Assertion (A) : Phenol is less acidic than alcohol.
Reason (R) : Phenoxide ion is more stable than alkoxide ion.
Assertion (A) : Phenol is less acidic than alcohol.
Reason (R) : Phenoxide ion is more stable than alkoxide ion.
Show Hint
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false, but Reason (R) is true.
The Correct Option is D
Solution and Explanation
The acidity of a compound \(\mathrm{HA}\) is decided by how easily it gives up \(\mathrm{H^+}\). The more stable the conjugate base \(\mathrm{A^-}\) is, the stronger the acid. So we compare the stability of the phenoxide ion with that of the alkoxide ion.
Step 2: Stability of the phenoxide ion.
In phenoxide ion the negative charge on oxygen is spread over the benzene ring by resonance. It sits on the ortho and para carbons as well as on oxygen. Spreading a charge over more atoms makes an ion more stable.
Step 3: Stability of the alkoxide ion.
In an alkoxide ion such as \(\mathrm{C_2H_5O^-}\) the charge stays on oxygen alone. The alkyl group also pushes electrons towards oxygen by its +I effect. This makes the charge denser and the ion less stable.
Step 4: Check the Reason (R).
By Steps 2 and 3, phenoxide ion is more stable than alkoxide ion. So (R) is TRUE.
Step 5: Check the Assertion (A).
A more stable conjugate base means a stronger acid, so phenol is MORE acidic than alcohols. The \(\mathrm{pK_a}\) of phenol is about 10, while that of ethanol is about 16. The assertion says the opposite, so (A) is FALSE.
Step 6: Why the other options are wrong.
Options 1 and 2 need (A) to be true, but it is false. Option 3 needs (R) to be false, but it is true. Only option 4 fits.
Final Answer:
(A) is false and (R) is true. \[ \boxed{\text{Option 4 (D)}} \]
Top CBSE CLASS XII Chemistry Questions
- ${AgNO_3}$ does not decompose where :
- Azeotropic mixture of water and HCl boils at 381.5 K. By distilling the mixture it is possible to obtain
Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines.
(i) (CH3 )2CHNH2 (ii) CH3 (CH2 )2NH2 (iii) CH3NHCH(CH3 )2
(iv) (CH3 )3CNH2 (v) C6H5NHCH3 (vi) (CH3CH2 )2NCH3 (vii) m–BrC6H4NH2
Give one chemical test to distinguish between the following pairs of compounds.
(i) Methylamine and dimethylamine
(ii) Secondary and tertiary amines
(iii) Ethylamine and aniline
(iv) Aniline and benzylamine
(v) Aniline and N-methylanilineAccount for the following:
(i) pKb of aniline is more than that of methylamine.
(ii) Ethylamine is soluble in water whereas aniline is not.
(iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide.
(iv) Although amino group is o– and p– directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
(v) Aniline does not undergo Friedel-Crafts reaction.
(vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines. (vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.
Top CBSE CLASS XII Chemical Reactions of Alcohols Phenols and Ethers Questions
Calculate the potential for half-cell containing 0.01 M K\(_2\)Cr\(_2\)O\(_7\)(aq), 0.01 M Cr\(^{3+}\)(aq), and 1.0 x 10\(^{-4}\) M H\(^+\)(aq).
- \((CH_3)_3C-OC_2H_5\) on reaction with HI gives
- Write the reaction involved in Kolbe's reaction.
- An organic compound ‘A’, with molecular formula $C_2H_6O$ reacts with active metals such as sodium to give compound ‘B’ and hydrogen gas. ‘A’ on treatment with iodine and sodium hydroxide gives ‘C’ and in presence of $H_2SO_4$ at $413$ K gives ‘D’ ($C_4H_{10}O$). ‘D’ on reaction with excess of HI gives ‘E’. Identify ‘A’, ‘B’, ‘C’, ‘D’ and ‘E’ and write all the reactions involved.
- Lucas reagent produces cloudiness immediately with :
Top CBSE CLASS XII Questions
- 2, b, c are in A.P. and the range of determinant $\begin{vmatrix}1&1&1\\ 2&b&c\\ 4&b^{2}&c^{2}\end{vmatrix}$ is [2, 16]. Then find range of c is
- A boy of mass 50 kg is standing at one end of a, boat of length 9 m and mass 400 kg. He runs to the other, end. The distance through which the centre of mass of the boat boy system moves is
- A capillary tube of radius r is dipped inside a large vessel of water. The mass of water raised above water level is M. If the radius of capillary is doubled, the mass of water inside capillary will be
- A circular disc is rotating about its own axis. An external opposing torque 0.02 Nm is applied on the disc by which it comes rest in 5 seconds. The initial angular momentum of disc is
- A circular disc is rotating about its own axis at uniform angular velocity \(\omega.\) The disc is subjected to uniform angular retardation by which its angular velocity is decreased to \(\frac {\omega}{2}\) during 120 rotations. The number of rotations further made by it before coming to rest is