Protonation

Example: acid-base reaction of acetic acid and water. Red arrows: deprotonation of acetic acid; green arrows: protonation of the acetate with formation of acetic acid.

In chemistry, protonation refers to the addition of protons ( hydrogen nuclei / cations ) to a chemical compound as part of an acid-base reaction . One or more positive charges are added to the target molecule, depending on the number of protons transferred . The compound that has taken up the protons is called the protonated compound . The opposite process, the splitting off of protons from a compound, is called deprotonation .

${\ displaystyle \ mathrm {HA \ + \ B \ \ rightleftharpoons \ A ^ {-} \ + \ HB ^ {+}}}$
Protonation of compound B by the acid HA , which is deprotonated in the process.

The prerequisite for the process of protonation is the presence of an acid and a base as defined by Brønsted and Lowry . The acid strength - represented by the pK S value - and the base strength ( pK B ) determine whether the balance on the side of the protonated or unprotonated compound is.

The protonation of a compound can be influenced by steric factors.

A positive charge is transferred with the proton, as in the following example, which shows the protonation of ammonia (NH 3 ) by hydrogen chloride (HCl):

${\ displaystyle \ mathrm {HCl \ + \ NH_ {3} \ \ rightleftharpoons \ Cl ^ {-} \ + \ NH_ {4} ^ {+}}}$
Hydrogen chloride gives off a proton to the ammonia molecule. This is a negatively charged are chloride - anion and a positively charged ammonium - cation formed.

Protonation is a reaction step that has been widely observed and used. It is often used to activate a chemical compound for subsequent reactions . But they are also used to ionize compounds , for example in the context of a mass spectrometric analysis.