Isodesmic reaction facts for kids

An isodesmic reaction is a special kind of chemical reaction. In these reactions, the types of chemical bonds that break in the starting chemicals (called reactants) are exactly the same as the types of bonds that form in the new chemicals (called products). Imagine you're building with LEGOs: an isodesmic reaction is like taking apart a car made of red bricks and using those exact same red bricks to build a house. The bricks (bond types) stay the same, just their arrangement changes! Scientists often use these reactions to understand and calculate energy changes in chemistry.

What is an Isodesmic Reaction?

An isodesmic reaction helps scientists study how much energy is involved when chemical bonds change. It's a way to simplify a reaction so they can focus on specific energy differences.

For example, think about this reaction:

CH₃^- + CH₃X → CH₄ + CH₂X^-

Here, 'X' can be fluorine (F), chlorine (Cl), bromine (Br), or iodine (I).

In this example, a methyl group (CH₃^-) takes a hydrogen atom from a methyl halide (CH₃X). This creates methane (CH₄) and a new ion (CH₂X^-). The important part is that the types of bonds (like carbon-hydrogen or carbon-X bonds) are similar on both sides of the reaction.

Energy Changes in Isodesmic Reactions

Scientists can use computers to figure out the energy changes in these reactions. For the example above, the energy change gets bigger as you go from fluorine to chlorine, then to bromine, and finally to iodine.

This tells us something important: the CH₂I^- ion (with iodine) is the most stable and least basic of these ions. Even though carbon-fluorine bonds are usually stronger than carbon-iodine bonds, the overall energy change in these specific isodesmic reactions shows a different trend. This can be due to things like steric strain, which is when atoms get in the way of each other because they are too big.

Homodesmotic Reactions

There's a similar term called a homodesmotic reaction. This type of reaction is even more specific than an isodesmic reaction.

In a homodesmotic reaction, not only do the types of bonds stay the same, but also:

• The way electrons are shared in the bonds (called orbital hybridization) is the same.
• The number of carbon-hydrogen (C-H) bonds is the same on both sides of the reaction.

Here is an example of a homodesmotic reaction:

c-(CH₂)₃ + 3 CH₃-CH₃ → 3 CH₃CH₂CH₃

Scientists use these very specific types of reactions to get very accurate information about the energy of molecules.

See also

• Gold Book IUPAC definition

In Spanish: Reacción isodésmica para niños