Type of Organic Questions Transition to Endemic Phase Workshop 04062022

ORGANIC CHEMISTRY WORKSHOP: PREPARING FOR FINAL EXAM IN TRANSITION TO ENDEMIC PHASES ‘I CAN SCORE CHEMISTRY’

TYPES OF QUESTIONS Interconversions of Organic Compounds Road-Maps Problems Distinguishing Organic Compounds Organic Acid-Base Reaction Mechanism

Interconversions of Organic Compounds This type of questions needs a broad coverage of organic reactions. Conversions not only test students on usage of reagents and conditions but also how such reagents and conditions may be change as need arises.

Interconversions of Organic Compounds Conversions are good exercises for students to practice on reactions. Since conversions involve transformations between functional groups, it will help you interrelate the various topics that you have studied.

Interconversions of Organic Compounds a) Conversion flow chart b) Conversion without guidance c) Conversion using Grignard Reagents

a) Conversion flow chart ❑ For each step in the conversion flow chart, you have to state: i. reagent(s) used, ii. the reaction conditions: ✓ Temperature Eg: ➢ 0-5 ºC for reaction of amines with HONO ➢ Reflux for reaction of hydrolysis of haloalkanes with NaOH(aq) ➢ Heat @ ∆ for oxidation of alkenes with concentrated and acidified KMnO4.

✓ Catalyst, if required. E.g: ➢ Ni for the hydrogenation of alkenes. ✓ Solvents, if required. E.g: ➢ Br2 in CCl4 for the halogenations of alkanes. Sometimes the reagent and condition(s) are given. You must give the structure of compound(s) that can react with the given reagent(s) and condition(s).

Example of question: I CH3CH2OH II III CH3CH(OH)CN CH3CH2Br CH3CHO IV CH3CH(OH)COOH II III I IV

b) Conversion Without Guidance ❑ This is a synthesis questions, only the starting material are given. You have to plan the synthesis to obtain the required product. Method of answering: a) Explain your synthesis step by step. b) Write down how you proceed with Step I by: ✓ Giving the reagents used, reactions conditions and the product obtained ✓ Writing a reaction equations for this step. c) Repeat (b) for Step II, III and so on, until the final product is obtained.

Example of question: Show how the following compound can be synthesised in the laboratory starting with methylbenzene.

MIND MAP OF AROMATIC COMPOUNDS TABLE 6.3 IN PAGE 232

Example of question: Show how the following conversions can be performed.

b) Conversion Using Grignard Reagent ❑ Grignard reagents react with carbonyl compounds to form alcohols. Methanal forms primary alcohols, other aldehydes form secondary alcohols, while ketones will form tertiary alcohols. Grignard reagents can lengthen the carbon chain of the resultant alcohols.

Example of questions: Outline with a flow chart to show how the following synthesis can be performed by using a Grignard reagent. methanol 2- propanol

Road-Maps Problems 1. Read the whole question carefully. 2. After getting a definite clue, you can work your way backwards to solve the other unknowns. ✓ Give your reasoning in identifying an organic compound. It should be based on descriptions given in the question about the reactions of the compound, any products obtained or from the results of a chemical test performed on the compound. ✓ Draw a structural formula for each unknown. ✓ Write equations for all reactions mentioned in the question.

Example of question: Compounds A and B are isomers with molecular formula C9H12O. A is optically active while B is optically inactive. On treatment with hot KMnO4/H+, A gives benzoic acid but B cannot be oxidized. Draw the structure of A and B.

Distinguishing Organic Compounds • Guidelines: a) State the reagents and conditions for the test. ➢ The reagent in a test must produce a change in one compound, for example a change in colour, but not in other compound) b) Describe the changes you would observe for the compound that is tested. c) Write a balanced equation for the test (if required). d) Deny the other compound shows these changes.

Example of question: • Consider the following pair of compounds. (1 mark) i. Name compound C. ii. Identify a reagent which could be used in a test-tube reaction to distinguish between C and D. In each case, state what you would observe with C and D. (3 marks)

Example of question: ii. Identify a reagent which could be used in a test-tube reaction to distinguish between C and D. In each case, state what you would observe with C and D. (3 marks)

Example of question: ii. Identify a reagent which could be used in a test-tube reaction to distinguish between C and D. In each case, state what you would observe with C and D. (5 marks)

Organic Acids - Bases This type of questions needs you to explain about the factors that influenced the acidity of organic compounds (alcohol, phenol and carboxylic acid) and basicity of organic compounds (amines) There are TWO (2) factors that will influence the acidity and basicity of organic compounds: 1. Resonance effect 2. Inductive effect.

Example of Question Arrange the following compounds in order of increasing acidity. Explain. Ethanoic acid, phenol, 2-chloroethanoic acid, ethanol

ANSWERS Increasing acidity: Ethanol, phenol, ethanoic acid, 2-chloroethanoic acid ✓ Ethanol is the least acidic because it cannot form any resonance structure. ✓ Phenol is more acidic than ethanol because the phenoxide ion is stabilized by the delocalization of electrons in the benzene ring. ✓ Ethanoic acid is more acidic than phenol and ethanol because the electrons are delocalized between the two electronegative oxygens. ✓ 2-chloroethanoic acid is the most acidic because it contains Cl which is an EWG which withdraw the electrons form carboxylate ion and stabilize it.

Example of Question Arrange the compounds D, E and F in order of decreasing pKa value. COOH H3C COOH Compound D Compound E (Benzoic acid) (p-methylbenzoic acid) Cl COOH Compound F (p-chlorobenzoic acid)

ANSWERS Ka ↑, pKa ↓  the more acidic Decreasing pKa value (increasing Ka @ increasing acidity): Compound E, compound D, compound F ✓ Compound E is the least acidic because it contains methyl which is an EDG which donates more electrons to the benzoate ion and destabilize it. ✓ Compound D does not contain any substituent group (reference) ✓ Compound F is the most acidic because it contains Cl which is an EWG which withdraw the electrons form benzoate ion and stabilize it.

Example of Question ➢Arrange aniline, ethanamine and N,N-diethylethanamine in a descending order of basicity. Explain.

ANSWERS Descending order of basicity: N,N-diethylethanamine, ethanamine, aniline ✓ N,N-diethylethanamine is the most basic because it contains greater number of alkyl groups (ethyl) which is an EDG that donate electrons to the alkylammonium ion and stabilizes it. ✓ Ethanamine is more basic than aniline because the alkyl groups (ethyl) which is an EDG that donates electrons to the alkylammonium ion and stabilizes it. ✓ Aniline is the least basic because the lone pair electron at N is delocalized inside the benzene ring, hence becomes less available to make bond with H+, therefore less basic.

Example of Question ➢Arrange the compounds below in order of increasing Kb value. NH2 NH2 NH2 NH2 cyclohexanamine aniline CH3 NO2 p-methylaniline p-nitrolaniline

ANSWERS Kb ↑, pKb ↓  the more basic Increasing Kb value (increasing basicity): p-nitroaniline, aniline, p-methylaniline, cyclohexanamine ✓ p-nitroaniline is the least basic because it contains nitro group which is an EWG that attracts more electrons from the arylammonium ion and destabilize it. ✓ aniline does not contain any substituent group (reference) ✓ p-methylaniline is more basic than aniline and p-nitroaniline because it contains methyl which is an EDG that donates more electrons to the arylammonium ion and stabilize it. ✓ Cyclohexanamine is the most basic because it contains alkyl group (cyclohexyl) which is an EDG that donates more electrons to the alkylammonium ion and stabilize it.

Reaction Mechanism This type of questions needs you to show the reaction mechanisms step by step using equations. The types of reaction mechanisms are: a) Free radical substitution (Alkane) b) Electrophilic addition (Alkene) c) Electrophilic aromatic substitution (aromatic compound) d) Nucleophilic substitution, SN1 and SN2 (haloalkane) e) Elimination (Alcohol) f) Nucleophilic addition (Carbonyl compounds) Some reaction also contains rearrangement especially the mechanisms that involve formation of carbocation, C+ a) 1,2-methyl shift b) 1,2-hydride shift

Example of Question Complete the reaction scheme below by drawing the structure of V, W, X, Y and Z. Next, state the type of mechanism for the reaction of propene with H2O, H+ and show the reaction mechanism for the formation of V. [10 Marks]

Example of question Compound X, C7H14 is optically active. On catalytic reduction of X over palladium, one mole equivalent of hydrogen was absorbed yielding compound Y, C7H16. Reaction of X with hot acidified solution of potassium permanganate gives ethanoic acid and compound Z, C5H10O2 which is an optically active carboxylic acid. a) Draw the structures for X, Y, and Z. b) Give the IUPAC name of X. c) Write the reaction equation for the reaction of X with HCl . Give the type of mechanism involves and state the rule used to determine the major product. Show the reaction mechanism for the formation of the major product. d) Suggest a chemical test to distinguish compound X from Y. e) State the observations and write the chemical equations.

Compound X, C7H14 is optically active. On catalytic reduction of X over palladium, one mole equivalent of hydrogen was absorbed yielding compound Y, C7H16. Reaction of X with hot acidified solution of potassium permanganate gives ethanoic acid and compound Z, C5H10O2 which is an optically active carboxylic acid. a) Draw the structures for X, Y, and Z.

Examples of questions: b) Give the IUPAC name of X.

c) Write the reaction equation for the reaction of X with HCl . Give the type of mechanism involves and state the rule used to determine the major product.



d) Suggest a chemical test to distinguish compound X from Y. e) State the observation and write the chemical equations.

Skills In Answering Questions Understanding clue words Clue words Meaning Define To give the exact meaning Distinguish between/ differentiate To give differences (e.g: colour when two compounds are tested) Explain in terms of To make clear by giving reasons in terms of given factors State/ Give/ Name Writing IUPAC names, type of mechanism (presented clearly and briefly) Draw/ Predict/ Show/ Give/ Determine/ Drawing the structural formula of a Identify/ Deduce compound