molecular formula of X is C 6 H 4 C l 2 1 c ½ for each structure ½ for each

Molecular formula of x is c 6 h 4 c l 2 1 c ½ for

This preview shows page 35 - 39 out of 53 pages.

molecular formula of X is C 6 H 4 C l 2 . [1] (c) [½] for each structure, [½] for each name. Comments: Some students would have struggled with the naming of the compounds using the IUPAC convention. A common mistake was to draw the same structure with a different orientation to pass it off as a different compound. (d) [1] for Y and [1] for equation. Comments: For mononitration of compound Y to give only one compound, it must possess a high degree of symmetry and only one possible chemical environment for the substitution of the nitro group. Common mistake: Answer produced a detailed description of the mechanism instead of an equation which showed the reaction with the reagent given.
Image of page 35
H2 Chemistry 9647 2015
Image of page 36
H2 Chemistry 9647 2015 PAPER 3 1 (a) (i) ROH + Na RONa + ½H 2 2ROH H 2 Amount of menthol = 2 x 1.32 x 10 2 = 2.64 x 10 2 mol Mass of menthol = 2.64 x 10 2 x 156 = 4.118 g % by mass of menthol in peppermint = 10 4.12 x 100 % = 41.2 % Note: Students need to recognise mole ratio = 1 menthol : ½ H2 and not 1:1. (ii) Chiral centres: 3 Number of optical isomers = 2 3 = 8 NEW SYLLABUS: enantiomers instead of optical isomers Note: General formula for number of stereoisomers = 2 n where n is the number of chiral centre or double bonds with different R groups. Common mistakes are to create six isomers (3 x 2) or nine isomers (3 2 ) (b) (i) (ii) Energy / kJ mol 1 Reaction pathway Menthone + Hydrogen Menthol H = −80 kJ mol 1 E a = +42 kJ mol 1 Nickel catalysed Platinum catalysed
Image of page 37