Aldehydes, Ketones And Carboxylic Acids

- Get the best out of this chapter by practicing the kind of problems asked in various exams.
1
Structures from Names, Names from Structures
Description:
Most important questions in this chapter can be asked from the IUPAC nomenclature of aldehydes, ketones and carboxylic acids. The structures of compounds can be given and you will have to write their IUPAC name, or the name can be given and you will have to write their structure. Sometimes, you will also be asked to write the common name.
How to Approach:
When structure is given and you have to write the IUPAC name:
  • Replace '-e' of the corresponding alkane with '-al' for aldehydes, with '-one' for ketones, and '-oic acid' for carboxylic acids.
  • Number the carbon chain such that the substituents (functional groups) get the lowest number.
When name is given and you have to write the structure:
  • Draw the parent chain with the correct number of carbons (as per corresponding alkane).
  • Write the carbonyl (CHO or CO) or carboxylic acid group (COOH) based on the locant mentioned in question for the functional group.
Let's practice now!
The IUPAC name of the compound is:
A
3 - Methyl - 5 - heptanone
B
5-Methylheptan-3-one
C
5 - Ethyl - 5 - hexanone
D
2 - Ethyl - 4 - hexanone
The structure of 6-hydroxyheptanal is:
A
B
C
D
2
Missing Reactants, Reagents, Products
Description:
Many questions from this chapter are concerned with the preparation reactions and other chemical properties. You may be given the reactants of a reaction and asked to predict the product, or draw the structure of the product. Some questions show reactions with missing starting material, intermediate or reagent, and you have to identify it.
How to Approach:
Be thorough with all the preparation reactions in the chapter, the reactants, reagents, intermediates if any, and also other chemical reactions. Here's a list of all the reactions from the chapter:
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Shall we practice?
Which of the following aldehydes undergoes Cannizzaro reaction?
A
B
C
D
Benzaldehyde can be prepared from benzene by passing vapours of _____ and _______ in its solution in presence of catalyst mixture of aluminium chloride and cuprous chloride. The reaction is known as:
A
, Resenmund reduction
B
, Gattermann - Koch reaction
C
, Clemmensen reduction
D
, alcohol, Wolff - Kishner reduction
3
Comparison of Physical Properties and Reactivities
Description:
You will be given specific properties of aldehydes, ketones and carboxylic acids and asked to arrange them in increasing or decreasing order. Questions may be related to reactivity, solubility, boiling points, melting points, dipole moment, etc.
How to Approach:
Pay attention to the type of compound in the question. Aldehydes and ketones have mostly similar properties, while those of carboxylic acid may differ. The properties of aldehydes and ketones are because of their carbonyl group, and those of carboxylic acids are due to the carboxyl carbon.
Here are few points that can come handy when you are solving questions:
  • Aldehydes and ketones have higher boiling points than other hydrocarbons but lower than alcohols of similar molecular mass
  • Carboxylic acids have higher boiling points than aldehydes, ketones and alcohols.
  • Only lower members of aldehydes, ketones and carboxylic acids are soluble in water
Here are some questions to practice:
Lower members of aliphatic carboxylic acids are soluble in water. This is due to:
A
water is non electrolyte
B
due to London forces
C
formation of hydrogen bonds with water
D
van der-waals interaction with water molecules
Which of the following carboxylic acids is highly insoluble in water?
A
Propanoic acid
B
Butanoic acid
C
Pentanoic acid
D
Decanoic acid
The boiling point of aldehyde and ketones are:
A
higher than those of alcohols of similar molecular masses
B
lower than those of alcohols of similar molecular masses
C
similar to the alcohols of similar molecular masses
D
similar to the alcohol of different molecular mass
4

Arrange in Ascending/Descending Order of Acidic Strength of Carboxylic Acid

Description:
You can expect questions that give the structures of different carboxylic acids and ask you to arrange them in ascending or descending order of acidic strength. Questions related to the effect of substituents on acidity are also asked. Some assertion and reasoning questions will also be asked for the same topic.
How to Approach:
For the acidity of carboxylic acids, remember these points:
  • Aromatic carboxylic acids are more acidic than aliphatic carboxylic acids
  • Substituents that stabilise the carboxylate anion increase acidity
  • Electron-withdrawing groups increase acidity
  • Electron-donating groups decrease acidity
  • For aliphatic carboxylic acids, the substituents that are closest to the functional group have the strongest effect.
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  • For aromatic carboxylic acids, substituents in ortho and para positions have the strongest effect.
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Let's try some questions now.
Which of the following orders regarding acidic strength is correct?
A
B
C
D
The weaker acid among the following is :
A
B
C
D
5

Distinguish between Pairs of Compounds Based on Chemical Tests

Description:
Some questions give a pair of compounds (aldehydes and ketones) and ask which test can be used to distinguish between the two. Sometimes, the results of a test are given and you have to identify which compound is an aldehyde and which is a ketone.
How to Approach:
Aldehydes and ketones differ in their oxidation reactions. Remember the different tests based on these oxidation reactions and how they can be used to distinguish between aldehydes and ketones.
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Let's put this to practice:
Acetone and acetaldehyde can be distinguished by using_______.
A
Grignard reagent
B
C
ammoniacal
D