Question

Identify the correct statement among the following. (a) A hydrocarbon of M.F. \(\mathrm{C}_{6} \mathrm{H}_{12}\) on reductive ozonolysis gave only one product which fails to react with Tollens reagent. The hydrocarbon could be 2,3 dimethyl but- 2 -ene. (b) Addition of bromine to trans-2-butene gives a racemic mixture of two optically active enantiomers. (c) But-1-yne and but-2-yne can be distinguished using ammoniacal cuprous chloride. (d) Ozonolysis of acetylene gives glyoxal.

Short answer

a) 2,3-dimethyl but-2-ene does not react with Tollens reagent. b) The addition of bromine to trans-2-butene leads to the formation of optically active enantiomers. c) Ammoniacal cuprous chloride can be used to differentiate between but-1-yne and but-2-yne. d) Ozonolysis of acetylene would give glyoxal as the product. Answer: (c) and (d) are the correct statements.

Step-by-step solution

Statement (a) Evaluation

Reductive ozonolysis involves the breakdown of a double Bond in organic compounds, followed by the addition of hydrogen to form different products. The given molecular formula is \(\mathrm{C}_{6} \mathrm{H}_{12}\). The product fails to react with Tollens reagent (which detects aldehydes), so it must not be an aldehyde. Therefore, the given hydrocarbon, 2,3-dimethyl but-2-ene, could be an accurate option.

Statement (b) Evaluation

trans-2-Butene is an alkene with a double bond between carbons 2 and 3. Addition of bromine to trans-2-butene leads to the formation of a dibromo product. Since the original trans-2-butene molecule is symmetric and has no chiral centers, the dibromo product formed should also have no chiral centers. Therefore, there will not be any optically active enantiomers formed. This statement is incorrect.

Statement (c) Evaluation

But-1-yne and but-2-yne are alkynes with triple bonds between carbons 1-2 and 2-3, respectively. Ammoniacal cuprous chloride (Cu\(_2\)Cl\(_2\)) is used to differentiate terminal and internal alkynes since it reacts with terminal alkynes only, forming a red precipitate of copper acetylide. But-1-yne being a terminal alkyne would react with ammoniacal cuprous chloride while but-2-yne (an internal alkyne) would not. This statement is correct.

Statement (d) Evaluation

Ozonolysis of acetylene (\({C_2H_2}\)) involves breaking the triple bond and formation of a carbonyl group at each carbon atom. Since there are two carbon atoms, the resulting molecule would have 2 carbonyl groups and the molecular formula \({C_2H_2O_2}\). This molecule is called glyoxal. Therefore, this statement is also correct. In conclusion, statements (c) and (d) are the correct statements among the given options.

Key concepts

Reductive Ozonolysis

Reductive ozonolysis is a useful technique in organic chemistry to break down alkenes by adding ozone, resulting in the formation of carbonyl compounds. During the process, the double bond in the hydrocarbon is cleaved, and ozone adds across it, forming ozonides. To convert these ozonides into more stable carbonyl compounds, a reductive agent like zinc in acetic acid is used.
This method is particularly significant because it provides important structural information about the original alkene. For example, if the ozonolysis of an alkene yields only one product which isn't reactive with Tollens' reagent, it suggests the formation of a ketone since Tollens' reagent typically detects aldehydes. Therefore, the reductive ozonolysis of 2,3-dimethyl but-2-ene would result in ketones, confirming statement (a) as potentially correct.

Tollens Reagent

Tollens' reagent is a classic chemical test utilized for detecting aldehydes. It comprises a solution of silver nitrate in ammonia that forms a complex, silver ammonia complex (\[ ext{[Ag(NH}_3^+]_2 ext{OH}^- \]). When reacted with an aldehyde, this reagent reduces to form a silver mirror on the reaction vessel's surface. This is indicative of the presence of an aldehyde group because ketones and most other functional groups do not react with Tollens' reagent.
In the context of reductive ozonolysis, if the product fails to react with Tollens' reagent, it’s likely not an aldehyde. This helps us correlate the findings with the intermediary or final products of the reaction, allowing chemists to draw conclusions about the structure of original organic compounds.

Alkanes and Alkynes

Both alkanes and alkynes are types of hydrocarbons but differ significantly in their bonding. Alkanes are saturated hydrocarbons with single bonds (e.g., ethane), while alkynes are unsaturated and contain at least one carbon-carbon triple bond (e.g., acetylene). Their chemical behaviors diverge due to these differences in bonding.
Alkynes can be categorized further into terminal and internal alkynes based on the position of the triple bond. A terminal alkyne has a triple bond at the end of the carbon chain, as in but-1-yne, while an internal alkyne has it in the middle, as seen in but-2-yne. This distinction is crucial because reagents like ammoniacal cuprous chloride can differentiate them. Terminal alkynes give positive reactions, forming a precipitate, indicating the presence of a terminal alkyne. Such differentiation is confirmed in statement (c) and helps chemists in identifying unknown compounds.

Bromination of Alkenes

Bromination is an important reaction in chemistry, where bromine (\( ext{Br}_2\)) is added to an alkene to form a dibromo compound. This electrophilic addition occurs across the double bond, allowing for halogenation which simplifies structural analysis.A key detail is the stereochemistry involved. For instance, bromination of trans-2-butene forms only one dibromide because of its inherent symmetry and lack of chiral centers, as evaluated in statement (b). Due to symmetry, the molecule cannot form optically active enantiomers. Hence, the bromination of such symmetric alkenes results in achiral products, debunking the idea that they would produce enantiomers.

Ozonolysis

Ozonolysis is a method designed to cleave alkenes or alkynes using ozone. In the case of alkynes, it efficiently breaks the triple bond and forms carbonyl compounds, typically resulting in either carboxylic acids or smaller aldehydes and ketones depending on the reduction step that follows. This transforms the structure into simpler, often more informative, compounds.
For example, when acetylene undergoes ozonolysis, the triple bond is broken, and each carbon atom forms a carbonyl group, resulting in glyoxal (\(C_2H_2O_2\)). This type of reaction is significant in identifying the arrangement of atoms in unsaturated hydrocarbons, as validated in statement (d). Through ozonolysis, chemists can gain insights into the positions of multiple bonds within a molecule.