This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Aldehydes and Ketones Chemical Reactions – 1”.
1. Aldehydes and ketones undergo __________ reactions.
a) electrophilic addition
b) electrophilic substitution
c) nucleophilic addition
d) nucleophilic substitution
Explanation: Aldehydes and ketones have a polar CO group which also has a double bond. The incoming nucleophile attacks the sp2 hybridised carbon, thus breaking the double bond and converting it into sp3, and forming a tetrahedral alkoxide intermediate. This undergoes another fast step to form an addition product.
2. Which of the following is added to the tetrahedral intermediate when it reacts with a proton from the reaction medium?
c) Both Nu– and H+
Explanation: A H+ proton is added to the negative O atom of the alkoxide intermediate to form an electrically neutral addition product. The Nu– is added in the first step itself when it attacks the electrophilic carbon centre in a slow step.
3. The nucleophilic addition reactions of aldehydes are carried out in ________ medium.
c) weakly basic
d) extremely basic
Explanation: When the incoming nucleophile is weak, the acidic medium helps in protonation by increasing the positive charge on the carbonyl carbon so that it is more susceptible to attack by relatively weaker nucleophiles.
4. What is the correct order of reactivity of the following towards nucleophilic addition?
a) Methanal > Ethanal > Acetone
b) Acetone > Ethanal > Methanal
c) Methanal > Acetone > Ethanal
d) Ethanal > Methanal > Acetone
Explanation: Methanal is the most reactive among aldehydes and ketones due to electronic and stearic reasons. Methanal (HCHO) does not have any alkyl groups that release electrons toward carbonyl carbon, thus making it more electrophilic. Also, presence of only two H atoms do not provide any hinderance to the attack of incoming nucleophile.
5. Which of the following is least reactive towards a nucleophilic attack?
c) Diisopropyl ketone
d) Ditert-Butyl ketone
Explanation: The stearic effects come into consideration when comparing these compounds. Ditert-Butyl ketone has two very bulky tert-butyl groups, one on either side of the carbonyl carbon. This increases the hinderance to the incoming nucleophile and decreases the reactivity.
6. What is the correct order of reactivity towards nucleophilic addition?
a) Benzaldehyde > Benzophenone > Acetophenone
b) Benzophenone > Benzaldehyde > Acetophenone
c) Acetophenone > Benzaldehyde > Benzophenone
d) Benzaldehyde > Acetophenone > Benzophenone
Explanation: Generally, the aromatic carbonyl compounds are less reactive than corresponding aliphatic compounds. From benzaldehyde to acetophenone to benzophenone, the number of electron releasing groups increases and the magnitude of positive charge on carbonyl carbon reduces, hence decreasing its reactivity towards nucleophile attack.
7. Which of the following is the most reactive towards hydrogen cyanide?
Explanation: Addition of hydrogen cyanide is a nucleophilic reaction. CH3 and COCH3 are electron releasing groups which decrease the positive charge on carbonyl carbon, whereas NO2 is an electron withdrawing group which increases the electrophilicity of carbonyl carbon, making it more reactive toward hydrogen cyanide than benzaldehyde.
8. Identify the catalyst in the nucleophilic addition of HCN to acetone.
Explanation: Acetone reacts with HCN to give a cyanohydrin of acetone. But this reaction proceeds very slowly with pure HCN. So, the reaction is carried out in a basic medium which removes the proton from HCN and produces CN– ion which is a stronger nucleophile and makes the reaction faster.
9. The hydrolysis of the addition product of acetone and methyl magnesium iodide gives _______
a) ethyl alcohol
b) isopropyl alcohol
c) tert-Butyl alcohol
Explanation: The methyl group of Grignard reagent is the nucleophile which initially get attached to the carbonyl carbon of acetone. This results in three methyl groups attached to the carbon atom and one OMgI group. This MgI is replaced by H+ on hydrolysis to give tert-Butyl alcohol. Similarly, methanal gives ethyl alcohol (1°) and other aldehydes give 2° alcohols.
10. The following is an addition product of the reaction between _______ and sodium bisulphite.
Explanation: Acetaldehyde reacts with NaHSO3 to form a crystalline bisulphite addition compound. First, the nucleophile HSO3– attaches to the carbonyl carbon, after which the hydrogen gets attached to the oxygen atom on proton transfer.
11. The following acetal shown is formed by the reaction between ______ and ______ in the presence of dry HCl.
a) methanal; methanol
b) methanal; ethanol
c) ethanal; methanol
d) ethanal; ethanol
Explanation: Ethanal and ethanol react in the presence of dry HCl to initially form an unstable hemiacetal having one ethoxy (OC2H5) and one hydroxy (OH) group. This further reacts with ethanol to form the gem-diethoxy compound, acetaldehyde ethyl acetal as shown.
12. Acetone reacts with ethylene glycol in dry HCl gas to form _________
c) cyclic acetals
d) cyclic ketals
Explanation: Dihydric alcohols like ethylene glycol react with aldehydes and ketones to directly form cyclic products known as cyclic acetals and cyclic ketals respectively. The cyclic ketals are also known as ethylene glycol ketals.
13. Which of the following products is formed when an aldehyde reacts with an amine in acidic medium?
b) Schiff’s base
Explanation: Amine is a nucleophile and a derivative of NH3 which react with aldehydes and ketones followed by dehydration to form compounds with C-N double bond called Schiff’s base or substituted imines.
14. Identify the N-substituted derivative of carbonyl compounds that are coloured compounds and are useful in the identification of aldehydes and ketones.
Explanation: 2,4-DNP derivatives are formed from the reaction between aldehydes or ketones and 2,4-Dinitrohydrazine. These products may be yellow, orange or ed coloured and help in characterisation of aldehydes and ketones.
15. Semicarbazones undergo acidic nucleophilic addition followed by dehydration to give N-substituted carbonyl derivatives.
Explanation: Semicarbazone is the name of the product formed from the rapid dehydration of the product of the reaction between semicarbazide and aldehyde or ketone in acidic medium. Semicarbazide is the reagent and Semicarbazone is the N-substituted carbonyl derivative.
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