Alcohols are polar molecules, a characteristic that defines their physical and chemical behavior. An alcohol is an organic compound characterized by a hydrocarbon structure (R group) to which a hydroxyl functional group (–OH) is attached. This –OH group introduces a significant and permanent charge separation into the molecule, distinguishing alcohols from non-polar hydrocarbons and explaining many of their properties.
Understanding Molecular Polarity
Molecular polarity arises from the uneven sharing of electrons between atoms within a chemical bond. This unequal distribution is caused by a property called electronegativity, which is an atom’s tendency to attract electrons toward itself. When atoms with different electronegativities bond, the shared electrons create a polar bond, resulting in partial negative (\(\delta-\)) and partial positive (\(\delta+\)) charges. The separation of these partial charges generates a net dipole moment.
For a molecule to be polar overall, it must contain polar bonds whose resulting dipoles do not cancel each other out due to molecular symmetry. The water molecule, for example, is highly polar because its bent shape prevents the dipoles from neutralizing each other.
The Role of the Hydroxyl Group
The hydroxyl group (-OH) is the specific structural feature responsible for alcohol’s polarity. The oxygen atom within this group is highly electronegative, significantly more so than the hydrogen atom and the carbon atom of the alkyl chain (R group). This difference causes the oxygen atom to strongly pull shared electrons toward itself, resulting in the oxygen acquiring a partial negative charge (\(\delta-\)), while the hydrogen atom attached to it carries a partial positive charge (\(\delta+\)).
This charge separation creates a strong, permanent dipole moment focused at the -OH group. This highly polarized bond allows alcohol molecules to participate in a special type of intermolecular attraction known as hydrogen bonding. Hydrogen bonds form when the partially positive hydrogen of one alcohol molecule is attracted to the partially negative oxygen of a neighboring molecule.
The Impact on Physical Properties
The polarity and hydrogen-bonding capability of the hydroxyl group affect the physical properties of alcohols. Alcohols have substantially higher boiling points compared to non-polar hydrocarbons of similar molecular weight. For example, ethanol (molecular weight 46) boils at \(78^\circ\text{C}\), while propane (molecular weight 44) boils at \(-42^\circ\text{C}\). More energy is required to overcome the strong intermolecular hydrogen bonds in alcohol than the weaker forces in propane.
Alcohol’s polarity also governs its solubility, following the principle that “like dissolves like.” The polar hydroxyl group is hydrophilic, meaning “water-loving,” and readily forms hydrogen bonds with water molecules. This strong interaction allows smaller alcohols, such as methanol and ethanol, to mix completely with water. Alcohols are also excellent solvents for other polar substances and some ionic compounds due to their ability to surround and stabilize charged species.
When Polarity Decreases
While the -OH group is polar, the alcohol molecule’s character is a balance between this polar head and the non-polar hydrocarbon tail. The alkyl chain (R group) is hydrophobic, meaning “water-fearing,” and contributes to weaker forces between molecules.
As the carbon chain length increases, the non-polar portion becomes proportionally larger and begins to dominate the molecule’s behavior. Consequently, the ability of the molecule to dissolve in water decreases rapidly. Alcohols with four or fewer carbon atoms are generally soluble in water, but those with five or more carbons, like pentanol, show reduced solubility. Past seven carbon atoms, the alcohol is essentially insoluble, as the size of the non-polar chain outweighs the effect of the single polar hydroxyl group.