The importance of a bond
Norbornadiene
Oeiras, 01.06.09
Being able to accurately measure the strength of chemical bonds is one of the most important goals in chemistry. Some bond strengths are particularly difficult to measure, such as the π-bond strength in a C=C double bond. This might seem nitpicking, but only at first sight (pun intended). A paper recently published in the Journal of Physical Chemistry A and involving ITQB researchers, describes an experimental alternative to the current methods of π-bond strength estimation, illustrated by the addition of tert-butoxyl radical to the highly strained norbornadiene molecule.
The proposed experimental method is based on the study by time-resolved photoacoustic calorimetry (PAC) of the reaction between tert-butoxyl radical and norbornadiene. PAC is a calorimetric technique that measures the heat involved in very fast reactions (such as that one), by allowing researchers to “listen” to them (see more information on PAC).
The tert-butoxyl radical is widely used in PAC studies to measure the strength of chemical bonds, because of its marked tendency to abstract hydrogens from other molecules, cleaving X–H bonds (X = C, N, O…) in the process (the strength of the bond is related to the heat of this reaction).
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The main character in this study is norbornadiene. This bicyclic hydrocarbon has an unusual structure, which imposes considerable strain on its framework, with dramatic consequences on the energy of its bonds, namely on the C–H bonds that tert-butoxyl radical usually craves for. Indeed, as every undergraduate chemistry student knows, primary C–H bonds are stronger than secondary and these are stronger than tertiary. In norbornadiene, however, the apex C–H bonds (secondary) are as strong as the C–H bonds in methane (primary) (440 kJ/mol, i.e. 27 kJ/mol higher than in the unstrained equivalent secondary C–H bonds in propane). And the strain at the bridgehead carbons is so high that the corresponding C–H bonds are even stronger (ca. 450 kJ/mol). In the presence of such strong C–H bonds, even the tert-butoxyl radical prefers to add itself to a C=C double bond (despite the fact that an addition reaction is entropically unfavorable)! |
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In conclusion, it is the unusual norbornadiene structure that is responsible for the atypical behavior of tert-butoxyl radical, thus providing a model reaction that can be used to measure the strength of the π-bond.
A bond at first sight
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To illustrate the importance of these numbers, consider the photochemical reaction that initiates the human vision process: the isomerization of cis-trans retinal, whose energy barrier is closely related to the π-bond strength of the C11=C12 double bond in 11-cis-retinal. This was estimated as ca. 160 kJ/mol, which is equivalent in energy to the red limit of the visible light; lower energy light is not enough to break the corresponding π-bond and drive the cis-trans isomerization in this molecule and that is why we cannot see past ca. 760 nm (the red limit). |
Original Paper:
J. Phys. Chem. A, Article ASAP, DOI: 10.1021/jp900089t
Energetics of tert-Butoxyl Addition Reaction to Norbornadiene. A Method to Estimate the π-Bond Strength of a Carbon-Carbon Double Bond
Paulo M. Nunes, Sílvia G. Estácio, Gustavo T. Lopes, Filipe Agapito, Rui C. Santos, Benedito J. Costa Cabral, Rui M. Borges dos Santos (ITQB), and José A. Martinho Simões (ITQB)
