How does the basicity of ethylamine compare to that of ammonia, and why?

Ethylamine is a stronger base than ammonia. This is because the ethyl group exerts a positive inductive effect, pushing electron density toward the nitrogen atom and making the lone pair more available to accept a proton.

Why is phenylamine a significantly weaker base than ammonia?

In phenylamine, the lone pair of electrons on the nitrogen atom is delocalised into the $\pi$ electron system of the benzene ring. This reduces the electron density on the nitrogen, making the lone pair less available to form a dative covalent bond with a proton.

What is the difference in water solubility between a primary amine and its corresponding chloride salt?

The chloride salt is generally much more soluble in water than the parent amine. This is because the salt is an ionic compound, allowing for strong ion-dipole interactions with water molecules, whereas the amine relies on weaker hydrogen bonding.

What is a common mistake when describing the effect of a benzene ring on the nitrogen lone pair?

A common error is stating that the benzene ring 'donates' electrons to the nitrogen. In fact, the nitrogen lone pair is 'withdrawn' or delocalised into the ring, which decreases the nitrogen's basic strength.

What happens to the characteristic 'fishy' smell of an amine when it reacts with hydrochloric acid?

The fishy smell disappears. This occurs because the volatile amine molecule is converted into a non-volatile ionic salt, which does not evaporate into the air to reach the nose.

Why is it incorrect to say that amines are Arrhenius bases in all contexts?

Amines are better described as Brønsted-Lowry bases because they accept protons rather than necessarily donating hydroxide ions. While they can produce $OH^-$ in water, their fundamental property is proton acceptance via the lone pair.

Define a 'dative covalent bond' in the context of amine basicity.

It is a covalent bond where both electrons in the shared pair originate from the same atom. In amines, the nitrogen provides both electrons from its lone pair to bond with an incoming $H^+$ ion.

What is the general formula for the salt formed when a primary amine ($RNH_2$) reacts with $HCl$?

The general formula is $RNH_3^+Cl^-$. The nitrogen atom accepts a proton to become a positive cation, which is ionically bonded to the chloride anion.

What is the 'positive inductive effect'?

It is the tendency of alkyl groups to push or donate electron density through sigma bonds toward a more electronegative atom or a positive center, such as the nitrogen in an amine.

How does the length of the alkyl chain affect the solubility of an amine salt?

As the alkyl chain length increases, the solubility of the salt decreases. The larger hydrophobic (non-polar) part of the molecule eventually outweighs the hydrophilic (polar) ionic head, making it less compatible with water.

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Advanced Organic Chemistry

Basic Properties

Basic Properties of Amines

Summary

Amines act as Brønsted-Lowry bases due to the lone pair of electrons on the nitrogen atom, which can accept a proton to form a dative covalent bond. Their basic strength is determined by the availability of this lone pair, which is influenced by the electronic effects of attached alkyl or aryl groups.

1. Definition & Core Concepts

2. Underlying Principles

Lone Pair Availability: The fundamental principle governing amine basicity is the electron density surrounding the nitrogen atom. A higher electron density makes the lone pair more attractive to protons, thereby increasing the strength of the base.
The Inductive Effect: Alkyl groups are electron-releasing, meaning they push electron density toward the nitrogen atom through sigma bonds. This positive inductive effect increases the availability of the lone pair compared to ammonia.
Delocalisation: In aromatic amines like phenylamine, the lone pair on the nitrogen atom overlaps with the $\pi$ electron system of the benzene ring. This delocalisation reduces the electron density on the nitrogen, making the lone pair significantly less available for bonding with a proton.

Diagram showing the trend of basicity from phenylamine (weakest) to ammonia to ethylamine (strongest), illustrating delocalisation and inductive effects.

3. Methods & Techniques

Ranking Basicity: To determine the relative strength of different amines, first identify the groups attached to the nitrogen. Aliphatic amines (with alkyl groups) are always stronger bases than ammonia, while aromatic amines (with aryl groups) are always weaker.
Comparing Aliphatic Amines: Generally, increasing the number of alkyl groups increases basicity due to the cumulative inductive effect. However, in aqueous solutions, tertiary amines can sometimes be weaker than secondary amines due to steric hindrance and reduced solvation of the resulting cation.
Predicting Reaction Products: When an amine reacts with a mineral acid, replace the lone pair on the nitrogen with a bond to $H^+$ and balance the charge with the acid's anion. The resulting structure is always an ionic salt.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions