18. Carboxylic acids and derivatives
A section of Chemistry, 9701
Listing 10 of 202 questions
Lactic acid, 2-hydroxypropanoic acid, CH3CH(OH)CO2H, occurs naturally in sour milk and in our muscles when we take hard exercise. Lactic acid is chiral and shows stereoisomerism. Draw fully displayed structures of the two optical isomers of lactic acid. Indicate with an asterisk (*) the chiral carbon atom in the lactic acid molecule. Lactic acid may be synthesised from ethanol by the following route. CH3CH2OH step 1 CH3CHO step 2 CH3CH(OH)CN step 3 CH3CH(OH)CO2H Give the reagentand essential conditionfor each step. reagentconditionstep 1 step 2 step 3 During exercise, lactic acid is produced in our muscles from pyruvic acid, CH3COCO2H. This reaction occurs in the presence of the enzyme lactic acid dehydrogenase. What type of chemical compound is the enzyme lactic acid dehydrogenase? How would you detect a small quantity of pyruvic acid in a sample of lactic acid? State the reagentyou would use and what would be seen in your test. reagentobservation How would you detect a small quantity of lactic acid in a sample of pyruvic acid? State the reagentyou would use and what would be seen in your test. reagentobservation What chemical reagent would be used to convert pyruvic acid into lactic acid? CH3COCO2H CH3CH(OH)CO2H
9701_s10_qp_23
THEORY
2010
Paper 2, Variant 3
View
Although there are many different types of food eaten around the world, animal fats and/or vegetable oils are commonly used in cooking. Animal fats and vegetable oils are usually glyceryl esters, that is esters of glycerol, propane-1,2,3-triol. CH2OH ⏐ CHOH ⏐ CH2OH Many animal fats contain esters of stearic acid, CH3(CH2)16CO2H. Vegetable oils often contain esters of oleic acid, CH3(CH2)7CHCH(CH2)7CO2H. Draw the structural formula of the glyceryl ester formed when one molecule of glycerol is completely esterified with stearic acid. What reagentwould you use, in a school or college laboratory, to obtain a small sample of oleic acid, C17H33CO2H, from the glyceryl ester present in a vegetable oil? Oleic acid is the cis isomer and elaidic acid the trans isomer of CH3(CH2)7CHCH(CH2)7CO2H. By using this formula, draw the structural formula of elaidic acid, clearly showing the stereochemistry. Oleic and elaidic acids are examples of mono-unsaturated acids. Many vegetable oils contain esters of polyunsaturated fatty acids. Such oils are often hydrogenated to form esters containing saturated or mono-unsaturated fatty acids. Suggest the meaning of the term polyunsaturated fatty acid. What reagent and conditionare used for the hydrogenation of an unsaturated fatty acid? reagent condition In cooking, unsaturated fats are often oxidised to give aldehydes or ketones. Give the structural formulae of the two aldehydes formed by the partial oxidation of the unsaturated fat below. In the structure, X, represents the rest of the fat molecule. CH3(CH2)7CHCH(CH2)7X Name the reagent you would use to show that the product contained either an aldehyde or a ketone. What change would be seen? reagent observation What reagent would you use to confirm the presence of an aldehyde? What change would be seen? reagent observation Animal fats and vegetable oils can become rancid because of oxidation. The rancid fat or oil has an unpleasant smell and taste. Antioxidants are used to prevent the spoilage of many foodstuffs by oxidation. One antioxidant that is widely used is vitamin C, ascorbic acid. HO HO H O O ascorbic acid OH HO How many chiral carbon atoms are present in one molecule of ascorbic acid? If none, write ‘none’. The ascorbic acid molecule contains three functional groups. Two of these are alcohol (primary and secondary) and alkene. What is the name of the third functional group?
9701_s11_qp_22
THEORY
2011
Paper 2, Variant 2
View
Questions Discovered
202