Formalism (chemistry)

In chemistry, a formalism is a model construct that is based on simple ideas and allows predictions to be made using simple rules of action. Many formalisms are of great importance to chemists because of their simplicity, their high predictive power, and their broad applicability. Due to their approximate character, formalisms often fail in the case of special problems.

In addition to referring to formalisms, the word formal is also used when it is to be made clear that an idea or explanation is plausible to a certain extent, but simplified and / or not real.

Examples

Examples of formalisms in chemistry are

• Oxidation numbers ( redox chemistry )
• Formal charges
• Electron arrows (mainly organic chemistry )
• Reaction orders
• Hückel molecular orbital theory
• fully localized bonds in valence structure theory , resulting therefrom
  • mesomeric limit formulas
  • The concept of localized charge (starting point for predicting fragmentation in mass spectrometry )

Formal responses

Formal reactions are chemical reactions that do not take place in reality, for example because the reactants do not react with one another or because one of the reactants is not stable. They are usually cited when there is a general pattern according to which substances of a class can be formed, but this is not applicable to a specific example, or when a compound is a derivative of a known basic structure but is not made from it.

For example, urethane is formally formed from isocyanic acid (unstable) and ethanol , triethyl orthoformate from orthoformic acid (unstable) and ethanol. Carboxamides form formally from amines and carboxylic acids ; the reaction does not take place (under normal pressure), because the reactants instead form a salt from the very unreactive ammonium and carboxylate ions in an acid-base reaction .