Alcohols , Phenols and Ethers

What is Alcohol?

In chemistry, an alcohol is any organic compound in which a hydroxyl group (-OH) is bound to a carbon atom of an alkyl or substituted alkyl group. The general formula for a simple acyclic alcohol is C_nH_2n+1OH. In common terms, the word alcohol refers to ethanol, the type of alcohol found in alcoholic beverages.

Ethanol is a colorless, volatile liquid with a mild odor which can be obtained by the fermentation of sugars. (Industrially, it is more commonly obtained by ethylene hydration-the reaction of ethylene with water in the presence of phosphoric acid.) Ethanol is the most widely used depressant in the world, and has been for thousands of years. This sense underlies the term alcoholism (addiction to alcohol).

Other alcohols are usually described with a clarifying adjective, as in isopropyl alcohol (propan-2-ol) or wood alcohol (methyl alcohol, or methanol). The suffix -ol appears in the "official" IUPAC chemical name of all alcohols.

There are three major subsets of alcohols: primary (1^°), secondary (2^°) andtertiary (3^°), based upon the number of carbon atoms the C-OH group's carbon is bonded to. Ethanol is a simple 'primary' alcohol. The simplest secondary alcohol is isopropyl alcohol (propan-2-ol), and a simple tertiary alcohol is tert-butyl alcohol (2-methylpropan-2-ol).

Structure of Alcohol

Alcohols are compounds of the general formula ROH, where R is any alkyl or substituted alkyl group. The group may be primary, secondary, or tertiary; it may be open-chain or cyclic; it may contain a double bond, a halogen atom, or an aromatic ring. For example

All alcohols contain the hydroxyl (-OH) group, which, as the functional group, determines the properties characteristic of this family. Variations in structure of the R group may affect the rate at which the alcohol undergoes certain reactions, and even, in a few cases, may affect the kind of reaction.

Compounds in which the hydroxyl group is attached directly to an aromatic ring are not alcohols; they are phenols, and differ so markedly from the alcohols that we shall consider them separately.

Classification of Alcohol

Monohydric alcohols : Containing two OH gp e.g. CH₃OH methyl alcohol

Dihydric alcohols :  Containing two OH gp e.g. HO-CH₂-OH

Polyhydric alcohols :  Containing more than two OH gp e.g. CH₂OHCHOHCH₂OH

Note : If two or more hydroxyl groups attached to same carbon atom, the compound is usually unstable and lose a molecule of water to give a stable and therefore two or more hydroxyl groups must be present on different carbon atoms.

We classify a carbon atom as primary, secondary, or tertiary according to the number of other carbon atoms attached to it. An alcohol can further classified according to the kind of carbon that bears the -OH group:

One reaction, oxidation, which directly involves the hydrogen atoms attached to the carbon bearing the -OH group, takes an entirely different course for each class of alcohol. Usually, however, alcohols of different classes differ only in rate or mechanism of reaction, and in a way are consistent with their structures. Certain substituents may affect reactivity in such a way as to make an alcohol of one class resemble the members of a different class. Benzyl alcohol, for example, though formally a primary alcohol, often acts like a tertiary alcohol. We shall find that these variations, too, are consistent with the structures involved.

Allylic alcohols:  In these alcohols, the —OH group is attached to a sp³ hybridised carbon next to the carbon-carbon double bond, that is to an allylic carbon. For example

Benzylic alcohols: In these alcohols, the —OH group is attached to a  sp³ —hybridised carbon atom next to an aromatic ring. For example

Vinylic alcohols: These alcohols contain —OH group bonded to a carbon-carbon double bond i.e., to a vinylic carbon or to an aryl carbon.

Structure of alcohols : In alcohols carbon as well as oxygen both show sp³ hybridization. In carbon all the four sp³ hybridized orbitals have on electron in each orbital whereas in oxygen two of the four sp³ hybridized orbitals have one electron & rest two orbitals have on electron pair in each. These two completely filled orbitals in oxygen atom do not take part in bonding and give rise to contraction in bond angles due to lone pair effect and therefore the bond angle C-O-H is 105^o which is lesser than the normal tetrahedral structure.

Nomenclature of Alcohol

For the simpler alcohols the common names, are most often used. These consist simply of the name of the alkyl group followed by the word alcohol. For example :

We should notice that similar names do not always mean the same classification; for example, isopropyl alcohol is a secondary alcohol, whereas isobutyl alcohol is a primary alcohol.

Finally, there is the most versatile system, the IUPAC. The rules are:

1. Select as the parent structure the longest continuous carbon chain that contains the -OH group; then consider the compound to have been derived from this structure by replacement of hydrogen by various groups. The parent structure is known as ethanol, propanol, butanol, etc., depending upon the number of carbon atoms; each name is derived by replacing the terminal -e of the corresponding alkane name by -ol.
2. Indicate by a number the position of the -OH group in the parent chain, generally using the lowest possible number for this purpose.
3. Indicate by numbers the positions of other groups attached to the parent chain.

Alcohols containing two hydroxyl groups are called glycols. They have both common names and IUPAC names.

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What are Phenols?

Phenols are aromatic compounds containing hydroxyl group directly attached to the nucleus. Phenols are compounds of the general formula ArOH, where Ar is phenyl, substituted phenyl, or some other aryl group (e.g., naphthyl). Phenols differ from alcohols in having the OH group attached directly to an aromatic ring.

Both phenols and alcohols contain the –OH group, and as a result the two families resemble each other to a limited extent. In most of their properties, however, and in their preparations, the two kinds of compounds differ so greatly that they well deserve to be classified as different families.

What are Ethers?

Ethers are compounds of the general formula R–O–R, Ar–O–R, or Ar–O–Ar.

To commonly name ethers we usually name the two groups that are attached to oxygen, and follow these names by the word ether:

If one group has no simple name, the compound may be named as an alkoxy derivative:

If the two groups are identical, the ether is said to be symmetrical (e.g., diethyl ether, diphenyl ether), if different, unsymmetrical (e.g., methyl tert-butyl ether, anisole).

Nomenclature : Ethers are supposed to have no functional group and thus IUPAC nomenclature does not provide their suffix name. These are named (in IUPAC) as alkoxy alkane, the smaller alkyl group along with oxygen is called alkoxy substituent.

Isomerism of Ethers

 Ethers show functional isomerism with alcohols.

        CH₃OCH₃ & CH₃CH₂OH

Ethers with at least four carbon atoms show metamerism due to different alkyl gps attached on bivalent O atom.

Structure  of Ether

 In CH₃OCH₃, the central oxygen atom is sp³ hybridized with two completely filled sp³ orbitals having lone pair of electrons and two half filled sp³hybridized orbitals. Also carbon atoms are sp³ hybridized and both the half filled sp³orbitals of O atom from strong s (C-O) bonds with half filled sp³ orbitals of two adjacent carbon atoms of alkyl gps.

The C-O-C bond angle is about 110^oC which is quite closer to 109^o 28' of sp³hybridized nature, inspite of the fact that lone pair of electrons must result in contraction in bond angles. This is because of the fact that presence of alkyl gps on O atom counterbalances the repulsion between the lone pair - bond pair electrons and leads to the bond angle nearer to 109^o 28'.

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