chemistry

Isodesmic reaction

Isodesmic reaction is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Isodesmic reaction rather than just read about it. In short: An isodesmic reaction is a chemical reaction in which the type of chemical bonds broken in the reactant are the same as the type of bonds formed in the reaction product. This type of reaction is often used as a hypothetical reaction in thermochemistry.

Key takeaways

Isodesmic reaction belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Isodesmic reaction to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Isodesmic reaction from memory before moving on to harder problems.

Reference excerpt

An isodesmic reaction is a chemical reaction in which the type of chemical bonds broken in the reactant are the same as the type of bonds formed in the reaction product. This type of reaction is often used as a hypothetical reaction in thermochemistry. An example of an isodesmic reaction is

CH3− + CH3X → CH4 + CH2X− (1) X = F, Cl, Br, I Equation 1 describes the deprotonation of a methyl halide by a methyl anion. The energy change associated with this exothermic reaction which can be calculated in silico increases going from fluorine to chlorine to bromine and iodine making the CH2I− anion the most stable and least basic of all the halides. Although this reaction is isodesmic the energy change in this example also depends on the difference in bond energy of the C-X bond in the base and conjugate acid. In other cases, the difference may be due to steric strain. This difference is small in fluorine but large in iodine (in favor of the anion) and therefore the energy trend is as described despite the fact that C-F bonds are stronger than C-I bonds. The related term homodesmotic reaction also takes into account orbital hybridization and in addition there is no change in the number of carbon to hydrogen bonds.

References IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "isodesmic reaction". doi:10.1351/goldbook.I03272

Worked examples

Example 1 — a first encounter with Isodesmic reaction

Start with the simplest possible case. Write down what Isodesmic reaction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Isodesmic reaction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Isodesmic reaction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Isodesmic reaction

In research: Isodesmic reaction appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Isodesmic reaction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Isodesmic reaction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computational chemistry, Thermochemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Isodesmic reaction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Isodesmic reaction in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Isodesmic reaction means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Isodesmic reaction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Isodesmic reaction in simple terms?

Why does Isodesmic reaction matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Isodesmic reaction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Isodesmic reaction.