Question

Question contains Statement-1 and Statement-2 and has the following choices (a), (b), (c) and (d), out of which ONLY ONE is correct. (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1 (b) Statement- 1 is True, Statement- 2 is True; Statement- 2 is NOT a correct explanation for Statement-1 (c) Statement- 1 is True, Statement- 2 is False (d) Statement-1 is False, Statement- 2 is True Statement 1 Alkynes are more reactive than alkenes in electrophilic addition reactions. and Statement 2 Electrophilic addition to both alkenes and alkynes involve a three membered cyclic intermediate and the bromonium ion type intermediate is less stable for an alkyne than for an alkene.

Short answer

If yes, is it because the three-membered cyclic intermediate is less stable for alkynes than for alkenes? Answer: Yes, alkynes are more reactive than alkenes in electrophilic addition reactions, and this is due to the three-membered cyclic intermediate being less stable for alkynes than for alkenes.

Step-by-step solution

Determine the correctness of Statement 1

Here, we need to determine whether alkynes are more reactive than alkenes in electrophilic addition reactions or not. Since alkynes are electron-rich and have greater electron density due to the triple bond, they are more reactive than alkenes in electrophilic addition reactions. So, Statement 1 is True.

Determine the correctness of Statement 2

Now, we need to determine whether electrophilic addition to both alkenes and alkynes involves a three-membered cyclic intermediate and that the bromonium ion type intermediate is less stable for an alkyne than for an alkene. In electrophilic addition reactions, indeed, a three-membered cyclic intermediate, such as a bromonium ion, forms as a result of the electrophile attacking the carbon-carbon double or triple bond. However, due to the greater electronegativity difference between the carbon atoms in an alkyne versus an alkene, a bromonium ion-type intermediate is less stable for alkynes. Therefore, Statement 2 is True.

Determine whether Statement 2 is a correct explanation for Statement 1

As we have established that alkynes are more reactive in electrophilic addition reactions due to their greater electron density, and we know that electrophilic addition involves a three-membered cyclic intermediate that is less stable for alkynes, we can say that Statement 2 provides a correct explanation for Statement 1. Both the statements are true, and Statement 2 is a correct explanation for Statement 1. Therefore, the answer is (a) Statement- 1 is True, Statement- 2 is True; Statement- 2 is a correct explanation for Statement-1.

Key concepts

Electrophilic Addition Reactions

Electrophilic addition reactions are vital in organic chemistry, particularly when discussing alkenes and alkynes. These reactions start when an electrophile, an electron-loving species, targets the electron-dense double or triple bonds found in alkenes and alkynes. The electrophile effectively "adds" across the bond, converting multiple bonds into single bonds and forming new compounds.

In such reactions, the electrophile attracts electrons from the carbon-carbon bonds, setting the stage for further chemical transformation. A distinctive feature of electrophilic addition reactions, especially with alkenes and alkynes, is the generation of an intermediate - often a cyclic one. These intermediates can influence the rate and outcome of the reactions significantly.

One common example is the process of halogenation, where halogens like bromine or chlorine add to the unsaturated hydrocarbons, yielding more saturated products. Understanding this mechanism is crucial as it forms the basis for various industrial synthesis processes. The details of these reaction mechanisms can further elucidate why certain compounds, like alkynes, react differently compared to others such as alkenes.

Alkynes and Alkenes Reactivity

Alkynes and alkenes are unsaturated hydrocarbons with varying reactivity levels in electrophilic addition reactions. An alkyne features a triple bond, while an alkene possesses a double bond. This
an additional bond means alkynes have more stored electron density, making them theoretically more reactive. This extra electron density attracts electrophiles more vigorously than alkenes.

Despite this, the actual reactivity in real-world conditions can sometimes differ. The reactivity of alkynes in generating highly stable intermediates during these reactions is contrasted by the stability often found in intermediates arising from alkenes. Alkenes, with their less tightly bound electrons, can stabilize positive charges more easily, sometimes leading to faster reactions in practice.

For students studying organic chemistry, getting to grips with these differences is essential. Understanding the nuances of why an alkyne might react more or less rapidly than an alkene at given conditions helps in predicting product formation and the reaction conditions required.

Cyclic Intermediates

In the world of organic chemistry, cyclic intermediates are often the hidden yet critical steps in many addition reactions. During electrophilic additions, after an electrophile attacks an unsaturated bond, a cyclic intermediate can often be formed. These intermediates are typically three-membered rings, like bromonium ions.

The stability of these cyclic intermediates varies between different types of unsaturated hydrocarbons. In the case of alkynes, these intermediates are generally less stable due to the strain involved in bending the linear geometry of triple bonds into a three-membered ring structure. For alkenes, forming such cyclic intermediates is less strained, partly due to their existing bent geometry from the double bond configuration.

This stability, or lack thereof, of cyclic intermediates directly affects the speed and pathway of the reaction. More stable intermediates favor reactions proceeding swiftly and cleanly, whereas less stable intermediates might rearrange or break down, altering the reaction outcome. Recognizing and accounting for the behavior of these intermediates provides insights into the mechanisms and potential outcomes of organic reactions. Such knowledge is beneficial in both academic and practical chemistry applications.