The figure shows the skeletal structure of hex-2-en-5-ynoic acid…
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The figure shows the skeletal structure of hex-2-en-5-ynoic acid. Its condensed formula is . Four atoms of the chain are labelled a, b, c and d. As in every skeletal structure, the hydrogen atoms on carbon are not drawn, so you must add them yourself. Each carbon makes four bonds in all.
The molecule has one open chain of six carbon atoms. It holds three kinds of multiple bond: a carbon-carbon triple bond, a carbon-carbon double bond and a carbon-oxygen double bond. A student wants to describe its bonding with valence bond theory and hybridisation. She also wants to know how much s character the hybrid orbitals on one of the labelled carbons have, because s character changes the size and strength of a bond.
- How many σ bonds does one molecule have?
- How many π bonds does it have?
- How many carbon atoms are hybridised?
- How many carbon atoms are hybridised?
- How many lone pairs does the whole molecule have?
- What is the percentage s character of the hybrid orbitals that carbon d uses for its σ bonds?
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. The skeleton and the condensed formula. The molecule has C, H and 2 O atoms: atoms in all.
Plan. Every pair of joined atoms has exactly one σ bond. The extra bonds of a multiple bond are π bonds. The number of σ partners of a carbon gives its hybridisation.
Step 1. (a) σ bonds. The bonds are 6 bonds to H (5 C-H and 1 O-H), 5 C-C bonds and 2 C-O bonds. That is σ bonds.
Step 2. (b) π bonds. The triple bond has 2, the C=C bond has 1 and the C=O bond has 1: π bonds.
Step 3. (c) and (d) Hybridisation. Carbon a (the COOH carbon) and both carbons of the C=C bond, b and the one next to it, have three σ partners each: atoms are . Carbon d and the end carbon of the triple bond have two σ partners each: atoms are . Carbon c has four σ partners, so it is .
Step 4. (e) Lone pairs. Each oxygen atom has two lone pairs: in all. Carbon and hydrogen have none.
Step 5. (f) s character. An orbital is made from one s and one p orbital, so its s character is .
Answer: (a) . (b) . (c) . (d) . (e) . (f) .
Check. An open chain of atoms needs σ bonds to hold it together. That is what we counted.
Watch out. Carbon c sits between a double bond and a triple bond, but it has four single bonds of its own. It is , not .
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