Triethyloxonium tetrafluoroborate, (CH3CH2)3O+ BF4-, is a solid with melting point 91–92 °C.


Show how this reagent can transfer an ethyl group to a nucleophile (Nuc:−) in an SN2 reaction.


What is the leaving group?


Why might this reagent be preferred to using an ethyl halide?


(Consult [TABLE 6-2] <IMAGE>)

Verified step by step guidance

1. Identify the structure of triethyloxonium tetrafluoroborate: (CH3CH2)3O+ BF4-. The oxonium ion (CH3CH2)3O+ is the active part in the reaction.
2. Recognize that the oxonium ion can transfer an ethyl group to a nucleophile (Nuc:−) through an SN2 mechanism. In an SN2 reaction, the nucleophile attacks the electrophilic carbon, leading to the displacement of a leaving group.
3. Determine the leaving group in this reaction. In the case of triethyloxonium ion, the leaving group is ethanol (CH3CH2OH), which is formed after the nucleophile attacks one of the ethyl groups.
4. Consider why triethyloxonium tetrafluoroborate might be preferred over an ethyl halide. One reason is that the oxonium ion is a more reactive electrophile than an ethyl halide, making the transfer of the ethyl group more efficient.
5. Another reason for preference is that the leaving group, ethanol, is a neutral and stable molecule, which can make the reaction more favorable compared to the leaving groups in ethyl halides, which are often halide ions.

Key Concepts

SN2 Reaction Mechanism

The SN2 (substitution nucleophilic bimolecular) reaction is a type of nucleophilic substitution where a nucleophile attacks an electrophile, resulting in the simultaneous displacement of a leaving group. This mechanism involves a single concerted step, where the nucleophile approaches the electrophile from the opposite side of the leaving group, leading to an inversion of configuration at the carbon center. Understanding this mechanism is crucial for analyzing how triethyloxonium tetrafluoroborate can transfer an ethyl group to a nucleophile.

Leaving Group Ability

In nucleophilic substitution reactions, the leaving group is the atom or group that departs with a pair of electrons, allowing the nucleophile to bond to the electrophile. A good leaving group is typically stable after departure and can stabilize the negative charge, such as halides or sulfonate groups. In the case of triethyloxonium tetrafluoroborate, the leaving group is the ethyl group, which is transferred to the nucleophile during the SN2 reaction.

Advantages of Triethyloxonium Tetrafluoroborate

Using triethyloxonium tetrafluoroborate as a reagent for ethyl group transfer offers several advantages over traditional ethyl halides. It provides a more stable and less reactive source of the ethyl group, reducing side reactions and improving selectivity. Additionally, the tetrafluoroborate anion is a non-nucleophilic counterion, which minimizes unwanted reactions, making this reagent particularly useful in sensitive synthetic applications.