Alcohols, Phenols and Ethers

Organic substances are divided into three categories: alcohol, phenol, and ether. These compounds have a wide range of industrial and home uses. Alcohol is formed when the hydroxyl (-OH) group binds to a saturated carbon atom. Ether is formed when alcohol is dehydrated. Based on the hydroxyl group, there are three kinds of alcohol: monohydric, dihydric, and trihydric. 

In hydrocarbon, when we replace one or more hydrogen with another group or groups of atoms a new substance is formed which is completely different from the previous compound. So in that manner, the organic compound in which one or more hydrogen atoms are replaced by hydroxyl groups are termed as alcohols or phenols. Hydroxyl derivatives of aliphatic hydrocarbons are alcohol whereas phenols are hydroxyl derivatives of aromatic hydrocarbons in which the hydroxyl group is directly attached to the carbon atom of the aromatic ring. However, the organic compounds in which the hydrogen atom of a hydrocarbon (aliphatic or aromatic) replace by an alkoxy (OR) or aryloxy(OAR) group are known as ethers.

Alcohol

Alcohols are chemical molecules in which the -OH group or hydroxyl group replaces the hydrogen atom of an aliphatic carbon. As a result, an alcohol molecule is made up of two parts: one with the alkyl group and the other with the hydroxyl group. It’s written as R-OH, with R denoting the alkyl group. 

The most well-known alcohol is ethanol, also known as ethyl alcohol, which is widely used in alcoholic beverages, as a preservative for biological specimens, as fuel (gasoline), and as a drug and paint solvent.

The suffix -ol is used to the parent chain of the alkane name when designating alcohols. The -OH functional group’s location is specified in the name. The parent chain is numbered at the end nearest to where the -OH is placed. C_nH_2n+1OH is the general formula for alcohol homologous series, where n=1, 2, 3,… 

Classification of Alcohols

Depending on how many –OH groups are connected to the carbon atom chain, alcohols can be classified into two different classes:

• On the basis of the carbon atoms that are linked to the hydroxyl group: The carbon and oxygen atoms are sp₃ hybridized in all of these kinds. 
  1. Primary (1°) Alcohols – The chemical is primary alcohol if the -OH group is connected to a primary carbon atom (1°, coupled to just one other carbon atom). As a result, the carbon atom carrying the -OH group is only connected to one neighbouring alkyl group. Primary alcohols include the following: ethanol, propan 1-ol, 2 methylpropan 1-ol, etc.
  2. Secondary (2°) Alcohols – When the -OH group is connected to a secondary carbon atom (2°, which is coupled to two additional carbon atoms), the chemical is classified as secondary alcohol. As a result, the carbon atom carrying the -OH group is directly connected to two neighbouring alkyl groups, which may be the same or different. The following are some examples: propan 2-ol, butan 2-ol, etc.
  3. Tertiary (3°) Alcohols – The compound is tertiary alcohol if the -OH group is connected to a tertiary carbon atom (3°, bonded to three additional carbon atoms). As a result, the carbon atom bearing the -OH group is directly bonded to three neighbouring alkyl groups that can be any combination of the same or different groups. Tertiary alcohols can be found in the following forms: 2 – methylpropan 2-ol, etc.
• Alcohols are categorised into three types based on the number of hydroxyl groups attached:
  • Monohydric Alcohols – Monohydric alcohols are those with only one hydroxyl group in their molecules. The kind of hybridisation of the carbon atom to which the hydroxyl group is connected further categorises monohydric alcohols as,
    • Alcohols containing C (sp²)–OH bond (Vinylic alcohols): The –OH group is connected to an sp² hybridised carbon atom, such as vinylic carbon, in these monohydric alcohols. Vinylic alcohols are another name for these alcohols.
    • Alcohols containing C (sp³) – OH bond:
      1. Benzylic Alcohols are the alcohols in which the -OH group is linked to the carbon that is sp³ hybridised which is present next to the aromatic ring. e.g. phenyl methanol, etc.
      2. Allylic Alcohols are the alcohols in which the hydroxyl group is attached to the sp³ hybridised carbon atom next to the carbon-carbon double bond, i.e., allylic carbon. Allylic alcohol may be primary, secondary and tertiary alcohols.
  • Dihydric Alcohols – Dihydric alcohols are alcohols with two hydroxyl groups in their molecules. Propylene glycol is an example of dihydric alcohol.
  • Trihydric Alcohols – Trihydric alcohols are alcohols with three hydroxyl groups in their molecules. Glycerol, for example.

Phenol

Phenol is a hydroxyl compound that is organic, aromatic and has one or more hydroxyl groups directly linked to the aromatic ring. C₆H₅OH is the chemical formula for phenol. It is a moderately acidic white crystalline substance that occurs as a byproduct of coal tar distillation. It’s a hygroscopic and volatile liquid. It has a sweet aroma and a harsh scorching flavour. It is utilised in the production of a wide range of vital goods, including polymers and antiseptics.

Classification of Phenol

Phenols are categorised into three categories based on the amount of hydroxyl (–OH) groups connected to the aromatic ring.

1. Monohydric phenols: Monohydric phenols are phenols with only one hydroxyl (–OH) group. e.g. O-cresol, etc.
2. Dihydric phenols: Dihydric phenols are phenols with two hydroxyls (–OH) groups. The phenols can be ortho, meta, or para derivatives. e.g. Catechol, Resorcinol, etc.
3. Trihydric phenols: Trihydric phenols are phenols with three hydroxyls (–OH) groups. e.g. Benzene- 1, 2, 3-triol.

Ethers

Ethers are a type of organic molecule in which two hydrocarbon groups (alkyl or aryl) are linked together by a single oxygen atom. The R-O-R′ formula is used to express it. The hydrocarbon group in the formula, R′, might be the same as or different from R. Ethers are formed when the hydrogen atom of an alcohol’s hydroxyl group is replaced with an alkyl or aryl group.

Classification of Ethers

Ethers are categorized as simple or symmetrical ethers depending on the alkyl or aryl group attached to an oxygen atom.

• Symmetrical Ethers – If the two alkyl or aryl groups attached to the oxygen atom are the same is known as symmetrical ethers.

e.g. CH₃-O-CH₃  (Dimethyl ether), CH₃CH₂-O-CH₂CH₃ (Diethyl ether). C₆H₅-O-C₆H₅ (Diphenyl ether), etc.

• Unsymmetrical ethers – If the two groups attached to the oxygen atom are different and is known as unsymmetrical ethers. 

e.g. CH₃-O-CH₂CH₃ (Ethyl methyl ether), C₆H₅-O-CH₃ (Methyl phenyl ether), C₆H₅-O-CH₂C₆H₅ (Benzyl phenyl ether), etc.

Sample Questions 

Question 1: How to differentiate between alcohol, phenol and ethers?

Answer:

In the case of alcohol (OH) group means a hydroxyl group attached to a carbon atom and in phenol hydroxyl group attached to benzene ring while in ether two carbon groups are attached by a single bond with oxygen.

Question 2: State whether phenol is an acid or a base?

Answer:

The nature of the phenol is acidic as it has a tendency to the hydrogen ions from its OH bond, after closing the hydrogen phenoxide ion is formed which is more stable.

Question 3: Why ethers are less dense than alcohols?

Answer:

As there is no hydrogen bond formed by ethers and has more boiling point than those of which forms hydrogen bond i.e., alcohols. So ethers are less dense than alcohols.

Question 4: What do you mean by denatured alcohol?

Answer:

The change in the property of alcohol referred to products adulterated with toxic additives which are unsuitable for the consumption of humans.

Question 5: In alcohol or phenol which is more soluble in water and why?

Answer:

Both alcohol and phenol are soluble in water as bonds can form hydrogen bonds with water but due to the presence of benzene ring means containing a large hydrocarbon group the solubility of phenol is less than alcohol.