Organic chemicals such as alcohol, phenol, and ether are widely used in both residential and industrial environments. These products range from perfumes to sanitisers to fuels and have a wide range of applications in our daily lives. In this article, we will look at their structure and classification in detail.

Alcohol

Alcohols are the organic compounds in which the hydrogen atom of an aliphatic carbon is replaced by the $\text{-OH}$ group or hydroxyl group. Hence, an alcohol molecule comprises of two parts; one containing the alkyl group and the other containing the hydroxyl group. It is represented as $\text{R-OH,}$ where $\text{R}$ is the alkyl group.

The most commonly known alcohol is ethanol or ethyl alcohol, widely used in alcoholic drinks, a preservative for biological specimens, fuel (gasoline),  and as a solvent for drugs and paints.

While naming alcohols, the suffix $\text{-ol}$ is added to the parent chain of the alkane name. The position of the $\text{-OH}$ functional group is indicated in the name. The numbering of the parent chain is done at the end closest to where the $\text{-OH}$ is located. The general formula of alcohol homologous series is ${\text{C}}_{\text{n}}{\text{H}}_{2\text{n}+1}\text{OH},$ where $\text{n}=1,2,3\dots .$

Structure of Alcohol

Similar to water, alcohols have an oxygen atom which is $\text{s}{\text{p}}^3$ hybridised with two nonbonding pairs of electrons. The oxygen atom is bonded to an $\text{s}{\text{p}}^3$ hybridised carbon atom of an alkyl group. The $\text{R-O-H}$ bond angle in alcohols is ${108.9}^{\circ }$ which is larger than $\text{H-O-H}$ bond angle in the water. This is due to the bulkiness of the alkyl groups present in alcohols.

There are three types of bonds in alcohol. $\left(\text{s}{\text{p}}^3\right)\text{C}–\text{H}\left(1\text{s}\right)$ sigma bond, $\left(\text{s}{\text{p}}^3\right)\text{C}–\text{O}\left(\text{s}{\text{p}}^3\right)$ sigma bond and $\left(\text{s}{\text{p}}^3\right)\text{O}-\text{H}\left(1\text{s}\right)$ sigma bond. This is diagrammatically represented as below.

Classification of Alcohols

Alcohols can be grouped into different classes depending on how many $–\text{OH}$ groups are attached to the chain of carbon atoms.

1. Classification based on which carbon atom is bonded to the hydroxyl group. In all these types, the carbon atom and the oxygen atom is $\text{s}{\text{p}}^3$ hybridised.

Primary (1°) Alcohols

If the $\text{-OH}$ group is attached to a primary carbon atom ($1^{\circ },$ bonded to only one other carbon atom), then the compound is a primary alcohol. Hence, the carbon atom that carries the $\text{-OH}$ group is attached to only one neighbouring alkyl group. Some examples of primary alcohols are: Methanol, $\text{C}{\text{H}}_3\text{OH},$ is classified as primary alcohol even though no alkyl groups are attached to the $\text{-OH}$ carbon atom.

Secondary (2°) Alcohols

If the $\text{-OH}$ group is attached to a secondary carbon atom ($2^{\circ },$ bonded to two other carbon atoms), then the compound is a secondary alcohol. Hence, the carbon atom that carries the $\text{-OH}$ group is attached directly to two neighbouring alkyl groups, which may be the same or different. Examples include the following:

Tertiary (3°) Alcohols

If the $\text{-OH}$ group is attached to a tertiary carbon atom ($3^{\circ },$ bonded to three other carbon atoms), then the compound is a tertiary alcohol. As a result, the $\text{-OH}$ group-carrying carbon atom is directly connected to three adjacent alkyl groups, which can be any combination of the same or different groups. The following are some examples of tertiary alcohols:

Based on the number of hydroxyl groups attached, alcohols can be classified as:

Monohydric Alcohols

Alcohols having only one hydroxyl group in their molecules are called monohydric alcohols.

Monohydric alcohols are further classified according to the type of hybridisation of the carbon atom to which the hydroxyl group is attached.

(1) Alcohols containing $\text{C}\left(\text{s}{\text{p}}^3\right)–\text{OH}$ bond: In these alcohols -the $\text{OH}$ group is attached to a $\text{s}{\text{p}}^3$ hybridised carbon atom of the alkyl group. These alcohols are represented as $\text{R-OH}\text{.}$ They are further classified as primary, secondary and tertiary alcohols in which $\text{-OH}$ group is attached to primary, secondary, and tertiary carbon atoms, respectively. (a) Allylic Alcohols: In these alcohols $\text{-OH}$ is attached to a $\text{s}{\text{p}}^3$ -hybridised carbon atom next to the carbon-carbon double bond, i.e., allylic carbon. Allylic alcohol may be primary, secondary and tertiary alcohols. (b) Benzylic Alcohols: In these alcohols -the $\text{OH}$ group is attached to a $\text{s}{\text{p}}^3$ -hybridised carbon atom next to an aromatic ring. The hydroxyl group $\left(–\text{OH}\right)$ is present in the side chain attached to an aromatic ring. Benzylic alcohol may be primary, secondary and tertiary alcohols. (2) Alcohols containing $\text{C}\left(\text{s}{\text{p}}^2\right)–\text{OH}$ bond (Vinylic alcohols): These monohydric alcohols have $–\text{OH}$ group attached to a $\text{s}{\text{p}}^2$ hybridised carbon atom, e.g., vinylic carbon. These alcohols are also called vinylic alcohols

Dihydric Alcohols

Alcohols having two hydroxyl groups in their molecules are called dihydric alcohols. For example – Propylene glycol

Trihydric Alcohols

Alcohols having three hydroxyl groups in their molecules are known as trihydric alcohols. For example – Glycerol

Phenol

Phenol is an organic, aromatic, hydroxyl compound in which one or more hydroxyl groups are directly attached to the aromatic ring. The chemical formula of Phenol is ${\text{C}}_6{\text{H}}_5\text{OH}.$ It’s a white crystalline solid that forms as a byproduct of coal tar distillation and is mildly acidic. It’s a hygroscopic and volatile substance. It smells sweet and has a strong burning flavour. It is used to make a variety of important products, including polymers and antiseptics.

Structure of Phenol

1. All carbon atoms comprising the aromatic ring of Phenol are $\text{s}{\text{p}}^2$ hybridised. Hence, the phenyl ring has a hexagonal planar structure with all bond angles ${120}^{\circ }$ and delocalised $\pi$-electrons distributed over the ring.
2. The hybridisation of the $\text{C-O}$ bond is $\text{s}{\text{p}}^2\text{-s}{\text{p}}^3,$ respectively, and the $\text{O-H}$ bond is formed from $\text{O}\left(\text{s}{\text{p}}^3\right)–\text{H}\left(1\text{s}\right)$ hybridisation.
3. As oxygen is more electronegative than carbon and hydrogen atoms, both the $\text{C-O}$ and $\text{O-H}$ bonds are polar and have a bent shape with a bond angle of ${109}^{\circ }.$

Phenol is weakly acidic. This is due to the fact that it rapidly loses the hydrogen atom, forming a phenoxide ion (phenolate) ion that is stabilised by resonance. The negative charge dispersion over the molecule can be seen using resonance structures or as a resonance hybrid, as shown below.

Classification of Phenol

Depending on the number of hydroxyl $\left(–\text{OH}\right)$ groups attached to the aromatic ring, phenols can be classified into three types

1. Monohydric phenols: Phenols that contain one hydroxyl $\left(–\text{OH}\right)$ group are called monohydric phenols.

2. Dihydric phenols: Phenols that contain two hydroxyl $\left(–\text{OH}\right)$ groups are called dihydric phenols. These phenols may be ortho-, meta- or para- derivative.

3. Trihydric phenols: Phenols that contain three hydroxyl $\left(–\text{OH}\right)$ groups are called trihydric phenols.

Ethers

Ethers belong to the class of organic compound in which two hydrocarbon groups (alkyl or aryl) are bonded to each other through the same oxygen atom. It is represented by the general formula ${\text{R-O-R}}^{\prime }$ as shown below.

The ${\text{R}}^{\prime },$ i.e. the hydrocarbon group in the formula, can be the same as $\text{R}$ or different.

Ethers are obtained when the hydrogen atom of the hydroxyl group in alcohols is replaced by an alkyl or aryl group. For example-

The oxygen and carbon in the $\text{C-O-C}$ bond is $\text{s}{\text{p}}^3$ hybridised. The repulsion between two lone pairs $\left(\text{lp}\right)$ on the oxygen atom results in a bent shape. The presence of bulky groups at both ends of the oxygen atom creates a steric hindrance and the $\text{bp-bp}$ repulsion results in a $\text{C-O-C}$ bond angle of about ${111.7}^{\circ }.$

Classification of Ethers

Depending on the groups at $\text{R}$ and $\text{R’,}$ ethers are classified into two types:

1. Simple ethers or symmetrical ethers: These ethers consist of the same alkyl group at both ends of the oxygen atom. For example-

2. Mixed ethers or asymmetrical ethers: These ethers consist of different alkyl groups at both ends of the oxygen atom. For example-

Summary

Alcohols and phenols are classified depending on the number of hydroxyl groups attached to the carbon atom. In contrast, ethers are classified on the basis of the hydrocarbon chain attached to both ends of the oxygen atom. Through this article, we learnt the classification of alcohols, phenols and ether, along with examples. We also learnt how alcohols and ethers are functional isomers of each other.

FAQs on Classification of Alcohols, Phenols and Ethers

Q.1. What is the classification of ether?
Ans: Based on the hydrocarbon (aryl or alkyl) chain present at both ends of the oxygen atom, ethers are classified into two types – Simple or Symmetrical ethers, Mixed or Asymmetrical ethers.
When the hydrocarbon (aryl or alkyl) chain present at both ends of the oxygen atom is the same, the ether so formed is called Simple or symmetrical ethers. Example – $\text{C}{\text{H}}_3\text{OC}{\text{H}}_3$
When the hydrocarbon (aryl or alkyl) chain present at both ends of the oxygen atom is different, the ether so formed is called Mixed or Asymmetrical ethers. Example -$\text{C}{\text{H}}_3\text{O}{\text{C}}_2{\text{H}}_5$

Q.2. What are alcohols? How are they classified?
Ans: Alcohols are the organic compounds in which the hydrogen atom of an aliphatic carbon is replaced by the $\text{-OH}$ group or hydroxyl group.
Based on the number of hydroxyl groups attached, alcohols are classified into monohydric (one $\text{-OH}$ group), dihydric (two $\text{-OH}$ group) and trihydric (three $\text{-OH}$ group) alcohols.
Based on which carbon atom is bonded to the hydroxyl group, alcohols are classified into primary $\left(1^{\circ }\right),$ secondary $\left(2^{\circ }\right),$ tertiary $\left(3^{\circ }\right),$ alcohols.

Q.3. What are trihydric alcohols? Give an example?
Ans: Alcohol that contains three hydroxyl groups is known as trihydric alcohol. For example, Glycerol is trihydric alcohol as it has three $\text{-OH}$ groups attached to the parent chain.

Q.4. What is the difference between alcohol and ether?
Ans: Both Ethers and alcohol have a similar structure and resemble water. In alcohol, one hydrogen atom of a water molecule is replaced by an alkyl group, whereas in an ether, both hydrogen atoms are replaced by alkyl or aryl groups.

Q.5. What type of isomers are alcohol and ether?
Ans: Alcohol and ethers are functional isomers. For example – Both dimethyl ether and ethanol have the same molecular formula ${\text{C}}_2{\text{H}}_6\text{O}$ but have different functional groups (the $–\text{O}–$ ether group and the $–\text{OH}$ alcohol group)