CHEM120 - ORGANIC CHEMISTRY WORKSHEET 1

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1 EM120 - RGANI EMISTRY WRKSEET 1 Some of the objectives To understand and know the hybridization concept Be able to distinguish different geometries, including basic bond lengths and angles within organic structures Name organic molecules Be able to identify different functional groups and name them. Know how to draw organic structures given the name or vice-versa, i.e. given the structure you should be able to name the compound Be able to use line notation and the condensed form to draw your structures as well as being able to draw the complete structure with all the atoms shown. You should be able to draw and name different kind of alcohols or amines, i.e. primary (1 ), secondary (2 ), tertiary (3 ) and quaternary (in the case of amines). Problems 1. Answer the following questions with respect to compounds (A) and (B): 5 (A) ( 3 ) ( 3 ) (B) 2 () (a) Draw the bond-line notation for the two compounds. (b) Identify the following substituents on structure 1(A): (i) an isopropyl group 1 Dr. V.. Nyamori

2 (ii) a tert-butyl group (iii) a sec butyl group (c) Identify all the functional groups in compounds (A) and (B). (d) Identify the hybridization of the numbered atoms in structures (A) and (B). 2. Name the compounds (a) to (d) below according to the IUPA rules. (a) 2 ( 3 ) 2 ( 2 3 ) 3 (b) (c) (d) 3. Write (i) structural formulae and (ii) draw the bond line notation for the following compounds: (a) 2,2,4-trimethylhexane (b) 3-ethyl-2,4,5-trimethyloctane (c) 2-bromo-2,4,6-trimethyloctane 4. (a) Explain why 2-sec-butyloctane is an incorrect IUPA name. (b) (i) Write a correct IUPA name and (ii) draw the bond-line notation for the intended compound. (c) Identify a primary (1 ), secondary (2 ), tertiary (3 ) and a quaternary (4 ) carbon atom in ( 3 ) 2 2 ( 3 ) 3. 2 Dr. V.. Nyamori

3 EM 120: RGANI EMISTRY: WRKSEET 2 Some of the objectives Be able to identify and name different isomers by using acceptable organic nomenclature rules, i.e. for constitutional, geometric and optical isomers. Be able to draw by using a perspective formula, Newman projections and Fischer projections. Problems 1. Name the following compounds by using the IUPA system. If any alcohols are present, indicate whether they are primary, secondary or tertiary. 2. Draw structures for: (a) 4-isopropyldecane (b) 2,2-dichloropropane 3 Dr. V.. Nyamori

4 (c) 3-heptyne (d) 5,5-dimethylnonane (e) 2-butanol (f) 2-methyl-2-propanol (g) 3-methylpentanal (h) 3-hydroxy-2-hexanone (i) octanoic acid (j) 4-bromo-5 hydroxynonanoic acid 3. There are four structural isomers with the molecular formula 4 9 Br. Draw structures for the four isomers. ne of these can exist as a pair of optical isomers which one is this? 4. The substance 2-butanol displays optical isomerism. Indicate the chiral carbon with a *. Draw three dimensional representations of the two enantiomers and label the isomers clearly. 5. Draw structures for the cis- and trans-isomers of 1-chloro-1-propene. 6. Determine the configuration of each of the following alkenes: 7. Draw the structure of trans- 1,3- dichlorocyclobutane. 4 Dr. V.. Nyamori

5 8. Explain why the cyclohexane boat conformation is much less stable than the chair conformation. 9. Find the stereogenic center (chiral centre) in i) 2 I 2 3 ii) BrF 3 iii) 3-methylcyclohexene iv) 2,3-dibromobutane 10. (a) Which of the following compounds is chiral? i) 1-bromo-1-phenylethane ii) 1-bromo-2-phenylethane (b) Draw three-dimensional structures (perspective formulae) for the two enantiomers of the chiral compound in problem 10(a). 11. The figure below shows Fischer projection structures of 2,3-dichlorobutane. lassify the following molecules of as either enantiomers, chiral, achiral or meso forms. Locate the planes of symmetry, if any, and assign the corresponding configurations (R) or (S) l l l l l l l l Dr. V.. Nyamori

6 EM 120: RGANI EMISTRY: WRKSEET 3 Some of the objectives Know what homolytic and heterolytic fission is and when each would occur. Know the significance of a full arrowhead and half an arrowhead and when to use each. You must know what a carbocation is. Be able to use arrows to show bond cleavage in a molecule. Know how radicals react to form bonds. Know how cations and anions react to form bonds. Know what nucleophiles and electrophiles are and be able to give examples of each. Be able to identify nucleophiles and electrophiles in a reaction. You must know how the carbonyl group reacts with nucleophiles and be able to write a mechanism for this reaction. Problems 1. Write reactions to show bond cleavage in each case: 3 3 F l l 2. ow many electrons are around carbon in each of the following species: Dr. V.. Nyamori

7 3. Identify the electrophile and nucleophile in the following reaction: Bl 3 + N 3 l 3 BN 2 4. Identify each of the following species as an electrophile or a nucleophile: I -, +, BF 3, -, 3 +, 3 -, 2 5. Write down the mechanism for the following reaction and identify the product that forms: 1. N Dr. V.. Nyamori

8 EM 120: RGANI EMISTRY: WRKSEET 4 Some of the objectives Be able to write a mechanism for the halogenation of alkanes. Be able to explain this free radical mechanism and be able to identify the initiation, propagation and termination steps. Be able to explain why a radical is a highly reactive species. Be able to explain the order of stability for 1, 2 and 3 radicals. Be able to explain how different groups stabilise or destabilise the radical. Be able to write a mechanism for the hydrohalogenation of an alkene. Be able to predict and explain why, which of two possible products will be formed in the greatest yield. Be able to explain why and know under what circumstances, enantiomers will form in electrophilic addition reactions. Know what substrates and reagents are involved in an ozonolysis reaction. Be able to predict the products of an ozonolysis reaction. You must know the conditions under which ozonolysis reactions occur. Problems 1. Write down the mechanism for the formation of 2 Br from the reaction of Br 2, 3 and light (hv). Identify the initiation, propogation and termination steps and briefly explain what occurs at each step. 2. What order of stability would you predict for a) 1, 2 and 3 carbocations; b) 1, 2 and 3 carbanions. Explain your answer in each case. 3. Label each of the following carbocations as 1, 2, 3 or methyl carbocation and then arrange them in order of increasing stability (least stable to most stable). 3 + ; ( 3 ) 3 + ; 2 + ; ( 3 ) Dr. V.. Nyamori

9 4. Label each of the carbon radicals as 1, 2, 3 and then arrange them in order of increasing stability (least stable to most stable). ( 3 ) 3 2 ( 3 ) 2 5. Which of the following carbocations is the more stable, l or 2 +. Explain your answer. 6. Write down the mechanism for the following reaction and predict which of the two possible products will be formed in the greatest yield. Write down the structures of the two possible products and explain your answer. 2 + Br 7. For each of the following reactions listed below, (a) write down the mechanisms, (b) write down the major and minor products in each case, and (c) indicate which of the products in (b) above would be enantiomers. 3 Br 3 Br Br 8. The following alkene is first treated with a) 3 at -78 followed by Zn and 2 and then with b) 3 at -78 followed by Write down the products that would form in each case. 9 Dr. V.. Nyamori

10 9. Write down the reagents and reaction conditions for the following reactions: Write down the structure of the missing substrate in the following reactions: 1. 3, -78 o 2. Zn, , -78 o Dr. V.. Nyamori

11 Some of the objectives EM 120: RGANI EMISTRY: WRKSEET 5 Be able to recognise when an oxidation or a reduction occurred by looking at the substrates and the products of the reaction. You must know the oxidising and reducing agents described in the module and what substrates they will oxidise or reduce as well as what products will be formed in each case. Problems 1. For each of the following reactions, suggest suitable reagents that could be used. Indicate whether the reaction is an oxidation or a reduction reaction Dr. V.. Nyamori

12 2. Write down the products for the following reactions. LiAl 4 2 r 3 / + 2 / + 2 /Pd l 3 Br 2 2 /Lindlar's catalyst 3 2 /Pd 12 Dr. V.. Nyamori

13 EM 120: RGANI EMISTRY: WRKSEET 6 Some of the objectives You must know what general elimination reactions and what S N 1 and S N 2 reactions stand for. ow many molecules are involved in the ratedetermining step of each of these S N 1 and S N 2 reactions. You must know the mechanisms for S N 1 and S N 2 reactions. You must know on what substrates each of these reactions can occur and what mechanism a strong or a weak base would favour. Be able to explain why S N 1 mechanisms cannot occur for primary alkyl halides but can occur for secondary and tertiary alkyl halides. Know how (a) the carbocation and (b) an electronegative atom affect the acidity of the alpha hydrogen and what the consequences of this are. Be able to identify the nucleophile, the electrophilic centre and the leaving group in a substitution reaction. Be able to determine which of two groups is the better base and be able to predict whether a substitution reaction will occur or not. If a reaction can occur, you must be able to predict the product. For nucleophilic substitution reactions, you must be able to predict when inversion of configuration takes place and when retention of configuration takes place. Problems 1. Write down the following elimination reactions and predict the product in each case. 2 2 Br 3 Br Na Dr. V.. Nyamori

14 2 2 3 Br Na 2 3 Br 2 ( 3 ) Br 2 2. Write down S N 2 mechanisms for the following reactions: l + Na 2 l + NaN 2 3. Draw the transition state for the reactions in 2 above. 4. ow many reactants are involved in the formation of the reaction intermediate in (a) an S N 1 type reaction; (b) an S N 2 type reaction? 5. Which of the following reactants will be able to undergo S N 1 type reactions? Give reasons for your answer. (a) 2 l (c) ( 3 ) 2 l (b) ( 3 )l (d) ( 3 ) 3 2 l 6. Write down the products of the following substitution reactions. 14 Dr. V.. Nyamori

15 NaBr + 3 Na Br 2 N 2 + l 3 7. Indicate which of the products in 6 above would occur with retention of configuration. Explain why. 15 Dr. V.. Nyamori

16 EM 120: RGANI EMISTRY: WRKSEET 7 Some of the objectives You must know why carboxylic acids, alcohols and terminal alkynes are weak acids and the consequences this has in organic reactions. Be able to predict when a strong base is required to remove the hydrogen of a weak acid. Be able to explain why alcohols and amines are bases in organic chemistry and what consequences this has for organic reactions. You must know why protonating the oxygen in alcohols leads to substitution reactions whereas protonating the nitrogen in amines does not. Problems 1. In which of the two molecules below would a strong base be needed to remove the proton. Explain your answer. ( 3 ) 3 pk a = 18 pk a = Add reagents to carry out the following reaction: 2 Br ow could the following molecule be synthesised from the given substrate? Write down reactions to show your answer Dr. V.. Nyamori

17 4. Verify that trifluoracetic acid (pk a 0.2) is a stronger acid than F (pk a 3.2). F 3 trifluoracetic acid 5. Which of the following molecules would be a better nucleophile, 3 N 2 (pka of 3 N 3 + is 10.7) or 3 (pk a of is -2.5)? 6. Would ethanol react with sodium ethoxide under neutral conditions? If not, explain why, and suggest what can be done to make the reaction work. 17 Dr. V.. Nyamori