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Brønsted-Lowry Acids and Bases

Brønsted-Lowry Acids and Bases

In 1923, the Brønsted–Lowry definition of acids and bases was proposed by Johannes Brønsted and Thomas Lowry. According to this theory, a Brønsted acid is defined as a species that donates a proton in a chemical reaction and gets converted to its conjugate base. A Brønsted base is defined as a species that accepts a proton in a chemical reaction and gets converted into its conjugate acid. These transfers of protons are caused by the displacement of electrons in these reactions, which is...

Bronsted-Lowry Acids and Bases

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The acid-base reaction class has been studied for quite some time. In 1680, Robert Boyle reported traits of acid solutions that included their ability to dissolve many substances, to change the colors of certain natural dyes, and to lose these traits after coming in contact with alkali (base) solutions. In the eighteenth century, it was recognized that acids have a sour taste, react with limestone to liberate a gaseous substance (now known to be CO2), and interact with alkalis to form neutral...

Titration in Nonaqueous Solvents

Titration in Nonaqueous Solvents

Most acid-base titrations are performed in an aqueous medium. In aqueous titrations, water competes with weaker acids or bases for proton donation or acceptance, leading to ambiguous endpoints in the titration curve. Water also affects the partial ionization of weak acids or bases. For example, water accepts a proton from acetic acid to form hydronium and acetate ions. The hydronium ion formed is a stronger acid than acetic acid, and the acetate ion is a stronger base than water. As a result,...

Water: A Bronsted-Lowry Acid and Base

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The reaction between a Brønsted-Lowry acid and water is called acid ionization. For example, when hydrogen fluoride dissolves in water and ionizes, protons are transferred from hydrogen fluoride molecules to water molecules, yielding hydronium ions and fluoride ions:

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Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids. Their reactions with water are:

Leveling Effect

Leveling Effect

In acid-base chemistry, the leveling effect refers to the limitation imposed by the solvent on the strength of acids and bases in solution. When a base stronger than the solvent's conjugate base is used, it deprotonates the solvent until the base is entirely consumed, making it ineffective against weaker acids. Conversely, an acid stronger than the solvent's conjugate acid protonates the solvent until the acid is depleted, rendering it ineffective against weaker bases. Essentially, the solvent...

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Parallel proton transfer pathways in aqueous acid-base reactions.

M J Cox¹, H J Bakker

¹FOM Institute AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands. jocelyn@amolf.nl

The Journal of Chemical Physics

|May 10, 2008

Summary

Proton transfer from HPTS to chloroacetate can occur directly or via water. This study reveals a new pathway where protons transfer to water first, then to the base, depending on base concentration.

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Area of Science:

Physical Chemistry
Spectroscopy
Chemical Dynamics

Background:

Proton transfer (PT) is fundamental in chemistry and biology.
Previous studies suggested direct proton transfer via water wires for carboxylate bases.

Purpose of the Study:

To elucidate the mechanism of proton transfer between 8-hydroxy-1,3, 6-pyrenetrisulfonic acid (HPTS) and chloroacetate.
To investigate competing proton transfer pathways.
To determine the role of solvent in proton transfer.

Main Methods:

Femtosecond mid-infrared spectroscopy was employed.
Vibrational responses of HPTS, its conjugate photobase, hydrated proton/deuteron, and chloroacetate were monitored.
Proton and deuteron transfer reactions were studied in aqueous solutions with varying base concentrations (0.25 M to 4 M).

Main Results:

Two competing proton transfer channels were identified.
For the weaker base chloroacetate, a pathway involving initial proton transfer to the solvent was observed.
The base concentration dependence of these competing channels was studied.

Conclusions:

Proton transfer mechanisms can vary depending on the base strength.
A solvent-mediated proton transfer pathway exists alongside direct transfer.
This finding offers new insights into proton transfer dynamics in solution.