Organic Chemistry/Ethers

Ethers

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).

Example ethers. MTBE is acyclic. THF is 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 S_N2, 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 S_N2 reaction with itself, creating a cyclic ether and a halogen anion.

Properties of ethers

Acyclic 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 S_N1 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 S_N2 mechanism. These reactions are often conducted at 100 degrees C.

Cyclic ethers

Two common cyclic ethers are epoxide, which has two carbons each bonded to the oxygen atom; and tetrahydrofuran, known by its abbreviation THF, which has four carbons and an oxygen atom.