction involves the newly formed water molecule detaching from the carbon chain, which is the common first step for both possible resulting molecule types.
Resulting Molecule Type 1: Unsaturated Hydrocarbons
When the acid-catalyzed dehydration is carried out at high temperatures, typically above \(140^\circ\text{C}\) to \(150^\circ\text{C}\), the major product formed is an unsaturated hydrocarbon, also known as an alkene. This outcome is favored because the increased thermal energy promotes an elimination reaction, where a molecule of water is removed from a single alcohol molecule. Primary alcohols, such as ethanol, require significantly higher temperatures, often reaching \(170^\circ\text{C}\) to \(180^\circ\text{C}\).
The molecule becomes an unsaturated hydrocarbon because the hydroxyl group is removed from one carbon atom, and a hydrogen atom is removed from an adjacent carbon atom. The carbon atoms then compensate for the missing bonds by forming a new carbon-carbon double bond.
The structure of the original alcohol influences the temperature needed, with secondary and tertiary alcohols requiring milder heat because their intermediate structures are more stable. If the carbon chain has multiple adjacent carbon atoms, the elimination reaction tends to favor the formation of the most substituted double bond, meaning the hydrogen is preferentially removed from the carbon atom bonded to the fewest other hydrogen atoms.
Resulting Molecule Type 2: Organic Oxides
The second type of molecule that can form from alcohol dehydration is an organic oxide, or ether, which is favored under milder temperature conditions, typically maintained below \(140^\circ\text{C}\). This reaction involves two separate alcohol molecules joining together while releasing a water molecule, known as a condensation reaction.
In this lower-temperature environment, one alcohol molecule is first protonated by the acid catalyst. Instead of the water molecule detaching immediately, a second, unreacted alcohol molecule acts as a nucleophile, attacking the carbon atom of the protonated alcohol. This attack displaces the water group, allowing the two alcohol carbon chains to link together via a single oxygen atom.
The resulting structure is represented by the general formula R—O—R’, where the oxygen atom acts as a bridge between the two carbon chains. This pathway is most effective when using primary alcohols.
Structural Differences Between the Products
The two resulting molecule types, unsaturated hydrocarbons and organic oxides, have distinct structural features that dictate their chemical behavior. Unsaturated hydrocarbons are characterized by the presence of at least one carbon-carbon double bond, which makes them highly reactive and prone to addition reactions.
In contrast, organic oxides are defined by the oxygen atom positioned between two carbon chains, forming a C—O—C linkage. The oxygen atom acts as a bridge, giving the molecule a bent geometry and a slight polarity. This structure results in molecules that are generally stable and chemically inert, making many ethers excellent solvents for organic compounds.