Ground-state, destabilisation

The discovery that tert-butylfluoroacetylene oligomerised spontaneously (ref. 1) could be explained by ground state destabilisation resulting from combining electronegative sp-carbon with fluorine. The "Electronegativity Effect" (ref. 2) produces maximal thermodynamic stability for substituents on carbon with complementary polarity, whereas equal polarity generally leads to destabilisation (Scheme 1, ref. 3). 

Contrary to conventional reactivity arguments, which imply that substitution at carbonyls by electronegative atoms reduces electron density at the carbonyl carbon and hence promotes addition to carbonyls, a systematic study of 13C NMR shift data for ester carbonyls shows that electron density is actually greater at such carbons (reactivity enhancement is actually due to destabilisation of the ground states of the esters by the electron-withdrawing substituents).132,133 Our observations are in line with those of Neovonen et al. Electron-withdrawing nitrogen in... 

Increased steric bulk of the diimine aryl substituents leads to faster reactions This is because the ground state of the resting state is destabilised relative to the transition state and the barrier to insertion thus becomes lower. 

One 11 Bond. It hardly needs saying that a n bond is not usually free to rotate. The n energy 2En that we saw in Fig 1.26 (280 kJ mol ) would be lost at the transition structure for rotation about the C—C bond, which would have the two p orbitals orthogonal. This value is higher than the energy normally available for a chemical reaction. For rotation about a n bond to become easy in the ground state, either the transition structures like diradical 2.101 or the zwitterion 2.102 must be stabilised or the planar structure 2.100 must be destabilised. 

E.s.r. spectra of this cation indicated that it has a triplet ground state [261,263]. Electrochemical determination of the pKp+ value were hampered by a number of uncertainties but indicate that it is exceedingly low, and much lower than the value for the allyl cation [264]. Calculations of the ionic heats of formation of the cyclopentyl, cyclopentenyl and cyclopentadienyl cations from the measured ionisation potentials of the corresponding radicals and the heats of formation of these radicals show that whereas introduction of one double bond stabilises the cation, introduction of the second double bond destabilises the system [265]. E.s.r. spectra of the pentaphenyl [258,259] and pentachloro [259,266] derivatives show that the former has a ground state singlet structure but with a triplet state less than 1 kcal.mol" higher, while the latter has a triplet ground state [252,259]. 

Solvatochromism is also seen with unsubstituted cyclopentadien-ylides [118,119,121], not only for pyridinium ylides but also in this case for phosphonium and sulphonium ylides. [125], It has been suggested that this solvatochromism is associated with intramolecular charge transfer [121,125] with the ionic ground-state becoming progressively destabilised, with respect to the excited state, in less polar solvents. 

The effect of bond weakening in the ground state would be expected to lead to an increase in reactivity in a dissociative process. However, in an associative process things will be more finely balanced, and the question arises if bond weakening destabilises the ground state more or less than it does the transition state. 

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