Edexcel Jun 2014 (IAL) Paper 2 Q21

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 21 In the human body, one chemical can have a number of different effects. An example is retinol, one of the substances in vitamin A. The structure of retinol is shown below. OH Retinol can be converted into retinal and a form of this is combined with an opsin molecule in the retina of the eye to make the light-sensitive pigment rhodopsin. When light enters the eye, the shape of the retinal molecule is changed, which results in it separating from the opsin molecule. The outcome of this separation is that we see light. Retinol can also be converted to retinoic acid, which is important in the body for the correct maintenance of mucous membranes. Failure to produce retinoic acid can lead to a condition known as dry eye, or xerophthalmia, which can result in blindness. The functions of this vitamin, and other nutrient molecules, illustrate the need for a healthy, balanced diet. (a) Retinol has an alcohol functional group. Classify the type of alcohol group in retinol and explain the meaning of the term functional group. (2)(b) Give the molecular formula of retinol. (2)18 *P42977A01824*<br /> (c) Retinol can be oxidized to the aldehyde, retinal. *(i) To illustrate the conversion of an alcohol to an aldehyde in the laboratory, a student suggested using the following apparatus and an excess of an oxidizing agent. Explain why this proposed method would have been unsuitable for the production of an aldehyde and explain what modifications are necessary for successful conversion. A new diagram is not required. water out pear-shaped flask open top Liebig condenser water in anti-bumping granules (3) HEAT(ii) The oxidizing agent suggested was sodium dichromate, Na2Cr2O7, in acidic solution. Complete the ionic half-equation below. Give the oxidation numbers of the chromium in the chromium species and state their colours. (5) Cr2O7 2(aq) + H+(aq) + e (cid:314) Cr3+(aq) + H2O(l) Oxidation NumberColour*P42977A01924* 19 Turn over<br /> *(iii) Describe two features on the infrared spectrum which could be used to determine whether the retinol has been completely converted to retinal. Select some of the following infrared data to support your answer. OH stretching in alcohols (variable, broad) at 37503200 cm1 OH stretching in carboxylic acids (weak) at 33002500 cm1 C=O stretching in aldehydes (strong) at C=O stretching in ketones (strong) at 17401720 cm1 17001680 cm1 C=O stretching in carboxylic acids, alkyl (strong) at 17251700 cm1 CH stretching in aldehydes (weak) at CH stretching in aldehydes (weak) at 29002820 cm1 27752700 cm1 (2)20 *P42977A02024*<br /> Retinal Retinol (iv) The structure of the retinal molecule that combines with opsin in the human body differs from the structure expected from the oxidation of retinol. The structures of retinal and retinol are shown below. OH As well as an oxidation, a structural change has occurred within the molecule. Circle only the part of the retinal molecule where this change has occurred. (v) The bond angle around each carbon atom in a carbon-carbon double bond is about 120. Explain the reason for this bond angle and state the name of the shape around each carbon atom. (1) (3) ReasonShape*P42977A02124* 21 Turn over<br /> (d) Complete the diagram below to show the skeletal formula of retinoic acid. *(e) Retinoic acid and retinol both have OH groups. Suggest one chemical reagent that you could use to test for the presence of an OH group which would work for both compounds. You may assume that both organic compounds are dissolved in suitable solvents. Give the positive observation for the test and state one necessary experimental precaution that you would make to reduce the risk from carrying out this test. (1) (3) ReagentObservationPrecaution(Total for Question 21 = 22 marks) TOTAL FOR SECTION C = 22 MARKS TOTAL FOR PAPER = 80 MARKS 22 *P42977A02224*<br />

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