Classes, structures and naming of Alcohols, Chemistry tutorial

Introduction:

Alcohols are the compounds in which hydrogen of an alkane has been substituted by an -OH group. They encompass -OH group (that is, a hydroxyl) bonded to a saturated alkane such as carbon atom R-OH where 'R' is any simple alkyl or substituted alkyl group. For illustration:

CH3OH      CH3CH2OH

Methanol    Ethanol

Classes of Alcohols:

Alcohols can have one or more hydroxyl group. It could be:

1) Monohydric: It contain only one hydroxyl group. For illustration: ethanol.

CH3-CH2-CH2-OH

Ethanol

2) Dihydric: It contains two hydroxyl groups. For illustration: ethane-1, 2-diol.

3) Trihydric: It contains three hydroxyl groups. For illustration: propane-1, 2, 3-triol.

Usually with more than one hydroxyl group, it is stated to be polyhydric.

Monohydric alcohols encompass the general formula CnH2n + 1OH. They can be categorized into three classes: primary (1o), secondary (2o) and tertiary alcohol (3o) according to the number of alkyl groups joined to the hydroxyl-bonded carbon atom.

a) Primary alcohol (1°):

It consists of one alkyl group attached to the hydroxyl bonded carbon atom. 

2389_Primary alcohol.jpg

Fig: Primary alcohol

For example: CH3OH     CH3CH2OH

                     Methanol      Ethanol 

b) Secondary alcohol (2o):

There are mainly two alkyl groups joined to the hydroxyl-bonded carbon atom.

1808_Secondary alcohol.jpg

Fig: Secondary alcohol

c) Tertiary (3o):

There are mainly three alkyl groups; however no hydrogen atom directly attached to the hydroxyl-bonded carbon atom.

881_Tertiary alcohol.jpg

Fig: Tertiary alcohol

Naming Alcohols:

a) Common names:

Simple alcohols are often known by their common names that comprises of the name of the alkyl group to which the -OH group is attached followed by the term 'alcohol'.

For illustration:

CH3OH               CH3CH2OH

Methyl alcohol    Ethyl alcohol

b) IUPAC names:

In the IUPAC system, alcohols are termed as alkanols and their names are derived from the parent alkane via replacing the 'e' from the parent hydrocarbon and adding the suffix 'ol'.

For illustration:

CH3CH2OH       CH3CH2CH2OH

Ethanol               Propanol

c) Glycols:

Dialcohols or diols are known as glycols. They are named as the derivatives of parent hydrocarbon by the -diol name ending added. The -OH groups are added as the prefixes.

For illustration:

HOCH2CH2OH

1, 2-Ethanediol

Physical Properties of Alcohols:

1) Physical Appearance:

Most of the simple alcohols are liquid at room temperature. Alcohols having more than twelve carbon atoms are waxy solid at room temperature.  

2) Melting and boiling point:

Similar to the parent hydrocarbon, the boiling points among the alcohols increase gradually by the increase in the number of carbon atoms. The boiling point of alcohols is extremely high as compared with that of the corresponding alkane as their capability to form hydrogen bond between the hydrogen atom of the hydroxyl group and the oxygen atom of the other molecule.

The influence of the -OH group though becomes less significant as the carbon skeleton rises.

The order of boiling points of the isomeric alcohols is as follows:

1° alcohol > 2° alcohols > 3° alcohols

3) Density:

The density of alcohols increases by increasing the relative molecular mass, though, branching can decrease this effect.

All the simple alcohols are less dense than water; however the aromatic homologues are slightly denser than water.

4) Solubility:

Hydrogen bonding in alcohols affects their solubility in water. Alcohols by a relatively short carbon skeleton are soluble in water. For illustration: methane, ethanol, propan-1-o1 and propan-2-o1 are fully miscible with water in all chains increases, the solubility of alcohols drop drastically.

All the alcohols are miscible by most organic solvents like hexane, benzene and ethanol; the smaller ones are helpful organic solvents.

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