1. Benzene reacts with concentrated nitric acid in the presence of concentrated sulphuric acid at about 50 º in an electrophilic substitution reaction to give nitrobenzene. (a) Give the equation representing the overall reaction. Give the equation representing the formation of the electrophile. (b) Give the mechanism for the reaction of the electrophile with benzene. In an experiment to determine the kinetics of this reaction, it is found that the attack of the electrophile on the benzene ring is the rate determining step. The following data shows the effect of changing the concentrations on the rate: [benzene] (relative) [electrophile] (relative) Rate (relative) 1 1 1 1 2 2 2 1 2 2 2 4 What is the order with respect to benzene.. the electrophile? Write a rate equation for the reaction. In some electrophilic substitution reactions of aromatic compounds, the rate determining step is the production of the electrophile. In such cases what would be the order of the reaction with respect to the aromatic compound? NT Exampro 1
(c) The equation representing the hydrogenation of ethene is 2 2 + 2 = 120 kj mol 1 Assuming that benzene consists of a ring with three double bonds, predict the enthalpy change for the reaction + 2 The enthalpy of hydrogenation of benzene is actually 205 kj mol 1. What can you deduce from this and your answer to part about the stability of the benzene ring? Use an enthalpy level diagram to illustrate your answer. The compound cyclo octatetraene has an enthalpy of hydrogenation of 480 kj mol 1; the molecule, unlike that of benzene, is not flat. Suggest in terms of the possibility of orbital overlap why cyclo octatetraene does not show the same type of stability as benzene, despite superficial similarities in their structures. (Total 14 marks) 2. (a) Three isomers A, B and B have the molecular formula 4 8 O. All three compounds give an orange precipitate with 2,4-dinitrophenylhydrazine reagent. B and also give a silver mirror when warmed with ammoniacal silver nitrate solution. Write the structural formulae of A, B and. Substance A Substance B NT Exampro 2
Substance (b) Substance A reacts with the Grignard reagent, 2 5 MgBr Give the equation for the preparation of this Grignard reagent... State the conditions for this preparation..... Write the structural formula of the product obtained when this Grignard reagent reacts with substance A. NT Exampro
(c) 2 5 MgBr reacts with carbon dioxide to form the acid 2 5 OO. This acid can be converted to propanamide in a two step process. 2 5 OO step 1 2 5 Ol step 2 2 5 ON 2 State the reagent required for each step. Step 1. (d) Step 2. 2 5 MgBr also reacts with ethanal to form substance D, which exists as a pair of optical isomers. Draw the structural formulae of these two isomers and use them to explain why these isomers exist. What is the difference in property between these isomers?.. (e) Write down the name and the structural formula of the organic compound formed when substance D is heated under reflux with a solution of potassium dichromate(vi) in dilute sulphuric acid. Name.. Structural formula. (4) State the colour of the solution remaining after this reaction... (Total 17 marks). (a) Write equations to show the reactions of the amino acid alanine, (N 2 )OO, NT Exampro 4
with: l.... NaO (b) (c).... Explain why alanine has a relatively high melting temperature (290 )................... Alanine exists as two optical isomers. Draw diagrams to show the structures of the two optical isomers. Explain how separate pure samples of each optical isomer can be distinguished from each other............. NT Exampro 5
(d) A mixture of isomeric alkenes is obtained when butan-2-ol is dehydrated. Draw diagrams to show the two structural isomers obtained when butan-2-ol is dehydrated. One of the above structural isomers can itself exist as two different stereoisomers. Draw diagrams to clearly illustrate these two stereoisomers, and name this type of stereoisomerism. Type of stereoisomerism.. Diagrams 4. The pain-relieving drug ibuprofen has the formula (a) 2 O A molecule of this compound is chiral. Explain, with the aid of diagrams, how two optical isomers result from chiral a centre. Identify the chiral centre in such a molecule and explain how these isomers could be distinguished from each other. 2 (Total 1 marks) (4) NT Exampro 6
(b) ompounds A and B, shown below, are used as intermediates in the manufacture of ibuprofen. O 2 2 A B O (c) Suggest how infra-red spectroscopy could be used to identify an impurity in a sample of ibuprofen which is known to be one of these two compounds. A table of infra-red absorbencies is given below. Bond Assignment Wavenumbers/cm 1 - alkanes alkenes, arenes 2850-2950 000-100 benzene ring arenes 1450-1650 =O O- aldehydes, ketones, esters, carboxylic acids free hydrogen bonded in alcohols or phenols hydrogen bonded in carboxylic acids 1680-1750 580-670 20-550 2500-00 Suggest a simple chemical test which would distinguish between A and B. Treatment of either A or B if heated with a mixture of aqueous sodium hydroxide and potassium manganate(vii) solution would produce the same compound. Suggest the structure of this compound. A tablet of ibuprofen contains a very small quantity of the drug and the remainder of the tablet material is unreactive. In an analysis 50 tablets were reacted with 100.0 cm of 1.00 mol dm aqueous sodium hydroxide, an excess. The ibuprofen reacted as a weak acid. When the reaction was complete, the remaining aqueous sodium hydroxide was titrated with 2.00 mol dm hydrochloric acid. 25.75 cm of the acid were required for neutralisation. alculate the mass, in mg, of ibuprofen in each tablet. (M r for ibuprofen = 206) alculate the number of molecules of ibuprofen that this tablet represents. (Avogadro constant h = 6.02 102 mol 1) (5) NT Exampro 7
(d) Ibuprofen is almost insoluble in aqueous solutions at p but if the p is raised to 8 then the ibuprofen becomes reasonably soluble. Account for these observations. (4) (e) Under the appropriate conditions ibuprofen reacts with ethanol to produce a substance with a characteristic odour. Identify the functional group in ibuprofen that would react with the ethanol. Suggest the conditions needed for the reaction to take place and identify the product. (Total 25 marks) 5. (a) 2-aminobutane, 2 (N 2 ), is a chiral molecule that can be optically active. It can be made from 2 (ON 2 ). What is a chiral molecule? ow is optical activity detected experimentally? (b) State the reagents and conditions necessary to convert 2 (ON 2 ) to 2-aminobutane....... (c) 2 (ON 2 ), D, can be made from 2-bromobutane in four steps shown below. i) O 2 Mg ii) l(aq) Br Grignard A B 2 ON 2 PL 5 2 D NT Exampro 8
Give the structural formulae of the three intermediate compounds A, B and. A B (d) Grignard reagents are often made in a solvent of dry ethoxyethane. Why must the ethoxyethane be dry? Give the reagents and conditions for the conversion of to D. Butanone can be made from 2-bromobutane by a synthetic route involving two steps, the first using aqueous sodium hydroxide and the second potassium dichromate(vi) solution acidified with dilute sulphuric acid. Give the structural formula of the intermediate compound in this synthetic route. NT Exampro 9
Butanone reacts with 2,4-dinitrophenylhydrazine solution but not with Fehling s solution. Why is this? (iv) Butanone also reacts with iodine in sodium hydroxide solution. What structural feature of butanone is shown by this reaction? Give the structural formulae of both the organic products from the reaction in. (Total 17 marks) NT Exampro 10