Question

I remember from class that a –OH group is a bad leaving group. How is it that this reaction will progress when bad leaving groups do not favor substitution reactions? (A partial discussion of the mechanism might help you here)

I think I added phosphoric acid instead of hydrochloric acid. So I went ahead and added the HCl. Would this lead to a different product? Explain why. Otherwise, explain what would happen and give the identity of the possible side product.)

Answer 1

First, the carbon-oxygen C-O bond in C-OH is relatively strong (~350kJ/mol) and difficult to break. The resulting products would be an organic cation R+ and a hydroxide ion OH-. If you were attempting to substitute the -OH with another nucleophile, (ie. bromide ion Br-), the use of NaBr would not produce a substituted product since the C-O bond would be stronger than the C-Br bond. In other words, hydroxide OH- is a stronger nucleophile than Br- and therefore a poorer leaving group.

To convert the -OH into a better leaving group, it is common to use a strong acid such as HCl or HBr. The acid will protonate the OH group forming C-OH2+ (a protonated OH group). The protonation greatly weakens the C-O bond and the bond easily breaks forming an organic cation R+ and a neutral water molecule. Any nucleophile stronger than water (ie. Br-) can now react with the organic cation forming a substituted product.