9701 · 13.3
Shapes of organic molecules; σ and π bonds flashcards
Revision flashcards for Cambridge 9701 Shapes of organic molecules; σ and π bonds (syllabus 13.3). Flip, recall, then mark a real past-paper question.
Card
What is a σ (sigma) bond?
A strong covalent bond formed by the direct, head-on overlap of atomic orbitals along the internuclear axis. All single covalent bonds are σ bonds.
Card
What is a π (pi) bond?
A covalent bond formed by the sideways overlap of adjacent, parallel p-orbitals, above and below the plane of the σ bond. It is weaker than a σ bond.
Card
How are the electrons distributed in a σ bond?
The electron density is concentrated on the line directly between the two bonded nuclei (the internuclear axis).
Card
How are the electrons distributed in a π bond?
The electron density is concentrated in two regions, one above and one below the plane of the nuclei.
Card
What is the composition of a C=C double bond?
One σ bond and one π bond.
Card
What is the composition of a C≡C triple bond?
One σ bond and two π bonds.
Card
Why is there free rotation around a C-C single bond?
Because it is a σ bond, which is symmetrical about the internuclear axis. Rotation does not break the orbital overlap.
Card
Why is there no free rotation around a C=C double bond?
Because rotation would break the sideways overlap of the p-orbitals that form the π bond, which requires a significant amount of energy.
Card
What is the shape and bond angle around each carbon in ethane ($C_2H_6$)?
Tetrahedral, with bond angles of approximately 109.5°. Each carbon has 4 bonding pairs and 0 lone pairs.
Card
What is the shape and bond angle around each carbon in ethene ($C_2H_4$)?
Trigonal planar, with bond angles of approximately 120°. Each carbon has 3 regions of electron density (2x C-H, 1x C=C).
Card
Describe the geometry of a benzene ($C_6H_6$) molecule.
It is a planar, regular hexagon. All C-C bond lengths are identical and all bond angles are 120°.
Card
What is a delocalised π system?
A system where π electrons are shared between more than two nuclei. In benzene, six p-orbitals overlap to form a continuous ring of electron density above and below the plane of the carbon atoms.