What are ethers in organic chemistry?

Ethers are a class of organic compounds that contain an oxygen atom bonded to two alkyl or aryl groups using a single covalent bond. In literature, they are identified with the suffix "-ether". For example, dimethyl ether or ethyl isopropyl ether.

A general simple chemical representation of ethers is R-O-R', where R and R' represent the alkyl or aryl groups bonded to the oxygen atom. R and R' can be the same or different entities.

Note: R or R' cannot be H; otherwise, it will become the hydroxyl group.

Ether functional group

Ethers are an interesting chemical compound, and they express different chemical and physical properties compared to alcohols, ketones, aldehydes, and carboxylic acids.

Looking at the geometric shape of the ether group, the central oxygen atom is sp3 hybridized. Thus, we have four sp3 orbitals. Two of the four are used for bonding with R or R' and the remaining two sp3 orbitals bear a lone pair of electrons. The bond angle between R-O-R' is less than 120°. This is due to the fact that two lone pairs of electrons exert more repulsion, which shrinks the R-O-R' angle. In dimethyl ether and diethyl ether, the angle is around 110°.

An oxygen atom weighs around 16, and carbon around 12. So, the molecular weight of any ether is always more than 16 + 12 * 2 = 40 g/mol. The simplest ether is dimethyl ether, i.e., two methyl groups attached to an oxygen atom, and has a molecular weight of 46 g/mol.

Ethers are a Lewis base since they have lone pairs on oxygen and have slightly basic pH values. On the other values, alcohols, aldehydes, and carboxylic acids are acidic in nature.

In alcohols (–OH), aldehydes (–CHO), and carboxylic acids (–COOH), an oxygen atom is directly attached to hydrogen. And oxygen is far more electronegative than hydrogen. As a consequence, these chemical groups can easily lose a proton (H+ ion). Whereas, in the ethers, no hydrogen is in direct contact with oxygen, which makes it difficult to remove a proton. Furthermore, the alpha hydrogens (hydrogens attached to the alpha carbon) are less acidic than that of aldehydes and ketones.

Ethers are non-polar compounds, and the molecules of an ether do not form strong hydrogen bonding, like in alcohols and carboxylic acids. As a result, they have lower boiling points than corresponding alcohols and carboxylic acids.

Nomenclature

Simple and smaller ethers are usually named by common terminology. The alkyl part with smaller carbon atoms comes first, for example, methyl ethyl ether. Here, methyl (–CH3) is a one-carbon group, while ethyl has two carbons (–CH2CH3). Methyl phenyl ether is another example. Methyl is smaller than phenyl, so it is placed first. When R is the same, we add the prefix di- to the group. As an example, diethyl ethyl (H3C–H2C–O–CH2–CH3) or diphenyl ether (C6H5–O–C6H5).

However, this common methodology does not work well as the complexity of an ether molecule increases. How do we name 1,4,7,10-Tetraoxacyclododecane using this method?

It has four oxygens and eight carbons, and it is a cyclic compound. We cannot name it with trivial techniques. So, we move toward IUPAC nomenclature.

IUPAC

The IUPAC nomenclature divides an ether molecule into two parts: alkoxy (–OR) and alkane (–R). Thus, methyl ethyl becomes methoxyethane, dimethyl ether becomes methoxymethane, and so on.

The table below mentions some common ethers with their IUPAC names.

Common name	IUPAC name
Dimethyl ether	Methoxymethane
Methyl ethyl ether	Methoxyethane
Diethyl ether	Ethoxyethane
Methyl phenyl ether	Methoxybenzene
Diphenyl ether	Phenoxybenzene
Diisopropyl ether	2-[(Propan-2-yl)oxy]propane

Production

Ethers are produced mainly by two industrial routes: dehydration of alcohols and Williamson ether synthesis.

Dehydration of alcohols

Alcohols (R–OH) closely relate to ethers. The hydroxyl group (–OH) of the alcohol can be converted into the ether functional group if we can replace H attached to O with –R. That's what we achieve by the dehydration of alcohols.

The general chemical equation is:

R–OH + R–OH → R–O–R + H2O

The reaction requires an acidic medium (sulphuric acid, H2SO4) and a temperature (≈of 125°C). The reaction mechanism is illustrated below:

As from the above reaction, the proton attacks a lone pair of electrons on oxygen. This destabilizes the carbon-oxygen bond. Seeing the opportunity, a lone pair on oxygen of another alcohol molecule bonds to form an intermediate complex. The complex breaks into an ether, releasing a water molecule and a proton.

One important thing to note is this method best works to produce symmetrical ethers. When a reacting mixture contains multiple alcohols, we will get a mixture of ethers as products. This is because multiple alcohols are protonated. For example, a methanol and ethanol mixture will give dimethyl ether, diethyl ether, and methyl ethyl ether.

CH3–OH + CH3CH3–OH + H+ + heat → H3C–O–CH3 + H3C–H2C–O–CH2–CH3 + H3C–O–CH2–CH3

Williamson synthesis

Williamson reaction is a nucleophilic displacement that can be used to produce asymmetrical ethers.

The reaction involves alkoxide and alkyl halide as reactants and ether and halide salt as products.

The general reaction is followed:

R–ONa + R'–X → R–O–R' + NaX

R–ONa is sodium alkoxide, R'X is alkyl halide, and NaX is sodium salt.

Sodium alkoxide can be easily prepared by treating the corresponding alcohol with sodium.

2 R–OH + 2 Na → 2 R–ONa + H2 (gas)

Let's take an example of methyl ethyl ether:

2 CH3–OH + 2 Na → 2 CH3–ONa + H2 (gas)

CH3–ONa + CH3CH2–Cl → CH3–O–CH2CH3 + NaCl

Chemical reactivity

In contrast to other functional groups, the ether group is relatively stable. The C–O bond of ether is stable and difficult to break. Thus, ethers have low reactivity. The low reactivity of ethers makes them very useful organic solvents. Diethyl ether is a very common solvent used in industries for the production of various intermediates, pharmaceuticals, plastics, and organic materials.

The chemical and physical properties of ethers also depend on the size and nature of the alkyl or aryl groups bonded to the oxygen atom.

Smaller ethers tend to be more volatile and less viscous than ethers with larger alkyl groups.

Ethers can show resonance structures when the conjugation system is present in the chemical structure. For example, in aromatic ethers, the oxygen atom is directly attached to the aromatic ring. The lone pairs of electrons on oxygen can be delocalized in the benzene ring, making them less accessible for reaction. This phenomenon further stabilizes aromatic ethers.