A common question on an organic chemistry exam is, "Draw the mechanism of this reaction." The following instructions should help you figure out how to solve this kind of problem.
Please note that memorizing these instructions will not teach you how to draw mechanisms. The only way you will learn to draw mechanisms is by practice. However, if you follow these instructions while you practice, you will quickly get the hang of drawing mechanisms.
Obey Grossman's rule. (Draw in all the H atoms and the bonds to those H atoms near the reactive centers.) You would not believe how helpful this step can be.
Balance the equation. If any atoms from the starting material are missing, they are probably in a coproduct that is not shown. Draw it.
Number the non-H atoms of the starting materials. Don't forget to number the atoms of any starting materials above or below the reaction arrow. Number all non-H atoms, including heteroatoms, leaving groups, etc.
Label the corresponding atoms in the products with the same numbers that they have in the starting materials. This task sounds trivial and babyish, but it is more difficult than it sounds. Just do it.
Identify which σ bonds break and which σ bonds make in the course of the reaction, and write them out. (Do not include π bonds; they will naturally end up in the right places if the σ bonds are correct.) For example:
Determine whether the mechanism is polar or free-radical chain.
If the mechanism is polar, determine whether the conditions are acidic or basic. If the mechanism is acidic, you will probably need to protonate a heteroatom or π bond as your first step. If the mechanism is basic, you may need to deprotonate an acidic atom as a first step, or maybe you'll promote an E2 elimination reaction.
If the mechanism is free-radical chain, you need to draw an initiation step and a propagation step. Signs that a free-radical chain mechanism is operative: air, O2, a peroxide (contains C–O–O–C) such as (BzO)2 or (t-BuO)2 (but not a hydroperoxide such as H2O2 or t-BuOOH), the presence of hν, or NBS or NCS and heat in the absence of a nucleophile. Rules for drawing free-radical chain mechanisms.
Draw electron-flow arrows starting at lone pairs or π bonds and ending at atoms, existing bonds, or incipient (newly forming) bonds. Electron-flow arrows show the movement of electrons. The electron-flow arrow should start at the electron source and end where the electrons are going. Do not start an electron-flow arrow at H+; H+ has no electrons, so an electron-flow arrow cannot start there. If you are making a new σ bond between two atoms, the electron-flow arrow should point to between the two atoms, because the incipient bond, which will contain the electrons, is located between the atoms.
As you proceed through the mechanism, focus on the list of bonds that you need to make or break, and cross off each bond in your list as you take care of it.