Ethers

Introduction

Ethers, like a colourless liquid, are pleasant-smelling. Generally less dense relative to alcohols, are less soluble in water, and lower boiling points. Any class of organic compounds and oxygen atoms bonded with two aryl or alkyl groups. Both ether meaning and alcohols are similar in structure to water and like in others. In an ether, both hydrogen atoms are exchanged, so in alcohol, one hydrogen atom of a water molecule is replaced by an alkyl group. As a result, as solvents for fats, oils, waxes, perfumes, resins, dyes, and hydrocarbons.  In medicines and pharmacology, especially for use as anaesthetics. Ethers are used as fumigants, pesticides, and insecticides.

Uses

For various chemical reactions, ethyl alcohol is an excellent solvent for extraction. For diesel engines and gasoline engines in cold weather, these are qualities of Ethyl ether. Used as a spray propellant as dimethyl ether. A gasoline additive boosts octane number and in the exhaust reduces the number of nitrogen oxide pollutants that is Methyl t-butyl.

Structure and bonding

Alcohols have a bond angle of 109°-28′, which is somewhat less than the tetrahedral angle. It’s caused by the repulsion of oxygen’s unshared electron pairs. The –OH group is connected to an aromatic ring’s sp hybridised carbon in phenols. The carbon-oxygen bond length in phenol (136 pm) is somewhat shorter than in methanol. This leads to a partial double bond nature of the aromatic ring due to the conjugation of the unshared electron pair of oxygen with it and the sp2 hybridised state of carbon to which oxygen is linked.

With reagents complexes of ethers

Nonbonding electron pairs but no hydroxyl group are strongly polar and enhance the formation and use of many reagents. 

Example:- to share its lone pair of electrons with the magnesium atom.

A toxic gas that is inconvenient and hazardous to use pure borane exists as its dimer, diborane (B2H6).

Only the cation is solvated by the crown of either of these crown-ether complexes.

For example, a bare MnO4- ion is a powerful oxidising agent made soluble in hexane (C6H14) by 15-crown-5.

Preparation of ethers:

1. Dehydration of R-OH:- 

In the presence of protic acids (H2SO4 and H3PO4 ), alcohol dehydrates. The reaction circumstances determine whether the reaction result is an alkene or an ether. In the presence of sulphuric acid, ethanol is dehydrated to ethene at 443 K.

Ethoxyethane is the major product at 413 K.

Mechanism:-

The formation of ether is a nucleophilic bimolecular reaction involving the attack of an alcohol molecule on protonated alcohols. To give an alkene is also associated with a substitution reaction that is acidic dehydration of alcohol to give an ether. So, the temperature should be less and less steric, hindering the alkyl chain.

To make an ether meaning, use a substitution reaction. The process is exclusively suited for producing ethers with primary alkyl groups. The alkyl group should be allowed to function freely, and the temperature should be maintained low. Otherwise, the reaction favours alkene production. When the alcohol is secondary or tertiary, the reaction follows the SN1 route, which you will study in higher grades. However, dehydration of secondary and tertiary alcohols to provide equivalent ethers fails because elimination competes with substitution, resulting in the formation of alkenes.

1. Williamson synthesis

It is a laboratory method for the preparation of unsymmetrical and symmetrical ethers. In this reaction, with an alkyl halide, sodium alkoxide reacts.

Substituted alkyl groups (secondary and tertiary) contain ethers prepared by this method. The primary alkyl halide reaction involves the SN2 attack of an alkoxide ion.

If the alkyl halide is primary, then the best results are obtained. Over substitution, elimination is completed. If tertiary alkyl halide is used in this process, no ether is formed.

For example:-  

Al Oxides react with alkyl halides leading to elimination reaction, and as per the nature of alkoxides are strong bases and nucleophilic. And phenols are converted to ethers through this method.

Physical property:-

1. The bond between carbon and oxygen in ethers is polar and has a net dipole moment.
2. Ethers have weak polarity; their boiling points do not appreciably affect as compared to those of the alkanes of comparable molecular masses but as much boiling points of alcohols.
3. In alcohols, due to hydrogen bonding a large difference in boiling points of alcohols.
4. With water resembles those of alcohols, the molecular mass miscibility of ethers.
5. Like alcohols, oxygen or ether can also form hydrogen bonds with water molecules. 

For example- n-pentane, ethoxyethane, but-1-of.

Boiling point:- 309.1, 307.6, 390  respectively.

When the alkyl halide is main, the outcomes are better. Elimination competes with substitution in secondary and tertiary alkyl halides. When a tertiary alkyl halide is utilised, the only reaction product is an alkene, and no ether is generated. The reaction of CH3ONa with (CH3)3C–Br, for example, yields just 2-methylpropene.

Example: Due to the more stable aryl oxygen bond at the alkyl oxygen bond, alkyl aryl ethers are cleaved. The reaction yields alkyl halide and phenol. In the same manner, ethers with two different alkyl groups. At high temperatures with concentrated HBr or HI, the order of reactivity of hydrogen halides is:-

HI> HBr > HCl. 

The halide formed is a tertiary halide; it only happens when one is of the alkyl group. A more stable carbocation is leaving, and the reaction follows the Sn1 mechanism.

An oxonium ion is formed by the protonation of ether methylphenyl, in the case of anisole. The bond between O-C6H5 is stronger than O-CH3 due to the partial double bond character because the carbon is in an sp2 hybridised state. To form CH3I when due to attack of (I negative ion) brakes by O-CH3. To give halide because the sp2 hybridised carbon of phenol does not react further cannot undergo nucleophilic substitution for conversion to the halide reaction needed.

     3- Electrophilic substitution

The –OR is ortho activating the aromatic ring towards electrophilic substitution and para directing. 

For example:-

1-Halogenation:- Undergoing through usual halogenation in the benzene ring is the property of phenyl alkyl hers.

For example:- while undergoing anisole bromination with bromine passing through acidic in ethanoic acid to obtain para isomer, the yield of 90% is obtained in the absence of iron (III) bromide catalyst.

2-Friedel- crafts reaction:- In this, the acyl and alkyl groups at para and ortho positions are introduced by reacting with acyl halide and alkyl halide in the presence of Lewis acid working as a catalyst in the process.

3-Nitration:-To yield a mixture of ortho and para nitro anisole when a mixture of anisole reacts with concentrated nitric acids and sulphuric acid.     

Conclusion

Dehydration of alcohols and Williamson synthesis can both be used to make ethers. Ethers have boiling temperatures similar to alkanes, and their solubility is equivalent to alcohols of the same molecular mass. Hydrogen halides can break the C–O link in ethers. The alkoxy group activates the aromatic ring and leads the entering group to ortho and para locations in electrophilic substitution. Diethyl ether has long been used as an anaesthetic for inhalation. However, it has been superseded as an anaesthetic by other compounds due to its sluggish impact and uncomfortable recovery period.