Reaction mechanisms require students to appreciate the electrical changes which are conjectured to accompany chemical reactions

And also to learn a new formalism where the movement of electrons or electron pairs is represented by full or half arrows, which may start and end on either atoms or bonds.

Reaction mechanisms

Chemists use reaction mechanisms to show what they think might be happening as molecules interact during chemical reactions.

When drawing reaction mechanisms, the chemist usually assumes:

  1. The reaction occurs in several distinct steps;
  2. That each step can be represented as the movement of electrons; and
  3. That sometimes electrons move as pairs, and sometimes they move individually.

Diagrams showing the steps in reaction mechanisms usually show the molecules and/or ions (shown by + and –) and/or radicals (shown by •) involved, as well as arrows showing the movement of electrons. Two types of arrows are used:

Reaction arrow examples

There are two questions in this exercise. The questions each consist of a central diagram showing the initial stage in a reaction mechanism, surrounded by a selection of suggestions for the result of that step. Your task in each case is to identify which of the diagrams gives the correct outcome of that reaction step. Draw a large arrow showing which diagram is correct, as in the example below.

Examples box and arrow

Try and explain your reason(s) for selecting the diagram you chose.

Reaction mechanism 1

Reaction mechanism 1

  • I selected this diagram because …

Reaction mechanism 2

Reaction mechanism 2

  • I selected this diagram because…

Reaction mechanism revealed

Reaction mechanism 1

The diagrams below show and explain the correct answer to the question about the ionic reaction mechanism.

Reaction mechanism 1 revealed

Reaction mechanism 2

The diagrams below show and explain the correct answer to the question about the free radical mechanism.

Reaction mechanism 2 revealed


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