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I was studying hydroboration from Clayden's Organic Chemistry [1, p. 1281] and the transition state had “‡” symbol in the upper right corner of the activated complex:

We know that this is not the whole story because of the stereochemistry. Hydroboration is a syn addition across the alkene. As the addition of the empty p orbital to the less substituted end of the alkene gets under way, a hydrogen atom from the boron adds, with its pair of electrons, to the carbon atom, which is becoming positively charged. The two steps shown above are concerted, but formation of the C–B bond goes ahead of formation of the C–H bond so that boron and carbon are partially charged in the four-centred transition state.

Hydroboration is regioselective

What does this symbol mean? I have seen it before as a superscript at the Gibbs energy symbol.

References

  1. Organic Chemistry; Clayden, J., Ed.; Oxford University Press: Oxford; New York, 2001. ISBN 978-0-19-850347-7.
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The symbol is called "double dagger" (sometimes also "double cross") and is used to denote transition state (a maximum in an energy diagram; also often denoted with "*" or "TS") or a related physical property. Note, however, that a transition state and an intermediate are two different terms.

The symbol has the peculiar origins: as written by H. Eyring, it's entirely the merit of the secretary of the department, Miss Lucy D’Arcy [1, p. 9]:

The symbol, ‡, first appeared in the second paper dealing with the activated complex. In that manuscript a star was used to designate the activated state but Miss Lucy D'Arcy, the departmental secretary, lacking a star used a plus and minus sign. Believing the typesetters would interpret this as a star it was left and by their option the cross, ‡, has become the almost universally used sign for the activated state.

Also, note that according to IUPAC recommendations [2, p. 1080]:

In accordance with previous IUPAC recommendations (IUPAC QUANTITIES (1988)) the symbol ‡ to indicate transition states ("double dagger") is used as a prefix to the appropriate quantities, e.g. $Δ^‡G$ rather than the more often used $ΔG^‡.$

In the future, I suggest to use services like Shapecatcher or Detexify to find the name of the symbol. Finding its usage in chemistry afterwards is normally a trivial task.

References

  1. Eyring, H. Models in Research. International Journal of Quantum Chemistry 1969, 3 (S3A), 5–15. DOI: 10.1002/qua.560030705.
  2. Muller, P. Glossary of Terms Used in Physical Organic Chemistry (IUPAC Recommendations 1994). Pure and Applied Chemistry 1994, 66 (5), 1077–1184. DOI: 10.1351/pac199466051077. (Free Access)
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The double dagger symbol is just to signify that something is or is related to the transition state of a reaction. It appears above Gibbs energy when it is referring to the Gibb's energy of activation (or the difference in Gibb's energy between the reactants and the transition state). It can also appear above the enthalpy symbol, H, when referring to the enthalpy of activation (or the difference in enthalpy between the reactants and the transition state).

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