ASRJC H2 Chem 8a. Intro to Organic Chem

2024 JC1 H2 Introduction to Organic Chemistry ©2024ASRJC/CHEM 1 ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY INTRODUCTION TO ORGANIC CHEMISTRY Important note: The Learning Outcomes (LOs) that are greyed out will be covered in subsequent Organic Chemistry chapters. Contents • Empirical, molecular and structural formulae • Functional groups and the naming of organic compounds • Common terms for organic reactions and reactivities • Shapes of organic molecules; σ and  bonds Learning Outcomes (LOs) Candidates should be able to: (a) interpret, and use the nomenclature, general formulae and displayed formulae of the following classes of compound: (i) hydrocarbons (alkanes, alkenes and arenes) (ii) halogen derivatives (halogenoalkanes and halogenoarenes) (iii) hydroxyl compounds (alcohols and phenols) (iv) carbonyl compounds (aldehydes and ketones) (v) carboxylic acids and derivatives (acyl chlorides and esters) (vi) nitrogen compounds (amines, amides, amino acids and nitriles) (b) interpret, and use the following terminology associated with organic reactions: (i) functional group (ii) degree of substitution: primary, secondary, tertiary and quaternary (iii) homolytic and heterolytic fission (iv) carbocation (v) free radical, initiation, propagation, termination (vi) electrophile (Lewis acid), nucleophile (Lewis base) (v) addition, substitution, elimination, condensation, hydrolysis (vi) oxidation and reduction [in equations for organic redox reactions, the symbols [O] and [H] are acceptable] (c) interpret, and use the following terminology associated with organic reactivities: (i) delocalisation (ii) electronic effect (electron–donating and electron–withdrawing effect) (iii) steric effect (steric hindrance) (d) describe sp3 hybridisation, as in ethane molecule, sp2 hybridisation, as in ethene and benzene molecules, and sp hybridisation, as in ethyne molecule (e) explain the shapes of, and bond angles in, the ethane, ethene, benzene, and ethyne molecules in relation to  and  carbon–carbon bonds (f) predict the shapes of, and bond angles in, molecules analogous to those specified in (e) (g) apply (b) and (c) to the understanding of mechanisms in terms of organic structure and bonding (h) recognise that the mechanism of polar reactions involve the flow of electrons from electron–rich to electron–poor sites References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Organic Chemistry, McMurry, Brooks/ Cole 3. Understanding Advanced Physical Inorganic Chemistry, Jeanne Tan and Kim Seng Chan

2024 JC1 H2 Introduction to Organic Chemistry ©2024ASRJC/CHEM 2 I. INTRODUCTION Organic chemistry is the branch of chemistry concerned with the study of compounds containing carbon. This includes the vast majority of carbon -containing compounds, except for compounds such as CO, CO2 and carbonates that are traditionally considered to belong to the field of inorganic chemistry. Every living organism is made of organic chemicals. The proteins that make up our hair, skin and muscles; the DNA that controls our genetic heritage; the food that nourish us; the clothes that keep us warm; and the medicines that heal us are mainly organic chemicals. Today, organic chemicals outnumber inorganic ones by 80:1, and number well over 10 million different compounds. The large variety of organic compounds arises due to 2 main reasons: 1. Carbon atoms can form strong bonds with other carbon atoms. Such bonding allows chains and rings of carbon atoms of various lengths to be produced. Example: the 4 different ways to arrange 4 linked carbon atoms C C CC C C C C C C C C C C C C Moreover, each carbon atom can form four bonds. This allows branched chains, multiple bonds and attachment of other atoms to occur, further increasing the number of structures possible. 4 single covalent bonds 2 single and 1 double bond 2 double bonds 1 single and 1 triple bond C C C C 2. Generally organic compounds are kinetically stable. Many organic reactions possess a high activation energy which must be supplied before the reaction will proceed. This is a result of the strong covalent bonds formed. These bonds have to be broken before reaction can occur, and breaking strong bonds require much energy. If the right conditions and catalysts can be found, it is possible to convert one organic compound into another. In subsequent topics, we will look at the reactions and properties of a number of organic compounds.

2024 JC1 H2 Introduction to Organic Chemistry ©2024ASRJC/CHEM 3 II. GENERAL PROPERTIES OF ORGANIC COMPOUNDS (a) Melting point and Boiling point Organic molecules generally have low melting points and boiling points. They are held by weak intermolecular forces of attraction (i.d.–i.d., p.d.–p.d. or hydrogen bonds) which can be overcome easily by heating. (b) Solubility Low solubility or insoluble in polar solvents (e.g. water) except when polar groups such as –OH, –COOH and –NH2 are present in the molecule. Most organic compounds are only soluble in non–polar organic solvents , such as benzene and ether. (c) Thermal stability Organic compounds are usually thermally unstable, decomposing into smaller molecules when heated to temperatures above 500 oC. Example, when heated, butane–1,4–dioic acid decomposes as follows: C O HO CH2CH2 C O OH C C C O O C O H H H H + H2O (d) Flammability Many organic compounds are flammable and burn exothermically in a plentiful supply of air to yield carbon dioxide and water. Thus most fuels are organic compounds (e.g. oil, petrol and natural gas), and their combustion is our main source of heat energy. e.g. C2H5OH + 3O2 2CO2 + 3H2O (e) Reactivity Rate of organic reactions are usually slow compared to inorganic reactions. They usually require heating or the use of a catalyst to speed up reactions in the laboratory or in the industry. This is due to the high activation energy to break strong covalent bonds. For example, concentrated H2SO4 is used as a catalyst in the esterification reaction. CH2H3C OH + H3C C O OH H3C C O O CH2 CH3 + H2O conc. H2SO4

2024 JC1 H2 Introduction to Organic Chemistry ©2024ASRJC/CHEM 4 III. HYBRIDISATION IN ORGANIC COMPOUNDS Covalent bonds are formed when atomic orbitals overlap. However, when one tries to derive the shape of a molecule using the overlap of atomic orbitals, sometimes its geometry does not correspond to the geometry as predicted by the VSEPR model (which is obs erved experimentally). For instance, in CH4, if all the bonds on CH4 are formed using s and p orbitals in C atom, since the three 2p orbitals that carbon used in bond formation are perpendicular to each other, wouldn’t three of the four C-H bonds be 90 o to each other? Why is there a tetrahedral arrangement of the four C-H bonds and all the bond angles 109.5° instead? Hybridisation is a solution that reconciles the overlap of atomic orbitals with the VSEPR model to explain what is observed experimentally. It is essentially the mathematical mixing of atomic orbitals to create new orbitals (known as hybrid orbitals) which are used to form sigma bonds in bonding. (This will be illustrated in the examples on pages 5 to 10) Combinations of different numbers of atomic orbitals give rise to different types of hybrid orbitals. The hybrid orbitals take up a distinctive shape and relative orientation, depending on the number and type of atomic orbitals that have been mixed. The total number of atomic orbitals on an atom remains constant, and so the number of hybrid orbitals on an atom equals the number of atomic orbitals mixed. In organic chemistry, our focus is hybridisation in molecules containing the carbon atom, although