Phenols, Alcohols and Carboxylic Acids - pKa Values

Definitions of the acid dissociation constant and pKa are given below the figures, together with the definition of some classes of organic acids .

In the table below, pK_a1 and pK_a2 for water solutions at 25°C are given together with boiling and melting point, density and molecular weight , as well as number of carbon, hydrogen and oxygen atoms in each molecule.

Below the table, figures showing the fractions of the different acid forms in aqueous solutions at varying pH are given for some common diprotic organic acids (values calculated from the tabulated pK_a s).

See also boiling and melting point of hydrocarbons, alcohols and acids , pKa of amines, diamines and cyclic organic nitrogen compounds and pKa of inorganic acids and bases , as well as acid base indicators , Acid-base properties of aqueous solutions of salts with ions from both acids and bases and Buffer solutions

For full table with Common Names, Boiling temperatures and Densities - rotate the screen!

An acid dissociation constant, K_a , is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation of acid–base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

HA + H₂O   =  A ^- +H₃ O ⁺

where HA is an acid that dissociates into A ^- , (known as the conjugate base of the acid) and a hydrogen ion which combines with a water molecule to make a hydronium ion.

The chemical species HA, A ^- and H₃ O ⁺ are said to be in equilibrium when their concentrations do not change with the passing of time. The dissociation constant is usually written as a quotient of the equilibrium concentrations (in mol/L), denoted by [HA], [A ^- ] and [H₃ O ⁺ ]

K_a = [A ^- ]*[H₃ O ⁺ ] / [HA]*[H₂O]

In all, but the most concentrated, aqueous solutions of an acid the concentration of water can be taken as constant and can be ignored. The definition can then be written more simply

HA = A ^- + H ⁺ and           K_a = [A ^- ]*[H ⁺ ] / [HA]

This is the definition in common usage. For many practical purposes it is more convenient to discuss the logarithmic constant, pK_a

pK_a = -log₁₀ K_a

The larger the value of pK_a , the smaller the extent of dissociation at any given pH - that is, the weaker the acid.

A weak acid has a pK_a value in the approximate range -2 to 12 in water.

Strong acids has pK_a values of less than about -2; the dissociation of a strong acid is effectively complete such that concentration of the undissociated acid is too small to be measured. pK_a values for strong acids can, however, be estimated by theoretical means.

After rearranging the expression defining K_a , and putting pH = -log₁₀ [H ⁺ ], one obtains

pH = pK_a + [A ^- ] / [HA]  and further

pH - pK_a = log [A ^- ] / [HA]

Then, a solution with 50% dissociation has pH equal to the pK_a of the acid.

Polyprotic acids are acids that can lose more than one proton. Then we have more than one dissiciation constant; K_a1 , K_a2 , etc..  and similar pK_a1 , pK_a2 , etc.

All data given in the figures apply to dilute aqueous solutions at ambient temperature.

The figure below shows the fractions of the different acid forms (H₂A, HA ^- and A ^2- for twoprotic acids) in aqueous solutions at varying pH :

Definition of organic compounds

Hydrocarbon: An organic compound consisting entirely of hydrogen and carbon.

Alkane: An acyclic saturated hydrocarbon, with the general formula C_n H_2n+2 . Also called paraffin.

Aromatic hydrocarbon : A cyclic (ring-shaped), planar (flat) molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. The simplest of the aromatics have 6 carbon atoms and contains 3 double bounds.  A one ring aromatic without any substituents is called benzene, with the formula C6H6.

Alkyl: An alkyl group is an alkane substituent missing one hydrogen, with general formula C_n H_2n+1.

Phenyl: An phenyl group is a benzene substituent missing one hydrogen, with general formula C₆ H₅.

Alcohol: An organic compound in which the hydroxyl functional group (–OH) is bound to a saturated carbon atom.

Alkanol: An alcohol where the hydroxyl group is bound to an alkyl group, with the general formula C_n H_2n+1 OH.

Phenol: An alcohol where the hydroxyl group is bound to a phenyl group, with the formula C6H5OH.

Phenols : A group of compounds consisting of a phenol molecule with one or more substiuents.

Diol: An organic compound containing two hydroxyl groups, -OH.

Triol: An organic compound containing three hydroxyl groups, -OH.

Carboxylic acid: an organic compound that contains a carboxyl group (C(=O)OH). The general formula of a carboxylic acid is R–COOH, with R referring to the rest of the molecule.

Paraffinic acid = Alkanoic acid: A carboxylic acid where the R is an alkyl group.

Dioic acid: A carboxylic acid with two acid groups (-COOH).

Benzoic acid: A carboxylic acid where the acid group is substituted to one carbon of a benzene ring.

Hydroxy acid: An carboxylic acid containing an additional hydroxy group (-OH), substistuted to another carbon atom than the acid group.