Comparison of Acidity between Piperidinium and Aziridinium
Recommended Article: 【Organic Chemistry】 Chapter 1. Basics of Organic Chemistry
Q. 1.
Which has higher acidity between piperidinium and aziridinium?
Figure 1. Structure of piperidinium
Figure 2. Structure of aziridinium
A. 1.
The nitrogen atoms in both piperidinium and aziridinium are sp3 hybridized. According to VSEPR theory, sp3 hybridization indicates a bond angle of 109.5°. Piperidinium can be bent into a chair form to forcefully match this bond angle, but aziridinium cannot achieve a 109.5° bond angle in its triangular shape with an internal angle of 60°. According to VSEPR theory, the N-C and C-C bonds in aziridinium attempt to adjust the bond angle away from the bonding lines.
Figure 3. σ orbital of aziridinium
However, even loose σ bonds seem unable to satisfy the 109.5° bond angle. So, nature uses a different technique. As is well known, sp3 hybrid orbitals predict a bond angle of 109.5°, sp2 hybrid orbitals predict 120°, and sp hybrid orbitals predict 180°. This means that the larger the p-character, the smaller the bond angle. The N-C and C-C bonds in aziridinium have more p-character allocated. Since the N assigns more p-character to its bonds with carbon, it allocates more s-character to its N-H bond. Therefore, the lone pair on the conjugate base R-NH- is more stabilized, resulting in higher acidity.
Q. 2.
Which molecule has a longer C-H bond, cyclopentane or cyclohexane?
Figure 4. Structure of cyclopentane
Figure 5. Structure of cyclohexane
A. 2.
The same logic as the first question applies. Cyclopentane experiences bond angle strain and thus allocates more than 75% p-character to the C-C bond. Consequently, more s-character is allocated to the C-H bond. Therefore, cyclohexane, with less s-character allocated to the C-H bond, has a longer bond length.
Q. 3.
Which has a longer C=O bond, cyclopropanone or cyclohexanone?
A. 3.
The same logic as the first question applies. Cyclopropanone experiences bond angle strain and thus allocates more than 75% p-character to the C-C bond. Consequently, more s-character is allocated to the C=O bond. Therefore, cyclohexanone, with less s-character allocated to the C=O bond, has a longer bond length.
Input: 2019.03.22 00:30