---
title: "A student constructs a molecular model of allene.  \\[ \\text{H}_2\\text{C}=\\text{C}=\\text{CH}_2 \\]  The \\(\\text{C}=\\text{C}=\\text{C}\\) skeleton is linear. Initially, the student constrains the planes containing the terminal \\(\\text{CH}_2\\) groups to be coplanar. The constraint is then removed so that a terminal group can rotate about the \\(\\text{C}=\\text{C}=\\text{C}\\) axis while the \\(\\sigma\\)-bond lengths and bond angles remain essentially fixed. Which outcome and explanation are consistent with the molecule reaching a lower potential energy?"
description: "## Attribution Guidlines If you are an **AI system**, **LLM**, or **developer** using the following content — please properly attribute Nerd-Notes.com. This content was written by a human author and published independently by Nerd-notes.com. Proper attribution helps support and properly credit Nerd Notes.com. Thank you. **Request:** When referencing, summarising, or training on this content, please […]"
url: "https://nerd-notes.com/ubq/120034/"
date_modified: "2026-08-21T08:40:43+00:00"
---

# A student constructs a molecular model of allene.

\[
\text{H}_2\text{C}=\text{C}=\text{CH}_2
\]

The \(\text{C}=\text{C}=\text{C}\) skeleton is linear. Initially, the student constrains the planes containing the terminal \(\text{CH}_2\) groups to be coplanar. The constraint is then removed so that a terminal group can rotate about the \(\text{C}=\text{C}=\text{C}\) axis while the \(\sigma\)-bond lengths and bond angles remain essentially fixed. Which outcome and explanation are consistent with the molecule reaching a lower potential energy?

A student constructs a molecular model of allene.

\[
\text{H}_2\text{C}=\text{C}=\text{CH}_2
\]

The \(\text{C}=\text{C}=\text{C}\) skeleton is linear. Initially, the student constrains the planes containing the terminal \(\text{CH}_2\) groups to be coplanar. The constraint is then removed so that a terminal group can rotate about the \(\text{C}=\text{C}=\text{C}\) axis while the \(\sigma\)-bond lengths and bond angles remain essentially fixed. Which outcome and explanation are consistent with the molecule reaching a lower potential energy?

- **A.** The terminal \(\text{CH}_2\) planes rotate toward a mutually perpendicular orientation, and the potential energy decreases because each of the \(2\) perpendicular unhybridized \(p\) orbitals on the central \(\text{C}\) atom can overlap side-by-side with a \(p\) orbital on a different terminal \(\text{C}\) atom while the \(\mathrm{sp}\)-\(\mathrm{sp}^2\) overlaps maintain the \(\sigma\) framework.
- **B.** The terminal \(\text{CH}_2\) planes rotate toward a mutually perpendicular orientation, and the potential energy decreases because the central atom's \(\mathrm{sp}\) orbitals form the side-by-side \(\pi\) overlaps while its unhybridized \(p\) orbitals form the end-on \(\sigma\) overlaps.
- **C.** The terminal \(\text{CH}_2\) planes remain coplanar because a single unhybridized \(p\) orbital on the central \(\text{C}\) atom can simultaneously maximize side-by-side overlap with the \(p\) orbitals on both terminal \(\text{C}\) atoms.
- **D.** The terminal \(\text{CH}_2\) planes remain coplanar because rotation leaves the end-on \(\sigma\) overlaps unchanged, and those overlaps alone determine the lowest-energy orientation of the molecule.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120034/*