15 This question is about hydrocarbons obtained from crude oil.
(a) Petrol used in cars has a high proportion of the alkane iso‑octane. Iso‑octane can be formed by heating octane to 400°C in the presence of a catalyst.
(i) Give the IUPAC name for iso‑octane.
(1)
ANSWER
2, 2-dimethyl-4-methylpentane
(ii) Name the industrial process used to convert octane into iso‑octane.
(1)
ANSWER
Reforming
(iii) Give a reason why petrol needs a high proportion of iso‑octane.
(1)
ANSWER
The process of reforming converts the straight chain octane into branched/cyclo iso-octane which helps them to burn smoothly in the engine; hence large proportion of iso-octane is needed.
(b) Another way to produce iso‑octane is by reacting iso‑butane with 2‑methylpropene
Draw the skeletal formulae of four isomers of 2‑methylpropene.
Include only one isomer that does not decolorize bromine water.
(4)
ANSWER
(c) 2‑methylpropene reacts with hydrogen bromide to form two possible products.
(i) Draw the mechanism for the formation of the major product of the reaction between 2‑methylpropene and hydrogen bromide, HBr.
Include curly arrows, and relevant lone pairs and dipoles.
(4)
ANSWER
(ii) Give the reason why your product in (c) (i) is the major product.
(1)
ANSWER
The tertiary (3o) carbocation is more stable than the primary (1o) carbocation.
(d) Another component of petrol, compound X, contains 92.3% carbon and 7.7% hydrogen by mass.
(i) Calculate the empirical formula of compound X.
You must show all your working.
(2)
ANSWER
Hence, empirical formula is CH.
(ii) A sample of compound X has a mass of 0.267g and vaporizes at 85.0°C and 104kPa to produce a gaseous sample with a volume of 98.0cm3
Calculate the molar mass of compound X.
You must show all your working.
[pV = nRT R = 8.31Jmol–1K–1]
(4)
ANSWER
Step 1: Calculating No. of Moles
Step 2: calculating Molecular mass
(iii) Deduce the molecular formula of compound X, using your answers to (d)(i) and (d)(ii).
(1)
ANSWER
Identifying the constant
Therefore,
