Question

why boric acid is a nonprotic acid?

Answer 1

Boric acid, also called hydrogen borate, boracic acid, orthoboric acid and acidum boricum, is a weak, monobasic Lewis acid of boron often used as an antiseptic, insecticide, flame retardant, neutron absorber, or precursor to other chemical compounds. It has the chemical formula H3BO3 (sometimes written B(OH)3), and exists in the form of colorless crystals or a white powder that dissolves in water. When occurring as a mineral, it is called sassolite.

Boric acid, or sassolite, is found mainly in its free state in some volcanic districts, for example, in the Italian region of Tuscany, the Lipari Islands and the US state of Nevada. In these volcanic settings it issues, mixed with steam, from fissures in the ground. It is also found as a constituent of many naturally occurring minerals – borax, boracite, ulexite (boronatrocalcite) and colemanite. Boric acid and its salts are found in seawater. It is also found in plants, including almost all fruits.[1]

Boric acid was first prepared by Wilhelm Homberg (1652–1715) from borax, by the action of mineral acids, and was given the name sal sedativum Hombergi ("sedative salt of Homberg"). However borates, including boric acid, have been used since the time of the Greeks for cleaning, preserving food, and other activities.

Answer 2

Boric acid is an acid because it is a Lewis acid; that is, it is an electron-pair acceptor. There is an empty p-orbital on the boron atom that longs for fulfillment. That said, one way that the boron atom can gain electrons is to enjoy a double bond with one of its oxygens, requiring one of the O atoms to release a H. The electron affinity of the "hungry" boron "weakens" the O-H bond and enables a proton to be removed by a Arrhenius base (see, we're mixing definitions here, which stinks). Now you're left with a molecule where the central boron is doubly bonded to one oxygen and singly bonded to two hydroxyl groups. The boron bears a formal negative charge but has a completed octet. Now the molecule has no atoms which violate the octet rule. The negatively charged boron atom repels the electronegative oxygen atoms, lengthening the B-O bond and resulting in a corresponding strengthening of the O-H bond. Because the O-H bond is stronger in the deprotonated form, removing another proton becomes very difficult. Another reason that a second deprotonation is nearly impossible is that it would result in adjacent atoms (an O and a B) bearing a formal negative charge; the coulombic penalty is too great to be feasable for this.

In summary, it is possible to practically remove one and only one proton from boric acid because the boron atom is electron-deficient and to fill its octet bears the resulting anionic charge. Subsequent removal of protons results in a strong electrostatic repulsion and also has to overcome a much stronger bond enthalpy of O-H than the first deprotonation.