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Chapter 23: Problem 22

Phosphorus, in the sume family as nitrogen, forms trivalent compounds called phosphines, which also have trigonal pyramidal geometry. The activation energy for pyramidal inversion of trivalent phosphorus compounds is considerably greater thin it is for trivalent compounds of nitrogen, with the result that a number of chiral phosphines have been resolved.

Short Answer

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Answer: The key difference between trivalent compounds of phosphorus (phosphines) and nitrogen lies in their activation energy for pyramidal inversion. Trivalent phosphorus compounds have a higher activation energy, making it less likely for them to undergo this process. This allows them to exist in different forms, leading to the formation of chiral compounds. Chiral phosphines can be resolved into their individual enantiomers, which can have different properties and activities, making this separation an important aspect in the study of these compounds.

Step by step solution

01

Understand the difference between nitrogen and phosphorus trivalent compounds

To begin, let's understand what trivalent compounds are. These are chemical compounds where the central atom is surrounded by three other atoms or groups. In this case, we have phosphines (trivalent phosphorus-containing compounds) and nitrogen-containing trivalent compounds.
02

Explain the geometry of these compounds

Both nitrogen and phosphorus trivalent compounds have a trigonal pyramidal geometry. This means that there are three atoms or groups surrounding a central atom in a roughly triangular shape, with the central atom slightly above the plane formed by the other atoms.
03

Discuss the activation energy for pyramidal inversion

The activation energy is the energy required for a reaction to occur. In this context, it's the energy required for pyramidal inversion - the process by which the molecule undergoes a change in its geometry. The inversion essentially involves the central atom 'flipping' through the plane. For trivalent phosphorus compounds, this activation energy is considerably greater than it is for trivalent nitrogen compounds.
04

Explain the consequence of having different activation energies

Because trivalent phosphorus compounds have a higher activation energy for pyramidal inversion, it's less likely for them to undergo this process. As a result, they can exist in different forms, which can in turn lead to them forming chiral compounds.
05

Elaborate on chiral phosphines

Chirality refers to the property of a molecule having a non-superimposable mirror image, similar to how left and right hands are non-superimposable. Due to the activation energy difference, chiral phosphines can be resolved, which means they can be separated into their individual enantiomers (non-superimposable mirror images). These enantiomers can have different properties and activities, making this separation an important aspect in the study of these compounds.

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Most popular questions from this chapter

The geometry of a nitrogen atom bonded to three other atoms or groups of atoms is trigonal pyramidal (Section 1.4). The sp, bybridized nitrogen atom is at the apex of the Pyramid, and the three groups bouded to it extend densaward to form the triangular base of the prramid. If we consider the unshared pair of electrons on nitrogen as a foeurth grosp, then the amrangement of "groups" around nitrogen is approximately tetrahedral. Berause of this geoenetry, an ansine with three different groups bonded to nitrogen is chiral and can exist as a pair of enantiomers, as illustrated by the nonsuperporable mirror images of ethyimethylamine. In assigning configuration to these enantiomers, the groxp of lowest prioxity on nitrogen is the unshared pair of electrons.

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.

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

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.

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.
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