I was going through a pupil’s A-level papers recently trying to find one extra mark so she could get the A* she was one mark away from. There was one question asking for reagents and conditions to convert a nitrile to an amine. One possible answer was using LiAlH4 dissolved in ether. The pupil had written LiAlH4 but not put ether. I had not seen this condition required before – although looking through past papers it had come up before in Jan 2004. So why is it important we use ether here and what is it? There is no other mention of it on the syllabus.
Ether is the common name for Ethoxyethane (also known as diethyl ether). Its formula is CH3CH2OCH2CH3, and it contains the ether functional group (-C-O-C-). Ether is a highly volatile liquid that can be used as a solvent but is dangerously flammable. What does volatile mean in chemistry and why does increasing volatility of organic substances mean they are more flammable?It has a lower boiling point than ethanol even though it is a larger molecule. Can you explain why? (Think about hydrogen bonding).
LiAlH4 is a reducing agent that can be used to convert carboxylic acids to alcohols, amides to amines and nitriles to amines (only the last reaction is required for the syllabus). The choice of solvent for LiAlH4 is important because it can react with some solvents. It reacts with the common solvents of water and ethanol in a similar way producing a gas. What bond in the water and ethanol must be broken by the LiAlH4 ? What gas do you think it would form? Why would Ether not react with the LiAlH4 in this way?
NaBH4 is another reducing agent we have met (used to convert carbonyls to alcohols). This is a less strong reducing agent and does not react with water and ethanol, so these can be used as a solvent for NaBH4 instead of ether.
Ether has had more uses than just as a solvent. It has anaesthetic properties and was one of the first general anaesthetics used in the 19th century. It is no longer used because it is too flammable and can cause nausea in the patient. It has also some history as a recreational drug. Hunter S Thompson in the cult novel ‘Fear and Loathing in Las Vegas’ makes a few references to it. “This is the main advantage of ether: it makes you behave like the village drunkard in some early Irish novel…total loss of all basic motor skills: Blurred vision, no balance, numb tongue-severance of all connection between the body and the brain. Which is interesting, because the brain continues to function more or less normally…you can actually watch yourself behaving in the terrible way, but you can’t control it.” Before you start thinking that sounds fun, ether will apparently attack your stomach lining if you drink it.
Fortunately we found the student the one mark she needed elsewhere on the paper and on getting it remarked by the exam board she gained her A*.