Question

Identify the correct statements among the following (a) Chlorination of \(\mathrm{n}\) -butane involves the attack by a free radical. (b) 2,3-Dimethylbutane can form only two monochloro derivatives (c) Between \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{3}\) (I) and \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{C} \equiv \mathrm{CH}\) (II), only (II) will react with \(\mathrm{CH}_{3} \mathrm{MgI}\). (d) When a mixture of ethane, ethylene and ethyne is passed into a solution of ammoniacal \(\mathrm{AgNO}_{3}\), the gases bubbling out will be a mixture of ethane and ethylene

Short answer

a) The chlorination of n-butane is a free radical reaction. b) There are four monochloro derivatives of 2,3-dimethylbutane. c) Only compound I will react with CH3MgI. d) Only ethyne reacts with ammoniacal AgNO3 among ethane, ethylene, and ethyne. Answer: The correct statements are (a), (c), and (d).

Step-by-step solution

Is the Chlorination of \(\mathrm{n}\)-butane a free radical reaction?

In the chlorination of \(\mathrm{n}\)-butane, the reaction includes homolytic cleavage of the \(\mathrm{Cl-Cl}\) bond, producing two chloride radicals. These free radicals can attack the hydrogen atom of \(\mathrm{n}\)-butane, and the resulting alkyl radical reacts with another chlorine molecule to generate the chlorinated product. Therefore, statement (a) is correct. #Step 2: Analyze statement (b)#

Number of monochloro derivatives of 2,3-Dimethylbutane

Let's examine the different carbon atoms in 2,3-dimethylbutane. The molecule has 3 types of carbons: two end methyl groups, one tertiary carbon in the middle, and a secondary carbon adjacent to the tertiary one. Chlorination could happen at any of these carbon atoms, resulting in 3 monochloro derivatives: 1- and 4-chloro-2,3-dimethylbutane, 2-chloro-2,3-dimethylbutane, and 3-chloro-2,3-dimethylbutane. Therefore, statement (b) is incorrect. #Step 3: Analyze statement (c)#

Reaction of I and II with \(\mathrm{CH}_{3} \mathrm{MgI}\)

Grignard reagents, like \(\mathrm{CH}_{3}\mathrm{MgI}\), are strong nucleophiles, and they can attack the electrophilic carbon atom of alkynes. In compound I, there are no protons on the carbon atom adjacent to the triple bond, so it can't undergo acid-base reaction with the Grignard reagent. In compound II, the terminal alkyne with an acidic proton will react with \(\mathrm{CH}_{3}\mathrm{MgI}\) to form an alkynide anion and then a new carbon-carbon bond. Therefore, statement (c) is correct. #Step 4: Analyze statement (d)#

Reaction of ethane, ethylene, and ethyne with ammoniacal \(\mathrm{AgNO}_{3}\)

Ammoniacal \(\mathrm{AgNO}_{3}\) solution, known as Tollen's reagent, is a mild oxidizing agent. When ethane, ethylene, and ethyne are passed into the solution, the ethyne will react, and ethane and ethylene will not. The reaction between ethyne and Tollen's reagent forms a silver acetylide precipitate and produces ethylene and ethane as gases. Therefore, statement (d) is correct. In conclusion, the correct statements are (a), (c), and (d).

Key concepts

Free Radical Reactions

A free radical reaction is a type of chemical reaction where atoms or molecules with unpaired electrons, known as free radicals, are formed and react with other substances. This process often includes three key steps: initiation, propagation, and termination.

During *initiation*, energy input, like heat or light, causes the homolytic cleavage of a bond, such as the Cl-Cl bond in chlorination, generating free radicals. In the case of chlorination of *n*-butane, this results in two chlorine radicals.

In the *propagation* step, these radicals attack stable molecules, creating new radicals. For example, chlorine radicals abstract hydrogen from *n*-butane, producing an alkyl radical and hydrochloric acid. The alkyl radical can then react with another chlorine molecule to produce chlorinated butane and another chlorine radical, continuing the chain reaction.

*Termination* occurs when two free radicals combine, ending the chain reaction. Free radical reactions are important in organic chemistry because they can efficiently form new bonds and introduce functional groups into molecules.

Grignard Reagents

Grignard reagents are a class of organomagnesium compounds, notably RMgX, where R is an organic group and X is a halogen. They are prized in organic synthesis for their ability to form carbon-carbon bonds.

These reagents are formed by reacting magnesium metal with an alkyl or aryl halide in an ether solvent, such as diethyl ether. They are strong nucleophiles and bases, meaning they can attack electrophilic carbons in compounds or deprotonate acidic hydrogens.

For example, when analyzing compounds I and II from the exercise, only compound II, with a terminal alkyne, reacts with \({\rm CH}_3{\rm MgI}\) because it has an acidic proton. This reaction proceeds through an acid-base mechanism, forming an alkynide anion, which then helps in constructing new carbon skeletons via subsequent reactions. Grignard reactions are essential tools in organic chemistry for adding depth and complexity to molecular structures.

Functional Group Transformations

Functional group transformations are processes that convert one functional group in a molecule to another. This is critical in organic synthesis, allowing chemists to build complex molecules from simpler ones.

In the exercise example, chlorination of hydrocarbons is a form of functional group transformation, converting a hydrogen atom into a chloro group. This not only changes the molecule's reactivity but also prepares it for further chemical reactions, such as nucleophilic substitution or elimination reactions.

Grignard reagents also facilitate functional group transformations. By converting alkyl or aryl halides into Grignard reagents, chemists can readily create alcohols, acids, and other valuable functional groups after adding to carbonyl compounds. This makes functional group transformations indispensable in the strategic planning of synthetic routes in chemistry. Such transformations enhance the versatility and applicability of molecules in producing drugs, materials, and other chemical innovations.