5.6 Amine Inversion and Chiral Molecules without Chiral Centers

Overview

This lecture covers amine inversion, chiral molecules without chiral centers, focusing on exceptions like allenes and biphenyls, and the requirements for chirality in these systems.

Amine Inversion

• Amines with sp³ hybridized nitrogen can undergo rapid inversion, flipping the lone pair to the opposite side.
• The activation energy for amine inversion is very low, making inversion rapid even at low temperatures.
• Due to rapid inversion, enantiomers of simple amines can't be isolated, so these molecules are not considered chiral.
• Bulky groups or ring-constrained amines can prevent inversion, but no clear cut-off exists for "bulky."
• Sulfur analogs (like sulfonium ions) do not rapidly invert, so they can be chiral.

Chiral Molecules With No Chiral Centers

Allenes

• Allenes have two adjacent double bonds (C=C=C) with orthogonal planes of substituents.
• The central carbon in allenes is sp hybridized; terminal carbons are sp²; none are chiral centers.
• An allene can be chiral if the two groups on each end are different from each other.
• If there is an internal mirror plane (sigma plane), the allene is achiral.
• Only allenes with even numbers of consecutive double bonds can show this chirality.

Biphenyls

• Biphenyls consist of two benzene rings joined by a single bond.
• If the ortho positions (adjacent to the bond) have large groups (three or four), rotation is restricted and rings are twisted.
• Biphenyls can be chiral if the two ortho substituents on each ring are different from each other.
• Presence of an internal mirror plane (identical ortho groups) makes the molecule achiral.
• All atoms are sp² hybridized, so there are no chiral centers even though the molecule may be chiral.

Key Terms & Definitions

• Chiral center — An atom (usually sp³) bonded to four different groups, leading to non-superimposable mirror images.
• Amine inversion — Rapid flipping of a nitrogen's geometry, exchanging the position of the lone pair and preventing fixed chirality.
• Allene — A molecule with two adjacent double bonds, potentially chiral if end groups differ.
• Biphenyl — Two benzene rings connected by a single bond; can be chiral due to restricted rotation and differing ortho substituents.
• Internal mirror plane (sigma plane) — A plane dividing a molecule into two mirror-image halves, indicating achirality.
• Enantiomers — Non-superimposable mirror image molecules.

Action Items / Next Steps

• Review optical activity as the next topic.
• Practice identifying chirality in allenes and biphenyls.
• Check your curriculum for coverage of biphenyl chirality.