CBSE Class 12 Chemistry (Organic Chemistry)
Chapter 8: Alcohols, Phenols and Ethers
20 Important Questions and Answers
1. What are alcohols? Classify alcohols based on the number of hydroxyl groups.
Answer:
Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a saturated carbon atom. Based on the number of hydroxyl groups present, alcohols are classified into three categories. Monohydric alcohols contain one hydroxyl group, such as ethanol. Dihydric alcohols contain two hydroxyl groups, such as ethane-1,2-diol (ethylene glycol). Trihydric alcohols contain three hydroxyl groups, such as propane-1,2,3-triol (glycerol). Alcohols can also be classified as primary, secondary, or tertiary depending on the carbon atom to which the hydroxyl group is attached. Their physical and chemical properties depend on the number and position of hydroxyl groups.
2. Explain the preparation of ethanol from ethene.
Answer:
Ethanol is prepared from ethene by the process of acid-catalysed hydration. In this reaction, ethene reacts with water in the presence of phosphoric acid catalyst under high temperature and pressure conditions. The double bond of ethene breaks, and the hydroxyl group and hydrogen atom are added across the double bond, forming ethanol. The reaction is represented as:
CH₂=CH₂ + H₂O → CH₃CH₂OH.
This method is widely used in industries because it gives a good yield of ethanol. Ethanol obtained by this process is used as a solvent, fuel additive, and raw material for the manufacture of various organic compounds.
3. Why do alcohols have higher boiling points than hydrocarbons of comparable molecular masses?
Answer:
Alcohols possess higher boiling points than hydrocarbons of similar molecular masses because of intermolecular hydrogen bonding. The oxygen atom in the hydroxyl group is highly electronegative and attracts the hydrogen atom strongly, creating a polar O–H bond. As a result, alcohol molecules are associated through hydrogen bonds. These strong intermolecular forces require more energy to break during boiling. In contrast, hydrocarbons contain only weak van der Waals forces between molecules. Therefore, more heat is needed to separate alcohol molecules, resulting in higher boiling points. The boiling point generally increases with molecular mass and decreases with branching in alcohol molecules.
4. Describe the reaction of alcohols with sodium metal.
Answer:
Alcohols react with active metals like sodium to form sodium alkoxides and liberate hydrogen gas. The hydrogen atom of the hydroxyl group is replaced by sodium due to the acidic nature of alcohols. The reaction can be represented as:
2ROH + 2Na → 2RONa + H₂↑.
For example, ethanol reacts with sodium to form sodium ethoxide and hydrogen gas. This reaction demonstrates the weak acidic character of alcohols. The evolution of hydrogen gas can be observed as bubbles during the reaction. The formation of alkoxides is important because alkoxides are useful intermediates in many organic synthesis reactions.
5. What is dehydration of alcohols? Explain with an example.
Answer:
Dehydration of alcohols is the removal of a water molecule from an alcohol to produce an alkene. This reaction is generally carried out by heating alcohol with concentrated sulphuric acid at about 443 K. For example, ethanol undergoes dehydration to form ethene:
CH₃CH₂OH → CH₂=CH₂ + H₂O.
The acid acts as a dehydrating agent and removes water from the alcohol molecule. Dehydration is an elimination reaction and is important for the industrial preparation of alkenes. The ease of dehydration follows the order: tertiary alcohols > secondary alcohols > primary alcohols because tertiary carbocations are more stable.
6. Why are phenols more acidic than alcohols?
Answer:
Phenols are more acidic than alcohols because the phenoxide ion formed after losing a proton is stabilized by resonance. In phenol, the negative charge on the oxygen atom is delocalized over the aromatic ring through resonance structures. This stabilization makes the loss of a proton easier. In alcohols, the alkoxide ion formed does not have resonance stabilization, and the negative charge remains localized on oxygen. Therefore, alcohols are less acidic. Phenol reacts with sodium hydroxide to form sodium phenoxide, whereas most alcohols do not react with sodium hydroxide. This difference clearly demonstrates the greater acidity of phenols.
7. Explain the preparation of phenol from chlorobenzene.
Answer:
Phenol can be prepared from chlorobenzene through Dow’s process. Chlorobenzene is heated with aqueous sodium hydroxide at high temperature (623 K) and high pressure. Sodium phenoxide is formed as an intermediate. On acidification with dilute hydrochloric acid, sodium phenoxide yields phenol.
C₆H₅Cl + NaOH → C₆H₅ONa + NaCl
C₆H₅ONa + HCl → C₆H₅OH + NaCl.
This method is widely used for the commercial production of phenol. Phenol obtained is an important industrial chemical used in the manufacture of plastics, medicines, dyes, and disinfectants.
8. What happens when phenol reacts with bromine water?
Answer:
Phenol reacts readily with bromine water due to the activating effect of the hydroxyl group attached to the benzene ring. The hydroxyl group increases the electron density at the ortho and para positions, making electrophilic substitution easier. When phenol is treated with bromine water, a white precipitate of 2,4,6-tribromophenol is formed.
C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr.
The reddish-brown colour of bromine water disappears during the reaction. This reaction serves as a characteristic test for phenol and demonstrates its high reactivity towards electrophilic substitution reactions.
9. What is the Kolbe reaction?
Answer:
The Kolbe reaction is an important reaction of phenol used for the preparation of salicylic acid. Sodium phenoxide reacts with carbon dioxide under pressure and moderate temperature to form sodium salicylate. Acidification of sodium salicylate produces salicylic acid.
Salicylic acid is an important compound used in the manufacture of medicines such as aspirin. The hydroxyl group in phenol activates the aromatic ring and directs substitution mainly to the ortho position. This reaction demonstrates the usefulness of phenol in synthesizing valuable aromatic compounds and is an important named reaction in organic chemistry.
10. Explain the Reimer–Tiemann reaction.
Answer:
The Reimer–Tiemann reaction is used for the introduction of a formyl group (-CHO) into the benzene ring of phenol. Phenol reacts with chloroform in the presence of aqueous sodium hydroxide. The reaction mainly produces salicylaldehyde (o-hydroxybenzaldehyde). The hydroxyl group directs substitution to the ortho position. After acidification, the aldehyde product is obtained. This reaction is important because it provides a method for synthesizing aromatic aldehydes from phenols. The Reimer–Tiemann reaction is widely studied due to its synthetic importance and its role in demonstrating electrophilic substitution in aromatic compounds.
11. What are ethers? Give their general structure.
Answer:
Ethers are organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups. Their general formula is R–O–R′, where R and R′ may be the same or different groups. Ethers are classified as simple ethers when both groups are identical and mixed ethers when the groups are different. For example, dimethyl ether (CH₃–O–CH₃) is a simple ether, while methyl ethyl ether (CH₃–O–C₂H₅) is a mixed ether. Ethers have relatively low boiling points compared to alcohols because they cannot form intermolecular hydrogen bonds among themselves.
12. Describe Williamson ether synthesis.
Answer:
Williamson ether synthesis is a common laboratory method for preparing ethers. In this reaction, an alkoxide ion reacts with an alkyl halide through nucleophilic substitution.
RONa + R′X → ROR′ + NaX.
For example, sodium ethoxide reacts with methyl bromide to produce methoxyethane. Primary alkyl halides are preferred because they undergo substitution easily. Secondary and tertiary alkyl halides often give elimination products. Williamson synthesis is versatile and allows the preparation of both symmetrical and unsymmetrical ethers. It is one of the most important methods for ether synthesis and has significant applications in organic chemistry.
13. Why are ethers less reactive than alcohols?
Answer:
Ethers are less reactive than alcohols because they lack the reactive O–H bond present in alcohols. In alcohols, the hydroxyl hydrogen can participate in many reactions such as oxidation and reaction with metals. Ethers contain a stable C–O–C linkage and generally do not undergo reactions easily under normal conditions. They are resistant to oxidation and reduction. However, ethers react with strong acids such as hydroiodic acid and hydrobromic acid, which can cleave the ether linkage. Due to their low reactivity, ethers are commonly used as solvents for organic reactions.
14. Explain the cleavage of ethers by hydrogen iodide.
Answer:
Ethers react with concentrated hydrogen iodide on heating to form alkyl iodides and alcohols or phenols. The oxygen atom of the ether is first protonated, making the C–O bond weak. The iodide ion then attacks the carbon atom and breaks the ether linkage. For example:
CH₃OC₂H₅ + HI → CH₃I + C₂H₅OH.
With excess HI, the alcohol formed may further react to produce another alkyl iodide. This reaction is important because it demonstrates the cleavage of the otherwise stable ether bond. Hydrogen bromide can also bring about similar cleavage reactions.
15. What is anisole? Discuss its nitration.
Answer:
Anisole is an aromatic ether having the structure C₆H₅OCH₃. It consists of a methoxy group attached to a benzene ring. The methoxy group is electron-releasing and activates the benzene ring towards electrophilic substitution. On nitration with a mixture of concentrated nitric acid and sulphuric acid, anisole forms a mixture of ortho-nitroanisole and para-nitroanisole. The para product is generally formed in greater quantity due to less steric hindrance. This reaction illustrates the directing influence of the methoxy group and the enhanced reactivity of anisole compared with benzene.
16. How can alcohols be oxidized?
Answer:
Alcohols undergo oxidation depending on their structure. Primary alcohols are oxidized first to aldehydes and then to carboxylic acids. Secondary alcohols are oxidized to ketones. Tertiary alcohols generally resist oxidation because they lack a hydrogen atom on the carbon bearing the hydroxyl group. Common oxidizing agents include acidified potassium dichromate and potassium permanganate. For example, ethanol is oxidized to ethanal and then ethanoic acid. Oxidation reactions are important in organic synthesis because they help convert alcohols into other useful functional groups with different chemical properties.
17. Why is phenol more reactive towards electrophilic substitution than benzene?
Answer:
Phenol is more reactive towards electrophilic substitution than benzene because the hydroxyl group donates electron density to the aromatic ring through resonance. This increases electron concentration at the ortho and para positions, making the ring more attractive to electrophiles. As a result, reactions such as bromination, nitration, and sulphonation occur more readily in phenol than in benzene. In many cases, phenol reacts without requiring a catalyst. The activating effect of the hydroxyl group significantly enhances the reactivity of the aromatic ring and influences the orientation of incoming substituents.
18. Differentiate between primary, secondary, and tertiary alcohols.
Answer:
Alcohols are classified according to the number of alkyl groups attached to the carbon atom bearing the hydroxyl group. In primary alcohols, the carbon attached to the hydroxyl group is bonded to one alkyl group, as in ethanol. In secondary alcohols, it is bonded to two alkyl groups, as in propan-2-ol. In tertiary alcohols, it is bonded to three alkyl groups, as in tert-butyl alcohol. These alcohols differ in reactivity, oxidation behavior, and dehydration tendencies. Primary alcohols oxidize to aldehydes, secondary alcohols to ketones, while tertiary alcohols are resistant to oxidation under ordinary conditions.
19. What are the uses of phenol?
Answer:
Phenol is an important industrial chemical with numerous applications. It is used in the manufacture of phenolic resins such as Bakelite, which are widely used in electrical and household products. Phenol is also used in the production of medicines, including aspirin and antiseptics. It serves as a starting material for dyes, explosives, and synthetic fibres. Dilute phenol solutions act as disinfectants and antiseptics. In laboratories and industries, phenol is an important reagent for organic synthesis. Due to its versatile properties, phenol plays a significant role in chemical and pharmaceutical industries.
20. State the important uses of ethanol.
Answer:
Ethanol is one of the most widely used alcohols in industry and daily life. It is used as a solvent for medicines, perfumes, paints, and varnishes because it dissolves many organic substances. Ethanol is an important raw material for the preparation of chemicals such as ethanoic acid, ethers, and esters. It is used as a fuel and as a fuel additive in petrol to reduce pollution. Ethanol is also used in alcoholic beverages and in pharmaceutical preparations. Due to its antiseptic properties, it is employed in sanitizers and disinfectants. Its wide range of applications makes ethanol economically important.