protischen

Protisch (German for 'protic'; in English, protic) describes substances capable of donating protons in chemical reactions. In solvent chemistry, protische solvents contain hydrogen atoms bonded to electronegative atoms, such as oxygen or nitrogen (for example O–H or N–H), and can participate in hydrogen bonding and proton transfer. The term contrasts with aprotische (aprotic) solvents, which lack exchangeable protons.

Protische solvents strongly solvate ions through hydrogen bonding, influencing reaction rates and mechanisms. Their ability to donate protons affects acidity and equilibria, often stabilizing charged intermediates and transition states. In nucleophilic substitutions and eliminations, protische media can reduce the reactivity of nucleophiles because the nucleophiles are heavily solvated, particularly for small, hard anions. This often favors reaction pathways such as SN1 over SN2. Conversely, aprotische solvents, which do not donate protons, tend to enhance nucleophilicity and can accelerate SN2 processes.

Common protische solvents include water, methanol, ethanol, formic acid, and acetic acid, as well as ammonia and alcohol-water mixtures. Their dielectric properties and hydrogen-bonding networks play crucial roles in solvation and catalysis. The choice between protische and aprotische media is a key consideration in reaction design, influencing solute acidity, nucleophile strength, reaction rate, and selectivity.

In German-language chemistry literature, protisch refers to this class of solvents or reagents, with protischen Eigenschaften describing their proton-donating behavior. The counterpart term is aprotisch, describing non-proton-donating media.