Sindh Mdcat Exclusive Course Chemistry Aldehydes And Ketones — Solved MCQs

Question 1

Which of the following will not undergo aldol condensation?

Accepted Answer

C. Formaldehyde — The aldehydes that have alpha hydrogen atoms undergo aldol condensation reaction

Question 2

Which one of the following reagents is used to distinguish between aldehydes and ketones?

Accepted Answer

B. Tollen’s Reagent — The Tollens' test is a reaction that is used to distinguish aldehydes from ketones, as aldehydes are able to be oxidized into a carboxylic acid while ketones cannot.Alkaline iodine is used in the iodoform test. When Iodine and sodium hydroxide are added to a compound that contains either a methyl ketone or a secondary alcohol with a methyl group in the alpha position, a pale yellow precipitate of iodoform or triiodomethane is formed. It can be used to identify aldehydes or ketones.A solution of bromine in CH2CL2 is used to either test for unsaturation or to check the ability to be oxidized for example in aldehydes. Bromine solution is naturally orange and when consumed in oxidation, it becomes colourless.2, 4 DNPH is used to detect ketones and aldehydes. A positive test is signalled by the formation of a yellow, orange or red precipitate of the dinitrophenylhydrazone.

Question 3

Ketones can be made by oxidation of:

Accepted Answer

C. Secondary Alcohols — Option A; aldehydes oxidize into carboxylic acids.Option B; primary alcohols oxidize into aldehydes and then to carboxylic acids.Option C; secondary alcohols oxidize into ketones.Option D; tertiary alcohols are resistant to oxidation.

Question 4

Phenylmethyl ketone can be converted into ethyl benzene in one step by using:

Accepted Answer

B. Zn(Hg)-HCl — The reaction is Clemmensen reduction. The reaction of aldehydes and ketones with zinc amalgam (Zn/Hg alloy) in concentrated hydrochloric acid, which reduces the aldehyde or ketone to a hydrocarbon, is called Clemmensen reduction.

Question 5

Which of the following does not give iodoform test?

Accepted Answer

D. Benzophenone — The reaction of iodine and base with methyl ketones is called the "iodoform test". It is used to probe the presence of a methyl ketone. However, Benzenophenone is a phenyl ketone and does not contain this group giving a negative test.

Question 6

Why is water added after the reduction process of aldehyde and ketone with LiAlH4?

Accepted Answer

A. Because water reacts violently with LiAlH4 — This is the correct option. LiAlH₄ is a highly reactive reducing agent. When water is added to a reaction mixture containing LiAlH₄, it reacts vigorously, leading to the release of hydrogen gas and the formation of aluminum hydroxide. Therefore, water is typically added after the reduction to safely quench the reaction and convert the reaction intermediates into alcohols.

Question 7

Which of the following reaction takes place when acetone reacts with HCN ?

Accepted Answer

C. Nucleophilic addition — HCN provides a nucleophile (CN⁻) that attacks the electrophilic carbonyl carbon of acetone. This forms a tetrahedral alkoxide intermediate. Protonation gives the cyanohydrin, a classic nucleophilic addition product.

Question 8

The common name for 2-propanone is:

Accepted Answer

B. Acetone — 2-Propanone is the systematic IUPAC name for the simplest ketone. Its widely used common name is acetone.

Question 9

Which type of reaction takes place when a carbonyl compound is treated with a mixture of NaCN and an acid?

Accepted Answer

C. Nucleophilic addition reaction — The given scenario is an example of a nucleophilic addition reaction, as the reactant, CN- is a nucleophile which adds across the carbonyl bond.Option A describes reactions undergone by alkenes, where an electrophile is added across the carbon-carbon double bond.Option B describes a reaction where one group of atoms is replaced by another group of atoms, but this is not the case here.Option D describes a reaction in which one element is replaced by another element in a compound.

Question 10

The general formula for an aldehyde is:

Accepted Answer

E. R-CHO — Aldehydes and ketones have a carbonyl group (C=O) as a functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon (RCOR’). The simplest ketone is acetone, which has two methyl groups attached to the carbonyl carbon (CH3COCH3).An aldehyde is similar to a ketone, except that instead of two side groups connected to the carbonyl carbon, they have at least one hydrogen (RCOH or R-CHO). The simplest aldehyde is formaldehyde (HCOH), as it has two hydrogens connected to the carbonyl group. All other aldehydes have one hydrogen bonded to the carbonyl group, like the simple molecule acetaldehyde, which has one hydrogen and one methyl group (HCOCH3).The carbonyl carbon in both aldehydes and ketones is electrophilic, meaning that it has a dipole due to the electronegativity of the attached oxygen atom. This makes the carbonyl carbon an ideal target for nucleophiles in a nucleophilic addition reaction. During this reaction, the nucleophile, or electron donor, attacks the carbonyl to form the tetrahedral intermediate. The negatively charged oxygen accepts a hydrogen ion to form a hydroxyl group.Typically, nucleophiles possess a negative charge or lone pair on a heteroatom, which can take several forms (OH-, RO-, CN-, R3C-, RNH2, ROH). For primary amines (RNH2), the reaction does not stop at the formation of the tetrahedral intermediate with a hydroxyl group. Rather, an elimination reaction occurs that produces a double-bonded carbon and nitrogen functional group known as an imine. Understanding the reactions that aldehydes and ketones can undergo provides a way to differentiate between these similar organic compounds.

Question 11

Phenylmethyl ketone can be converted into ethyl benzene in one step by using:

Accepted Answer

B. Zn(Hg)-HCl — The reaction is Clemmensen reduction. The reaction of aldehydes and ketones with zinc amalgam (Zn/Hg alloy) in concentrated hydrochloric acid, which reduces the aldehyde or ketone to a hydrocarbon, is called Clemmensen reduction.

Question 12

Treatment of propionaldehyde with Dil. the NaOH solution gives:

Accepted Answer

C. CH3 CH2 CH (OH) CH(CH3) CHO — When propionaldehyde (CH3CH2CHO) reacts with a dilute NaOH (sodium hydroxide) solution, it undergoes a nucleophilic addition reaction known as an aldol condensation. In this reaction, the aldehyde group of propionaldehyde is attacked by the hydroxide ion (OH-) from the NaOH solution.

Question 13

Which of the following reactions is NOT shown by ketones?

Accepted Answer

B. Reaction with Fehling solution — Fehling's solution is a mixture of Rochelle salt (blue color) and copper sulphate (chso4). It is an oxidizing agent which is used to diffentiate between aldehydes and ketones. Ketones generally do not react with Fehling's solution. A red colored precipetate is formed when Fehling's solution is reacted with aldehyde.Reaction with HCN:Ketones when added with HCN give hydroxynitriles. HCN will be added with the carbon-oxygen double bond to produce hydroxynitriles.Rxn with NaHso3:Ketones generally get added with NaHSO3 by the process of nucleophilic addition.Rxn with 2,4 dinitrophenol hydrazine:Aldehydes and ketones on reaction with this generally form orange/yellow precipitates.

Question 14

Which of the following reagent is used to separate and purify carbonyl and non carbonyl compounds?

Accepted Answer

C. NaHSO3 — Sodium bisulfite reacts selectively with carbonyl compounds to form stable, crystalline bisulfite adducts. Non-carbonyl compounds do not react, allowing easy separation. Regenerating the carbonyl from the adduct gives purified carbonyl compounds.

Question 15

Ketone reacts with Grignard reagent to form _ in acidic media?

Accepted Answer

A. Tertiary alcohol — The correct answer is a tertiary alcohol. When a ketone, which has a carbonyl group (C=O) bonded to two carbon-containing groups, reacts with a Grignard reagent, the carbonyl carbon gets attacked. This nucleophilic addition forms an alkoxide intermediate. Upon protonation in acidic conditions, the alkoxide converts into an alcohol. As the carbonyl carbon is already attached to two other carbon groups, the alcohol formed is tertiary.Isopropyl alcohol is incorrect as it forms from acetaldehyde, not a ketone. A primary alcohol is produced from formaldehyde, and a carboxylic acid forms from carbon dioxide with Grignard reagents, making these options irrelevant for this reaction.

Question 16

The following reaction is an example of:

Accepted Answer

B. Acylation of benzene — Since the acyl group is added to the benzene as a result of this reaction process we can say that it is an acylation of benzene.

Question 17

Carbonyl group undergoes:

Accepted Answer

B. Nucleophilic addition reaction — t, the carbonyl group (C=O) undergoes nucleophilic addition reactions rather than electrophilic addition reactions. In a nucleophilic addition reaction, a nucleophile (electron-rich species) attacks the electrophilic carbon atom of the carbonyl group, leading to the formation of a new bond and subsequent addition of the nucleophile. One of the most common nucleophilic addition reactions involving carbonyl compounds is the reaction with a nucleophilic reagent, such as a primary or secondary amine. The lone pair of electrons on the nitrogen atom of the amine attacks the carbon atom of the carbonyl group, resulting in the formation of an imine or a Schiff base. Another important nucleophilic addition reaction is the reaction of carbonyl compounds with nucleophilic organometallic reagents, such as Grignard reagents or organolithium compounds. The nucleophile attacks the electrophilic carbon, breaking the carbon-oxygen double bond and forming a new carbon-carbon bond. This reaction is commonly used to synthesize alcohols, as the addition of water or an alcohol source after the initial reaction generates the alcohol product. Other nucleophiles, such as hydride ions (H-) or cyanide ions (CN-), can also add to the carbonyl carbon, resulting in the formation of alcohols (via reduction) or cyanohydrin derivatives, respectively. These are just a few examples of nucleophilic addition reactions involving the carbonyl group. The specific reaction conditions and mechanisms can vary depending on the nature of the carbonyl compound and the nucleophile used. In a carbonyl group, -CHO, the carbon is double bonded to an oxygen which is electronegative and so it pulls electron density, acquiring a partial negative charge. At the same time, the carbon acquires a partial positive charge. Nucleophiles are electron donors so they prefer the electron-deficient carbon centre and attack it. Furthermore, carbonyls undergo addition reactions because the double bond between Carbon and Oxygen can break, setting an electron free to bond with another group, the nucleophile. We can therefore establish that carbonyls (ketones and aldehydes) undergo nucleophilic addition reactions, hence, option B is correct.

Question 18

What is the color of the precipitate when Brady’s reagent is used to test a benzaldehyde?

Accepted Answer

D. Red — Brady’s reagent (2,4-dinitrophenylhydrazine) reacts with the carbonyl group of benzaldehyde. It forms a 2,4 -dinitrophenylhydrazone, which is an insoluble red-orange/red precipitate. This confirms the presence of an aldehyde or ketone group.

Question 19

Which of the following can be oxidized to an aldehyde?

Accepted Answer

D. Ethanol — Ethanol can be oxidized to produce acetaldehyde, which is an aldehyde. The oxidation can be carried out using an oxidizing agent such as potassium permanganate or chromium trioxide. The other options, 2-Ethanoic acid, 2-Ethanoate, and Diethyl ether, do not contain a functional group that can be oxidized to form an aldehyde.

Question 20

Select the reagent X from the following choices for this conversion:

Accepted Answer

B. Acidified Potassium dichromate (VI) — The reaction above is oxidation of a secondary alcohol into a ketone. As such, an oxidizing agent must be chosen for reagent X. Among the options, 'Potassium dichromate (VI)' is an oxidizing agent so, the correct option is B. Another answer could be 'potassium manganate (VII)', but it is not mentioned in any of the options. The other options do not oxidize alcohols.

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