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Chapter 23: Problem 21

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.

Short Answer

Expert verified
Answer: Pyramidal inversion is a process that occurs in chiral amines, where the three groups connected to a nitrogen atom with a lone pair of electrons switch positions, turning the molecule "inside out." This inversion affects the chirality of the molecule, causing it to become a racemic mixture (an equal mixture of enantiomers). The rate of nitrogen inversion varies depending on the amine, with quaternary ammonium ions being unable to undergo inversion due to the lack of a lone pair of electrons on the nitrogen atom.

Step by step solution

01

Understanding Pyramidal Inversion

Pyramidal inversion is a process that occurs in chiral amines (molecules containing a nitrogen atom bonded to three distinct groups and a lone pair of electrons). During this process, the three groups connected to the nitrogen atom essentially switch positions, turning the molecule "inside out." This inversion affects the chirality of the molecule, causing it to become a racemic mixture (an equal mixture of enantiomers).
02

Activation Energy and Nitrogen Inversion

To undergo pyramidal inversion, a certain amount of energy, known as activation energy, is required. For simple amines, this value is approximately 25 kJ/mol (6 kcal/mol). In the case of ammonia, which is a simple amine with three hydrogen atoms attached to the nitrogen atom, the rate of nitrogen inversion at room temperature is about \(2 \times 10^{11} \mathrm{~s}^{-1}\). This rapid rate leads to a constant change in chirality.
03

Rate of Inversion in Simple Amines

Though the rate of nitrogen inversion in ammonia is strikingly high, the inversion rate in other simple amines is less rapid. However, it is still fast enough to make resolution (separating enantiomers) practically impossible.
04

Quaternary Ammonium Ions and Their Properties

Quaternary ammonium ions are formed when a nitrogen atom is bonded to four distinct groups, leaving no space for a lone pair of electrons. Because the nitrogen atom in these ions has no lone pair electrons, pyramidal inversion is not possible. Therefore, quaternary ammonium ions can display stable chirality, enabling resolution of their salts.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Chiral Amines
Chiral amines are intriguing molecules that play a crucial role in various biological systems and synthetic drugs. The chirality in amines arises when a nitrogen atom is bonded to three different substituents, creating a non-superimposable mirror image of itself, much like your left and right hands.

This characteristic asymmetry in chiral amines leads to two forms of the molecule, known as enantiomers, each with a distinct spatial arrangement. While these molecules may share the same molecular formula, their physical properties can behave differently, especially when interacting with other chiral substances, which has profound implications in the field of pharmaceuticals.
Activation Energy
Activation energy is the minimum quantity of energy that is needed to initiate a chemical reaction. Think of it as a metaphorical 'hill' that the reacting particles must climb over before they can transform into products. In the context of chiral amines, an energy input of approximately 25 kJ/mol (6 kcal/mol) is required to trigger pyramidal inversion. This energy need is vital to understand as it determines how quickly a reaction, such as the inversion of nitrogen-containing compounds, will occur under normal conditions, affecting the purity and resolution of chiral substances.
Racemic Mixture
A racemic mixture is a 50:50 blend of two enantiomers of a chiral molecule. In this evenly balanced mix, the physical properties of the enantiomers cancel each other out, leading to a net effect that is different from either pure enantiomer.

For example, while one enantiomer of a drug might have therapeutic effects, its mirror image might be inactive or potentially harmful. Racemic mixtures are often the result of processes such as pyramidal inversion which continuously interchange one enantiomer to another, leading to a dynamic equilibrium where both forms are equally present.
Nitrogen Inversion
Nitrogen inversion is a fascinating phenomenon where the atom at the apex of a nitrogen-containing pyramid (for instance, in an amine) momentarily inverts its spatial configuration. This rapid flipping, comparable to an umbrella turning inside out on a windy day, happens because the nitrogen atom has a pair of non-bonding electrons that can distort the molecule's geometry.

At room temperature, ammonia, a simple amine, exhibits a nitrogen inversion rate of approximately 2x10^11 times per second. This high frequency occurs too quickly for the enantiomers to be isolated, therefore rendering these forms of amines as a racemic mixture under ordinary conditions.
Quaternary Ammonium Ions
Quaternary ammonium ions stand out as they are ineligible for pyramidal inversion. In contrast to regular amines, the central nitrogen atom in quaternary ammonium ions is bonded to four substituents, leaving no room for a lone pair of electrons to enable the inversion process.

As a result, these ions maintain a fixed spatial arrangement, which can lead to stable chirality - an essential quality allowing the resolution, or separation, of different enantiomers. This stability is a valuable trait, particularly in the pharmaceutical industry where the separation of enantiomers can be critical for the production of effective and safe medications.

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Most popular questions from this chapter

The hasicitydecreasing effect of nitro substitution in the s-position is almost entirely the result of its inductive effect, whereas that of nitro substitution in the 4.position is attributable to both inductive and resonance effects. In the case of para suhstitution (as well as ortho substitution), delocalization of the lone pair on the amino nitrogen involves not only the carbons of the aromatic ring but alwo oxy gen atoms of the nitro group.

An ion containing a nitrogen atom bonded to axy combination of four alkyl or aryl groups is classified as a quaternary (4") ammonium ion. Compounds containing such ions have properties characteristic of salts. Cetylpyridinium chloride is used as a topical antiseptic and disinfectant.

Proton transfer from water or other acid to pyridine does not involve the electrons of the aromatic sextet. Why, then, is pyridine a considerably weaker base than aliphatic amines? The answer is that the unshared pair of electrons on the pyridine nitrogen lies in a relatively electronegative \(x f^{2}\) hybrid orbital, whereas in aliphatic amines, the unshared pair lies in an spr hybrid orbital. This effect decreases markedly the basicity of the electron pair on an sp²-hybridized nitrogen compared with that on an \(s p^{3}\) hytvridized nitrogen. There are two nitrogen atoms in imidazole, each with an unshared pair of electrons. One unshared pair lies in a \(2 p\) orbital and is an integral part of the \((4 n+2)\) \(\pi\) electrons of the aromatic system. The other unshared pair lies in an spr hybrid orbital and is mot a part of the aromatic sextet; this pair of electrons functions as the proton acceptor.

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.

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.
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