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Chapter 2: Problem 74

The correct decreasing order of the acidity of hydrogen in acetylene, phenol, alcohol, ethylene and acetic acid is (a) Acetic acid \(>\) Phenol \(>\) alcohol \(>\) acetylene \(>\) ethylene (b) Phenol \(>\) acetic acid \(>\) alcohol \(>\) acetylene \(>\) ethylene (c) Acetic acid \(>\) alcohol \(>\) phenol \(>\) acetylene \(>\) ethylene (d) Acetic acid \(>\) acetylene \(>\) phenol \(>\) alcohol \(>\) ethylene

Short Answer

Expert verified
Question: Arrange the following molecules in decreasing order of acidity: acetylene, phenol, alcohol, ethylene, acetic acid. Answer: Acetic acid > Phenol > Alcohol > Acetylene > Ethylene

Step by step solution

01

Review the concept of acidity

Acidity of a molecule is generally evaluated by the ease with which it donates a proton (H+). A more acidic substance will have a lower pKa value, since it more readily donates its proton.
02

Identify the acidic hydrogens in each molecule

We need to identify the acidic hydrogens in each molecule - the hydrogens attached to the most electronegative atoms or found in highly polar functional groups. Acetylene: H-C≡C-H (hydrogen attached to the sp-hybridized carbon atoms) Phenol: \(\displaystyle \ce{C6H5-OH}\) (hydrogen attached to the oxygen atom in the hydroxyl group) Alcohol: R-OH (hydrogen attached to the oxygen atom in the hydroxyl group) Ethylene: H\(_2\)C=CH\(_2\) (no significantly acidic hydrogen) Acetic acid: CH\(_3\)COOH (hydrogen attached to the oxygen atom in the carboxylic acid group)
03

Compare the stability of the conjugate bases

The acidity of a molecule can also be compared by the stability of their conjugate bases (anions). Higher stability of the conjugate base leads to higher acidity. Several factors can influence the stability of a conjugate base, such as resonance stabilization and inductive effects. Acetylene conjugate base: HC≡C⁻ Phenol conjugate base: \(\displaystyle \ce{C6H5-O}^{-}\) (stabilized by resonance with the aromatic ring) Alcohol conjugate base: R-O⁻ Ethylene conjugate base: H\(_2\)C=CH⁻ (not significantly stabilized) Acetic acid conjugate base: CH\(_3\)COO⁻ (stabilized by resonance with the carbonyl group)
04

Arrange the molecules in the decreasing order of acidity

Based on the factors mentioned above, such as the electronegativity of atoms, functional groups, and stability of conjugate bases, we can now arrange the molecules in the decreasing order of acidity: 1. Acetic acid (carboxylic acid group with resonance-stabilized conjugate base) 2. Phenol (phenol group with resonance-stabilized conjugate base) 3. Alcohol (hydroxyl group with non-resonance-stabilized conjugate base) 4. Acetylene (hydrogens attached to sp-hybridized carbon atoms) 5. Ethylene (no significantly acidic hydrogen) The correct decreasing order of acidity is: Acetic acid \(>\) Phenol \(>\) alcohol \(>\) acetylene \(>\) ethylene So, the correct answer is option (a).

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Most popular questions from this chapter

Identify the name of reactions (A) - (D) given below (A) \(2 \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{I} \stackrel{\text { Na /ether }}{\longrightarrow} \mathrm{C}_{4} \mathrm{H}_{10}+2 \mathrm{NaI}\) (B) (C) \(\mathrm{R}_{2} \mathrm{CuLi} \stackrel{\mathrm{R}^{\prime} \mathrm{X}}{\longrightarrow} \mathrm{R}-\mathrm{R}\) (D) \(\mathrm{CH}_{3}-\mathrm{CO}-\mathrm{CH}_{3} \stackrel{\mathrm{Zn}-\mathrm{Hg} / \mathrm{HCl}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (a) (A) Ulmann reaction (B) Electrolytic cleavage (C) Grignard reaction (D) Wolff-kishner reduction (b) (A) Wurtz reaction (B) Kolbe's electrolysis (C) Corey-House synthesis (D) Clemmensen reaction (c) (A) Wurtz reaction (B) Decarboxylation (C) Frankland reaction (D) Clemmensen reduction (d) (A) Wurtz-Fittig reaction (B) Decarboxylative coupling (C) Corey-House synthesis (D) Clemmensen reduction

Reaction of trans-2-phenyl-1-bromocyclohexane with hot alcoholic \(\mathrm{NaOH}\) gives (a) 3 -phenylcyclohexene (b) 1 -phenylcyclohexene (c) 2 -bromophenyl cyclohexane (d) 2 -phenyl cyclohexanol

Alkynes are less reactive than alkenes towards electrophilic reagents. But the alkyne chemistry is useful for organic synthesis, in the form of hydration of the triple bond, formation of metalacetylides, selective reduction of alkynes and few other related reactions. Acetylene and HCHO react together in the presence of \(\mathrm{NaOCH}_{3}\) to give (a) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{OH}\) (b) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{HOH}_{2} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{CH}_{3}-\mathrm{CHOH}-\mathrm{CHOH}-\mathrm{CH}_{3}\)

Which of the following alkenes exhibit geometrical isomerism? (a) 2,4 -hexadiene (b) 2 -methyl-1-butene (c) 1 -methylcyclopentene (d) 1,3 -butadiene

An organic compound ( \(3.0 \mathrm{~g}\) ) gave on complete combustion, \(3.476 \mathrm{~g}\) of \(\mathrm{CO}_{2}\) and \(1.422 \mathrm{~g}\) of water. The molecular weight of the compound is 228 . Give molecular formula of the compound.
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