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Chapter 4: Problem 13

In acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\), the \(\mathrm{OH}\) hydrogen is more acidic than the \(\mathrm{CH}_{3}\) hydrogens. Explain.

Short Answer

Expert verified
Answer: The OH hydrogen in acetic acid is more acidic than the CH₃ hydrogens because, upon removal, the resulting acetate ion forms a stable resonance structure that easily delocalizes the negative charge. In contrast, removing a hydrogen from the methyl group results in an unstable radical or negative charge that cannot be stabilized through resonance or delocalization.

Step by step solution

01

Understanding acidic hydrogen

First, we need to know which hydrogen (H) atoms in the acetic acid molecule qualifies as acidic. An acidic hydrogen is one that can be easily removed from the molecule as a proton (\(\mathrm{H}^+\)) and leave a stable ion.
02

Identify possible acidic hydrogen atoms

In acetic acid, the molecule has a structure \(\mathrm{CH}_{3}\ce{-}\mathrm{C}(\ce{=O})\ce{-}\mathrm{OH}\). The carbon atom in the methyl group (CH₃) is bound to three hydrogen atoms, and the carbon atom in the carboxyl group (COOH) is bound to an \(\mathrm{OH}\) hydrogen.
03

Analyze the stability of the ions formed after the hydrogen atoms are removed

Removing the OH hydrogen (deprotonation) from the carboxyl group (COOH) results in an acetate ion with a stable resonance structure. The negative charge on the oxygen atom in the ion can be easily delocalized between the two oxygen atoms, increasing the stability of the ion as shown below: \(\ce{-O-C(=O)-}\leftrightarrow\ce{=O-C(-O)-}\) On the other hand, removing a hydrogen atom from the methyl group results in a carbon-centered radical or negative charge that is much less stable and not able to stabilize through resonance or delocalization.
04

Relate the stability of the formed ions to their acidity

In general, the more stable the resulting ion after a hydrogen atom is removed, the easier it is for the compound to give up that hydrogen ion, thus increasing its acidity. The formation of the acetate ion after the removal of the OH hydrogen atom in acetic acid is a stable structure due to resonance. However, the removal of a hydrogen atom from the methyl group results in an unstable radical or charge, making it difficult for the CH₃ hydrogens to be considered acidic. Therefore, the OH hydrogen in acetic acid is more acidic than the CH₃ hydrogens.

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Most popular questions from this chapter

In cach pair, sclcct the strongcr acid. (a) Pyruvic acid \(\left(\mathrm{p} K_{\mathrm{a}} 2.49\right)\) and lactic acid \(\left(\mathrm{p} K_{\mathrm{a}} 3.08\right)\) (b) Citric acid \(\left(\mathrm{p} K_{\mathrm{a} 1} 3.08\right)\) and phosphoric acid \(\left(\mathrm{p} K_{\mathrm{a} 1} 2.10\right)\)

Benzoic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\left(\mathrm{p} K_{\mathrm{a}} 4.19\right)\), is only slightly soluble in water, but its sodium salt, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COO}^{-} \mathrm{Na}^{+}\), is quite soluble in water. In which solution(s) will benzoic acid dissolve? (a) Aqueous \(\mathrm{NaOH}\) (b) Aqueous \(\mathrm{NaHCO}_{3}\) (c) Aqueous \(\mathrm{Na}_{2} \mathrm{CO}_{3}\)

If the \(\Delta G^{\circ}\) for a reaction is \(-4.5 \mathrm{kcal} / \mathrm{mol}\) at \(298 \mathrm{~K}\), what is the \(K_{\text {eq }}\) for this reaction? What is the change in entropy of this reaction if \(\Delta H^{\circ}=-3.2 \mathrm{kcal} / \mathrm{mol}\) ?

Write an equation to show the proton transfer between each alkene or cycloalkene and HCl. Where two carbocations are possible, show each. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}\) (b) C1=CCCCC1 2-Pentene Cyclohexene

Predict the position of equilibrium, and calculate the equilibrium constant, \(K_{\text {cq, }}\), for each acid-base reaction. (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}+\mathrm{CH}_{3} \mathrm{COOH} \rightleftharpoons \mathrm{CH}_{3} \mathrm{NH}_{3}^{+}+\mathrm{CH}_{3} \mathrm{COO}^{-}\) \(\begin{array}{ccc}\text { Methylamine Aceticacid } & \begin{array}{c}\text { Methylammonium } \\ \text { ion }\end{array} & \begin{array}{c}\text { Acetate } \\ \text { ion }\end{array}\end{array}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{NH}_{2}{ }^{-}\) Ethoxide ion Ammonia Ethanol Amide ion
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