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Chapter 5: Problem 2

Account for the fact that aldehydes are more reactive than ketones in nucleophilic additions.

Short Answer

Expert verified
Answer: Aldehydes are more reactive than ketones in nucleophilic addition reactions due to their molecular structure, which results in a greater electron deficiency at the carbonyl carbon. This makes the carbonyl carbon in aldehydes more prone to attack by nucleophiles than the carbonyl carbon in ketones.

Step by step solution

01

Understanding the Structure of Aldehydes and Ketones

Aldehydes and ketones are both carbonyl compounds with a C=O double bond. The difference is in the groups bonded to the carbonyl carbon. In aldehydes, the carbonyl carbon is bonded to a hydrogen atom and an alkyl or aryl group (R), while in ketones, the carbonyl carbon is bonded to two alkyl or aryl groups (R and R'). Aldehyde: R-CHO Ketone: R-CO-R'
02

Polar Nature of the Carbonyl Group

In both aldehydes and ketones, the carbonyl group (C=O) is polar in nature. This is because oxygen is more electronegative than carbon, leading to an uneven distribution of electron density, with a partial negative charge on the oxygen atom and a partial positive charge on the carbonyl carbon atom. This polarization makes the carbonyl carbon susceptible to attack by nucleophiles (electron-rich species).
03

Comparing Electron Density in Aldehydes and Ketones

Since aldehydes and ketones only differ in the groups attached to the carbonyl carbon, the difference in reactivity is dependent on how the attached groups influence the electron density around the carbonyl carbon. In aldehydes, one of the attachments is a hydrogen atom (R-CHO), which is electron-neutral. In ketones, there are alkyl or aryl groups (R-CO-R') on both sides of the carbonyl carbon. Alkyl and aryl groups have an electron-donating effect due to the inductive effect and can donate electron density towards the carbonyl carbon, making it less positive.
04

The Reactivity of Aldehydes and Ketones in Nucleophilic Additions

Now, we can account for why aldehydes are more reactive than ketones in nucleophilic additions. The carbonyl carbon in aldehydes is more positively charged than in ketones because it has only one electron-donating group (compared to two in ketones). Consequently, aldehydes have a greater electron deficiency at the carbonyl carbon, making it more susceptible to attack from nucleophiles. In summary, aldehydes are more reactive than ketones in nucleophilic addition reactions because of their molecular structure, which results in a greater electron deficiency at the carbonyl carbon. This makes the carbonyl carbon in aldehydes more prone to attack by nucleophiles than the carbonyl carbon in ketones.

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Most popular questions from this chapter

Identify the product (b) in the following sequence of reactions. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CHO} \stackrel{\mathrm{H}_{2} \mathrm{NOH}}{\longrightarrow}(\mathrm{A}) \stackrel{\mathrm{P}_{+} \mathrm{O}_{\mathfrak{l}}}{\longrightarrow}(\mathrm{B})\) (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CN}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{NH}\) (c) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}=\mathrm{NOH}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\)

Salicylic acid is more acidic than o-methoxybenzoic acid though both have electron releasing groups i.e., \(-\mathrm{OH}\) and \(-\mathrm{OCH}_{3} .\) This is because (a) \(-\mathrm{OH}\) is a stronger electron releasing group compared to \(-\mathrm{OCH}_{3}\). (b) salicylate ion is stabilized by intramolecular hydrogen bonding. (c) salicylic acid has two replaceable hydrogen atoms. (d) \(-\mathrm{OCH}_{3}\) has a stronger steric effect to prevent the stabilization of the carboxylate anion.

Wolff-Kishner reduction of benzophenone gives (a) benzene (b) toluene (c) diphenylmethane (d) ethylbenzene

The increasing order of the rate of HCN addition to the following carbonyl compounds is O=Cc1ccccc1 O=Cc1cccc(Cl)c1 \(\mathrm{Ph}-\mathrm{CO}-\mathrm{Ph}\) (IV) (II) (III) (a) II > IV < III < I (b) IV \(<\) II \(<\) III \(<\mathrm{I}\) (c) \(\mathrm{IV}<\mathrm{I} \leq \mathrm{III}<\mathrm{II}\) (d) \(\quad \mathrm{IV}<\mathrm{III}<\mathrm{II}<\mathrm{I}\)

Directions: Each question contains Statement- 1 and Statement- 2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement- 1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement- 1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement- 1 is False, Statement- 2 is True Statement 1 Acetals and ketals are unstable in aqueous acid conditions, but quite stable to alkaline conditions. and Statement 2 A weaker base like (OH) cannot displace a stronger base like (OR) found in acetals and ketals.
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