AS Level Chemistry · An introduction to AS Level organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Carbon chains can be straight, branched or cyclic (joined in a ring). The shape around each carbon atom depends on how its orbitals are mixed, which is called hybridisation.
A σ bond forms when two orbitals overlap end-on, directly between the two nuclei. Every single bond is a σ bond. A π bond forms when two p orbitals overlap sideways, above and below the line between the nuclei. A double bond is one σ and one π bond; a triple bond is one σ and two π bonds.
Count the atoms joined to a carbon atom: four means sp3, three means sp2, two means sp. A planar molecule, such as ethene, has all its atoms in one plane.
Students often count a double bond as two π bonds, or forget the C–H bonds. Every bond contains exactly one σ bond, and each C–H bond must be counted too.
Answer: 6 σ bonds and 2 π bonds; hybridisation sp3, sp, sp.
Try each one, then open the answer.
Answer: A. Propene has six C–H bonds, one C–C single bond and one C=C double bond. Every bond contains one σ bond, giving 6 + 1 + 1 = 8 σ bonds, and the second bond of the C=C is the single π bond.
Answer: D. 2-methylpentane has a methyl side-chain on C2 of a five-carbon chain. Hexane and hex-1-ene are straight chains, and cyclohexane is cyclic.
Answer: C. Each carbon in ethene is sp2 hybridised with three regions of electron density in a plane, giving angles of about 120°. The tetrahedral 109.5° applies to sp3 carbons such as those in ethane.
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