The Probe Experiments

The first 36 main group elemental hydrides, H2 to BaH2, are probed with five experiments involving hydrogen species. These five experiments are:

• Add a proton or two, H⁺
• Remove a proton, H⁺
• Add a hydride ion or two, H^–
• Remove a hydride ion or two, H^–
• Remove a hydrogen radical or two, H^•

To and/or from the first 36 main group elemental hydrides:

 

The main group elemental hydrides and the products of the five hydrogen probe experiments can be grouped into periodic "arrays". There are 23 such arrays:

8  arrays are 1x1 dots
4  arrays are 1x4 series
6  arrays are 1x5 series (Group 1 and 2 cation series are counted only once)
5  arrays are 4x4 planars

Arrays always consist of sets of isoelectronic species:

"Two or more molecular entities are described as isoelectronic if they have the same number of valence electrons and the same structure, i.e. number and connectivity of atoms, but differ in some of the elements involved." IUPAC

It follows that, isoelectronic species have the same Lewis structure and/or frontier molecular orbital (FMO) topology.

This is important because species can only be expected to have similar, mutually predictable, reaction chemistry if they are isoelectronic. And isoelectronic arrays usually exhibit linear reactivity traits, something we shall explore in detail over the next few web pages.

Arrays which are linear in both structure and reactivity are defined as being:

congeneric (or "of the same family" or here)

The 23 arrays associated with the main group elemental hydrides and generated by the five hydrogen probe experiments exhibit a few, general types of reactivity behaviour:

9     of the arrays are Lewis acids
8    of the arrays are Lewis bases
2     of the arrays are Lewis acid/base complexes
2     of the arrays are radicals
2     of the arrays are metallic

These are colour coded:

• All the species generated by the five hydrogen probe experiments are common with well established structure and reaction chemistry.

• The species generated by the five hydrogen probe experiments make excellent surrogates for other cations, anions and radicals. For example, the [BH4]^– ion has many similarities with the [BF4]^– ion.

• Protons, H⁺, hydride ions, H^–, and hydrogen radicals, H•, make excellent structural and reactivity probes because they are electronically simple moieties. Hydrogen interactions minimise the often convoluted orbital/orbital interactions observed with heavier cations, anions and radicals.

• Hydrogen species have such a low mass that they are able to partially quantum tunnel between species (states). Transfer reactions involving H⁺, H^– and H• are kinetically fast and the thermodynamic product product is formed. Complicating activation energy effects are minimal. For example:

• The Brønsted acid reaction, proton or H⁺, transfer, is a thermodynamically controlled process.

• The mechanistically equivalent nucleophilic substitution, R⁺ transfer, is kinetically controlled, and:

• The reaction may require heating in a polar solvent.
• Side reactions are likely.
• The kinetics may be first order, SN1, or second order, SN2.

• The supply and removal of H⁺ to/from a species equates with Brønsted acidity and the pKa of a Brønsted acid gives information about both the Brønsted acid and about the conjugate [Brønsted AND Lewis] base. The fact that methane has a pKa 46 tells us about the proton donating ability of CH4 AND about the proton abstracting ability of the conjugate base, H3C^–. There is a rule:

• Weak Brønsted acids have strong conjugate Brønsted bases.
• Strong Brønsted acids have weak conjugate Brønsted bases.

• Methane is such a weak protonating [Brønsted] acid that many would not consider it to be acidic at all. But the conjugate base the methyl anion, H3C^–, in the form of methyl lithium, LiCH3, is amongst the strongest proton abstracting [Brønsted] bases known. It is a "super-base".

• The element to hydrogen bond length, X-H, is a subtle and versatile proxy for a kind of reactivity.

Subtle, because the parameter often tells us something about the intrinsic hardness of an atomic centre X, a term which will be strictly defined later, here.

Versatile, because the X-H bond can be disconnected , as indicated by the disconnection arrow =>, in three ways to give information about conjugate anions, X^–, cations, X⁺, and radicals, X^•

The X–H molecular distance (bond-length) parameter is difficult to obtain by physical experiment because hydrogen atoms cannot be seen in x-ray crystal analysis, the most common direct method of determining the three dimensional structure of molecules. To observe hydrogen atoms, and hence X–H distances, it is necessary to perform neutron diffraction experiments.

However, X–H distances can be readily and accurately calculated using ab initio molecular orbital (MO) software such as Spartan by Wavefunction.

• The five hydrogen probe experiments are a subset of the 10 simplest probe experiments, described here.

© Mark R. Leach 1999-2008

Queries, Suggestions, Bugs, Errors, Typos...

If you have any:

Queries
Comments
Suggestions
Suggestions for links
Bug, typo or grammatical error reports about this page,

please contact Mark R. Leach, the author, using mrl@meta-synthesis.com

This free, open access web book is an ongoing project and your input is appreciated.