VJC H2 Chem Chpt 13 Alkenes Lecture Notes

VICTORIA JUNIOR COLLEGE CHEMISTRY DEPARTMENT Mr Chang Siang Wen [chang.siangwen@vjc.sg] H2 ORGANIC CHEMISTRY: ALKENES Lecture Outline 1 Introduction 2 Physical Properties of Alkenes 3 Preparation of Alkenes 3.1 Dehydration of Alcohols 3.2 Dehydrohalogenation of Halogenoalkanes 4 Reactions of Alkenes 4.1 Electrophilic Addition 4.11 Addition of hydrogen halides 4.12 Addition of halogens in CCl4 4.13 Addition of halogens in water 4.14 Addition of water 4.2 Reduction 4.3 Oxidation 4.31 Combustion 4.32 Formation of diol (mild oxidation) 4.33 Formation of carboxylic acids, ketones and/or CO2 (strong oxidation) 5 Tests for Alkenes 6 Summary Assessment Objectives Candidates should be able to: (a) explain the general reactivity of alkenes towards electrophilic reagents/electrophiles (b) describe the chemistry of alkenes as exemplified, where relevant, by the following reactions of ethene: (i) electrophilic addition of water/steam, hydrogen halides and halogens (ii) reduction via catalytic hydrogenation (catalytic addition of hydrogen) (iii) oxidation by cold, alkaline solution of manganate(VII) ions to form the diol (iv) oxidation by hot, acidified solution of manganate(VII) ions leading to the rupture of the carbon–to–carbon double bond in order to determine the position of alkene linkages in larger molecules (c) describe the mechanism of electrophilic addition in alkenes, using bromine with ethene as an example (d) apply Markovnikov’s rule to the addition of hydrogen halides to unsymmetrical alkenes, and explain the composition of products in terms of the stability of carbocation intermediates References 1 A–level Chemistry by EN Ramsden 2 Chemistry – Longman A–level Guides by JGR Briggs 3 Understanding Advanced Organic and Analytical Chemistry by Tan J. and Chan K. S. 4 Chemistry – The Molecular Nature of Matter and Change by Silberberg Video Resources Electrophilic addition mechanism of alkenes Test for alkenes by bromine water Lecture 1 2 Pages 1–9 10-16 Complete by 25 Aug 3 Sep Tutorial Qns 1–4 5–7

2 1 Introduction • Alkenes belong to the homologous series of hydrocarbons containing a C=C bond. • General formula: CnH2n • The names of all alkenes end with ‘–ene’. The simplest alkene is ethene, CH2=CH2. • Isomerism occurs in alkenes containing more than three carbon atoms and occurs in three ways. (i) Constitutional / Chain isomers due to different arrangement of carbon atoms. (ii) Constitutional / Position isomers due to different position of C=C bond. (iii) Cis–trans isomers due to restricted rotation / lack of free rotation about C=C bond. E.g. C4H8 has four isomers (including both constitutional and cis–trans isomers). CH3CH2CH=CH2 CH2=C(CH3)2 but–1–ene 2–methylpropene cis–but–2–ene trans–but–2–ene • Nomenclature Step 1: Identify the parent chain (longest continuous carbon chain containing the C=C bond), name the compound after it by replacing ‘–ane’ ending from alkane containing the same number of carbon atoms by ‘–ene’. E.g. CH2=CH2 CH3CH=CH2 ethene propene Step 2: Identify the position of the C=C bond. Number the carbon atoms consecutively from the end of the parent chain which gives the lower number for the position of the C=C bond. Indicate the position of the C=C bond by placing the number before the syllable ‘–ene’. 5 4 3 2 1 1 2 3 4 5 6 E.g. CH3CH2CH=CHCH3 CH3CH=CHCH2CH2CH3 pent–2–ene hex–2–ene Name the cis or trans isomer as it is displayed. (See cis–but–2–ene and trans–but–2–ene as shown above.) Step 3: Identify the alkyl substituent (s) (−CnH2n+1) attached to the parent chain and indicate the position(s) of the substituent(s) based on the numbering system obtained in step 2. Arrange the substituents in alphabetical order , ignoring the prefixes such as di– or tri–.

3 E.g. 5CH3 4C CH3 H 3CH 2CH 1CH3 4CH3 3C CH3 CH3 2CH 1CH2 4–methylpent–2–ene 3,3–dimethylbut–1–ene Self–practice 1 (Check your answers on SLS Alkenes Lesson 1) Draw the structures for the following molecules. (a) 4,5–dimethylcyclohexene (b) cis–hex–3–ene Self–practice 2 (Check your answers on SLS Alkenes Lesson 1) Give the IUPAC names for the following molecules. (a) (b) CH2 CHCHCHCH3 CH2CH3 CH3 ________________________________ _______________________________ You can now attempt Tutorial Question(s): Q1 – Q2 2 Physical Properties of Alkenes • The structure and bonding of alkenes are similar to that of alkanes. Hence, they share similar physical properties with alkanes. I. Solubility and Density • Alkenes are non–polar. They are insoluble in polar solvents such as water but soluble in non–polar solvents such as CCl4. • Alkenes float on water and thus are less dense than water. II. Boiling and Melting Points • Boiling point increases with number of carbon atoms as more energy is required to overcome the increasing strength of instantaneous dipole– induced dipole interactions due to increasing number of electrons. • Ethene, propene and butene are gases at room temperature, but higher members of alkenes are liquids and then solids. • Branched chain isomers have lower boiling points than their straight chain isomers. Branched chain isomers are more spherical with less surface

4 area of contact between molecules for electron interactions, resulting in a decrease in strength of instantaneous dipole– induced dipole interactions. • For cis–trans isomers, due to the larger net dipole moment in the cis isomer, it is a polar molecule and is held by stronger permanent dipole–permanent dipole interactions and has a higher boiling point than the non–polar trans isomer. E.g. cis trans µ > 0 Debye µ = 0 Debye • However, the trans isomer has a higher melting point than the cis isomer due to the closer packing of the trans isomer molecules. 3 Preparation of Alkenes 3.1 Dehydration of Alcohols R C H C H H H OH R C H C H H + H2O • Type of reaction: Elimination • Reagents & conditions: Heat with excess concentrated H2SO4 OR • Reagents & conditions: Heat with Al2O3 Example Dehydration of ethanol (CH3CH2OH) using heated Al2O3 Vapour of ethanol is passed over heated Al2O3 to form ethene. CH3CH2OH → CH2=CH2 + H2O Al2O3 ethene gas collected wool soaked in ethanol Making Thinking Visible Question: Why doesn’t the closer packing of the trans isomer molecules lead to the trans isomer having a higher boiling point than the cis isomer? Answer: In the liquid state, the molecules are not rigidly held in fixed positions and they are not very close together as in the solid state. Thus, the packing of molecules is not influential to account for differences in boiling points.

5 3.2 Dehydrohalogenation of Halogenoalkanes R C H X C H H H + KOH (in alcohol) reflux R C C H H H + KX + H2O • Type of reaction: Elimination • Reagents & conditions: Heat with KOH in ethanol (or NaOH in ethanol) • The major product formed is the more stable product which is the more substituted alkene. E.g. There are two ways to r emove one HBr molecule from the following halogenoalkane. major product minor product (more substituted alkene) (less substituted alkene) • How to identify the more substituted alkene? The more substituted alkene is the one with more alkyl groups (–CnH2n+1) bonded to the C atoms in the C=C bond (inside the dotted boxes) as shown in the example above. The enthalpy change of hydrogenation of the