NJC 9 Polymer Part 2

National Junior College SH1 H1 Chemistry 1 Polymers (Part 2) – Reactions of Functional Groups Learning Outcomes Students should be able to: describe the chemistry of the following classes of compounds: (i) alkanes (exemplified by ethane) as being generally unreactive except in terms of combustion and substitution by chlorine (ii) alkenes (exemplified by ethene) in terms of combustion and addition reactions with bromine (in CCl4) and hydrogen (iii) halogenoalkanes (exemplified by bromoethane) in terms of substitution reaction to alcohols and elimination reactions to alkenes (iv) aldehydes (exemplified by ethanal) and ketones (exemplified by propanone) in terms of their reduction to primary and secondary alcohols respectively; and oxidation of aldehydes to carboxylic acids (v) alcohols (exemplified by ethanol) in terms of combustion, oxidation to carboxylic acids and elimination to alkenes (vi) carboxylic acids (exemplified by ethanoic acid) in terms of conden sation with alcohols to form esters (in the presence of concentrated sulfuric acid), and amines (exemplified by ethylamine) to form amides (in the presence of dicyclohexylcarbodiimide, DCC) (vii) esters (exemplified by ethyl ethanoate) and amides (exemplified by ethanamide) in terms of hydrolysis with acids and bases

National Junior College SH1 H1 Chemistry 2 Success Criteria: • I can understand alkanes are generally unreactive except it undergoes o combustion reaction to form CO2 and H2O o substitution reaction by chlorine to form halogenoalkane C CH H H H H H ethane 1 Alkanes Alkanes are saturated hydrocarbons. (i.e. contains only C−H and C−C single bonds) All the C atoms in alkanes forms 4  bonds, therefore every C atom has a tetrahedral shape. methane butane CH4 They are generally unreactive, except in terms of combustion and substitution by chlorine or bromine. Reasons: o They are non -polar molecules (Since C and H have very similar electronegativity). o Since C−C and C−H bonds are very strong, large amount of energy is required to break the C−C and C−H bonds in order for reaction to occur. 1.1 Physical Properties Since the electronegativity of C and H are similar (i.e. negligible electronegativity difference), C–H bond is non –polar, alkanes molecules are non –polar and are held together by weak instantaneous dipole–induced dipole attraction. (a) Boiling and melting point Boiling and melting points increases as number of carbon atoms in the alkane increases. This is because incresing amount of energy is required to overcome the increasing strength of instantaneous dipole-induced dipole interaction between molecules as the size of electron cloud increases. Straight–chain alkanes: C1 to C4 are gases, C5 to C17 are liquids, C18 and above are solids at r.t.p. Boiling points of branched alkanes are lower than the boiling point of the corresponding straight –chain alkanes. Due to branching of hydrocarbon chain, the molecules are more spherical and have smaller surface area of contact between molecules . T hus, there is weaker instantaneous dipole–induced dipole attraction between the molecules. pentane (b.p. 36 °C) 2,2-dimethylpropane (b.p. 10 °C)

National Junior College SH1 H1 Chemistry 3 (b) Solubility Alkanes are insoluble in water as the molecules cannot form ion-dipole interaction nor hydrogen bonds with water molecues. They are soluble in organic solvents. 1.2 Combustion In complete combustion, hydrocarbons burn in excess O2 to form CO2, H2O and release heat. CxHy (g) + (x + 4 y ) O2 (g) ⎯→ x CO2 (g) + 2 y H2O (l) E.g. C2H6 (g) + 7 2 O2 (g) ⎯→ 2 CO2 (g) + 3 H2O (l) Note: R is used to represent an alkyl group. e.g. methyl (–CH3), ethyl (–CH2CH3) 1.3 Substitution reaction C H + Cl2 C Cl + HClUV light halogenoalkanesalkane Type of reaction Substitution Reagent and condition Cl2(g), UV light R−H + Cl2 ⎯→ R−Cl + HCl For example, when methane reacts with chlorine in the presence of UV light, one of H atoms of methane is substituted by a Cl atom. CH4 + Cl2 ⎯→ CH3Cl + HCl CH3Cl undergoes further substitution with chlorine to produce a mixture of alkyl chlorides: CH3Cl + Cl2 ⎯⎯→ CH2Cl2 + HCl (disubstitution) CH2Cl2 + Cl2 ⎯⎯→ CHCl3 + HCl (trisubstitution) CHCl3 + Cl2 ⎯⎯→ CCl4 + HCl (tetrasubstitution) If methane is used in excess, Cl2 limiting, the major product is CH3Cl (chloromethane). If Cl2 is used in excess, methane limiting, the major product is CCl4 (tetrachloromethane) These substitutions take place very rapidly in the presence of UV light. However, no reaction will take place in the dark at room temperature. UV UV

National Junior College SH1 H1 Chemistry 4 Success Criteria: • I can understand alkene undergoes o combustion reaction to form CO2 and H2O o addition reactions with bromine (in CCl4) to form dibromoalkane o reduction reactions with hydrogen to form alkane 2 Alkenes Alkenes are unsaturated hydrocarbons because of the presence of C=C double bond. The C atom in C=C has a trigonal planar shape. C C H H H H ethene CH2=CHCH2CH3 CH3CH=CHCH3 CH2=CHCH=CH2 but-1-ene but-2-ene buta-1,3-diene 2.1 Physical Properties (a) Boiling and melting point Alkenes, like all hydrocarbons, have relatively low boiling and melting points as the alkene molecules have weak instantaneous-dipole induced-dipole interaction between molecules. Boiling and melting points increases as number of carbon atoms in the alkene increases. This is because incresing amount of energy is required to overcome the increasing strength of instantaneous dipole-induced dipole interaction between molecules as the size of electron cloud increases. (b) Solubility Alkenes are insoluble in water as the molecules cannot form ion-dipole interaction nor hydrogen bonds with water molecues. They are soluble in organic solvents. 2.2 Combustion In complete combustion, hydrocarbons burn in excess O2 to form CO2, H2O and release heat. C2H4 (g) + 3 O2 (g) ⎯→ 2 CO2 (g) + 2 H2O (l)

National Junior College SH1 H1 Chemistry 5 Video of reaction of alkene with Br2 Note: Distinguishing tests are chemical reactions for specific functional group that gives a significant observation (e.g. change in colour, production of precipitate or gas) 2.3 Addition reaction Unsaturated alkene undergoes addition reaction to form saturated compound. Less energy is required to break the  bond in the C=C bond as the side-way overlap of atomic orbitals is less effective than head-on overlap in  bond. Type of Bond C—C () C=C ( + )  bond in C=C Bond Energy/ kJ mol−1 350 610 260 Hence, alkenes tend to undergo reactions that involve the breaking of the weaker  bond in the C=C bond, leaving the  bond intact. The  bond electrons are used to form two new  bonds during addition reaction to form one product. (a) Addition reaction With Br2 (Distinguishing test for alkene) C C H H Br H H Br Inert organic solvent e.g. CCl4 C C H H H H + Br2 Type of reaction Addition Reagent and condition Br2 in inert organic solvent (CCl4) Observation Orange-red Br2 decolourises. (b) Catalytic addition (reduction) reaction With H2 C C HH H H + C C HH H H H HH2 Pt(s) catalyst (or Ni(s) catalyst, heat) Alkenes are reduced to alkanes by the addition of hydrogen gas. Type of reaction Reduction Reagent and condition H2(g), Pt(s) catalyst or H2(g), Ni(s) catalyst, heat Industrial use for addition of H2 to alkene: Used in the food industry to make a large variety of manufactured goods, like spreads and shortenings, fr