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.
What does this symbol mean? I have seen it before as a superscript at the Gibbs energy symbol.
- Organic Chemistry; Clayden, J., Ed.; Oxford University Press: Oxford; New York, 2001. ISBN 978-0-19-850347-7.