Question

This section contains multiple choice questions. Each question has 4 choices (a), (b), (c) and (d), out of which ONLY ONE is correct. An organic compound on elemental analysis is found to contain two chlorine atoms. This compound is treated with caustic soda and subsequently with \(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{OH}\) to get a silver deposit. Then we can say that (a) The two \(\mathrm{Cl}\) atoms are attached to an aromatic ring. (b) The two \(\mathrm{Cl}\) atoms are on adjacent carbons of an aliphatic chain. (c) The two \(\mathrm{Cl}\) atoms are on the same carbons of an aliphatic chain. (d) One chlorine is on a tertiary carbon atom.

Short answer

Answer: The correct statement is: "The two Cl atoms are on adjacent carbons of an aliphatic chain."

Step-by-step solution

Reaction with caustic soda (NaOH)

The first step involves the reaction of the organic compound with NaOH. Caustic soda reacts with halogens like chlorine, forming water-soluble salts. This reaction can break the carbon-chlorine bond and create an anion that can proceed to the next step.

Reaction with Tollen's Reagent (\(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{OH}\))

The second step involves the reaction of the anion produced in step 1 with Tollen's Reagent (\(\left[\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}\right] \mathrm{OH}\)). This reagent is generally used to detect the presence of aldehyde or ketone functional groups in organic compounds. It can also react with halide anions to form a silver deposit (silver metal) and an ammonium salt. If a silver deposit is observed after the reaction, it means that the halogen (chlorine in this case) was present in the organic compound and was successfully ionized in the first step. Now that we understand the reactions, we can proceed to evaluate the statements to find the correct answer.

Evaluate Statement (a)

"The two Cl atoms are attached to an aromatic ring." In this case, the reaction with NaOH would not break the carbon-chlorine bond, as the bond is stronger due to resonance stabilization. Thus, no silver deposit would be observed, so statement (a) is not correct.

Evaluate Statement (b)

“The two Cl atoms are on adjacent carbons of an aliphatic chain.” If two chlorine atoms are bonded to adjacent carbons in an aliphatic chain, the reaction with NaOH would lead to the formation of a dihaloanion. This dihaloanion will react with Tollen's Reagent, resulting in the silver deposit observed in the exercise. Thus, statement (b) seems to be a suitable answer.

Evaluate Statement (c)

"The two \(\mathrm{Cl}\) atoms are on the same carbons of an aliphatic chain." As both chlorine atoms are on the same carbon atom, it forms a highly polarized and reactive bond, which could be easily broken with NaOH. This would lead to the formation of a halogen anion which would react with Tollen's Reagent, creating a silver deposit. This scenario matches the given description, so statement (c) cannot be ruled out.

Evaluate Statement (d)

"One chlorine is on a tertiary carbon atom." There is no conclusive information about the presence of chlorine on a tertiary carbon atom, and, moreover, it does not give any indication about the second chlorine's position or bonding. Thus, statement (d) cannot be considered as the correct answer.

Choosing the correct statement

After evaluating all the given statements, we can conclude that both statement (b) and statement (c) fit the description in the exercise. However, only one statement can be correct. Based on the silver deposit's requirement, statement (b) seems more plausible, as it specifies that both Cl atoms are on adjacent carbons of an aliphatic chain. In statement (c), the compound may not react with Tollen's Reagent to give a silver deposit as a consequence of steric hindrance. Therefore, statement (b) is the correct choice.

Key concepts

Elemental Analysis in Organic Compounds

Understanding the composition of organic molecules forms a fundamental aspect of organic chemistry. Elemental analysis is a process used to determine the percentage composition of each element within an organic compound. By knowing the elemental makeup, chemists can infer molecular structures and identify unknown substances.

This method often involves complete combustion of the organic substance to convert all its elements to the most stable forms: carbon to CO2, hydrogen to H2O, nitrogen to N2, sulfur to SO2, and halogens to corresponding halides. The masses of these products are measured to calculate the amount of each element present in the original sample.

In the context of our exercise, the organic compound is found to contain two chlorine atoms. This crucial information helps deduce potential reactions and the structure of the molecule, by considering various functional groups and their reactivity with other chemicals such as caustic soda in subsequent steps of analysis.

Reactions with Caustic Soda

Caustic soda, also known as sodium hydroxide (NaOH), is a strong base and a versatile reagent in organic chemistry. When an organic compound containing halogens like chlorine is treated with NaOH, it undergoes a nucleophilic substitution reaction where the halogen atom is replaced by a hydroxide ion. This substitution can occur via different mechanisms depending on the structure of the organic compound.

In aliphatic chains, the carbon-halogen bond can be cleaved to form a water-soluble salt and an alcohol. If the halogens are on adjacent carbons or a carbon atom is tertiary, the reactivity and the type of reaction may vary. This step is crucial for the preparatory process in certain analytical tests, such as the Tollen's Reagent Test, where the resultant anion plays a role in the final chemical reaction observed.

Tollen's Reagent Test

Tollen's reagent, a solution of silver nitrate in ammonia, is a classic chemical test for aldehydes. The reagent is a mild oxidizing agent that turns into a shiny silver mirror when reduced by an aldehyde. However, it's also sensitive to other reducing agents.

In this test, the silver ion in Tollen's reagent is reduced to metallic silver, often seen as a deposit or mirror on the test tube. The presence of a silver deposit in our exercise indicates the reaction between the anion produced from the caustic soda step and the Tollen's reagent, leading us to deduce the nature of the halogen bonding in the organic compound.

Specifically, this confirms that the chlorine atoms in the compound were able to form anions that could react with the reagent. It's important to note that the test does not work with chlorides attached to an aromatic ring due to the stability of the aromatic system. This is vital in eliminating possible structures of the organic compound and determining the correct answer for our multiple-choice question.