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Chapter 23: Problem 48

Like ammonia, all amines are weak bases, and aqueous solutions of amines are basic. The following acid-base reaction between an amine and water is written uxing curved arross to emphasize that, in these proton-transfer reactions, the unshared pair of electrons on nitrogen forms a new covalent bond with hydrogen and displaces hydroxide ion.

Short Answer

Expert verified
Answer: The unshared electron pair on the nitrogen atom plays a crucial role in the acid-base reaction between an amine and water. It allows the nitrogen to act as a nucleophile, attacking the hydrogen atom in the water molecule and forming a new covalent bond, resulting in an ammonium ion (NR4+) and a hydroxide ion (OH-). This demonstrates the weak base behavior of amines in aqueous solutions, as they accept protons (H+) through the unshared electron pair on the nitrogen atom.

Step by step solution

01

Identify the reactants

In this reaction, we need an amine and water. A general formula for an amine can be written as NR3 (where R can be hydrogen or an alkyl group). We also need a water molecule (H2O).
02

Illustrate the initial state of the reactants

Before the reaction takes place, we have an amine with an unshared electron pair on the nitrogen atom. This can be represented as: NR3: → H2O At this point, there is no bond between the nitrogen and hydrogen.
03

Draw curved arrows to represent the electron movements

We now illustrate the movement of electrons in the reaction using curved arrows. The lone pair of electrons on the nitrogen attacks the hydrogen atom in the water molecule, and at the same time, the bond between the hydrogen and oxygen breaks, forming a hydroxide ion (OH-) and leaving the oxygen with its unshared electron pair as well. This can be shown as: NR3: + H2O → NR3:-H ↔ H-O-H
04

Show the final product of the reaction

After the reaction is complete, the nitrogen in the amine has formed a new bond (a covalent bond) with the hydrogen atom, and a hydroxide ion is displaced. This results in the ammonium ion (NR4+) and a hydroxide ion (OH-). The final products can be written as: NR3-H^+ + OH- This reaction shows how amines, like ammonia, behave as weak bases in aqueous solutions, due to their ability to accept protons (H+) through the unshared electron pair on the nitrogen atom.

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Most popular questions from this chapter

All aliphatic amines have about the same hase strength, \(\mathrm{p} K_{\mathrm{2}}\) of the conjugate acid \(10.0-11.0\), and are slightly stronger bases than ammonia. The increase in basicity compared with ammonia can be attributed to the greater stability of an alkylammonium ion, as for example \(\mathrm{RCH}_{2} \mathrm{NH}_{3}{ }^{+}\)compared with the ammonium ion, \(\mathrm{NH}_{4}{ }^{+}\). This greater stability arises from the electron- releasing effect of alky groups and the resulting partial delocalization of the positive charge from nitrogen onto carbon in the alkylammonium ion.

Common names for most aliphatic amines are derived by listing the alkyl groups bonded to nitrogen in alphabetical order in one word ending in the suffix awine, that ix, they are named as alkylamines.

As a result of pyramidal inversion, a chiral amine quite literally tarns itself inside out, like an umbrella in a strong wind, and in the process becomes a racemic mixture. The activation energy for pyramidal inversion of simple amines is about \(25 \mathrm{~kJ}\) ( \(6 \mathrm{kal}) / \mathrm{mol}\). For ammonia at roon temperature, the rate of nitrogen inver sion is approximately \(2 \times 10^{11} \mathrm{~s}^{-1}\). For simple amines, the rate is less rapid but nonetheless sufficient to make resolution impossible. Pyramidal imversion is not possible for quaternary ammonium ions, and their salts can be resolved.

An N-HNN hydrogen bond is weaker thiun an \(\mathrm{O}-\mathrm{H}-\mathrm{O}\) hydrogen bond because the difference in electronegativity hetween nitrogen and hydrogen \((3.0\) \(2.1=0.9)\) is less than that between oxygen and hydrogen \((3.5-2.1-1.4)\). The ef= fect of intermolecular hyvlrogen bonding can be illustrated by comparing the boiling points of methylamine and methamol. Both are polar molecules and interact in the pure liquid hy hydroggen bonding. Because hydrogen bonding is stronger in methanol than in methylamine, methanol has the: higher boiling point.

Show reagents and conditions to convert toluene to 3-hromo-4.methylphenol.
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