Why does a double bond restrict rotation?
You will be able to: Count sigma and pi bonds and connect side-on overlap to restricted rotation.
Why does a double bond restrict rotation?
A single C–C connection can rotate without losing its main end-on overlap. A C=C bond also relies on sideways p-orbital overlap; twisting one end out of alignment disrupts that overlap.
A useful starting point: How do sp, sp² and sp³ connect to local geometry? →
Words and symbols before equations
- Sigma bond σ
- Bonding from overlap along the internuclear axis.
- Pi bond π
- Bonding from side-on overlap of parallel p orbitals.
- Restricted rotation
- Rotation that is energetically unfavorable because bonding overlap would be lost.
- Geometric isomers
- Distinct arrangements such as cis and trans when rotation is restricted and each double-bonded C has two different substituents.
What this picture assumes
Schematic p-orbital axes and alignment index cos²(twist), not probability density, bond energy, or a freely rotating stable double bond. A forced twist changes internal geometry; camera rotation does not.
Read the picture in three steps
- Read the species and labels first. A Lewis line represents two electrons; a spatial stick indicates connectivity. Use the stated quantities and units for numerical comparisons.
- At 0° twist, cos²(twist) = 1. Parallel p axes can overlap side-on. This index is not a bond energy.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
A single bond has one sigma interaction. A double has one sigma and one pi; a triple has one sigma and two pi interactions in the localized model. Do not label every line in a double bond a sigma bond.
Rotating about a single bond usually preserves its sigma overlap, though real rotational barriers may remain. Twisting a double bond misaligns the p orbitals and costs substantial energy.
For 1,2-dichloroethene, each carbon is attached to H and Cl, so same-side and opposite-side Cl arrangements are distinct cis/trans forms. Ethene itself has two identical H substituents at each C, so it does not have that pair of geometric isomers.
A worked example, step by step
Count sigma and pi bonds in H₂C=CH₂.
- There are four C–H single bonds, each with one sigma bond.
- The C=C connection contributes one more sigma bond.
- Its second interaction is one pi bond.
- Total: five sigma and one pi. The pi overlap helps keep the atom arrangement planar near the double bond.
A double bond is one sigma plus one pi, not two sigma bonds. Restricted rotation alone does not guarantee geometric isomers.
Does ethene have cis and trans forms?
Compare with an explanation
No. Each double-bonded carbon has two identical H substituents, so exchanging their positions does not create distinct forms.
Predict. Change one thing. Explain.
Use the schematic twist control to compare aligned p orbitals with a forced 90° twist. The indicator represents overlap alignment, not a measured energy or an allowed free rotation of a stable double bond.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
At 0° twist, cos²(twist) = 1. Parallel p axes can overlap side-on. This index is not a bond energy.
Schematic p-orbital axes and alignment index cos²(twist), not probability density, bond energy, or a freely rotating stable double bond. A forced twist changes internal geometry; camera rotation does not.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using electron accounting, electrostatic interactions or spatial geometry. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionCount sigma and pi bonds in H–C≡C–H. Explain one distinction between rotating a physical single bond and rotating the camera, and state the condition for cis/trans forms at a C=C bond.
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Compare with the answer and four-point rubric
- 1 point: Two C–H sigma bonds plus one C–C sigma bond give three sigma bonds.
- 1 point: The triple bond adds two pi bonds.
- 1 point: Camera rotation preserves all internal geometry; internal bond rotation changes relative atom positions.
- 1 point: For a C=C cis/trans pair, each carbon must have two different substituents.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1How many sigma bonds connect any one bonded atom pair?
One in the localized single/double/triple model.
RECALL 2Why does a pi bond restrict rotation?
Side-on overlap is lost when the orbitals become misaligned.
RECALL 3Is the displayed alignment index a bond energy?
No. It is a qualitative geometric illustration.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why does a double bond restrict rotation?
- Single: 1σ, 0π. Double: 1σ, 1π. Triple: 1σ, 2π.
- Cis/trans requires distinguishable substituents at both double-bonded carbons.
Remember: A double bond is one sigma plus one pi, not two sigma bonds. Restricted rotation alone does not guarantee geometric isomers.
Conditions: Schematic p-orbital axes and alignment index cos²(twist), not probability density, bond energy, or a freely rotating stable double bond. A forced twist changes internal geometry; camera rotation does not.
Refresh Kid · AP Chemistry Unit 2 · Objectives 2.7.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 2.7, objectives 2.7.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 2: Compound Structure and Properties, Topics 2.1–2.7. The focused lesson breakdown is Refresh Kid’s editorial sequence. Models and original practice are teaching materials, not official AP questions. Numerical potential curves, ion comparisons and orbital-alignment indices state their approximations. Five- and six-domain shapes are included; d-orbital hybridization and molecular-orbital diagrams are not required here. GitHub’s 3D website examples, including the Three.js Mars camera-control example, informed the use of rotatable scenes. Our scientific geometry and viewer code are original; no repository artwork or tutorial code was copied. The self-hosted Three.js library retains its MIT license. Camera rotation does not alter chemistry. See also the official clarifications.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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