HCI_H2_CHEM_P3 Prelim

This document consists of 10 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 13S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9647/03 22 September 2014 2 hours INSTRUCTIONS TO CANDIDATES Write your name and class on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Begin each question on a new piece of paper. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. Write down the question numbers for the questions that you have attempted on the cover page provided.

2 © Hwa Chong Institution 2014 9647 / 03 / C2 Prelim 2014 Answer any four questions. 1 (a) Describe and explain the trend observed for the reactions of chlorine, bromine and iodine with hydrogen. [2] (b) Bromoalkanes can be synthe sised by reacting alkanes and bromine in the presence of UV light or heat. One example of such reaction is shown below: + Br2 UV or heat Br + HBr reaction 1 (i) Describe the mechanism for reaction 1. Indicate any unpaired electrons using a dot (). (ii) State how monosubstituted product can be favoured for reaction 1. [4] (c) Another way to synthesise bromoalkanes is by reacting alkenes with hydrogen bromide, as shown in reaction 2 below. This is known as a radical addition reaction, where HBr is added across the alkene in a reaction involving radicals. + HBr Br reaction 2ROOR (R = alkyl group) major product (i) By considering the electrophilic addition of HBr on alkenes, s uggest why the major product of reaction 2 is unusual. The compo und ROOR in reaction 2 is used to generate a bromine radical, which will attack the alkene C=C bond to produce a radical intermediate as shown below: + Br Br intermediate The radical intermediate will then abstract a hydrogen atom from HB r to form the major product in a subsequent step. (ii) Explain why the bromine radical would attack the C=C bond. (iii) Draw the structure of another possible radical intermediate . C ompare the relative stabilities of the two radical intermediates to account for the major product formed. (iv) Hence, propose a three-stage reaction scheme for the following transformation, naming the reagents and conditions used in each step. Draw the str