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Answer the following.
The hydroxy derivatives of aliphatic hydrocarbons are called alcohols. Ethanol is perhaps the most…
The hydroxy derivatives of aliphatic hydrocarbons are called alcohols. Ethanol is perhaps the most important aliphatic alcohol. During COVID-19 pandemic alcohol-based sanitizers became an essential protective tool worldwide, reducing the spread of the virus. Nowadays, it is also used as a substitute for petrol in internal combustion engines.
The boiling points of alcohols are considerably higher than those of the corresponding ethers due to the presence of intermolecular hydrogen bonding in alcohols and it increases with increase in molecular mass.
For isomeric alcohols the boiling point follows the order:
Primary ($1^\circ$) alcohol > Secondary ($2^\circ$) alcohol > Tertiary ($3^\circ$) alcohol, which is evident from the following data:
Aliphatic alcohols behave as weak acids and ionize to a small extent. Their acidic strength is even less than that of both water and phenol. Phenols are hydroxy derivatives of arenes and have varied application in manufacturing of dyes, plastics, drugs, explosives, etc. At different concentrations, phenols are used both as a disinfectant and an antiseptic.
| Compound | Propan-1-ol | Butan-1-ol | Butan-2-ol | 2-methyl-propan-2-ol |
|---|---|---|---|---|
| Boiling point | 370K | 391K | 371K | 356K |
(i)[1.0]
The boiling point of 2-methyl-propan-2-ol is much lower than that of butan-1-ol, although both have the same molecular formula. Give a reason to explain such observation.
(ii)[1.0]
Although alcohols are weaker acids than water, phenol is more acidic than alcohols. Justify.
(iii)[1.0]
In what form (i.e. concentration), does phenol act as a disinfectant or an antiseptic?
Answer
Answer (i)
Official answer keyBecause of branching, 2-methylpropan-2-ol has a smaller surface area (more compact, nearly spherical) than butan-1-ol, so the van der Waals forces between its molecules are weaker and its boiling point is lower.
Answer (ii)
Official answer keyBecause the phenoxide ion is resonance stabilised, as the negative charge is spread over the benzene ring. The oxygen of phenol attracts the O-H electron pair strongly, weakening the O-H bond so that $\mathrm{H^+}$ is released more easily. An alkoxide ion has no resonance, and the $+I$ effect of the alkyl group makes it less stable.
Answer (iii)
Official answer keyA 0.2% solution of phenol acts as an antiseptic, whereas a 1% solution is used as a disinfectant.
From ISC 2027 Specimen Chemistry Paper 1, question 1(C).