Question

The correct -I effect order is (A) \(-\mathrm{F}>-\mathrm{Cl}>-\mathrm{Br}>-\mathrm{I}\) (B) \(-\mathrm{F}>-\mathrm{Cl}>-\mathrm{CN}>-\mathrm{NO}_{2}\) (C) \(-\mathrm{F}>-\mathrm{NO}_{2}>-\mathrm{OH}>-\mathrm{COOH}\) (D) \(-\mathrm{CN}>-\mathrm{F}>-\mathrm{NO}_{2}>-\mathrm{I}\)

Short answer

The correct -I effect order is (A) \(−F>−Cl>−Br>−I\), as it accurately represents the decreasing electron-withdrawing strength of the halogens due to their decreasing electronegativity values.

Step-by-step solution

Understand the inductive effect

The inductive effect occurs due to the difference in electronegativity between two atoms in a molecule or ion. It is a permanent effect present in a molecule, and it causes a polarization of the electron cloud resulting in either electron-withdrawing or electron-donating nature.

Analyze Option A

Option A involves the order of the halogens: \(−F>−Cl>−Br>−I\). It's important to note that as the electronegativity value decreases going down the halogen group, the inductive effect becomes weaker.

Analyze Option B

Option B includes the following groups: \(−F>−Cl>−CN>−NO_{2}\). Here, we have a combination of halogens and other groups. Comparing these groups according to their electron-withdrawing ability, we notice that \(−NO_{2}\) has a stronger -I effect than \(−F\), and \(−CN\) has a significant -I effect as well, making this order incorrect.

Analyze Option C

Option C involves the following groups order: \(−F>−NO_{2}>−OH>−COOH\). Comparing these groups, we observe that despite having two electronegative atoms, the \(−COOH\) group has a resonance effect, which somewhat neutralizes its -I effect. Thus, the order for option C is incorrect.

Analyze Option D

Option D states the following order: \(−CN>−F>−NO_{2}>−I\). Comparing these groups, we can see that the \(−CN\) and \(−NO_{2}\) groups are strong electron-withdrawing species, while the \(−I\) group is electron-donating. The order for option D is incorrect since the electron-withdrawing effects of the \(−F\) and \(−NO_{2}\) groups should be closer to each other, and the electron-donating effect of \(−I\) should be at the end of the sequence.

Conclusion

After analyzing all the options, we can determine that option A \((−F>−Cl>−Br>−I)\) represents the correct -I effect order, as it correctly takes into account the electron-withdrawing nature of the halogens and their decreasing electronegativity values in the order given.

Key concepts

Electronegativity and inductive effect

Understanding the inductive effect in organic chemistry begins with a fundamental concept: electronegativity. Electronegativity is the measure of an atom's ability to attract electrons towards itself. When atoms with different electronegativities form a covalent bond, the shared electrons are pulled closer to the more electronegative atom. This creates a permanent polarization of the bond, with partial charges developing on the atoms.

This polarization is what's known as the inductive effect. It's a key factor in determining the behavior of molecules in various chemical reactions. An important point to note is that the inductive effect can be either electron-withdrawing (-I effect) or electron-donating (+I effect), depending on the nature of the substituent bonded to the chain.

• An electron-withdrawing group (EWG) increases the positive character of the adjacent atom by pulling electron density towards itself.
• An electron-donating group (EDG) decreases the positive character of the adjacent atom by pushing electron density away from itself.

Knowing this principle helps you predict reactivity and stability of intermediate species which are vital to understanding mechanisms in organic chemistry.

-I effect order

The -I (Inductive effect) order refers to the ranking of atoms or groups based on their ability to pull electron density towards themselves along a carbon chain, thus causing a chain reaction of shifting electrons. This order is influenced by the electronegativity of the atom or group and its ability to delocalize charge.

In the sample exercise, the comparison between halogens reveals that the most electronegative halogen, fluorine, has the strongest -I effect. This understanding extends to other substituents too. For example, the nitrile (-CN) and nitro (-NO2) groups are known for their strong electron-withdrawing nature.

Recognizing the correct order is important for predicting chemical behavior. A correct -I effect order, such as the one in Option A, is based on solid principles of electronegativity and not merely memorized lists. This understanding aids students in reasoning out the effect, rather than relying on rote knowledge.

Comparative analysis of functional groups

Functional groups in organic chemistry are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The behavior of these functional groups can often be predicted by analyzing their inductive effects and considering resonance if applicable.

Take the carboxylic acid (-COOH) group, for example. Despite being quite electronegative due to the oxygen atoms, its -I effect is less pronounced because it engages in resonance, which allows the charge to be delocalized over the structure.

Different functional groups can be compared based on their inductive effects by considering both electronegativity and resonance. Such a comparative analysis is essential for understanding and predicting the reactivity of organic molecules and their intermediates in synthetic pathways. Learning how to weigh these factors against each other accurately allows students to rationalize the role of functional groups in complex organic reactions.