IJC H2 Chem 2012 Prelim P3 Soln

2012 H2 Prelim 2 Essay solutions_13 aug   2    Step 2 (ii) The bromine atom in the product from b(i) is very reactive towards nucleophilic reagents. The bromine atom in 1–bromopropene is unreactive towards nucleophilic reagents. Suggest an explanation for the unreactivity of the bromine atom in 1–bromopropene. [2] In 1-bromopropene, the p orbital of Br overlaps with the π orbital of adjacent C atom, strengthening the C – Br bond. Thus, the C – Br bond is too strong to be broken and hence unreactive towards nucleophilic reagents. OR In 1-bromopropene, the C atom is adjacent to the double bond and hence less electron deficient and thus less susceptible to nucleophilic attacks. (c) para-Methoxyamphetamine first came into circulation in the early 1970s and went by the street names of "Chicken Powder" and "Chicken Yellow" and was found to be the cause of a number of drug overdose deaths in the United States and Canada at that time. It can be synthesised by the following route. (i) Suggest reagents and conditions for steps II and III. II: CH3Br / CH3I/ CH3Cl, heat with reflux

2012 H2 Prelim 2 Essay solutions_13 aug   3    III: NH3 in ethanol, heat in a sealed tube (ii) What type of reaction is step II? Nucleophilic substitution (iii) Suggest why compound J is converted into K before step II is carried out. To generate a stronger nucleophile, phenoxide ion. (iv) What type of stereoisomerism does para-methoxyamphetamine exhibits? Draw the structures of the stereoisomers of para-methoxyamphetamine. Optical isomerism C CH2 H NH2 CH2N CH2 H mirror CH3 OO CH3 [6] (d) Compound P is a neutral, sweet-smelling liquid with molecular formula C 5H8O2. It reacts with hot sulfuric acid to give a single compound Q, C 5H10O3. Q has two stereoisomers and gives a pale yellow precipitate with alkaline iodine and compound R. Deduce the structures of P, Q and R. P undergoes acidic hydrolysis with hot sulfuric acid to give Q. P is an ester. Q contains CH3CH(OH) since it undergoes mild oxidation with alkaline iodine. Q only contains 1 chiral carbon since it has two stereoisomers.

2012 H2 Prelim 2 Essay solutions_13 aug   4    [5] [Total:20]

2012 H2 Prelim 2 Essay solutions_13 aug   5      2 (a) When a precipitate is formed, ∆Gθ ppt, in kJ mol -1, is given by the following expression. ∆Gθ ppt = 1000 K log 2.303RTsp (i) Given that the Ksp value of BaF 2 is 1.70x10 -6 at 298K, calculate ∆Gθ ppt, in kJ mol-1, for BaF2. ∆Gθ ppt = [2.303 x 8.31 x 298 x log (1.70x 10-6)] ÷ 1000 = -32.9 kJ mol-1 (ii) The standard enthalpy change of formation of BaF 2 is −858 kJ mol-1. Use your answer in (a)(i) to calculate ∆Sθ ppt, in J mol -1 K-1 for the formation of the precipitate at 298K. ∆G = ∆H –T∆S -32.9 = -858 – (298) ∆S ∆S = -2.77 kJ mol-1K-1 = -2770 J mol-1K-1 (iii) E