2024 VJC Alkenes Tutorial

1 Victoria Junior College 2024 JC1 H2 Organic Chemistry Tutorial: Alkenes Summary of Reactions Complete the following table. 1 Preparation of Alkenes Starting compound Reaction type Reagents & Conditions Products formed CH3CH2CH2OH Elimination Excess conc. H2SO4, heat OR Al2O3, heat CH3CH(Br)CH2CH3 2 Reactions of Alkenes Starting compound Reaction type Reagents & Conditions Carbon-containing product(s) CH3CH2CH=CH2 HBr(g) CH3CH2CH(Br)CH2Br Br2(aq) CH3CH2CH(OH)CH3 CH3CH2CH2CH3 Oxidation cold KMnO4 in NaOH(aq) KMnO4 in H2SO4(aq), heat Note: If no heat is written for a reaction, it means that the reaction can take place at room temperature.

2 Introduction 1 Write down the skeletal formula for each of the following. (a) 4–bromopent–1–ene (b) cis–2,2–dimethylhex–3–ene 2 Give the IUPAC name for each of the following. (a) (CH3)2C=CHCH3 (b) Reaction mechanism 3 (a) Name the type of reaction and describe the mechanism for the reaction between but–1–ene and hydrogen bromide to form 2 –bromobutane. Show all charges and relevant lone pairs and show the movement of electron pairs using curly arrows. (b) State the type of hybridisation in the alkene carbon atom of but –1–ene and the reactive carbon atom in the intermediate formed in the mechanism. (c) By referring to the shape of the reactive carbon atom in the intermediate formed in the mechanism, e xplain why compound formed from the reaction between but–1–ene and hydrogen bromide has no effect on plane-polarised light. (d)(i) Write an equation, giving the structural formula of the major organic product, for the reaction between but–1–ene and aqueous bromine under room temperature. (ii) [EXTENSION] Describe the mechanism written in (d)(i). Show all charges and relevant lone pairs and show the movement of electron pairs using curly arrows. Electrophilic addition of alkenes 4 Hydrogen iodide undergoes an addition reaction with propene forming 2 –iodopropane. When propene is bubbled through iodine monochloride, ICl, dissolved in a suitable solvent, a similar reaction occurs. Which product will be present in the greatest yield? A B C D CH3CHCH2Cl Cl CH3CHCH2I I CH3CHCH2I Cl CH3CHCH2Cl I [N13/I/21] Oxidative cleavage of alkenes 5 The following compound is used as an alarm pheromone by some aphids. Which product is obtained from the reaction of this pheromone with excess hot concentrated acidic KMnO4? A CH3COCH2CHO B CH3COCH2CH2CO2H C HCHO D HO2CCH2COC(CH3)=CH2

3 6 An alkene, X, has the molecular formula C5H10. On oxidation with hot acidified potassium manganate(VII) solution, X gives rise to CH3(CH2)2CO2H as the only organic compound. Deduce the structure of X, and using [O] notation, write the equation for the above reaction. 7 An alkane B is a component of petrol. When treated with bromine under free radical conditions it produces a mixture of bominated compounds, including C, C 6H12Br2. Dehydrobromination of C produces D, C6H10. When D is oxidised by hot concentrated KMnO4, three compounds, CO 2, CH3CO2H and CH 3COCO2H, are formed in equimolar amounts. *Strategy: Draw a flowchart. Easier to understand all the reactions. Compare the difference among all molecular formulae in deducing the structures. (a) Suggest reagents and conditions for the conversion of C to D. (b) Suggest a structure for compound D, and hence deduce the structure of compound B. (c) There are four possible isomers of C 6H12Br2 that could give D on dehydrobromination. Draw the structures of two of them. (d) Suggest, with a reason, the number of stereoisomers of compound D. [N06/III/6(b)]

4 Supplementary Questions 1 Reaction of ethene with bromine in the presence of aqueous sodium chloride gives a mixture of products. Which pair of products will be included in this mixture? A CH2OH–CH2Br CH2Cl–CH2OH B CH2Br–CH2Br CH2OH–CH2Cl C CH2Br–CH2Br CH2Cl–CH2Cl D CH2Br–CH2Br CH2Cl–CH2Br 2 The diagram shows the structure of the naturally–occurring molecule cholesterol. Cholesterol reacts with cold, dilute acidified KMnO 4. Four students made claims about this reaction. W The cyclohexene ring becomes saturated. X The cyclohexene ring is broken open. Y The number of chiral centres increases by 1. Z The number of chiral centres increases by 2. Which students’ claims are correct? A W and Y B W and Z C X and Y D X and Z [N12/I/22] 3 Predict the carbon –containing product(s) obtained by subjecting each of the following alkenes under the following conditions. cold, alkaline KMnO4 hot alkaline KMnO4 Conc. H2SO4 followed by H2O CH3CH2CH=CH2 (CH3)2C=CHCH3 4 A hydrocarbon, on heating with an excess of hot concentrated acidic KMnO 4(aq), produces HO2CCH2CH2CH2COCH3. What could the hydrocarbon be? 1 CH3 2 CH3 3 CH3 CH3 H CH3 HO H H H H3C cholesterol

5 5 How many different alkenes, including cis-trans isomers, could be produced when CH3CHBrCH2CH3 is reacted with hot ethanolic NaOH? How could one of these isomers be distinguished from the rest through a chemical test? In each case identify the products of any reaction. 6 A hydrocarbon of molecular formula C 4H10 undergoes chlorination to form a mixture of monosubstituted products of molecular formula C4H9Cl. Compounds A and B of molecular formula C 4H9Cl undergo elimination to form compounds C and D respectively. Only compound C exists as a pair of cis–trans isomers E and F. Compound D reacts with hot acidified KMnO 4(aq) to yield propanoic acid, CH3CH2CO2H, and carbon dioxide as the only carbon–containing products. (a) State the reagents and conditions for the formation of compounds C and D. (b) Identify the compounds A, B, D, E and F. 7 Suggest a structural formula for each of the compounds A – D in the following schemes. Cl2 A C6H13Cl NaOH in ethanol heat + a mixture of these two products formed Cl2 B C10H18Cl2 NaOH in ethanol heat only one product formed C C7H12 NaOH in ethanol heat only one product formed Br2 (in an inert solvent) D C7H12Br2 [H2 paper: N10/III/1(d)]