CAIE A-Level Chemistry 13.3 Organic Shapes and Sigma/Pi Bonds

Practise relating sp, sp2 and sp3 hybridisation to molecular shape, bond angles, planarity and sigma/pi bonding.

Exam points

assign sp, sp2 or sp3 hybridisation from the number and arrangement of electron domains
state the corresponding linear, trigonal planar or tetrahedral geometry and approximate bond angle
count sigma and pi bonds and represent side-on p-orbital overlap in a multiple bond

Covered syllabus points

13.3.1—Organic molecules as either straight-chained,13.3.2—Shape of, and bond angles in, molecules 13.3.3—Arrangement of σ and π bonds in molecules 13.3.4—Terms: planar when describing the arrangement

[Maximum number: 4]

$4 \vec{P}, Q$ and R all have the molecular formula $\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}$ . They are all structural isomers of each other.
P and Q each contain an oxygen atom bonded directly to a carbon atom that is $\mathrm{sp}^{2}$ hybridised. R contains an oxygen atom bonded directly to a carbon atom that is $\mathrm{sp}^{3}$ hybridised.

(a)

Explain how $\mathrm{sp}^{2}$ and $\mathrm{sp}^{3}$ hybridisation can occur in carbon atoms.
$\mathrm{sp}^{2}$ hybridisation
$\mathrm{sp}^{3}$ hybridisation

[ 2 ]

$\mathrm{sp}^{2}$ overlap of (2)s with two (2)p (atomic) orbitals

$\mathrm{sp}^{3} \quad$ overlap of (2)s with all three (2)p (atomic) orbitals

(b)

State the bond angles normally associated with each type of hybridisation in carbon atoms.
$\mathrm{sp}^{2}$ $\mathrm{sp}^{3}$

[ 2 ]

$\mathrm{sp}^{2}=116^{\circ}-124^{\circ}$ $\mathrm{sp}^{3}=106^{\circ}-112^{\circ}$

[Maximum number: 2]

Fig. 5.2 shows two reactions of W to produce organic compounds Y and Z.

Fig. 5.2

Deduce the number of sigma ( $\sigma$ ) bonds and pi ( $\pi$ ) bonds present in Z.
number of $\sigma$ bonds
number of $\pi$ bonds

number of $\sigma$ bonds =14

number of $\pi$ bonds =2

[Maximum number: 1]

Quinone is an unsaturated molecule.

quinone

Which statement about quinone is correct?

Quinone is non-planar and has an overall dipole moment.

Quinone is non-planar and does not have an overall dipole moment.

Quinone is planar and has an overall dipole moment.

Quinone is planar and does not have an overall dipole moment.

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Covered syllabus points

13.3 Shapes of organic molecules; σ and π bonds question 1

Question (a)

Question (b)

13.3 Shapes of organic molecules; σ and π bonds question 2

13.3 Shapes of organic molecules; σ and π bonds question 3