Question

Arrange the following groups in order of decreasing inductive effects: \(\mathrm{NO}_{2}, \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}, \mathrm{CH}_{3}, \mathrm{OCH}_{3}, \mathrm{Br}\) (a) \(\mathrm{NO}_{2}>\mathrm{Br}>\mathrm{OCH}_{3}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{CH}_{3}\) (b) \(\mathrm{NO}_{2}>\mathrm{Br}>\mathrm{OCH}_{3}>\mathrm{CH}_{3}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) (c) \(\mathrm{NO}_{2}>\mathrm{OCH}_{3}>\mathrm{Br}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{CH}_{3}\) (d) \(\mathrm{NO}_{2}>\mathrm{OCH}_{3}>\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{Br}>\mathrm{CH}_{3}\)

Short answer

(a) \(\mathrm{NO}_{2} > \mathrm{Br} > \mathrm{OCH}_{3} > \mathrm{C(CH}_{3})_{3} > \mathrm{CH}_{3}\)

Step-by-step solution

Understanding Inductive Effect

Inductive effect refers to the electron-withdrawing or electron-donating behavior of substituents relative to hydrogen. Groups with strong electron-withdrawing abilities have a strong -I (negative inductive) effect, while strong electron-donating groups have a +I (positive inductive) effect.

Analyzing the Groups

The given groups need to be analyzed based on their electron-withdrawing or donating tendencies:- \(\mathrm{NO}_{2}\) is a strong electron-withdrawing group due to its resonance and electronegativity, thus having a strong -I effect.- \(\mathrm{Br}\) is also electron-withdrawing but less so than \(\mathrm{NO}_{2}\).- \(\mathrm{OCH}_{3}\) can have a slight electron-withdrawing effect primarily due to the oxygen atom.- \(\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) (tert-butyl) is an electron-donating group with a +I effect, but weaker than alkyl groups like \(\mathrm{CH}_{3}\) (methyl).- \(\mathrm{CH}_{3}\) is an electron-donating group with a +I effect, generally stronger than the tert-butyl group's donating effect.

Arranging Groups by Inductive Effect

Arrange the groups from strongest -I effect (electron-withdrawing) to strongest +I effect (weakest electron-withdrawing to electron-donating): 1. \(\mathrm{NO}_{2}\) > 2. \(\mathrm{Br}\) > 3. \(\mathrm{OCH}_{3}\) > 4. \(\mathrm{C}\left(\mathrm{CH}_{3}\right)_{3}\) > 5. \(\mathrm{CH}_{3}\)

Key concepts

Electron-withdrawing groups

In organic chemistry, electron-withdrawing groups are atoms or groups of atoms that pull electron density away from other areas of a molecule. This occurs due to differences in electronegativity and often involves resonance effects. For instance, the nitro group (\(\mathrm{NO}_2\)) is a classic example of a strong electron-withdrawing group. It can stabilize negative charges or electron-rich sites by delocalizing the charge through the molecule. This is essential in reactions like electrophilic aromatic substitution, where the Reactivity of aromatic compounds can be dramatically affected by such groups.
When you hear about the "inductive effect," think about how these groups increase the electron density at different parts of the molecule, making it more susceptible to certain reactions.

• Electron-withdrawing groups stabilize carbanions.
• They increase the acidity of nearby hydrogen atoms.
• Inductive effects diminish with distance from the electron-withdrawing group.

Electron-donating groups

Electron-donating groups (EDGs) are the molecular groups that push electron density toward other parts of the structure. These groups are characterized by their ability to release electrons, typically through resonance or hyperconjugation. Common EDGs include alkyl groups like methyl (\(\mathrm{CH}_3\)) and hydroxyl (\(\mathrm{OCH}_3\)), which are known for their +I (positive inductive) effect. In a reaction, they often enhance nucleophilicity by contributing extra electron density to a reaction site.
Understanding these groups is crucial in predicting the outcome of many organic reactions:

• They lower the acidity of a compound by destabilizing the conjugate base.
• Enhances the nucleophilicity (electron-donating ability) of certain sites.
• Generally increases the reactivity of a compound in electrophilic aromatic substitutions.

Chemical Resonance

Resonance is a key concept in organic chemistry that often accompanies inductive effect discussions. It is the idea that some molecules can be represented by multiple valid Lewis structures. These structures are not distinct but a blend that describe a hybrid state of the molecule. This blending allows atoms to delocalize electrons, providing greater molecule stability.
For electron-withdrawing groups like \(\mathrm{NO}_2\), resonance can significantly increase their withdrawing power by spreading negative charge across the molecule, further stabilizing it. This phenomenon holds true for other groups like carbonyls, where alternating single and double bonds can delocalize charge effectively.
Resonance affects:

• The stability of intermediates in many reactions.
• The strength of acids and bases due to charge delocalization.
• The physical properties, such as color and reactivity, of compounds.

Electronegativity

Electronegativity plays a pivotal role in the inductive effect, as it measures an atom's ability to attract and hold onto electron density. In organic molecules, atoms with a high electronegativity, such as oxygen, nitrogen, and halogens, are fundamental to electron-withdrawing effects.
For example, in \(\mathrm{Br}\), its relatively high electronegativity makes it an effective electron-withdrawing group through the inductive effect. This can influence reaction pathways and the stability of reaction intermediates by altering electron distributions.
Key elements to consider about electronegativity:

• Determines the direction and magnitude of electrical dipoles in molecules.
• Impacts molecule polarity and solubility.
• Guides the interpretation of chemical behavior in reactions by predicting bond formations.

Organic Chemistry

Organic chemistry is the study of carbon-based compounds which incorporates principles like the inductive effect, resonance, and electronegativity to understand molecule behavior. It involves the investigation of organic molecules' structure, properties, composition, reactions, and preparation.
These concepts allow chemists to predict how molecules will react and interact with one another within complex molecular frameworks. Inductive effect influences the distribution of electrons within molecules, impacting chemical reactivity, stability, and acidity.
Understanding organic reactions and processes involves:

• Mastering the relationships between structure and chemical reactivity.
• Learning about functional groups and their chemical behavior.
• Applying core principles of resonance, electron-withdrawing and -donating effects to predict reactivity.

Thus, at the foundation of organic chemistry lies an interplay of these principles, helping explain vast chemical phenomena from biological metabolism to industrial synthesis.