3 Alkane(organic Chem) NJC

National Junior College SH1 H2 Chemistry 1 3 HYDROCARBONS - ALKANES AND CYCLOALKANES 3.1 INTRODUCTION Alkanes are the simplest of the homologous series of organic compounds and are also the least reactive hydrocarbons. Alkanes are saturated hydrocarbons as they contain only C −H and C −C single () bonds with a maximum of 4  bonds per carbon. All C atoms in alkanes are sp3 hybridised with each C atom in a tetrahedral geometry. Alkanes can be classified as acyclic (aliphatic, straight and branched chains) or cyclic closed rings. 3.1.1 Acyclic (non-cyclic) Alkanes Acyclic alkanes have the general formula CnH2n+2 and contain zig-zag chains of C atoms. Acyclic alkanes can be classified as straight and/or branched chains. methane butane 3-methylpentane CH4 Straight chain Straight chain Branched chain 3.1.2 Cyclic Alkanes (closed rings alkanes) Cyclic alkanes (cycloalkanes) have the general formula CnH2n (same as alkenes). Molecular formula Name Skeletal formula C3H6 cyclopropane C4H8 cyclobutane C5H10 cyclopentane C6H12 cyclohexane .

National Junior College SH1 H2 Chemistry 2 3.2 NOMENCLATURE OF BRANCHED CHAIN ISOMERS (IUPAC SYSTEM) Success criteria: • I can deduce the structure of an alkane from its given IUPAC name (and vice versa) The names of alkanes end with the letters ‘-ane’. The following are the names for the first 6 straight-chain alkanes. n Molecular formula Name Condensed Structural formula Skeletal formula 1 CH4 methane CH4 2 C2H6 ethane CH3CH3 3 C3H8 propane CH3CH2CH3 4 C4H10 butane CH3(CH2)2CH3 5 C5H12 pentane CH3(CH2)3CH3 6 C6H14 hexane CH3(CH2)4CH3 With branching, atoms or groups of atoms that make up the side chains have to be considered a substituent. The IUPAC name of an organic compound consists of a parent (or stem), a suffix and as many prefixes as necessary. Prefix(es) Parent Suffix How to name an organic compound 1 Identify the longest continuous carbon chain for the parent name. 2 Numbers carbon atoms in the longest chain nearer to the first branch point 3 Identify and label numbers on the substituents. If two or more substituents are present, cite them in alphabetical order. The number prefixes di-, tri-, tetra-, etc. are ignored in alphabetising. Substituents Longest continuous carbon chain bearing the principal (most important) functional group Principal functional group

National Junior College SH1 H2 Chemistry 3 Checkpoint 1 1 Determine if each alkane is named correctly using IUPAC nomenclature. If the naming is incorrect, give the correct IUPAC name. Organic compound Is naming correct? (Y / N) Correct IUPAC name (if relevant) 2-methyl-4-ethylpentane 2,2-methylpropane 4,4-dimethyl-3-ethylpentane 2,2,4-trimethylpentane 2 Draw the skeletal formula of 2-ethyl-1,1-dimethylcyclohexane.

National Junior College SH1 H2 Chemistry 4 3.3 ISOMERISM Alkanes with 4 or more carbon atoms exhibit structural isomerism due to branching of the hydrocarbon chains. Optical isomerism can also occur in higher members of the alkane series. Remember? The strength of instantaneous dipole - induced dipole (id-id) interactions between molecules increases with increasing size of electron cloud. Molecules of branched alkanes have a smaller surface area of contact between adjacent molecules than that of their straight-chain isomers, resulting in a weaker id-id interaction. For dissolution to take place, solute-solvent interaction should be stronger or comparable to solute-solute and solvent-solvent interaction. 3.4 PHYSICAL PROPERTIES OF ALKANES Success criteria: • I can explain the difference in melting point and boiling point between alkanes. • I can understand the solubility of alkanes in different types of solvent. Alkane molecules are made up of only C and H atoms which have similar electronegativities. The C−H bond is considered non-polar with a very small dipole moment. Thus, alkane molecules are held together by weak instantaneous dipole- induced dipole interactions. 3.4.1 Boiling and Melting point The boiling and melting points of alkanes increase as the number of carbon atoms in the molecules increases due to the increase in Mr and hence the increasing size of electron cloud. Formula Name Mr Melting point / oC Boiling point / oC State (at r.t.p) CH4 Methane 16 –182 –162 gas C2H6 Ethane 30 –183 –89 gas C3H8 Propane 44 –188 –42 gas C4H10 Butane 58 –138 0 gas C5H12 Pentane 72 –130 36 liquid C6H14 Hexane 86 –95 69 liquid Branched-chain alkanes usually have lower boiling points than their corresponding straight-chain alkanes. The greater the degree of branching, the lower the boiling point of the compound. 3.4.2 Solubility Alkanes are insoluble in polar solvents (e.g. water) but soluble in non-polar solvents (e.g. CCl4, benzene). 3.4.3 Density Density of alkanes increases down the homologous series. Mr is larger and stronger instantaneous dipole -induced dipole interactions between molecules bring the alkane molecules closer to each other. Larger mass and smaller volume results in higher density. However, density of alkanes decreases with more branching in the molecules of similar Mr values due to poorer packing (branched isomers occupy bigger space than straight chain).

National Junior College SH1 H2 Chemistry 5 3.4.4 Viscosity Viscosity is a measure of the internal resistance to the flow of a fluid which form friction between neighbouring molecules. The viscosity of liquid alkanes increases with increasing Mr as the strength of intermolecular instantaneous dipole-induced dipole interactions increases. 3.5 CHEMICAL PROPERTIES OF ALKANES 3.5.1 Reactivity Success criteria: • I can explain why alkanes are generally unreactive towards most reagents. Alkanes are generally unreactive and chemically inert towards most chemical reagents such as acids, bases, oxidising and reducing agents. Alkanes lack chemical reactivities as: (i) the C–H bonds are non-polar (C and H have similar electronegativities) (ii) C−C and C−H bonds are rather strong. As a result, alkane do not have electron rich or electron poor centres to attract reactive charged species. This renders alkanes unreactive towards ionic and polar reagents. Instead, alkanes react with oxygen (combustion) and halogens involving electrically neutral free radicals (substitution), the two most important reactions. 3.5.2 Combustion Success criteria: • I can write balanced equation for alkanes to undergo combustion. • I can deduce the identity of a hydrocarbon based on combustion data Alkanes burn exothermically in excess oxygen when ignited. (i) Limited supply of oxygen Incomplete combustion of alkanes results in the production of CO and/or C (as black soot) and water. e.g. 2CH4 + 3O2 → 2CO + 4H2O CH4 + O2 → C (soot) + 2H2O (ii) Excess supply of oxygen Complete combustion of alkanes results in the production of CO 2 and H2O as the only products. e.g. CH4 + 2O2 → CO2 + 2H2O ∆H= −890 kJ mol–1

National Junior College SH1 H2 Chemistry 6 General equation for the combustion of hydrocarbon: CxHy + (x + 4 y ) O2 → xCO2 + 2 y H2O Many alkanes, such as methane (natural gas), propane, butane, are used as fuels due to their ease of burning which are highly exothermic. Alkanes burn readily in the gaseous state. Hence, solid and liquid alkanes must be vapourised before they will burn. Higher members of alkanes have a lower volatility and burn less readily to give sooty flames. Checkpoint 2 State the oxidation number of carbon in each molecule. CO HCHO CH3CH2OH oxidation state o