Alcohols Phenols And Ethers IIT JEE

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IIT JEE Main exam Alcohols , Phenols and Ethers notes

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 CnH2n+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 105o 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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Nomenclature : Common name Carbinol name IUPAC name
CH3OH methyl alcohol carbinol methanol
CH3CH2OH ethyl alcohol methyl carbinol ethanol
CH3CH2CH2OH n-propyl alcohol ethyl carbinol propanol-1
CH3CHOHCH3 isopropyl alcohol dimethyl carbinol propanol-2
CH3CH2CH2CH2OH n-butyl alcohol propyl carbinol butanol-1
(CH3)2CHCH2OH isobutyl alcohol isopropyl carbinol 2-methylpropanol-1
CH3CH2CHOHCH3 sec.butyl alcohol ethylmethylcarbinol butan-2-ol
(CH3)3COH t-butyl alcohol trimethyl carbinol 2-methylpropan-2-ol

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.

Formula Trivial Name IUPAC Name
CH3OCH3 Dimethyl ether Methoxymethane
CH3OCH2CH3 Methyl ethyl ether Methoxyethane
CH3OCH2CH2CH3 Methyl propyl ether 1-Methoxypropane
Methyl isopropyl ether 2-Methoxypropane
CH3CH2OCH2CH3 Diethyl ether Ethoxyethane

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 110oC which is quite closer to 109o 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 109o 28'.

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