3 Top Mistakes in Organic Mechanisms

Organic mechanisms cause headaches for students every year. Students rarely understand the ideas and instead learn them by rote. This is a result of the syllabus teaching a concept without teaching the the underlying concepts. And this comes from the underlying concepts being too complex to include in an A-level specification. The result: year in and year out students produce woolly answers to mechanistic questions. Unfortunately, the specifications of the major exam boards are unlikely to change any time soon and we are faced with making the best of what we have. What are the three biggest mistakes in mechanisms and how can we fix them?

Mistake 1: arrows not starting at electrons

As taught, curly arrows represent a movement of electrons. To gain maximum credit, the student must make explicit which pair of electrons (or single electron in free-radical substitution) are moving and to do this, the arrow must start at a pair of electrons. Too often, students lose marks because their arrows start in some vague region of space.

Mistake 2: careless charge balancing

If an atom forms a new bond or breaks an existing bond, the charge on that atom will change. Only if an atom both has a bond formed to it and simultaneously breaks an existing bond, will its charge remain the same. Students lose marks here because they are trying to remember the specific reaction. This is why I find it better to teach the concepts which apply to all reactions (arrow from a species, charge becomes more positive, etc.) before starting any discussion of specific reactions.

Mistake 3: "magic" appearances

This occurs when a student is trying to force a mechanism and parachutes in a species to make the answer fit. Unfortunately, if the species doesn't appear in the reaction equation, it cannot be used in the mechanism (unless it is generated from the reactants). On examination, these species would often destroy the reactants. Unfortunately, one exam board actually promotes this practice.

How AQA encourages bad chemistry

First, I am not trying to discredit AQA here. All exam boards simplify things at A-level which students who go on to complete chemistry degrees find are not quite true. This though, is one example which I feel they could have left out. Propanone is reduced to propan-2-ol using an aqueous solution of sodium tetrahydridoborate (sometimes called sodium borohydride). The reaction proceeds via a nucleophilic addition-elimination mechanism. So far, so good. The specification takes it a step further, getting students to use the hydride ion in the mechanism. The hydride ion is extremely unstable as it is incredibly basic. It cannot be formed from sodium tetrahydridoborate and, if it could, it would instantly deprotonate the water. The result would be an alkaline solution of propanone.

Writing mechanisms is a challenge and one students have to grapple with every year. Would you like us to produce a mechanisms course? If so, let us know in the comments. If you are finding mechanisms hard and would like some 1:1 support, get in touch about booking some sessions.