Acid-Base chemistry is where the discussion of mechanism begins in most Organic 1 courses. While this topic is covered in General Chemistry, its application in Organic seems to cause problems as the structures of the acids and bases involved are generally more complicated than the simple examples seen in the first year. The ideas, however, are the same.

Initially we are looking at protic acids in which the H will be bonded to a more electronegative element (e.g. OH in water and alcohols) or a bigger element (e.g. Br in HBr). HBr is a stronger acid than water because the H-Br bond is weaker and easier to break (363 kj/mol, with very different sized atoms) than the O-H bond (467 kj/mol, atoms closer in size), but also because of the relative conjugate base stabilities. Br is much better at dealing with a negative charge (extra electrons) than O because it is bigger and the charge (lone pair) has more volume to occupy. The Br anion is thus much easier to form and is much more stable than hydroxide (and later alkoxide) so this reaction favours the right-hand side completely.