How will you distinguish between Aldehyde and Ketone by chemical test?

Tollen's reagent test is used to distinguish Aldehyde and Ketone. Tollen's reagent Ag NH 3 2 + OH - is ammonical silver nitrate solution. When Tollen's reagent warmed with an aldehyde, it gets reduced to metallic silver. This metallic silver gets deposited on the inner wall of the test tube to form a silver mirror. Therefore, this test is also called silver mirror test. Whereas ketones do not give silver mirror test. 2 Ag NH 3 2 + + 3 OH - + RCHO Aldehyde → RCOO - + 2 Ag Silver mirror + 4 NH 3 + 2 H 2 O

Arrange the following carboxylic acid in ascending order of their acidity. Benzoic acid, 4 - methoxybenzoic acid, 4 - nitrobenzoic acid.

Benzoic acid contain carboxyl group attached to the benzene ring. The acidic strength of a substituted benzoic acid depends on the nature of the substituent group. The electron-releasing groups such as - CH 3 tend to decrease the acidic strength of benzoic acid. Whereas electron-withdrawing group such as - NO 2 tend to increase the acidic strength of benzoic acid. Therefore, ascending order of acidity of given compound is 4 - methoxybenzoic acid < Benzoic acid < 4 - nitrobenzoic acid.

Alkanoic acid have the highest boiling points. Explain.

Alkanoic acid (Carboxylic acid having alkyl group/ aliphatic carboxylic acid) have the highest boiling points because of following two reasons: 1. The - OH group of the - COOH group in an acid is more strongly polarised due to the presence of carbonyl oxygen adjacent to it. 2. Dimer structure of alkanoic acid: The presence of carbonyl group in acid permits the formation of H - bonds through - OH and the carbonyl oxygen. Thus, an alkanoic acid molecule can enter hydrogen bonding at two different points. This leads to the formation of cyclic dimers in acids.

Arrange the following compounds in the descending order of their reactivity towards nucleophilic substitution reaction. CH 3 CHO , HCHO , CH 3 COCH 3

Based on the + I effect (Inductive effect), and the steric hindrance due to the alkyl groups present on carbonyl carbon, the reactivity towards nucleophilic substitution reaction is as follows: HCHO Formaldehyde > CH 3 CHO Acetaldehyde > CH 3 COCH 3 Acetone Alkyl groups are electron-releasing groups ( + I effect). The presence of alkyl groups on the carbonyl carbon will lower the extent of positive charge on the carbon. As a result, the presence of alkyl groups will lower the reactivity of the carbonyl group. As number of alkyl group increases, steric hindrance increases which decreases the reactivity. Therefore, acetone is less reactive.

Write IUPAC name and chemical formula of Acetone.

The chemical formula of Acetone is: CH 3 COCH 3 . It contains a ketone functional group. Its IUPAC name is propan-2-one. It is the simplest and smallest ketone. It is a colourless, highly volatile and flammable liquid with a characteristic pungent odour.

Aldehydes are more reactive than ketones towards nucleophilic addition reactions. Explain.

Aldehydes are more reactive than ketones towards nucleophilic addition reactions due to the following reasons. (i) Steric effect- The carbonyl carbon of the group of the ketone is attached with two alkyl groups while in aldehyde, the carbonyl group is attached with only one alkyl group. Therefore, the steric hindrance in ketone is more than aldehyde. As a result, the carbonyl carbon of ketone becomes less reactive towards nucleophilic addition reactions. (ii) Inductive effect- There are two alkyl groups are attached with carbonyl carbon in the ketone. So, due to the positive inductive effect of alkyl groups, decreases the electrophilicity of the carbonyl carbon more effectively in ketones than aldehydes.

Arrange the following haloacids in increasing order of their acidity. FCH 2 COOH , ClCH 2 COOH , BrCH 2 COOH

The acidity of the carboxylic acids depends on the stability of the conjugate base (carboxylate ion). The electron-withdrawing groups, withdraw the electrons from the carbons to which it is attached, and this effect is transmitted throughout the chain. As a result, the electrons of O - H bonds are withdrawn more strongly towards the oxygen atom, thus, the release of proton becomes easier. Therefore, the acidity of carboxylic acids increases. As the electronegativity of the electron-withdrawing group increases, the acidity of carboxylic acids increases. The increasing order of the given acids of their acidity. BrCH 2 COOH < ClCH 2 COOH < FCH 2 COOH

Why carboxylic acids are more acidic than phenol?

When carboxylic acid loses a proton, it forms a carboxylate ion. The acidity of carboxylic acids depends on the stability of carboxylate ions. The conjugate base of phenol is a phenoxide ion, and it is also stabilised by the resonance of the benzene ring. The carboxylate ion shows resonance by the effective delocalisation of negative charge which makes it stable. The stability of carboxylate ions is more than phenoxide ions therefore, carboxylic acids are more acidic than phenol.

E M B I B E

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR CHEMISTRY>Chapter 12 - Aldehydes, Ketones and Carboxylic Acids>Rajasthan Board-2018>Q 6

EASY

12th Rajasthan Board

IMPORTANT

Earn 100

Draw the resonating structures of Urea.

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