Question

Which of the following pairs will give effervescence with aqueous sodium hydrogen carbonate solution? (a) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\) (b) \(\mathrm{CH}_{3} \mathrm{CONH}_{2}\) and \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\)

Short answer

Answer: (a) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\) and (d) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\)

Step-by-step solution

Identify acidic compounds

Generally, compounds with acidic hydrogens (H⁺) will react with NaHCO₃ to produce CO₂ and cause effervescence. Let's examine each compound in the given pairs to find out if there are acidic hydrogens present. (a) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\) * \(\mathrm{CH}_{3} \mathrm{COCl}\): Acyl chloride, a strong acid with an acidic hydrogen. * \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\): Ketone, not acidic. (b) \(\mathrm{CH}_{3} \mathrm{CONH}_{2}\) and \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\) * \(\mathrm{CH}_{3} \mathrm{CONH}_{2}\): Amide, not acidic. * \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\): Ether, not acidic. (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\) * \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\): Phenol, weak acid with an acidic hydrogen. * \(\mathrm{CH}_{3} \mathrm{COOCH}_{3}\): Ester, not acidic. (d) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\) * \(\mathrm{CH}_{3} \mathrm{COCl}\): Acyl chloride, a strong acid with an acidic hydrogen. * \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\): Ether, not acidic.

Find the pair with an acidic compound reacting with NaHCO₃

Now that we have identified the acidic compounds in each pair, we can see that acyl chloride (\(\mathrm{CH}_{3} \mathrm{COCl}\)) and phenol (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\)) are the acidic compounds in the given pairs. Acyl chloride will react with NaHCO₃ to produce CO₂. Phenol has an acidic hydrogen, but it is a weak acid and does not react with NaHCO₃.

Identify the pair that will give effervescence

Based on our analysis, effervescence will occur in the presence of the acyl chloride (\(\mathrm{CH}_{3} \mathrm{COCl}\)). (a) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}\) (d) \(\mathrm{CH}_{3} \mathrm{COCl}\) and \(\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2} \mathrm{O}\) Out of the two options that include acyl chloride, both pairs will give effervescence with aqueous sodium hydrogen carbonate solution. Therefore, the correct pairs are (a) and (d).

Key concepts

Acidic Hydrogens in Organic Compounds

Understanding acidic hydrogens in organic compounds is crucial when predicting reactions with bases such as sodium hydrogen carbonate (NaHCO₃). Generally, these hydrogen atoms are attached to electronegative atoms like oxygen or nitrogen, or to a carbon that is part of an unsaturated bond.

In the context of the given exercise, organic compounds such as acids, phenols, and certain acyl chlorides contain hydrogen atoms that can be released as protons (H⁺) in a reaction. This release of protons is responsible for the characteristic effervescence seen when these compounds react with NaHCO₃, as the bicarbonate ion acts as a weak base capturing these protons to form carbon dioxide gas and water.

Chemical Properties of Acyl Chlorides

Among the most reactive carbonyl compounds are the acyl chlorides, also known as acid chlorides. They are characterized by the presence of a carbonyl group (a carbon double-bonded to an oxygen) attached to a chlorine atom. Their chemical properties include high reactivity towards nucleophilic agents such as water, alcohols, and even weak bases like NaHCO₃.

This reactivity is due to the high electrophilicity of the carbon atom in the carbonyl group, which in turn is heightened by the good leaving-group quality of the chloride ion. When acyl chlorides come into contact with NaHCO₃, they rapidly react, releasing CO₂ gas, hence the effervescence observed in these reactions.

Carbonyl Compound Reactivity

The reactivity of carbonyl compounds such as ketones, aldehydes, and esters varies depending on the compound's structure and the surrounding atoms. In particular, the carbon atom of the carbonyl group is electrophilic, but not all carbonyl compounds will react with NaHCO₃ to produce effervescence.

For instance, ketones and esters lack the acidic hydrogen that is crucial for the reaction with NaHCO₃ to form CO₂. Therefore, although they contain a carbonyl group, they do not exhibit the same reactivity as acyl chlorides in the presence of sodium hydrogen carbonate.

Effervescence in Chemical Reactions

Effervescence refers to the escape of gas from a liquid solution, creating a fizzing or bubbling effect. This usually occurs in chemical reactions that produce a gas as one of the products.

In the context of the given problem, effervescence is indicative of a chemical reaction taking place between an acid, such as an acyl chloride, and NaHCO₃. The reaction liberates CO₂ gas, observable as bubbling in the solution. This visual cue is a common and useful indicator to confirm the presence of acidic hydrogens in organic compounds and to gauge their reactivity with basic substances like sodium hydrogen carbonate.