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 t