CBSE Class 12 Chemistry Alcohols Phenols And Ethers Question Bank

Question. Which of the following is a secondary alcohol?
a) 2-methylbutan-2-ol
b) 3-methylbutan-1-ol
c) 2-methylbutan-1-ol
d) 3- methylbutan-2-ol
Answer. D

Question. Reaction between acetone and methylmagnesiumchloride followed by hydrolysis will give :
a) Isobutylalcohol
b) Isopropyl alcohol
c) sec-butyl alcohol
d) tert-butyl alcohol
Answer. D

Question. Which of the following will not be soluble in NaHCO₃
a) 2,4,6 trinitrophenol
b) Benzoic acid
c) o- Nitrophenol
d) Benzene sulphonic acid
Answer. B

Question. Which of the following reagents may be used to distinguish between phenol and benzoic acid ?
a) Neutral FeCl₃
b) Aqueous NaOH
c) Tollen’s reagent
d) Molisch reagent
Answer. A

 

SHORT ANSWER TYPE QUESTIONS

Question. How simple or symmetrical is different from mixed or unsymmetrical?
Answer. Ethers are classified as simple or symmetrical, if the alkyl or aryl groups attached to the oxygen atom are the same, and mixed or unsymmetrical, if the two groups are different. Diethyl ether, C₂H₅OC₂H₅ , is symmetrical ether whereas C₂H₅OCH₃ and C₂H₅OC₆H₅ are unsymmetrical ethers.

Question. Give a chemical test to distinguish between the following pairs of compounds: Methanol and phenol
Answer. Phenol gives a violet colouration with FeCl₃ solution while methane does not.

Question. The carbon– oxygen bond length (136 pm) in phenol is slightly less than that in methanol. Give reason?
Answer. The carbon– oxygen bond length (136 pm) in phenol is slightly less than that in methanol. This is due to (i) partial double bond character on account of the conjugation of unshared electron pair of oxygen with the aromatic ring and (ii) sp² hybridised state of carbon to which oxygen is attached.

Question. Why the bond angle in ether is slightly greater than the tetrahedral angle?
Answer. 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.

Question. Why the boiling point of 2- Methylpropan-2-ol is less than that of butanol?
Answer. The boiling points of alcohols and phenols increase with increase in the number of carbon atoms (increase in van der Waals forces). In alcohols, the boiling points decrease with increase of branching in carbon chain (because of decrease in van der Waals forces with decrease in surface area).

Question. Arrange the following sets of compounds in order of their increasing boiling points:
Answer. (a) Pentan-1-ol, butan-1-ol, butan-2-ol, ethanol, propan-1-ol, and methanol.
(b) Pentan-1-ol, n-butane, pentanal, ethoxyethane. (a) Methanol, ethanol, propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol.
(b) n-Butane, ethoxyethane, pentanal and pentan-1-ol.

Question. Why alcohols act as Bronsted bases?
Answer. Alcohols act as Bronsted bases. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors.

Question. Why p-Nitro phenol is more acidic than Cresols?
Answer. In substituted phenols, the presence of electron withdrawing groups such as nitro group enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalization of negative charge in phenoxide ion when substituent is at ortho or para position. On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength.

Question. Of the two hydroxy organic compounds ROH and R’OH, the first one is basic and other is acidic in behaviour. How is R different from R’?
Answer. When R = alkyl, ROH behaves as a bronsted base and when R’ = aryl, R’OH behaves as a bronsted acid

Question. Give a chemical test to distinguish between 2-Pentanol and 3-Pentanol.
Answer. 2-pentanol gives Iodoform test with yellow ppt. of Iodoform while 3-pentanol does not give this test.

Question. Explain the following observations :
(i) The boiling point of ethanol is higher than that of methoxymethane.
(ii) Phenol is more acidic than ethanol.
(iii) o- and p-nitrophenols are more acidic than phenol.
Answer. (i) Due to presence of intermolecular H-bonding, associated molecules are formed, hence ethanol has high boiling point while methoxymethane does not have intermolecular H-bonding.
(ii) Phenol on losing H⁺ ion forms phenoxide ion, and ethanol on losing H+ ion forms ethoxide ion.
Phenoxide ion is more stable than ethoxide ion as phenoxide ion exists in resonance structure. Due to this phenol is more acidic than ethanol.
(iii) Both o- and p-nitrophenols contain the NO₂ group which is an electron withdrawing group. Due to -R and -I effect of the -NO₂ group, electron density in the OH bond of substituted phenol decreases and hence the loss of proton becomes easy and therefore more acidic.

 

 

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