Question

Identify set of compound in which first compound react by higher rate with HBr than second compound. (A) \(\mathrm{HC} \equiv \mathrm{CH}, \mathrm{CH}_{2}=\mathrm{CH}_{2}\) (B) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{O}-\mathrm{C}_{6} \mathrm{H}_{5}, \mathrm{C}_{6} \mathrm{H}_{\mathrm{i1}}-\mathrm{O}-\mathrm{C}_{6} \mathrm{H}_{11}\) (C) (D)

Short answer

The correct answer is set (A), as acetylene (HC≡CH) reacts at a higher rate with HBr than ethene (CH2=CH2) due to its higher electron density.

Step-by-step solution

Reactivity of Acetylene and Ethene with HBr

Acetylene and ethene react with HBr through electrophilic addition due to the presence of a π bond. The reaction occurs more quickly at a more electron-rich π bond, likely due to its higher electron density. Since acetylene (HC≡CH) has a triple bond, it contains two π electrons, whereas ethene (CH2=CH2) has just a single π electron. Therefore, acetylene is more electron-rich and reacts more rapidly with HBr.

Reactivity of Diphenyl Ether and Dicyclohexyl Ether with HBr

Diphenyl ether (C6H5-O-C6H5) and dicyclohexyl ether (C6H11-O-C6H11) fall under the category of aryl ethers. Both of these compounds are stable and resistant to reactions due to resonance effects. They are unable to participate in an electrophilic addition reaction with HBr. Based on the analysis above, we can conclude that:

Conclusion

Only the pair of compounds in set (A), acetylene (HC≡CH) and ethene (CH2=CH2), shows different reactivity with HBr, where acetylene reacts at a higher rate than ethene. Thus, the correct answer is set (A).

Key concepts

Reactivity with HBr

In organic chemistry, electrophilic addition reactions are a pivotal concept, notably involving the interaction of hydrogen bromide (HBr) with compounds containing unsaturated bonds. In such reactions, the π electrons in a multiple bond attract the electrophilic hydrogen, leading to an addition reaction. Among the various carbon-carbon bonds, the reactivity with HBr primarily depends on the electron density within the π bond.
For compounds like acetylene ( HC≡CH ) and ethene ( CH_2=CH_2 ), the reaction with HBr varies due to differences in electron density and bond structure. Acetylene, with its triple bond, consists of two π bonds, making it richer in electrons compared to ethene, which contains just one π bond. As electrons play a significant role in attracting an electrophile such as HBr, acetylene reacts more swiftly than ethene.
Understanding the nuances of electrophilic addition is crucial for predicting reaction outcomes based on molecular structure and electron distribution in π bonds.

π Bonds

Pi bonds (π bonds) form the backbone of electrophilic addition reactions due to their relatively high electron density. They arise from the sideways overlap of p orbitals, as opposed to sigma bonds (σ bonds), which result from head-on overlapping.
Here's what makes π bonds unique:

• High electron density available above and below the plane of the nuclei of the bonding atoms.
• They are generally weaker than σ bonds due to their side-on overlap configuration.
• They are prone to attack by electrophiles such as HBr, which seek electron-rich areas to bond.

In the context of reactivity, more π bonds, like those seen in acetylene, can result in greater reactivity due to increased electron availability. This trait is crucial not only in the stability and reactivity of organic compounds but also in predicting how substances interact with each other during chemical reactions.

Aryl Ethers

Aryl ethers, such as diphenyl ether ( C_6H_5-O-C_6H_5 ) and dicyclohexyl ether ( C_6H_{11}-O-C_6H_{11} ), present an interesting aspect of chemical stability due to their resonance effects and structural properties. These compounds contain an ether linkage, where an oxygen atom connects to an aryl or cyclic group, contributing to their unique reactivity, or lack thereof, with certain reagents.
Here are some key points about aryl ethers:

• They are comparatively less reactive than aliphatic ethers due to the delocalization of electrons across the aromatic ring, a phenomenon known as resonance.
• The oxygen in ethers stabilizes the molecule through lone pair interactions but does not actively engage in the addition reactions typical of unsaturated hydrocarbons.
• This stability renders aryl ethers resistant to electrophilic addition, including reactions with HBr, unlike their saturated counterparts.

Thus, their robust stability underscores why aryl ethers do not participate in electrophilic addition reactions with HBr, behaving distinctly from acyclic ethers or unsaturated bonds.