Naming of Alcohols - JEE Important Topic

Most people know that alcohol has many uses in our daily lives as solvent, disinfectant, and in the preparation of medicines. However, many do not know that this substance is toxic in its purest form or in high concentration solutions. The laboratory has containers labelled "Absolute Ethanol", "95% Ethanol", and "Denatured Ethanol". Laboratory ethanol is not suitable for consumption under any circumstances. There are many different types of alcohols such as methanol, isopropyl alcohol, phenol (aromatic alcohol), butanol, etc. All these alcohols have a functional group -OH attached to the carbon atom.

Alcohol Classification

Alcohols can be classified as primary (1o), secondary (2o), or tertiary (3o), depending on the number of alkyl substituents attached to the carbon attached to the O-H group.

• Primary Alcohols 

In primary (1°) alcohols, carbons with -OH groups bind to only one alkyl group. The abbreviation for ethanol is ‘-ol’. For example: propan-1-ol, 2-methylpropan-1-ol.

Note that it does not matter how complex the pendant alkyl group is. In each case, there is only one bond from the CH₂ group with the -OH group to the alkyl group. There is one exception to this. Methanol and CH₃OH are counted as primary alcohols, but carbons with -OH groups do not have an alkyl group attached.

• Secondary Alcohol 

In secondary (2°) alcohol, the carbon-containing -OH group is directly bonded to two alkyl groups. The two alkyl groups can be the same or different. 

For example: Propan-2-ol, butan-2-ol, and pentane-3-ol.

• Tertiary Alcohol 

In tertiary (3°) alcohol, the carbon atom carrying the -OH group is directly attached to the three alkyl groups. This can be any combination of the same or different alkyl groups.

Nomenclature of Alcohols

• The IUPAC nomenclature for alcohols is based on the name of the parent alkane chain.

• The longest chain containing a hydroxyl (OH) group is considered the parent chain. 

• Remove the trailing -e from the parent alkane chain name and add the suffix -ol.

• Number the parent alkane chains so that the number of hydroxyl groups is as low as possible. 

• The old IUPAC rules originally preceded the parent chain name with a hydroxyl group number. 

• However, the new rule is numbered before the suffix -ol.

• Number the substituents according to the position of the parent chain.

• Then list the substituents in alphabetical order.

• When naming a cyclic structure with a hydroxyl group, -OH is considered to be on the first carbon.

• If you have more than one alcohol, use di, tri, etc. after the parent's name and before ol. 

• -e is not removed from the parent chain name, even if the prefix is ​​used, 2,3-Hexane-di-ol.

• If an alkene and alcohol are present in the molecule, they are named as follows (alkene site)-(parent chain prefix + en)-(hydroxyl site)-ol.

Example:

1. METHANOL - $C{{H}_{3}}OH$

2. ETHANOL - $C{{H}_{3}}C{{H}_{2}}OH$

3. 2,2-dimethyl-1 - propanol - \[{{(C{{H}_{3}})}_{3}}CC{{H}_{2}}OH\]

4. But-3-en-1-ol - $C{{H}_{2}}=CH-C{{H}_{2}}-C{{H}_{2}}OH$ 

Nomenclature of Phenol

• Identify the position of the hydroxyl group attached to the benzene ring.

• Benzene rings attached to multiple hydroxyl groups are prefixed with Greek numbers such as Di, Tri, Tetra to indicate the number of similar hydroxyl groups attached to the benzene ring. 

• When two hydroxyl groups are attached to the adjacent carbon atoms of the benzene ring, it is said that the benzene-1,2-diol, hydroxyl group is attached. 

• For example, if a methyl group is attached to the 4th carbon atom of a hydroxyl group, the compound is called 4-methylphenol.

• Depending on the position of the substituted functional group with respect to the hydroxyl group, ortho (if the functional group is attached to an adjacent carbon atom, (1,2)), para (if the functional group is attached to the third, (1-4)), meta (when the functional group is attached to the second carbon atom from the hydroxyl group, (1,3)).

• Examples:
  
  

Phenol C₆H₅–OH

Uses of Alcohol

• The word "alcohol" contained in alcoholic beverages refers to ethanol (CH₃CH₂OH).

• Ethanol is usually sold as industrial denatured alcohol. 

• Ethanol burns to produce carbon dioxide and water as shown in the following formula and can be used as a stand-alone fuel or in a blend with gasoline (gasoline).

${{C}_{2}}{{H}_{5}}OH+3{{O}_{2}} \longrightarrow 2C{{O}_{2}}+3{{H}_{2}}O$

• Ethanol is often used as a solvent. It is relatively safe and can be used to dissolve many organic compounds that are insoluble in water. For example, it is used in many perfumes and cosmetics.

• Methanol also burns to produce carbon dioxide and water:

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

Most methanol is used in the production of other compounds such as metanal (formaldehyde), acetic acid, and methyl esters of various acids. In most cases, these are converted to other products.

Summary

Alcohols and phenols are organic compounds with at least one hydroxyl group attached to saturated and aryl carbons, respectively. Enols are a related third class of compounds with hydroxyl groups attached to vinyl carbon. Alcohol molecules contain one or more hydroxyl (OH) groups that replace hydrogen atoms along the carbon chain. The structure of the simplest alcohol, methanol (methyl alcohol), can be derived from the structure of methane by replacing one of H with OH.

A compound in which a hydroxyl group is attached to an aromatic ring is called phenol. The chemical behaviour of phenol differs from that of alcohol in some respects. Therefore, it makes sense to treat phenol as a group that is similar but characteristically different.