When 1 -fluorobutane is dissolved in superacid at low temperature, it is possible to observe a carbocation. However, the \({ }^{13} \mathrm{C}\) NMR spectrum of that carbocation shows only two signals. (a) How many signals did you expect to see in the spectrum? (b) What carbocation do you think is observed? Although part (a) might be easy for you, part (b) actually poses a vexing problem. Can you write a mechanism for formation of the carbocation that shows only two signals in its \({ }^{13}\) C NMR spectrum? What problems appear as you try to write the mechanism?

1-Fluorobutane is a simple organic compound with the molecular formula C4H9F. It consists of a four-carbon chain with a fluorine atom attached to the first carbon. In chemical notation, its structure is written as CH3-CH2-CH2-CH2F. Each carbon atom in this molecule has a unique environment, so in a normal 13C NMR spectrum, you would expect to see four distinct signals corresponding to the four distinct carbon atoms:

• CH3
• CH2 (adjacent to CH3)
• CH2 (adjacent to CH2F)
• CH2F

However, when 1-fluorobutane is subjected to specific conditions, this changes.

Superacids are acids that are stronger than 100% sulfuric acid. These extremely strong acids, such as fluoroantimonic acid, can protonate nearly any molecule. When 1-fluorobutane is exposed to superacid conditions, the molecule can undergo significant changes.

In this scenario, superacid conditions facilitate the formation of a carbocation, a positively charged ion where a carbon atom has an incomplete octet. The superacid protonates the fluorine group, initiating the process of carbocation formation and subsequent rearrangement.

This intense acidic environment enables unusual reactions that might not occur under milder conditions.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique to determine the structure of organic compounds. 13C NMR spectroscopy focuses on the carbon atoms within a molecule. Each distinct carbon environment in a molecule will produce a unique signal in the NMR spectrum.

For 1-fluorobutane, we initially expect to observe four signals because there are four unique carbon environments in its structure. However, under superacid conditions, the observed 13C NMR spectrum of the resulting carbocation shows only two signals. This implies a change in the molecular structure has occurred, resulting in equivalent carbon environments, hence fewer distinct signals.

Carbocation rearrangement is a common phenomenon in organic chemistry where a carbocation (positive ion) changes its structure to form a more stable cation. In the case of 1-fluorobutane in a superacid environment, the molecule undergoes rearrangement to form a more stable carbocation.

Typically, rearrangements occur to achieve a tertiary carbocation, as tertiary carbocations are more stable than primary or secondary ones. Fluorine, being highly electronegative, facilitates the formation of a carbocation by leaving a carbon atom positively charged upon protonation.

The formation of a cyclopropylmethyl carbocation from the initial 1-fluorobutane structure is an example of such a rearrangement.

A cyclopropylmethyl carbocation is a highly stable structure given its resonance stability. This structure forms when a rearrangement leads to the formation of a three-membered cyclopropane ring with a positively charged carbon attached to it.

In the rearrangement of 1-fluorobutane under superacid conditions, the intermediate carbocation rearranges to form a cyclopropylmethyl carbocation.

Due to resonance, the three carbons in the cyclopropane ring are equivalent, resulting in symmetry. In 13C NMR, this symmetry collapses multiple signals into fewer distinct peaks, hence only two signals:

• One for the three equivalent carbons in the ring
• One for the carbon attached to the ring

This explains why the 13C NMR spectrum of the carbocation formed from 1-fluorobutane shows only two signals.