Alcohols

Physical Properties, Uses, and Acidity

Alcohols exhibit both dipole-dipole forces and hydrogen bonding. Phenols are significantly more acidic than alcohols because of resonance.

An alcohol contains an OH{-}{\rm{OH}} group (a hydroxyl group) attached to a carbon chain. The hydroxyl group generally gives the molecule its characteristics, and the alkyl group modifies those properties. A hydrogen bond is a weak intermolecular bond between a hydrogen atom of one molecule and a highly electronegative atom (usually fluorine, oxygen, or nitrogen) of a nearby molecule. An intermolecular force is an attractive or repulsive force between a molecule and a nearby molecule, atom, or ion. Intermolecular forces between atoms affect how they interact. Oxygen is highly electronegative, and this makes the hydroxyl group highly polar. The hydroxyl group that characterizes alcohols makes these compounds capable of hydrogen bonding to other alcohol molecules, neutral molecules, and anions.

Intermolecular forces include forces of repulsion and attraction. Since alcohols have hydrogen bonds and dipole-dipole forces, they have larger intermolecular forces than their corresponding hydrocarbon. Alcohols are typically liquids because of the hydrogen bonds formed between molecules. As the number of carbon atoms increases in an alcohol, its boiling point increases. Alcohols are often used as solvents because of their increased propensity to form hydrogen bonds. Alcohols with fewer than five carbons are soluble in water. Alcohols with six or more carbons are generally insoluble in water because of the nonpolar alkyl group. Smaller alcohols are less dense than water.

Alcohols are common in nature, medicine, and industry. Alcohols are often used in perfumes and as a preservative in pharmaceuticals. Alcohols come from a wide range of sources and are used in many applications.

Methanol (CH3OH), also known as "wood alcohol," is used as a fuel and, in a combustion reaction, produces carbon dioxide and water. Methanol is an industrial chemical used to produce other chemicals and a wide range of products. Methanol derives from coal, biomass, solid wastes, and biogas. Ethanol (C2H5OH) is the alcohol found in alcoholic beverages, and it is formed as a product of sugar fermentation by yeast. Ethanol is used as an additive in some fuels, such as gasoline, and as starting material for the production of ether. Isopropyl alcohol (C3H7OH), commonly referred to as "rubbing alcohol," is often found in the household and is used as a disinfectant.

A useful quantity for describing the strength of an acid is pKa, which is the negative log of the acid dissociation constant, Ka:
pKa=logKaKa=[H+][A][HA]\begin{aligned}{\rm{p}}K_{\rm{a}}=&-{\rm{log}}K_{\rm{a}}\\K_{\rm{a}}=\;&\frac{[\rm{H}^+][\rm{A}^-]}{\rm{[HA]}}\end{aligned}
In this equation, HA is an acid and A is its conjugate base. A lower pKa indicates a higher acid strength. Brønsted-Lowry acids are compounds that donate a hydrogen ion, also called a proton or H+. Alcohols are weak acids, which have a pKa around 15–20. For example, ethanol has a pKa of 15.9, which is about as acidic as water. Alcohols donate the proton from the hydroxyl group and leave a negatively charged ion, called an alkoxide, on the original molecule.
ROHRO+H+{\rm {R{-}OH}}\rightarrow{\rm R{-} O}^-+{\rm {H}}^+
However, alcohols can accept a proton from strong acids such hydrogen sulfate or hydrogen chloride, forming a protonated alcohol.
ROH+H2SO4ROH2++HSO4{\rm {R{-}OH}}+{\rm {H_2SO}}_4\rightarrow{\rm {ROH}_2}^{+}+{\rm{HSO}_4}^{-}
Phenol (C6H5OH) has a highly stable aromatic benzene ring with a hydroxyl group attached. Aromaticity is a property of select fully conjugated cycloalkenes, which are cycloalkanes with alternating single and double bonds. Aromaticity enhances the stability of a molecule because of the ability of the electrons in the π\pi molecular bonding orbitals to delocalize and produce resonance structures. Therefore, phenol is a much stronger acid than an alcohol such as cyclohexanol, because of the stabilization the resonance structure gives to phenol's conjugate base. The negative charge on phenol's conjugate base is delocalized into the ring structure, which increases its stability. Any component that increases the stability of the conjugate base increases the acidity of those compounds. Consequently, phenols are much more acidic because of the resonance structures that are drawn for the conjugate base and have a pKa around 10.

Phenol has many of the same physical properties as alcohols. However, phenol contain a six-carbon benzene ring and is only moderately soluble in water, so it is not often used as a solvent. Phenol is used in low concentrations as an antiseptic, is used to create epoxies for paint, sunscreens, and cosmetics, and was once used as an antiseptic in surgery.