Nascent hydrogen

Nascent hydrogen
Names
Systematic IUPAC name
Hydrogen[1] (substitutive)
Hydrogen(•)[2] (additive)
Identifiers
12385-13-6 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:29235
ChemSpider 4515072 YesY
PubChem [https://pubchem.ncbi.nlm.nih.gov/compound/5362549

pubchem from PubChem 5362549 pubchem from PubChem]

Properties
H
Molar mass 1.01 g·mol−1
Reacts
0 D
Thermochemistry
114.715-114.719 J K−1 mol−1
217.992-218.004 kJ mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Nascent hydrogen is purported to consist of a chemically reactive form of hydrogen that is freshly generated, hence nascent. Molecular hydrogen (H2), which is the normal form of this element, is unreactive toward organic compounds, so a special state of hydrogen was once invoked to explain certain kinds of hydrogenations. Mechanistic understanding of such reactions is now available, and the concept of nascent hydrogen is discounted, even ridiculed.[3][4]

Example, the Bouveault–Blanc reduction

Reductions of esters to give alcohols using a mixture of sodium and alcohols is called the Bouveault–Blanc reduction. It is an old reaction that has largely been superseded by alternative methods. At the time of popularity, the process caused much puzzlement because esters are unreactive toward hydrogen. It was also known that sodium reacts with alcohols to release H2. it was concluded that some freshly generated ("nascent") hydrogen was responsible for this remarkable reaction. Subsequent studies have shown that this reaction proceeds via electron-transfer from metallic sodium to the ester substrate followed by protonation of the reduced intermediate.[5] The evolution of hydrogen by the reaction of sodium and alcohol is purely a competitive reaction, the sole benefit being that in the presence of sufficient alkoxide, the sodium/alcohol reaction slows.

Making atomic hydrogen

It takes 4.476 electron volts to disassociate an ordinary H2 hydrogen molecule. When the atoms recombine, they liberate this energy. An electric arc or ultraviolet photon can generate atomic hydrogen.

Uses of atomic hydrogen

The atomic hydrogen torch uses it to generate very high temperatures near 4,000°C for welding. Hydrogen is a powerful reducing agent which eliminates the need for flux to prevent oxidation of the weld.

Atomic hydrogen determines the frequency of hydrogen masers which are used as precise frequency standards. They operate at the 1420 MHz frequency corresponding to an absorption line in atomic hydrogen.

NASA has investigated the use of atomic hydrogen as a rocket propellant. It could be stored in liquid helium to prevent it from recombining into molecular hydrogen. When the helium is vaporized, the atomic hydrogen would be released and combine back to molecular hydrogen. The result would be an intensely hot stream of hydrogen and helium gas. The lift-off weight of rockets could be reduced by 50% by this method.[6]

See also

References

  1. ↑ "Hydrogen atom - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information.
  2. ↑ "hydrogen(.) (CHEBI:29235)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute.
  3. ↑ Laborda, F.; E. Bolea; M. T. Baranguan; J. R. Castillo (2002). "Hydride generation in analytical chemistry and nascent hydrogen: when is it going to be over?". Spectrochimica Acta Part B: Atomic Spectroscopy. 57 (4): 797–802. Bibcode:2002AcSpe..57..797L. doi:10.1016/S0584-8547(02)00010-1. ISSN 0584-8547. Retrieved 2009-05-01.
  4. ↑ V. Fobos, A. K. L. Yuen, A. F. Masters, T. Maschmeyer "Exploring the Myth of Nascent Hydrogen and its Implications for Biomass Conversions" Chem. Asian J. 2012, 7, 2629 – 2637. doi:10.1002/asia.201200557
  5. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
  6. ↑ NASA/TM—2002-211915 : Solid Hydrogen Experiments for Atomic Propellants, 2002 (Web archive)

Further reading

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