22. Zingiberene is the compound that gives ginger its characteristic flavor. Its IUPAC name is 2-methyl-5-(6-methylhept-5-en-2-yl) cyclohexa-1,3-diene.
(a) On the structure of zingiberene, draw a circle around the ‘2-methyl’ group referred to in the IUPAC name.
(1)
ANSWER
(b) Deduce the molecular formula of zingiberene.
(2)
ANSWER
Total number of Carbon = 15
Total number of Hydrogen = 24
Hence, C15H24 is the answer.
(c) When zingiberene reacts with excess hydrogen bromide, there are a number of possible products. The structure of the major product is shown.
(i) Name the type and mechanism of the reaction.
(1)
ANSWER
Electrophilic Addition Reaction
(ii) The diagram shows a simplified structure of zingiberene, in which part of the molecule is represented by A. Complete the mechanism for the reaction of zingiberene with one molecule of hydrogen bromide. Include curly arrows, and any relevant dipoles and lone pairs.
(4)
ANSWER
(iii) For the reaction in (c)(ii) there are two possible products:
Explain why I is the major product, by referring to your mechanism.
(2)
ANSWER
I is formed via a tertiary carbocation and II is formed via a secondary carbocation. Tertiary carbocations are more stable than secondary.
(d) Zingiberene reacts with hydrogen gas in the presence of a catalyst.
(i) Identify the catalyst, by name or formula.
(1)
ANSWER
Nickel (Ni), Platinum (Pt) /Palladium (Pd)
(ii) 2.0mol of zingiberene react completely with hydrogen at 150°C and a pressure of 120kPa.
Calculate the minimum volume of hydrogen needed under these conditions, stating your units.
[Ideal gas equation is pV = nRT Gas constant (R) = 8.31Jmol−1K−1]
(4)
ANSWER
