PROPERTIES OF ETHERS, EPOXIDES AND THIOETHERS
Ethers consist of ansp3 hybridized oxygen linked to two carbon atoms by a single σ bond. Alkyl ethers are ethers where two alkyl groups are linked to the oxygen. Aryl ethers are ethers where one or two aromatic rings are attached to the oxygen. Since ethers cannot form hydrogen bonds, they have lower boiling points than comparable alcohols, and similar boiling points to comparable alkanes. However, hydrogen bonding is possible with protic solvents which means that ethers have water solubilities similar to alcohols of equivalent molecular weight. Ethers are relatively unreactive since they have weak nucleophilic and electrophilic centers.
Epoxides are three-membered cyclic ethers which are more reactive than other cyclic or acyclic ethers due to the ring strain inherent in three-membered rings. They will react with nucleophiles by an SN2 reaction at the electrophilic carbons.
Thioethers are the sulfur equivalents of ethers. The polarizable sulfur can stabilize a negative charge on an adjacent carbon making protons attached to that carbon acidic.
The presence of an ether or epoxide is indicated by C–O stretching absorp-tions in the IR spectrum. Supporting evidence can be obtained from the chemical shifts of neighboring groups in the 1H and 13C nmr spectra. It is important to consider other spectroscopic evidence as well as the molecular formula before deciding whether an ether or epoxide is present.
Ethers consist of an oxygen linked to two carbon atoms by σ bonds. In aliphatic ethers (ROR), the three atoms involved are sp3 hybridized and have a bond angle of 112°. Aryl ethers are ethers where the oxygen is linked to one or two aromatic rings (ArOR or ArOAr) in which case the attached carbon(s) is sp2 hybridized.
The C–O bonds are polarized such that the oxygen is slightly negative and the carbons are slightly positive. Due to the slightly polar C–O bonds, ethers have a small dipole moment. However, ethers have no X–H groups (X heteroatom) and cannot interact by hydrogen bonding. Therefore, they have lower boiling points than comparable alcohols and similar boiling points to comparable alkanes. How-ever, hydrogen bonding is possible to protic solvents resulting in solubilities similar to alcohols of comparable molecular weight.
The oxygen of an ether is a nucleophilic center and the neighboring carbons are electrophilic centers, but in both cases the nucleophilicity or electrophilicity is weak (Fig. 1). Therefore, ethers are relatively unreactive.
Epoxides (or oxiranes) are three-membered cyclic ethers and differ from other cyclic and acyclic ethers in that they are reactive to various reagents. The reason for this reactivity is the strained three-membered ring. Reactions with nucleophiles can result in ring opening and relief of strain. Nucleophiles willattack either of the electrophilic carbons present in an epoxide by an SN2 reaction (Fig. 2).
Thioethers (or sulfides; RSR) are the sulfur equivalents of ethers (ROR). Since the sulfur atoms are polarizable, they can stabilize a negative charge on an adjacent carbon atom. This means that hydrogens on this carbon are more acidic than those on comparable ethers.
The IR spectra of ethers are characterized by C–O stretching absorptions. An aliphatic ether tends to have an absorption in the region 1150–1070 cm−1 which is often stronger than surrounding peaks. Alkyl aryl ethers tend to give two relatively strong absorptions, one in the region 1275–1200 cm−1 and the other in the region 1075–1020 cm−1. The C–O stretching absorption for epoxides occurs in the region 1260–1200 cm−1. C–O Stretching absorptions are also possible for carboxylic acids and esters, as well as for alcohols and phenols. Therefore it is important to consider other evidence before deciding whether an ether or epoxide is present. For example, if the molecular formula only has one oxygen, this rules out the possibility of an acid or an ester. If there are no D2O exchangeable protons in the 1H nmr spectrum, this rules out alcohols and phenols.
The 1H and 13C nmr spectra of an ether do not give direct evidence of the func-tional group but may indicate its presence indirectly by the chemical shifts of neighboring groups. For example, the methyl group of a methyl ether appears at 3.3 ppm in the 1H spectrum and at 59 ppm in the 13C spectrum.
The protons of an epoxide show characteristic signals at 2.5–3.5 ppm in the 1H nmr spectrum.