The large variation in acidity of diethyl ether cation induced by internal rotation about a single covalent bond
Yoshiyuki Matsuda1,2, Tomoya Endo1, Naohiko Mikami1
1†Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki-Aoba 6-3, Aoba-ku, Sendai, 980-8578, Japan.
Abstract:
In the IR spectrum of the diethyl ether cation, an extraordinarily intense band, with an extremely broad bandwidth, was observed at 2700 cm(-1), much lower frequency than normal CH stretch frequencies. This band is assigned to the stretch band of the CH bond, which is hyperconjugated with the singly occupied molecular orbital of the oxygen atom. The hyperconjugation causes the delocalization of the σ electron of the CH bond so that it enhances the acidity of the CH bond as well as the CH stretch band intensity. Theoretical simulation shows that the strength of hyperconjugation varies greatly with internal rotation of the ethyl group, and this is reflected in the large width of the observed CH stretch band. These results indicate that the DEE cation drastically changes its property from aprotic to highly acidic by the rotational isomerization of the ethyl group.
Related Concept Videos
Acidity of 1-Alkynes
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement
Inductive Effects on Chemical Shift: Overview
Ethers to Alkyl Halides: Acidic Cleavage
Crown Ethers


