A-LevelChemistryHydrocarbons9701/22 · Oct/Nov 2021 · Q317 Marks

Compound T is an isomer of C₆H₁₂. [Figure 3.1 shows the structure of T, 2,3-dimethylbut-2-ene]

(a)Name T.[1]
(b)Draw the skeletal formula of a structural isomer of T that shows cis-trans (geometrical) isomerism.[1]
(c)(i)On the diagram, draw the orbitals that represent the pi (π) bond that is also present in T. [Figure 3.2][1]
(c)(ii)State the hybridisation of the two carbon atoms between which the pi (π) bond forms.[1]
(d)(i)Give the reagent(s) and conditions for reaction 1.[1]
(d)(ii)State and explain how 2,4-dinitrophenylhydrazine (2,4-DNPH) can be used to detect the presence of V as a product of reaction 2.[2]
(d)(iii)The progress of reaction 2 can be monitored by infrared spectroscopy. The absorption caused by O-H bonds is always present because water is used as a solvent. Identify two absorptions, and the bonds responsible for these absorptions, whose appearance will change significantly during the reaction.[2]
(e)(i)V and W are colourless and soluble in water. State what you would observe in reaction 3.[1]
(e)(ii)Reaction 3 is a redox reaction. Identify which of the reactants is reduced in this reaction.[1]
(e)(iii)Construct an equation for reaction 4. Use [H] in the equation to represent an atom of hydrogen from NaBH₄. C₆H₁₂O +[1]
(e)(iv)X is a mixture of two optical isomers. Draw the two optical isomers in the boxes provided.[2]
(e)(v)Both optical isomers of X can be dehydrated to form a single product, Y. Give the reagent(s) and conditions required for reaction 5.[1]
(e)(vi)Y can form an addition polymer Z. Draw one repeat unit of Z.[1]
(e)(vii)Reaction 6 does not proceed quickly at room temperature. Suggest why this is the case.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[1] (2,3-)dimethylbut-2-ene
[1] (Diagram of cis or trans isomer of C6H12 alkene, e.g., hex-2-ene or 3-methylpent-2-ene)
[1] (Diagram showing two p-orbitals overlapping to form a pi bond, perpendicular to sigma bond plane on the C=C carbons)
[1] sp²
[1] cold dilute acidified potassium manganate(VII)
[2] M1 (2,4-DNPH will produce a) red / orange / yellow precipitate
[2] M2 V has a carbonyl group
[2] M1 C-O in range 1040-1300 (cm⁻¹)
[2] M2 C=O in range 1670–1740 (cm⁻¹)
[1] yellow precipitate
[1] Iodine / I2
[1] C6H12O + 2[H] → C6H14O
[2] M1 One correct 3-dimensional representation of X
[2] M2 3-d structure which represents the enantiomer of their X
[1] heat AND concentrated H2SO4 / concentrated H3PO4 / concentrated sulfuric [(VI)] acid / concentrated phosphoric[(V)] acid
[1] (Diagram of repeat unit of polymer from 3,3-dimethylbut-1-ene, i.e., [-CH(C(CH3)3)-CH2-] )
[1] high activation energy