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Alcohols, Phenols, and Ethers — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistryAlcohols, Phenols, and Ethers4 Marks
Question

Read the passage given below and answer the following questions:

Ethers are readily cleaved by HI or HBr at 373K to form an alcohol and an alkyl halide.

$\text{R}-\text{O}-\text{R}+\text{HX}\xrightarrow{373\text{K}}\text{R}-\text{X}+\text{R}-\text{OH}$

$\text{R}-\text{OH}+\text{HX}\xrightarrow{373\text{K}}\text{R}-\text{X}+\text{H}_2\text{O}$

Mixed ether, containing primary or secondary alkyl group, when heated with hydrogen halide, the lower alkyl group forms halide and higher will form an alcohol. Tertiary alkyl ether when heated with hydrogen halide gives tertiary alkyl halide.

The following questions are multiple choice questions. Choose the most appropriate answer:

  1. Among the following ethers, which one will produce methyl alcohol on treatment with hot concentrated HI?

  1. $\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{C}-\text{O}- \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3 $

  2. $\text{CH}_3-\text{CH}-\text{CH}_2-\text{O}-\text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ |\\\ \ \ \ \ \ \ \ \ \ \ \text{CH}_3$

  3. $\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{O}-\text{CH}_3$

  4. $\text{CH}_3-\text{CH}_2-\text{CH}-\text{O}-\text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3$

  1. When CH2 = CH - O - CH2 - CH3 reacts with one mole of HI, one of the products formed is:
  1. Ethane.
  2. Ethanol.
  3. Iodoethene.
  4. Ethanal.
  1. (CH3)3COCH3 and CH3OC2H5 are treated with hydroiodic acid. The fragments obtained after reactions are respectively:

  1. (CH3)3CI + CH3OH; CH3I + C2H5OH

  2. (CH3)3CI + CH3OH; CH3OH + C2H5I

  3. (CH3)3COH + CH3I; CH3OH + C2H5I

  4. CH3I + (CH3)3COH; CH3I + C2H5OH

  1. Which of the following ether is unlikely to be cleaved by hot cone. HBr?

✓

Answer

  1. (a) $\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{C}-\text{O}- \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3 $

Explanation:

$\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\text{CH}_3-\text{C}-\text{O}\text{CH}_3+\text{HI}\xrightarrow{\text{S}_\text{N}1}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\ \ \ \ \ \ \ \ \ \ \ \ \ \text{(Hot andconc.)}$

  1. (b)

Explanation:

  1. (d) Ethanal.

Explanation:

  1. (a) (CH3)3CI + CH3OH; CH3I + C2H5OH

Explanation:

When mixed ethers are used, the formation of alkyl iodide depends on the nature of alkyl groups. Methyl iodide is formed when one group is methyl and the other a primary or secondary alkyl group. Here reaction follows SN2 mechanism and because of the steric effect of the larger group, I- attacks the smaller (Me) group.

$\text{CH}_3\text{OC}_2\text{H}_5+\text{HI}\rightarrow\text{CH}_3\text{I}+\text{C}_2\text{H}_5\text{OH}$

When the substrate is a methyl t-alkyl ether, the products are t-RI and MeOH. Here, reaction follows SN1 mechanism and formation of products is controlled by the stability of carbocation. Since carbocation stability order is:

$3^\circ>2^\circ>1^\circ>\stackrel{+}{\hbox{ CH}}_3,$ therefore alkyl halide is always derived from tert-alkyl group.

$\ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\ \ \ \ \ \ \ \ \ \ \ \ \ \ | \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |\ \\\text{CH}_3-\text{C}-\text{O}-\text{CH}_3+\text{HI}\xrightarrow[\text{S}_\text{N}1]{373\text{K}}\text{CH}_3-\text{C}-\text{I}+\text{CH}_3\text{OH}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ |\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ |\\ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{CH}_3\\ _\text{rert-Butyl methyl ether}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ _\text{tert-Butyl iodide} $

  1. (a)

​​​​​​​Explanation:

Diphenyl ethers are not cleaved by HBr or HI.

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Reason: 2º alkyl halides are less reactive than 1º towards SN1 reactions.

  1. Assertion: Polar solvent slows down SN2 reactions.

Reason: CH3-Br is less reactive than CH3Cl.

  1. Assertion: Benzyl bromide when kept in acetone- water it produces benzyl alcohol.

Reason: The reaction follows SN2 mechanism.

  1. Assertion: Rate of hydrolysis of methyl chloride to methanol is higher in DMF than in water.

Reason: Hydrolysis of methyl chloride follows second order kinetics.

  1. Assertion: SN1 reaction is carried out in the presence of a polar protic solvent.

Reason: A polar protic solvent increases the stability of carbocation due to solvation.

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  1. 3.7
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$\text{Fe}^{2+}+\text{Ce}^{4+}\rightleftharpoons\text{Ce}^{3+}+\text{Fe}^{3+}$

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  1. 7.6 × 1012
  2. 6.5 × 1010
  3. 5.2 × 109
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Ag|Ag+ (satd. Ag2CrO4 soln) || Ag+ (0.1 M) | Ag

is 0.164V at 298 K, is:

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  3. 1.158 × 10-12 mol3 L-3
  4. 4.135 × 10-12 mol3 L-3
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Due to intermolecular hydrogen bonding, the boiling points of alcohols and phenols are much higher than those of corresponding haloalkanes, haloarenes, aliphatic and aromatic hydrocarbons. Among isomeric alcohols, the boiling points follow the order: primary > secondary > tertiary. Boiling points of ethers are much lower than those of isomeric alcohols. The solubility of alcohols in water decreases as the molecular mass of alcohols increases. Amongst isomeric alcohols solubility increases with branching. The solubility of phenols in water is much lower than that of alcohols. Lower ethers such as dimethyl ether and ethyl methyl ether are soluble in water, but the solubility decreases as the molecular mass increases.

In these questions (Q. No. i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.

  1. Assertion and reason both are correct statements and reason is correct explanation for assertion.
  2. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
  3. Assertion is correct statement but reason is wrong statement.
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Reason: Alcohols and ethers are isomeric in nature.

  1. Assertion: The solubility of phenols in water is much lower than that of alcohols.

Reason: Phenols do not form H-bonds with water.

  1. Assertion: Among n-butane, ethoxyethane, 1-propanol and 2-propanol, the increasing order of boiling points is, 1-butanol < 1-propanol < ethoxyethane < n-butane.

Reason: Boiling point increases with increase in molecular mass.

  1. Assertion: Dimethyl ether and diethylether are soluble in water.

Reason: As the molecular mass increases, solubility of ethers in water decreases.

  1. Assertion: Butan-2-ol has higher boiling point than 2-methylpropan-2-ol.

Reason: Amongst isomeric alcohols, the boiling points decreases with branching.

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