The main group elemental hydrides – as a set – are subjected to a series of experiments with nature's simplest chemical probes: H+, H– & H•. Patterns of structure and reactivity behaviour are found.
The Probe Experiments
The first 36 main group elemental hydrides, H2 to BaH2, are probed in 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 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", or as we shall call them "congeneric 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 consist of sets of species that are isoelectronic and/or show group periodicity:
"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/periodic species will 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/periodic. And isoelectronic/periodic arrays usually, but do not always, 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 arrays are Lewis acids
8 arrays are Lewis bases
2 arrays are Lewis acid/base complexes
2 arrays are radicals
2 arrays are metallic
These are colour coded:
Why Are The Main Group Elemental Hydrides and The Five Hydrogen Probe Experiments So Interesting?
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.
While, 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.
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.
Ten Simple Probe Experiments
The five hydrogen probe experiments are a sub-set of 10 simple probe experiments, using five simple probes:
- Photon, hv
- Electron, e–
- Proton (hydrogen cation), H+
- Hydride ion (hydrogen anion), H–
- Hydrogen radical (hydrogen atom), H•
Each of these five probe species can – in principle – be added or removed form a species, which results in 10 possible probe experiments, however, any single species not be able to partake in all 10 of the probe experiments. We will be using methane as our subject molecule:
Add A Photon to Methane
Adding a photon gives an excited state methane, which may relax in a number of ways. The effect will depend upon the wavelength (energy) of the photon. Infrared radiation of the correct wavelength will cause the methane molecule to vibrate. Ultra violet radiation of the correct wavelenth will result in electronic excitation, or possibly ionisation.
Geometry optimisation using density function theory at the pBP/DN** level shows excited state methane to be tetrahedral.
The study of electromagnetic radiation interacting with matter is the science of spectroscopy and is outside the scope of this web book.
Remove A Photon from Methane
A photon can only be removed from an excited species. Thus, the above process can be deemed to occur when an excited methane loses a photon (ie, the reverse of the methane plus photon reaction).
Add An Electron to Methane
Adding an electron to methane gives the methane radical anion. The species is an anion because it has an excess electron and it is a radical because the extra electron is unpaired.
The methane radical anion is known in the gas phase. Radical anions are rouienly generated and studied in the technique of negative ion mass spectrometry.
Geometry optimisation using density function theory at the pBP/DN** level shows the methane radical anion to be tetrahedral.
Remove An Electron from Methane
Removing an electron from methane gives the methane radical cation. The species is a cation because is has one electron less that is required for neutrality, and it is a radical because it has an unpaired electron.
Radical cations are routienly generated in the gas phase and studied in the technique of mass spectrometry.
Geometry optimisation using density function theory at the pBP/DN** level shows the methane radical cation to be tetrahedral.
Add A Proton to Methane
Methane (and other alkanes) can be protonated in solution by George Olah type "super-acids", for example H+[SbF6]–. Protonated methane is called the carbonium ion.
Geometry optimisation using density function theory shows the carbonium ion to be trigonal bipyramidal.
Remove A Proton from Methane
While it is not possible to remove a proton from methane to generate the methyl anion, H3C–, the reverse reaction is well know. Methyl lithium, CH3Li, is a well known strong base which violently reacts with water to give methane.
Geometry optimisation using density function theory shows the carbanion to be trigonal pyramidal. The same geometry is predicted with VSEPR.
Add A Hydride Ion to Methane
This reaction is not known for methane. However borane, BH3, reacts with hydride ion to give the tetrahydroborate ion, [BH4]–.
Geometry optimisation using density function theory shows the [H5C]– ion to be trigonal bipyramidal.
Remove A Hydride Ion from Methane
While it is not possible to directly remove a hydride ion from methane to generate the methyl cation, [CH3]+ (the carbenium ion), the reverse reaction is well know. Methyl iodide, a proxy for the methyl cation, reacts with lithium anuminium hydride [a source of hydride ion] to give methane.
Geometry optimisation using density function theory shows the carbenium ion to be planar. The same geometry is predicted with VSEPR.
Add A Hydrogen Radical to Methane
This reaction is not known for methane. Geometry optimisation using density function theory shows the H5C• radical to be trigonal bipyramidal.
Remove A Hydrogen Radical from Methane
A hydrogen radical can be abstracted from methane by a chlorine radical to give a methyl radical, and this is one of the steps in the free radical halogenation of methane to methyl chloride.
Geometry optimisation using density function theory at the pBP/DN** level shows the methyl radical to be slightly distorted to trigonal pyramidal.