NJC J1 Alkene tutorial questions

National Junior College SH1 H2 Chemistry 1 ALKENES 4.1 INTRODUCTION Alkenes form a homologous series of aliphatic hydrocarbons with the carbon- carbon double bond as the distinguishing feature. Alkenes with one C=C bond present in the structure have the general formula, CnH2n. Because of its carbon -carbon double bond, an alkene is thus referred to as an unsaturated hydrocarbon. Ethene (C2H4) (presence of C=C) – unsaturated) Ethane (C2H6) (all C-C single bonds – saturated) 4.1.1 Nomenclature of Alkenes Success criteria: I can deduce the structure of an alkene from its given IUPAC name (and vice versa). Every name of organic compounds consists of Prefix ⎯ Parent ⎯ Suffix Alkenes have the suffix –ene to identify the C=C bond functional group present in them. When the alkene contains two or three C=C double bonds, it is known as di-ene and tri-ene respectively. • Number the chain from the end closest to the double bonds. • Lowest number assigned to C=C when it is part of cyclic ring. Cis-trans isomers of alkenes to be distinguished by using the prefix cis- or trans-. Checkpoint 1 Give the structural formulae for the following compounds: (a) (b) trans-pent-2-ene 2-chloro-3-methylpenta-1,4-diene

National Junior College SH1 H2 Chemistry 2 FYI: Crystal lattice: the 3D structural arrangement of atoms, ions or molecules in a crystalline material 4.2 PHYSICAL PROPERTIES OF ALKENES Success criteria: • I can explain the difference in boiling point and melting point between cis and trans isomers. Similar to alkanes • Insoluble in water and soluble in non-polar (organic) solvents • Less dense than water • Boiling point increases with increasing carbon number • Branching lowers boiling point Comparing cis- vs trans- isomers of 1,2-dichloroethene cis-1,2-dichloroethene trans-1,2-dichloroethene Structural formula Boiling point / oC 60.3 47.5 Melting point / oC −81 −49 Explanation of the difference in boiling points between cis -1,2-dichloroethene and trans-1,2-dichloroethene: Cis-1,2-dichloroethene molecules are polar and has permanent dipole-permanent dipole attractions between the molecules which is stronger than instantaneous dipole-induced dipole attractions between trans -1,2-dichloroethene molecules. Stronger intermolecular forces require more energy to overcome. Explanation of the difference in melting points between cis -1,2-dichloroethene and trans-1,2-dichloroethene: Trans-isomers pack better into a crystal lattice , leading to more intermolecular forces per unit volume . They generally will have higher melting points than their respective cis-isomers.

National Junior College SH1 H2 Chemistry 3 4.3 PREPARATION OF ALKENES Success criteria: • I can state the reagents and conditions to convert o alcohol to alkene o halogenoalkane to alkene • I recognize that elimination of H−X from halogenoalkanes and H−OH from alcohol is only possible between two adjacent carbon atoms. • I can give the structure of the alkene formed from the elimination reaction. 4.3.1 Elimination Reactions (i) Elimination of water from alcohols C C H H HH H OH C C H H H H + H2O excess conc. H2SO4 heat Type of reaction Elimination Reagent and condition concentrated H3PO4, heat or Al2O3(s), heat (or excess conc. H2SO4, heat) Note: Elimination reaction will cause the degree of unsaturation (no. of  bond) in the product to increase by 1 Note: HCl eliminated undergoes acid-base reaction with KOH to form KCl and H2O (ii) Elimination of HX from halogenoalkanes C C H H CH3H H Cl C C H H H CH3 + KCl ethanol heat+ KOH + H2O Type of reaction Elimination Reagent and condition ethanolic KOH/NaOH, heat A NOTE: When group of atoms such as H & Cl or H & OH are eliminated from different pairs of adjacent carbons , a mixture of alkene isomers is produced. The relative proportion of each alkene will depend on Zaitsev’s Rule. Explanation: Btw cis- and trans- alkenes, bulky groups like alkyl groups are best placed far apart from each other to reduce steric strain. Thus, the trans-isomer is more stable and formed in greater quantity. Zaitsev’s Rule (for your info) The more highly substituted alkenes are more stable and hence more readily formed in an elimination reaction. C C R H H H C C R H R H C C R H H R C C R R R H C C R R R R Decreasing stability > >> > conc H3PO4

National Junior College SH1 H2 Chemistry 4 Experimental Evidence (For your info) The relative chemical stability of the isomers of an alkene can be easily measured by determining their heats of combustion. The exothermic combustion reaction converts isomers of butene to same set of oxidised products; carbon dioxide and water. Differences in chemical stabilities of the isomers of butene result in different amounts of energy released during combustion. The more stable the isomer, the less exothermic is its combustion reaction. Note: KOH is a base which reacts away the acidic HCl molecule from the Halogenoalkane. Conc H2SO4 is a dehydrating agent which can react H2O away from the alcohol molecule. Checkpoint 2 Consider the following molecules. Give the structures of the possible products formed when each undergoes elimination. Identify the major product, where applicable. (a) (b) Conc. H3PO4

National Junior College SH1 H2 Chemistry 5 4.4 REACTIONS OF ALKENES Success criteria: • I can state the reagents and conditions for the reactions of alkenes. • I can deduce the structure of alkenes when the organic products and reagents and conditions are given. • I can write balanced equations for the reactions. 4.4.1 Combustion Alkenes, being hydrocarbons, undergo complete combustion in excess oxygen to form carbon dioxide and water. C2H4 (g) + 3 O2 (g) ⎯→ 2 CO2 (g) + 2 H2O (l) 4.4.2 Reduction with hydrogen to form alkane Type of reaction Reduction Reagent and condition H2(g), Ni(s) catalyst (Or H2(g), Pt(s) catalyst) Alkenes Combustion Reduction Electrophilic Addition X2(in CCl4) X = Br, Cl X2(aq) X = Br, Cl HX(g) X = Br, Cl, I H2O(g) Oxidation Mild oxidation Vigorous oxidation Ni catalyst

National Junior College SH1 H2 Chemistry 6 4.4.3 Electrophilic Addition Reactions Success criteria: • I can explain why alkenes are reactive towards electrophiles. • I understand why alkenes tend to undergo addition reactions. (i) Reactivity of Alkene The C=C bond in alkenes is made up of one  and one  bond. • Sigma bond: head-on overlap between two 2sp2 hybrid orbitals • Pi bond: side-way overlap between two unhybridised 2p orbital The structure of C=C results in the following reactivity: • Susceptible to Reactions with Electrophiles The  electrons are found in the regions above and below the plane of the molecule. Being less tightly bound to the carbon nuclei, the presence of  electrons causes a region of relatively high electron density at C=C, thus alkene is more likely to react with electrophiles such as HBr and Br2 molecules. Recall: C=C bond consist of a sigma bond (formed via head on overlap of hybrid orbitals) and  bond (formed via side- way overlap of unhybridised orbitals). Note: How to tell if a reaction is an addition reaction? One clue is the reaction between 2 reactants (one unsaturated) to form 1 product. 2 Reactants → 1 Product • Tendency to Undergo Addition Reactions In addition, less energy is required to break the  bond in the C=C bond as the side -way overlap of atomic orbitals is less effect ive than head-on overlap ones. Type of Bond C—C () C=C ( + ) C—