Alcohols, Phenols and Ethers Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by -OH group. These classes of compounds find wide applications in industry as well as in day-to-day life. For instance, have you ever noticed that ordinary spirit used for polishing wooden furniture is chiefly a compound containing hydroxyl group, ethanol. The sugar we eat, the cotton used for fabrics, the paper we use for writing, are all made up of compounds containing -OH groups. The common name of an alcohol is derived from the common name of the alkyl group and adding the word alcohol to it. For example, \(\mathrm{CH_3OH}\) is methyl alcohol. The simplest hydroxy derivative of benzene is phenol. It is its common name and also an accepted IUPAC name. As structure of phenol involves a benzene ring, in its substituted compounds the terms ortho (1,2-disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted) are often used in the common names. In ethers, the four electron pairs, i.e., the two bond pairs and two lone pairs of electrons on oxygen are arranged approximately in a tetrahedral arrangement. The bond angle is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky (-R) groups.
What is the common name for the following structure given below?
Show Hint
Find the relative positions of the two OH groups: they are para (1,4).
Step 1: Understanding the Concept: The structure is a benzene ring with two -OH groups. Such compounds are called dihydric phenols. The common name depends on where the two groups sit on the ring.
Step 2: Key Formula or Approach: Find the relative positions of the two -OH groups. Ortho is 1,2, meta is 1,3 and para is 1,4. Then match the position with the common name.
Step 3: Read the structure: In the drawing one -OH is at the top carbon and the other is at the bottom carbon. They are on opposite corners of the ring. That is the 1,4 or para position. So the compound is benzene-1,4-diol.
Step 4: Check option (1): Resorcinol is benzene-1,3-diol, with the groups in the meta position. Our compound is para. So option 1 is WRONG.
Step 5: Check option (2): Catechol is benzene-1,2-diol, with neighbouring -OH groups (ortho). So option 2 is WRONG.
Step 6: Check option (3): Hydroquinone is benzene-1,4-diol. This matches the drawing. So option 3 is CORRECT.
Step 7: Check option (4): m-cresol is 3-methylphenol. It has one -OH and one -CH3 group, not two -OH groups. So option 4 is WRONG.
Final Answer: The compound is benzene-1,4-diol, whose common name is hydroquinone. This is option 3.\[ \boxed{\text{Hydroquinone}} \]
