Beta Carotene OCR Chemistry June 2017 Question

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Report Thread starter 2 years ago
Hi there,
I wanted to ask a question that came up in my mock exam and hence in last year's June 2017 paper.

β-Carotene is a naturally occurring unsaturated hydrocarbon found in carrots.

A β-carotene molecule contains 40 carbon atoms, has two rings, and a branched chain. 0.0200mol of β-carotene reacts with 5.28dm3 of hydrogen gas to form a saturated hydrocarbon.

Using molecular formulae, construct a balanced equation for this reaction. Include relevant calculations and reasoning.

Now, I worked out that the moles of Hydrogen gas were \frac{5.28}{24} = 0.220 and that \frac{0.220}{0.0200} = 11 so 11 moles of Hydrogen gas reacted per mole of Beta-Carotene.

Now, using this information, I needed to work out the original formula of Beta-Carotene. I know the general formula is CnH2n+2 so with 40 carbon atoms, this would be C40H82.

Now, I know it has "carbon rings" but how does the prove these rings are not benzene rings? They are alicyclic rings if you search for them on wikipedia but I thought they were aromatic rings, so why not?

I attached how the question appeared on my exam for reference.
I also included the mark scheme for reference.

Many thanks.

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