Question:
Basic strength of alkylamines in aqueous phase is not decided by
Basic strength of alkylamines in aqueous phase is not decided by
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Remember that the basicity order of amines is different in the gaseous phase vs. the aqueous phase. In the gas phase, only the inductive effect matters (3\(^\circ\)>2\(^\circ\)>1\(^\circ\)). In the aqueous phase, the combination of inductive, solvation, and steric effects leads to a non-linear trend (e.g., for methylamines: 2\(^\circ\)>1\(^\circ\)>3\(^\circ\)).
Updated On: Apr 23, 2026
- Inductive effect
- Solvation effect
- Steric hindrance
- Hyperconjugation effect
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The Correct Option is D
Solution and Explanation
Step 1: Understanding the Basicity of Alkylamines:
The basicity of an amine in the aqueous phase depends on the availability of the lone pair of electrons on the nitrogen atom for donation to a proton (H\(^+\)) and the stability of the resulting conjugate acid (alkylammonium ion) in water.
Step 2: Analyzing the Factors Affecting Basicity in Aqueous Phase:
Several factors collectively determine the basic strength of alkylamines in water:
(A) Inductive Effect (+I Effect): Alkyl groups (like -CH\(_3\), -C\(_2\)H\(_5\)) are electron-donating. They push electron density towards the nitrogen atom, increasing the availability of the lone pair and thus increasing basicity. Based on this effect alone, the order of basicity would be: Tertiary (3\(^\circ\))>Secondary (2\(^\circ\))>Primary (1\(^\circ\))>Ammonia. This factor is crucial.
(B) Solvation Effect (Hydration): After an amine accepts a proton, it forms a positively charged alkylammonium ion (RNH\(_3\)\(^+\), R\(_2\)NH\(_2\)\(^+\), R\(_3\)NH\(^+\)). This ion is stabilized by hydrogen bonding with surrounding water molecules. The more hydrogen atoms attached to the nitrogen in the cation, the greater the extent of hydrogen bonding and the more stable the ion. - Primary amine conjugate acid (RNH\(_3\)\(^+\)) has 3 H-atoms for H-bonding. - Secondary amine conjugate acid (R\(_2\)NH\(_2\)\(^+\)) has 2 H-atoms. - Tertiary amine conjugate acid (R\(_3\)NH\(^+\)) has 1 H-atom. Thus, the order of stabilization by solvation is 1\(^\circ\)>2\(^\circ\)>3\(^\circ\). This effect enhances the basicity of primary and secondary amines relative to tertiary amines. This is a very important factor in the aqueous phase.
(C) Steric Hindrance: Bulky alkyl groups around the nitrogen atom can physically block the lone pair, making it difficult for a proton to approach (steric hindrance to protonation). They also hinder the solvation of the conjugate acid. This effect is most pronounced in tertiary amines and tends to decrease their basicity. This factor is also significant.
(D) Hyperconjugation Effect: Hyperconjugation involves the delocalization of \(\sigma\)-electrons into an adjacent empty or partially filled p-orbital or a \(\pi\)-orbital. It is primarily used to explain the stability of carbocations, alkenes, and free radicals. While it might have a very minor, indirect influence, it is not considered a primary or deciding factor in determining the basic strength of amines. The basicity is dominated by the interplay of the inductive effect, solvation, and steric hindrance.
Step 3: Conclusion:
The inductive effect, solvation effect, and steric hindrance are the three major factors that decide the basic strength of alkylamines in the aqueous phase. The hyperconjugation effect is not a direct or significant factor in this context.
Step 4: Final Answer:
The basic strength of alkylamines in the aqueous phase is not decided by the hyperconjugation effect.
The basicity of an amine in the aqueous phase depends on the availability of the lone pair of electrons on the nitrogen atom for donation to a proton (H\(^+\)) and the stability of the resulting conjugate acid (alkylammonium ion) in water.
Step 2: Analyzing the Factors Affecting Basicity in Aqueous Phase:
Several factors collectively determine the basic strength of alkylamines in water:
(A) Inductive Effect (+I Effect): Alkyl groups (like -CH\(_3\), -C\(_2\)H\(_5\)) are electron-donating. They push electron density towards the nitrogen atom, increasing the availability of the lone pair and thus increasing basicity. Based on this effect alone, the order of basicity would be: Tertiary (3\(^\circ\))>Secondary (2\(^\circ\))>Primary (1\(^\circ\))>Ammonia. This factor is crucial.
(B) Solvation Effect (Hydration): After an amine accepts a proton, it forms a positively charged alkylammonium ion (RNH\(_3\)\(^+\), R\(_2\)NH\(_2\)\(^+\), R\(_3\)NH\(^+\)). This ion is stabilized by hydrogen bonding with surrounding water molecules. The more hydrogen atoms attached to the nitrogen in the cation, the greater the extent of hydrogen bonding and the more stable the ion. - Primary amine conjugate acid (RNH\(_3\)\(^+\)) has 3 H-atoms for H-bonding. - Secondary amine conjugate acid (R\(_2\)NH\(_2\)\(^+\)) has 2 H-atoms. - Tertiary amine conjugate acid (R\(_3\)NH\(^+\)) has 1 H-atom. Thus, the order of stabilization by solvation is 1\(^\circ\)>2\(^\circ\)>3\(^\circ\). This effect enhances the basicity of primary and secondary amines relative to tertiary amines. This is a very important factor in the aqueous phase.
(C) Steric Hindrance: Bulky alkyl groups around the nitrogen atom can physically block the lone pair, making it difficult for a proton to approach (steric hindrance to protonation). They also hinder the solvation of the conjugate acid. This effect is most pronounced in tertiary amines and tends to decrease their basicity. This factor is also significant.
(D) Hyperconjugation Effect: Hyperconjugation involves the delocalization of \(\sigma\)-electrons into an adjacent empty or partially filled p-orbital or a \(\pi\)-orbital. It is primarily used to explain the stability of carbocations, alkenes, and free radicals. While it might have a very minor, indirect influence, it is not considered a primary or deciding factor in determining the basic strength of amines. The basicity is dominated by the interplay of the inductive effect, solvation, and steric hindrance.
Step 3: Conclusion:
The inductive effect, solvation effect, and steric hindrance are the three major factors that decide the basic strength of alkylamines in the aqueous phase. The hyperconjugation effect is not a direct or significant factor in this context.
Step 4: Final Answer:
The basic strength of alkylamines in the aqueous phase is not decided by the hyperconjugation effect.
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