The electron-deficient central carbon atom has only six electrons, which it uses to form three σ bonds, and therefore also carries an empty p orbital. Any carbocation will have a planar carbon atom with an empty p orbital. Think of it this way: only fi lled orbitals contribute to the energy of a molecule, so if you have to have an unfilled orbital (which a carbocation always does) it is best to make that unfilled orbital as high in energy as possible to keep the filled orbitals low in energy. p orbitals are higher in energy than s orbitals (or hybrid sp, sp², or sp³ orbitals for that matter) so the carbocation always keeps the p orbital empty.

We know that the t-butyl cation is stable enough to observe because of the work of George Olah, who won the Nobel Prize for Chemistry in 1994. The challenge is that carbocations are very reactive electrophiles, so Olah’s idea was to have a solution containing no nucleophiles. Any cation must have an anion to balance the charge, so the important advance was to find anions, consisting of a negatively charged atom surrounded by tightly held halogen atoms, which are just too stable to be nucleophilic. Examples include BF4 −, PF6 −, and Sb6 −. The fi rst is small and tetrahedral and the others are larger and octahedral. In these anions, the negative charge does not correspond to a lone pair of electrons (they are like BH₄ − in this respect) and there is no orbital high enough in energy to act as a nucleophile. By using a non-nucleophilic solvent, liquid SO₂, at low temperature, Olah was able to turn alcohols into carbocations with these counterions. This is what happens when tert-butanol is treated with SbF5 and HF in liquid SO₂. The acid protonates the hydroxyl group, allowing it to leave as water, while the SbF5 grabs the fluoride ion, preventing it from acting as a nucleophile. The cation is left high and dry.

The proton NMR of this cation showed just one signal for the three methyl groups at 4.15 ppm, quite far downfield for C–Me groups. The ¹³C spectrum also showed downfi eld Me groups at 47.5 ppm, but the key evidence that the cation was formed was the shift of the central carbon atom, which came at an amazing 320.6 ppm, way downfield from anything you have met before. This carbon is very deshielded—it is positively charged and extremely electron deficient.

From Olah’s work we know what the t-butyl cation looks like by NMR, so can we use NMR to try to detect it as an intermediate in substitution reactions? If we mix t-BuBr and NaOH in an NMR tube and let the reaction run inside the NMR machine, we see no signals belonging to the cation. But this proves nothing. We would not expect a reactive intermediate to be present in any significant concentration. There is a simple reason for this. If the cation is unstable, it will react very quickly with any nucleophile around and there will never be any appreciable amount of cation in solution. Its rate of formation will be much slower than its rate of reaction.

مواضيع ذات صلة

A closer look at steric effects

Adjacent C=C or C=O π systems increase the rate of SN2 reactions

An adjacent C=C π system stabilizes a carbocation: allylic and benzylic carbocations

Alkyl substituents stabilize a carbocation

A closer look at the SN1 reaction

? How can we decide which mechanism (SN1 or SN2) will apply to a given organic compound

Significance of the SN1 rate equation

Significance of the SN2 rate equation

Mechanisms for nucleophilic substitution

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres