Problem 143 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

The correct order of decreasing basic character is (i) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (ii) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (iii) \(\left(\mathrm{C}_{6} \mathrm{H}_{5}\right)_{2} \mathrm{NH}\) (iv) \(\mathrm{C}_{6} \mathrm{H}_{11} \mathrm{NH}_{2}\) (a) \(\mathrm{ii}>\mathrm{i}>\mathrm{iii}>\mathrm{iv}\) (b) \(\mathrm{iv}>\mathrm{ii}>\mathrm{i}>\mathrm{iii}\) (c) \(\mathrm{iv}>\mathrm{iii}>\mathrm{ii}>\mathrm{i}\) (d) \(\mathrm{iv}>\mathrm{ii}>\mathrm{iii}>\mathrm{i}\)

Step-by-Step Solution

Answer

The correct order is (iv) > (ii) > (i) > (iii), so option (b) is correct.

Step 1: Understanding Basic Strength

The basic character of an amine is determined by its ability to donate a lone pair of electrons from the nitrogen atom. The more readily an amine can donate its lone pair, the stronger base it is. Factors such as electronic effects (inductive or resonance) from substituents affect this ability.

Step 2: Analyze the Effects of the Aromatic Ring

In aromatic amines like aniline ({C}_{6}H_{5}NH_{2}), the lone pair on nitrogen is delocalized into the benzene ring, decreasing its availability to donate the lone pair, thus reducing the basicity compared to aliphatic amines. Analyzing the effects, if the nitrogen is directly attached to the aromatic ring (as in {C}_{6}H_{5}NH_{2}) or if there are more phenyl groups (as in {(C}_{6}H_{5})_{2}NH), the basicity decreases.

Step 3: Compare the Different Compounds

- (iv) C_{6}H_{11}NH_{2} (Cyclohexylamine) is an aliphatic amine and shows higher basicity due to the absence of electron-withdrawing resonance effects. - (ii) C_{6}H_{5}CH_{2}NH_{2} (Benzylamine) has the nitrogen atom only weakly influenced by the aromatic ring because of the methylene bridge (CH₂), which slightly decreases basicity compared to aliphatic amines but less than a direct attachment. - (i) C_{6}H_{5}NH_{2} (Aniline) and (iii) (C_{6}H_{5})_{2}NH (Diphenylamine) are less basic due to the direct resonance effect of the phenyl group(s) reducing the electron pair availability on the nitrogen.

Step 4: Determine the Order of Basic Strength

Based on the analysis: - (iv) is the most basic due to being an aliphatic amine with no resonance effects. - (ii) is next due to a weaker interaction with the aromatic ring. - Comparing (i) and (iii), (i) is more basic than (iii) because (iii) has more resonance delocalization with two phenyl groups. Thus, the order of decreasing basicity is: (iv > ii > i > iii).

Key Concepts

Amine Basicity Ranking

The basicity of amines is a fundamental concept in organic chemistry that refers to the ability of an amine to accept protons by donating a lone pair of electrons from its nitrogen atom. When ranking amines based on basicity, we examine how easily these amines can donate this lone pair. This ability can be influenced by the presence of substituents, as well as electronic effects like inductive and resonance effects. Amines with substituents that increase electron density on the nitrogen atom tend to be more basic.

• Aliphatic amines, like cyclohexylamine (\[ \mathrm{C}_{6} \mathrm{H}_{11} \mathrm{NH}_{2} \]), generally exhibit higher basicity compared to aromatic amines due to the absence of electron-withdrawing effects.
• Benzylamine (\[ \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2} \]) has a methylene bridge separating the nitrogen from the aromatic ring, maintaining moderate basicity due to reduced aromatic influence.
• Aniline (\[ \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \]) and diphenylamine (\[ \left( \mathrm{C}_{6} \mathrm{H}_{5} \right)_{2} \mathrm{NH} \]) experience decreased basicity due to resonance effects with the aromatic ring.

Electron Donation in Amines

In amines, the nitrogen atom has a lone pair of electrons that can be donated, acting as a Lewis base. The ease with which this lone pair can be donated is a key factor determining an amine's basicity. Electron donation efficacy is influenced by the surrounding groups: groups that donate electrons (electron-donating groups or EDGs) enhance basicity.- **Aliphatic Amines**: In aliphatic amines, the nitrogen is attached to uninterrupted carbon chains that typically provide electron supply, making it easier for nitrogen to donate its lone pair.- **Benzylamine**: This amine has a bridge (\[ \mathrm{CH}_{2} \]) between the aromatic ring and nitrogen, allowing some electron donation without strong resonance effects from the aromatic ring, maintaining moderate basicity.The better the electron donation to nitrogen, the stronger the base it becomes, thus increasing the amine's overall basicity.

Effect of Aromatic Rings on Basicity

Aromatic rings, such as benzene, have a significant impact on the basicity of amines. In aromatic amines, the lone pair of electrons on the nitrogen atom may become part of the extended pi-electron system of the aromatic ring through resonance. This delocalization of electrons decreases the availability of the lone pair for donation, reducing basicity.

• **Aniline**: Here, the nitrogen's lone pair gets partly "trapped" by the benzene pi-system, reducing its ability to function as a base.
• **Diphenylamine**: With two aromatic rings, the resonance effect is even stronger, further decreasing the basicity compared to aniline.

Therefore, the more phenyl groups attached directly to the nitrogen, the lower the basicity, as the lone pair becomes less available due to resonance delocalization.

Electronic Effects on Nitrogen

Electronic effects significantly impact an amine's ability to donate electrons from the nitrogen's lone pair, altering its basicity. These effects include both inductive and resonance contributions. - **Inductive Effect**: This involves the donation or withdrawal of electron density through sigma bonds. In cases like cyclohexylamine, which lacks significant resonance effects, the nitrogen retains a higher electron density, making these amines more basic. - **Resonance Effect**: This effect takes place when the lone pair on nitrogen overlaps with an adjacent pi-system, common in aromatic compounds, such as aniline. The lone pair's delocalization through resonance reduces its availability to be donated, thus decreasing basicity. Understanding these electronic effects helps in predicting and explaining the basicity of various amines and their behavior in chemical reactions.