Ethers. They constitute a very important class of compounds due to the extremely widespread nature. Low molecular weight esters are fruit-pleasing odor liquids, much of the fragrant smells of various fruits and flowers are due to the esters they contain. Thus the isoamyl acetate is in the banana , the pentyl butyrate in apricot , and ethyl acetate in pineapple tropical.
On the other hand, the esters of long chain linear aliphatic acids constitute the oils , fats and waxes that are so abundant in the vegetable and animal kingdoms.
Summary
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- 1 Features
- 2 Nomenclature
- 3 Properties
- 1 Chemical Properties
- 2 Physical Properties
- 4 Reactions
- 1 Acid-catalyzed opening
- 2 Aperture by nucleophile
- 5 Obtaining
- 1 Dehydration of alcohols
- 2 From Alcoholates
- 6 Types of ethers
- 7 Application
- 8 Sources
characteristics
Most ethers are volatile, light, and flammable liquids, soluble in alcohols and other organic solvents. From a chemical point of view, they are inert and stable compounds; The alkalis or acids not readily attack. They are closely related to alcohols and are obtained directly from them. The most typical and most widely used compound in this group is common ether or ethyl ether , usually called ether.
Classification of the ethers according to the type of radical
They can be considered the result of replacing the hydrogen of the OH group of alcohols with a hydrocarbon radical. Depending on the type of these radicals, the ethers can be:
- Aliphatic, R — O — R (the two alkyl radicals).
- Aromatic, Ar-O-Ar (the two aryl radicals).
- Mixed, R — O — Ar (one alkyl and one aryl radical).
Ethers are called symmetric when the two radicals are the same and, asymmetrical, if they are different.
Nomenclature
How to name the ethers
To name the ethers we have two alternatives:
- First: Consider the alkoxy group as a substituent (R being the simplest radical).
- Second: Cite the two radicals that are attached to the O in alphabetical order and then the word ether.
Properties
Chemical properties
Ethers have very little chemical reactivity due to the difficulty of breaking the C-O bond. Therefore, they are widely used as inert solvents in organic reactions. In contact with air they undergo a slow oxidation in which very unstable and little volatile peroxides are formed. These are dangerous when an ether is distilled, as they are concentrated in the residue and can lead to explosions. This is avoided by storing the ether with sodium wire or adding a small amount of a reducer (SO 4 Fe, LiAIH 4 ) before distillation.
Physical Properties
Structurally, the ethers can be considered as derivatives of water or alcohols, in which one or two hydrogens, respectively, have been replaced by carbon residues.
The angular structure of the ethers is well explained by assuming a sp3 hybridization in oxygen , which has two unshared pairs of electrons. It cannot establish hydrogen bonds with itself, and its boiling and melting points are much lower than the reportable alcohols. A very special case is epoxides , which are three-membered cyclic ethers. The ring contains a lot of tension, although somewhat less than in cyclopropane .
Ethylene epoxide
But the presence of oxygen, which polarizes the bonds, and the existence of tension, makes epoxides, unlike normal ethers, very reactive and extremely useful in synthesis.
Because the angle of the CoC bond is not 180º, the dipole moments of the two CO bonds do not cancel; consequently, the ethers have a small net dipole moment (for example, 1.18 D for diethyl ether).
This weak polarity does not appreciably affect the boiling points of the ethers, which are similar to those of alkanes of comparable molecular weights and much lower than those of isomeric alcohols. Compare, for example, the boiling points of n-heptane (98 ° C), methyl n-pentyl ether (100 ° C) and hexyl alcohol ( 157 ° C). The hydrogen bonds that hold the alcohol molecules firmly together are not possible for the ethers, since the ethers only have hydrogen attached to carbon.
On the other hand, the ethers have a solubility in water comparable to that of alcohols: both diethyl ether and n-butyl alcohol, for example, have a solubility of about 8 g per 100g of water. The solubility of lower alcohols is due to hydrogen bonds between water and alcohol molecules; the solubility of the ethers in water is likely due to the same cause.
Reactions
Ethers are not reactive except for epoxides . Epoxide reactions pass through the cycle opening. This opening can be acid catalyzed or nucleophilic opening.
Types of ethers
Crown ethers : There are ethers that contain more than one ether functional group (polyethers) and some of these form cycles; These polyethers are called crown ethers. They can be synthesized in different sizes and are usually used as ligands, to selectively complex (by size) alkali cations. Oxygens interact with the cation, which is placed in the center of the cycle, forming a complex. Such compounds exist in nature. They usually serve as transport of alkaline cations so that they can cross cell membranes and thus maintain optimal concentrations on both sides. For this reason, they can be used as antibiotics, such as valinomycin.. Other related compounds are cryptates, which contain, in addition to oxygen atoms, nitrogen atoms. Cryptates and crown ethers are often called ionophores .
Polyethers : Polymers containing the ether functional group can be formed. An example of formation of these polymers: R-OH + n (CH2) O! RO-CH2-CH2-O-CH2-CH2-O-CH2-CH2-O- .. The best known polyethers are epoxy resins, which are mainly used as adhesives. They are prepared from an epoxide and a dialcohol.
Epoxides or oxiranes : Epoxides or oxiranes are ethers where the oxygen atom is one of the atoms in a cycle of three. They are thus heterocyclic compounds. The three cycles are very stressed, so they react easily in opening reactions, both with bases and with acids.
Silicon ethers : There are other compounds in which the functional group is not RO-R ‘, oxygen being attached to two carbons, but they are still called ethers. For example, silicon ethers, where the general formula is RO-Si, that is, oxygen is attached to a carbon and a silicon atom. There are still a couple of nonbonding electrons. These compounds are called silicon ethers .
Application
There are multiple applications for these compounds. The most widely used is as solvents for oils and fats . Other applications are: General anesthetic. Extract medium to concentrate acetic acid and other acids. Carrier medium for dehydration of ethyl and isopropyl alcohols. Solvent of organic substances (oils, fats, resins, nitrocellulose, perfumes and alkaloids). Initial fuel for Diesel engines.
