SRJC H2 Chem 2012 Prelim P3 Soln

SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CLASS CHEMISTRY 9647/03 Preliminary Examination Paper 3 Free Response Questions 21 August 2012 Suggested Solutions 1 (a) Aqueous sodium hydroxide is an important reagent in many organic reactions where the hydroxide ion can act as both a base and a nucleophile. In the Reimer-Tiemann reaction, phenol is heated with chloroform in the presence of aqueous sodium hydroxide to give 2-hydroxybenzaldehyde, compound B . The reaction involves the formation of a reactive intermediate, CC l2, through an acid- base reaction. CHCl3 + OH- ® CCl2 + H2O + Cl – chloroform OH OH OH + CCl2 CHCl2 NaOH (aq) CHO I heat II A B (i) State the type of reaction that occurred in reaction I. Electrophilic substitution (ii) Suggest a chemical test to differentiate compounds B and A under room conditions. Add 2,4-DNPH to B and A separately. B: Orange precipitate formed A: No orange precipitate formed Note: Tollens’ reagent and aqueous NaOH tests are not allowed, as both require heating, which is not “room conditions”

(iii) Compound B can undergo further reaction to form the product below. Name and describe the mechanism involved, using curly arrows to show the movement of electrons and indicating all charges. Mechanism: Nucleophilic addition OH C O H OH C H OH CN (iv) Comment on the relative acidity of phenol and compound B. [9] Presence of electron withdrawing –CHO attached on the benzene ring. Negative charge of the phenoxide ion of compound B will be further dispersed. Phenoxide ion of compound B is more stabilised. Compound B is a stronger acid as compared to phenol. (b) Besides its exten sive use in organic chemistry, aqueous sodium hydroxide is also used in photographic development. The black-and-white (B/W) photography process involves four steps: Step 1 Photo-taking: Interaction of light with silver halides (AgX) on film. Step 2 Developing: Redox reactions to develop the actual image. Step 3 Fixing: Changing the solubility of chemical compounds with different conditions to preserve the image on the film. Step 4 Printing: Shining light through the film onto photosensitive paper, producing the final photograph using the same chemistry as step 1 to 3. When light is shone on the film during the phototaking process, a very small number of X- ions in AgX are oxidised. OH C OH CN H B OH CHO d+ d- CN- . . slow OH C O H CN - fast H–CN + CN- . .

(i) A small amount of silver is produced as a resu lt of the oxidation reaction of X-. The silver produced catalyses the reduction of the surrounding AgX to black silver metal in the presence of a reducing agent, D, during the developing process.