ASRJC 2024 H2 Chem Alkenes Notes

2024 JC1 H2 Alkenes 1 2024/ASRJC/CHEM ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY HYDROCARBONS: ALKENES Content/Outline Alkenes (exemplified by ethene) 1 Introduction 2 Nomenclature 3 Isomerism 4 Physical Properties 5 Preparation of alkene - Cracking - Elimination, including Saytzeff’s rule 6 Chemical Property 7 Reactions of alkene - Electrophilic addition, including Markovnikov’s rule - Oxidation - Reduction 8 Chemical Tests for alkene 9 Electronic Effect (Inductive Effect) Learning Outcomes 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 (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 the carbocation intermediates References 1. Chemistry for Advanced Level. Peter Cann & Peter Hughes 2. Chemistry (2nd edition). Chris Conoley and Phil Hill 3. Chemistry in Context. Hill & Holman 4. Understanding Advanced Organic and Analytical Chemistry, K.S Chan and Jeanne Tan

2024 JC1 H2 Alkenes 2 2024/ASRJC/CHEM 1 INTRODUCTION Alkenes are unsaturated hydrocarbons which contain one or more C=C double bonds in their structures. Alkenes have the general formula CnH2n. Cycloalkenes have the general formula CnH2n-2. (Note that cycloalkanes have the same general formula as alkenes e.g. C 3H6 represents propene as well as cyclopropane.) Each C atom of the C=C is __________________________. Hence the geometry (shape) about each of the double bonded C is trigonal planar. Two of the three sp2 hybrid orbitals of the carbon atom overlap head–on with the 1s orbitals of the two hydrogen atoms to form two C–H sigma () bonds. The remaining sp 2 hybrid orbital overlaps head –on with the sp 2 hybrid orbital of the neighbouring carbon atom to form a C–C sigma () bond. The unhybridised 2p orbital, which is perpendicular to the plane of the inter –nuclear axis , overlaps side–on with the 2p orbital of the neighbouring carbon atom to form a pi () bond. The  electron cloud lies above and below the plane of the inter–nuclear axis. bonding in ethene molecule x x H H x x H H Unhybridised 2p orbital sp2 hybrid orbital 1s orbital Displayed formula of ethene C C H H HH 120o 120o 120o Three sp2 hybrid orbitals lie in a trigonal plane  bond H H H H  bond Bonding in an ethene molecule

2024 JC1 H2 Alkenes 3 2024/ASRJC/CHEM 2 NOMENCLATURE The name of an alkene is derived from an alkane with same number of carbon atoms by changing the suffix from –ane to –ene. The longest continuous carbon chain that contains the C=C is named as the parent chain. The position of the double bond and any substituents are indicated by numbering the carbon atoms in the chain, as shown below. n Molecular formula Name Structural formula 2 C2H4 ethene CH2=CH2 3 C3H6 propene CH2=CHCH3 4 C4H8 but–1–ene CH2=CHCH2CH3 but–2–ene CH3CH=CHCH3 2–methylpropene CH2 C CH3 CH3 If the molecule has two C=C double bonds, it is named as a diene. Structural formula Appropriate IUPAC name CH2=CH–CH=CH2 buta–1,3–diene CH2=CHCH2CH=CH2 penta–1,4–diene cyclohexa–1,4–diene 4 1 2 3 2 1 3 1 2 3 4 2 3 4 1 5 1 2 2 3 4 5 6

2024 JC1 H2 Alkenes 4 2024/ASRJC/CHEM Cl CH3 More examples on nomenclature Compound Appropriate IUPAC name Inappropriate names CH3CH2CH=CHCH3  pent–2–ene  pent–3–ene The position of the double bond in the chain is indicated by the smallest number possible. CH3CH2CH2 C CH CH2CH3 H CH2  3–ethylhex–1–ene CH3CH2CH2 C CH CH2CH3 H CH2 3–propylpent–1–ene The parent chain must be the longest continuous carbon chain that contains the C=C. 4–ethylhex–5–ene The position of the double bond in the chain is indicated by the smallest number possible. Cl CH3  2–chloro–3– methylcyclohexa–1,4– diene 1–choro–2–methylcyclohexa– 3,6–diene The position of the double bond in the chain is indicated by the smallest number possible.  Cl CH3 The number assigned to the C=C must be in consecutive order. (e.g. 1 & 2. Not 1 & 6) 1–choro–6–methylcyclohexa– 1,4–diene The position number assigned to the substituent should be the lowest number possible. 3–methyl–2–chlorocyclohexa– 1,4–diene The substituent should be arranged in alphabetical order. 1 2 3 4 5 1 2 3 4 5 6 6

2024 JC1 H2 Alkenes 5 2024/ASRJC/CHEM 3 ISOMERISM IN ALKENES Two types of isomerism to consider in alkenes: • Constitutional isomerism • Cis–trans isomerism 3.1 Constitutional isomerism Isomers that have the same molecular formula but different structural formulae. Constitutional isomerism in alkenes can arise: - due to different positions of the C=C double bond in the carbon chain E.g. - due to different degree of branching in the carbon chain E.g. 3.2 Cis–trans isomerism Isomers that have the same molecular and structural formula but different spatial arrangements due to the restricted rotation about a bond. Cis–trans isomerism exists in alkenes as a result of the restricted rotation about a C=C double bond due to the presence of  bonds. Criteria for alkenes to exhibit cis–trans isomerism (i) Presence of C=C double bond to prevent free rotation about the double bond (ii) Have two different groups / atoms attached to each C atom of the C=C double bond E.g. cis−but−2−ene C C CH3 HH CH3 trans−but−2−ene C C H H CH3 CH3 –CH3 (or H) on same side of C=C –CH3 (or H) on different sides of C=C but–1–ene CH2=CHCH2CH3 but–2–ene CH3CH=CHCH3 2–methylpropene CH2 C CH3 CH3 is a constitutional isomer of but–1–ene and but–2–ene.

2024 JC1 H2 Alkenes 6 2024/ASRJC/CHEM Exercise 3.1 Which of the following cannot exhibit cis–trans isomerism? A CH3 Cl C H C H B C C H CH3 Br CH2CH3 C C C Cl CH3 H CH3 D C C Br CH3 H Cl

2024 JC1 H2 Alkenes 7 2024/ASRJC/CHEM 4 PHYSICAL PROPERTIES (Recall and apply Chemical Bonding concepts here) 4.1 Boiling point/volatility 1. Boiling point increases as the number of carbon atoms increases (as with alkanes) Chain length of alkene Physical state at room temperature C2 – C4 gas C5 – C15 liquid > C15 waxy solid Example Name Molecular formula Boiling point / C ethene C2H4 –102 propene C3H6 –48 but–1–ene C4H8 –6.5 pent–1–ene C5H10 30 • As the size of electron cloud increases, the ease of distortion of the electron cloud increases. • This results in stronger instantaneous dipole–induced dipole (id –id) attractions between the alkene molecules. • Hence, more energy is required to overcome the stronger id –id attractions between the molecules. 2. For the same molecular formula, b ranched chain alkenes have lower boiling point s than their corresponding straight chain alkenes (as with alkanes). Example Alkene Structural formula Boiling Point pent–1–ene CH3CH2CH2CH=CH2