Ethers can be derived from alcohols. The functional group of ethers is R-O-R (instead of R-O-H in an alcohol). Ethers can be viewed as a water molecule in which both H atoms are replaced with alkyl groups. Ethers may exist in straight chain carbons (acyclic) or as part of a carbon ring (cyclic).
Preparation of ethers
Synthesis of acyclic ethers
Most acyclic ethers can be prepared using Williamson's synthesis. This involves reacting an alkoxide with a haloalkane. As stated previously, alkoxides are created by reacting an alcohol with metallic sodium or potassium, or a metal hydride, such as sodium hydride (NaH). To minimize steric hindrance and achieve a good yield, the haloalkane must be a primary haloalkane. This is because the mechanism is SN2, where the oxygen atom does a backside attack on the carbon atom with the halogen atom, causing the halogen atom to leave with its electrons.
Synthesis of cyclic ethers
You can also use the Williamson synthesis to produce cyclic ethers. You need a molecule that has a hydroxyl group on one carbon and a halogen atom attached to another carbon. This molecule will then undergo an SN2 reaction with itself, creating a cyclic ether and a halogen anion.
Properties of ethers
Naming acyclic ethers
Name the two sides of the ether as substituents, then add the word "ether" at the end. For example, CH3-O-CH3 is dimethyl ether. CH3-O-C(CH3)3 can be called methyl tert-butyl ether (MTBE) or tert-butyl methyl ether (TBME).
Cleavage of acyclic ethers
Acyclic ethers can be cleaved by a strong acid, typically HI or HBr, but not HCl. The acid breaks the ether apart into an alcohol and an alkyl halide (a haloalkane.) Cleavage of ethers by an acid was first seen by Alexander Butlerov in 1861, when he discovered that hydroiodic acid causes 2-ethoxypropanoic acid to break apart into iodoethane (ethyl iodide) and lactic acid (2-hydroxypropanoic acid.) The mechanism used in acidic cleavage of ethers depends on whether they have primary, secondary, or tertiary groups attached to oxygen. If one of the carbons attached to the central oxygen atom is tertiary, benzylic (contains benzene ring), or allylic (contains carbon-carbon double bond), then the cleavage will occur via an SN1 or an E1 mechanism. The E1 mechanism leads to an alcohol and an alkene instead of an alkyl halide. These reactions often take place around 0 degrees C. On the other hand, if both groups attached to the central oxygen atom are primary or secondary, the reaction takes place via an SN2 mechanism. These reactions are often conducted at 100 degrees C.