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Acids and Bases pH – a measure of acidity The pH scale is useful in that it lets us express acidity by numbers that are not exponentials The strength of acids and bases A strong acid is one that is completely ionized in water. A weak acid is one that ionizes in water to the extent of less than 100%. 1 2 1811 - Humphry Davy Historical development Questioned Lavoisier's theory, noting that hydrochloric acid (HCl) did not contain oxygen yet is an acid. Soon thereafter, several more acids without oxygen were found. e.g. HBr, HF, HI 1661 - Robert Boyle Characterized acids and alkalies (bases) as the following: Acids: Sour taste; Corrosive Change litmus (石蕊) (dye extracted from lichens) from blue to red Become less acidic when combined with alkalies. Alkalies (Bases): Feel slippery Change litmus from red to blue Become less alkaline when combined with acids. 1838 - Justig Liebig Suggested that acids contain one or more hydrogen atoms which can be replaced by metal atoms to produce salts. e.g. HSCN is an acid because the H atom can be replace by a metal to form a salt, such as NaSCN. 1884 – Svante Arrhenius acids and bases: 1778 - Antoine Lavoisier (He stated the first version of the law of conservation of mass) Acids: compounds that produce hydrogen ions (H+ or H3O+) in water Bases: compounds that form OH- when they dissolve in water Believed that all acids contained oxygen after studying several acids e.g. H2SO4 - sulfuric acid, HNO3 - nitric acid 3 4 1 A base is a proton acceptor 1923 - Johannes Brønsted and Thomas Lowry Bronsted-Lowry acids and bases An acid is a proton donor A base is a proton acceptor hydronium ion Identifying the acid-base pair 5 ammonium ion Questions: A. Which of the following compounds can function as Bronsted acid? NaH H2S Fe(H2O)6]3+ B. Which of the following compounds can function as Bronsted base? CH3OH ClPPh3 6 Water as an acid and a base 7 8 2 Generalizations The stronger the acid, the weaker the conjugate base. Hydronium ion is the strongest acid that can exist in aqueous solution. Hydroxide ion is the strongest base that can exist in aqueous solution. The position of equilibrium lies to the side of the weaker acid 9 10 Weak acids and acid ionization constants 11 12 3 13 Weak bases and base ionization constant 14 The relationship between the ionization constants of acids and their conjugate bases 15 16 4 Some typical Bronsted acids Non-oxygen containing acids, e.g. HX (X = halide) Aqua acids: Fe(H2O)6]3+ Which one is a stronger Bronsted acid? (charge effect) Ca(H2O)62+ v.s. Mg(H2O)62+ Fe(H2O)62+ v.s. Fe(H2O)63+ Oxoacids and hydroxoacids O O O S OH OH Cl OH O O OH HO Si OH OH OH B HO OH H2SO4 and B(OH)3 (or Si(OH)4): which one is more acidic? Ka1 H2SO4 H+ + HSO4Ka2 HSO4- H+ + SO42- 17 B B HO OH HO OH OH O O O S Molecular structures and the strength of oxo and hydroxoacids O OH + + H + H+ 18 OH O S O OH More oxo-ligands give higher acidity OpE(OH)q Op+1E(OH)q-1- + H+ Stability of Op+1E(OH)q-1- increases when p ___ Pauling's rules The pKa value of H3PO3 was reported to be 1.8. Predict its structure. (1) For the oxoacid OpE(OH)q, pKa ≈ 8-5p (2) The successive pKa values of polyprotic acids (q>1), increase by 5 units for each successive proton transfer. 19 20 5 1923 - Gilbert N. Lewis Lewis acids and bases Some typical examples Ag+ (1) Metal complexes H+ + OH- H2O Co2+ + 6 H2O [Co(H2O)6]2+ (2) A molecule with an incomplete octet can complete its octet by accepting an electron pair. 21 22 (3) Valence shell rearrangement Hard and soft Lewis acids and bases Mx+ + y X(4) Expand the octet SiF4 + 2F- [SiF6]2(5) Charge transfer I2 + I- I3- I [MXy]x-y Equilibrium constant I I I I K= I [MXy]x-y [Mx+ ] [X-]y C O To some Lewis acids (cations), the basicity order of halids is F- > Cl- > Br- > IBr To others, the order is F- < Cl- < Br- <I- Br 23 24 6 1963 - Ralph G. Pearson Hard soft acid-base principle (HSAB principle) Hard acids/bases: hold electrons tightly and are not readily polarized Soft acids/bases: do not hold electrons tightly. Their valence electron cloud are easily distorted or polarized The HSAB principle: A hard acid prefers to combine with a hard base and a soft acid prefers to combine with a soft base. Some general observations: (1) Hardness increases across a period and decreases down a group (2) Hard bases are associated with O, N and F (3) Hard acids are those with higher charges (4) Many transition metals are soft acids 25 26 Exercises Why? Hard acid-base interactions are predominantly electrostatic; soft acidbase interactions are predominantly covalent. Lewis acids: Ru2+ versus Os2+ Na+ versus Cu+ Os2+ versus Os3+ Mg2+ versus Ca2+ Lewis bases: Cl- versus BrCO versus NH3 NH3 versus PH3 Chemical consequences of hardness More examples The tendency of soft acids to bond to soft bases and of hard acids to bond to hard bases explains certain aspects of the Goldschmidt classification of the elements into 4 types. (2 of the classes are the lithophile elements and the chalcophile elements.) (1) W(CO)6 is air stable but W(NH3)6 is unknown Lithophile elements: Li, Mg, Ti, Al, Cr (hard cations which are found in association to the hard base O2-) Chalcophile elements: Cd, Pb, Bi, Ag (soft cations, which are found in associaion with the soft bases S2-, Se2-) 27 (2) ZnO + 2 LiMe ZnMe2 + Li2O K > 1 or < 1 ? (3) MeHgI + HCl MeHgCl + HI K > 1 or < 1 ? (4) AgCl2- + 2 CN- Ag(CN)2- + 2 Cl- K > 1 or < 1 ? 28 7 Organic chemistry and Polymers What is Organic Chemistry? It is defined as the study of hydrocarbons (compounds of hydrogen and carbon) and their derivatives 7 million Organic Compounds 1.5 million Inorganic Compounds Animal and plant matter, Foods, Pharmaceuticals, Cosmetics, Fertilizers, Plastics, Petrochemicals, Clothing Carbon, the element of life on earth Has Four Bonding Electrons; Unique Strong Covalent Bonds; Strong Single, Double and Triple Bonds Average Bond Energies (kJ mol-1) C-C 607 Si-Si 230 C-H C-N 754 Si-N 470 C-O 416 336 Si-H Si-O 323 368 O-Si-O = Sand and Rocks 29 30 Hydrocarbons CH3 CH3 Alkanes (saturated hydrocarbons), CnH2n+2 H H C H H methane CH3 H H H C C H H H ethane CH3CH3 CH3CH2CH2CH2CH CH CH CH3 H H H H H C C C C H H H H H H H H H H H C C C C C H H H H H H propane butane pentane CH3CH2CH3 CH3CH2CH2CH3 H H H H C C C H H H H CH2 CH2 CH CH3 CH3 CH3CH2CH2CH2CH3 butane H H H C C H H H Ethane Skeletal structure of only carbon atoms propane Rotation about single covalent bonds occurs freely. The energy barrier is small. The position of hydrogen atoms relative to one is thus constantly changing C1 – C4 n-alkanes are all gases Methane main component of natural gas Propane and butane often stored as compressed gases 3D – models show that because of the tetrahedral carbon atoms the chains are zig-zagged and not at all straight pentane 31 32 8 Non-polar molecules, which are less dense than water. Alkanes are immiscible with water making two layers. Newman projections Van-der Waals or dipole-dipole attractive forces, and not H-bonding (as in polar molecules) are the main intermolecular forces Alkanes show regular increases in bpt and mpt as molecular weight increases down the homolgous series These weak intermolecular forces operate over small distances, arising because the electron distribution within molecules at any given instance is not uniform. Resulting in tiny electrical attractions between molecules. These temporary dipoles hold alkanes as liquids or solids, and must be overcome in order to vaporize a liquid or melt a solid (wax) 33 Isomers – the have the same molecular formula, but a different structures 34 Petroleum: a complex mixture of hydrocarbons (linear or branched) Structural Isomers – same molecular formula, but atoms are bonded in different orders. Refining petroleum – fractional distillation – separation according to boiling points H H 3C C CH 3 CH3 Fractional distillation of crude oil C4H10 – has two isomers, n-butane and isobutane (2methylpropane) Natural Gas (C1-C4) Isobutane H3 C CH Gasoline (C 4-C 12) Bpt (40-200 ºC) CH2 CH3 CH3 Isopentane (2-methylbutane) Has the same molecular formula as n-pentane, C5H12 Petroleum Kerosene (C12-C 16) Bpt (200-250 ºC) CH3 H3C C CH3 CH3 Have different Physical Properties, Mpt, Bpt, densities, Heating oil, Lubricating oil (C15-C 18) Bpt (250-300 ºC) Neopentane (2,2-dimethylpropane) 35 36 9 37 38 Octane rating The octane rating is a measure of the autoignition resistance of gasoline (petrol) and other fuels used in spark-ignition internal combustion engines. Straight-chain alkanes are a pure fuel, because of engine knock. 2-methylheptane 23 39 n-hexane 25 2-methylhexane 44 1-heptene 60 n-pentane 62 1-pentene 84 n-butane 91 cyclohexane 97 benzene 101 toluene 112 40 10 Cycloalkanes CnH2n Cyclohexane H cyclopropane H cyclobutane H H H H H H chair conformation H H cyclopentane H equatorial position H axial position Alkyl halides (haloalkanes) ring strain exists in these three molecules Cl Br cyclohexane (no ring strain) 1-Bromobutane Cl Cl 2-Chloropropane or Isopropyl chloride C Cl CH3 CH Cl CH2 CH2 Br Tetrachloromethane Cl 3-Bromo-1-chlorobutane F Cl Cl C F Cl Trichlorofluoromethane (Freon-11) Cl C F Cl Chlorofluorocarbons (CFCs) Refrigerant Gases, Ozone Depletion, More H’s more degradable Dichlorodifluoromethane(F reon-12) 41 Alkenes – unsaturated compounds CnH2n H2C CH2 Ethene H3C CH CH2 H Propene H 1-Butene 1-Pentene C C 42 CisCis-Trans Isomerization of Retinal (視網膜) H H 1-Hexene Bond rotation? 1,3-Butadiene 3-methyl-1,4-pentadiene R R R H H H H R cis- Opsin (視蛋白) trans- Present in Rhodopsin (視網膜色素) 43 44 11 Reactivity π−bond lobes represent areas of high electron density Therefore, the π− bond is susceptible to attack by electron deficient molecules, called electrophiles, E+ Mechanism C Slow C H C + X C H X ADDITION REACTIONS C E+ + C A B A C C fast B C C X H H H C C X H OSO3H H H C C C C X Alkyl Halides H C C Oxidation of alkenes Alcohols H2C CH2 X X X C C X X The electrophile is a Lewis acid, its accepted a pair of electrons, the simplest Lewis acid is H+ The nucleophile is a Lewis base, its donated a pair of electrons OSO3H Alkyl hydrogen Sulfate OH C H X- is the nucleophile, and the carbocation is the electrophile OH H+ C KMnO4, -OH, H2O H2C CH2 + MnO2 OH OH Dihaloalkanes 45 C Hydrogenation Slow C C O C O H H OSO2OH Markovnikov’s Rule H C H Br CH3 S O H2 adds to the C atoms of a double/triple bond. A catalyst (e.g. Pt or Ni) is used to speed up the reaction. H O Fast H2C 46 H2C H C C H OSO3H H atom adds to the carbon atom which already has the most H atoms H C Adding H2 to double bonds in vegetable oils produces: compounds with higher melting points. solids at room temperature such as margarine, soft margarine, and shortening. CH3 Br Markovnikov addition Product Unsymmetrical Alkene 47 48 12 H Alkynes CnH2n-2 H3 C C C H C C H Ethyne (acetylene) + HBr H3C C C H Propyne H 3C H C C H Br Markovnikov product Alkynes are high energy compounds H C C H triglyceride (三酸甘油酯 ) Vegetable oils + 2 CO2 + H2O 2.5 O2 Welding (焊接 ) gas 49 Benzene and aromatics 50 Benzene and aromatics H H H C C C C C C H H H Kekulé was the first to suggest a sensible structure for benzene. Benzene is one of the main building blocks of the petrochemical industry. The global market grew from 4 million tonnes in 1960 to World production (million tonnes) over 30 million tonnes in 2000. Today benzene is mainly used as an intermediate to make other chemicals. Its most widely-produced derivatives include styrene, which is used to make polymers and plastics, phenol for resins and adhesives (via cumene), and cyclohexane, which is used in the manufacture of Nylon. World production 30 Western Europe 7.53 North America 7.23 South America 0.89 Asia 8.49 Kekulé said that he dreamt the structure of benzene – so called Kekulé structure of benzene (1865) 51 High Carbon content – burn with a smoky flame Delocalisation, Resonance -Stabilise molecules, so make them less reactive π-Electron Density Rings above and below the plane of the ring – Susceptible to electrophilic attack Michael Faraday first isolated benzene in 1825 52 13 Bonding 53 54 Derivatives of benzene Aromatic compounds Benzene and its derivatives are said to be aromatic - a term coined because of the strong fragrance of some of the derivatives of benzene 1. 2. 3. 4. p O- H N H HO OH O 2 3 m Must be cyclic Must be planar Each atom of the ring must have a p orbital and these p orbitals must be perpendicular to the plane of the ring Must contain 4n+2 π− electrons (where n = 0, 1, 2, ...) –Hückel Rule N+ 1 o Rules for Aromaticity O Br CH3 4 Electrophilic Aromatic Substitution H E H H H H E X H H H H + H H X H Electrophilic attack – Slow Rate Determining Step n = 1 , 6π π electrons E H Naphthalene 10 π Anthracene Phenanthrene 14 π sp3 E H E H E H E Transition State or Wheland Intermediate Delocalised Cyclohexadienyl cation 55 56 14 Alcohols, phenol and ethers Halogenation: examples of electrophilic substitution Cl Cl2, AlCl3 Alcohols and Ethers can be regarded as derivatives of water in which one or two of the H atoms has been replaced by an alkyl group Br Br2, FeBr3 Water, H2O Br Br H FeBr4 + H Br FeBr3 O 0.96 Ao Methanol, CH3OH 0.96 Ao H O H C H H 104.5o H 1.43 Ao Saturated molecules are sp3 hybridized 108.5o Methoxymethane, CH3OCH3 Nitration NO2 1.43 Ao NO2 H NO2 H C O C H H H H H Nitrobenzene 109.5o 1.10 Ao o 111.7 Alcohols are found to have much higher bpt than those of alkanes or haloalkanes of comparable size, e.g. Methanol (65oC), Chloromethane and Methane are gases ; Ethanol (78.5oC), Chloroethane (12oC) and Ethane is a gas 57 hydroxy or alcohol group CH3 OH CH3 Methyl alcohol (methanol) CH2 CH3 OH Ethyl alcohol (ethanol) CH3 CH3 CH OH 58 Phenols are stronger acids than alcohols CH2 CH2 OH OH Propyl alcohol (propanol) CH2 pKa = 18 CH CH2 CH3 H O CH2 OH CH3 Isopropyl alcohol O OH pKa = 10 O O 2-Ethyl-1-butanol Polyhydroxy alcohols are alcohols that possess more than one hydroxyl group CH2 CH3 CH2 HO OH CH CH2 HO OH CH2 HO HO Cl F OH 1,2-Ethanediol 1,2-Propanediol 1,2,3-Propanetriol (ethylene glycol) (propylene glycol) (glycerol) Extremely Toxic Resonance Stabilised Phenoxide anion CH CH2 Ethers CH3 CH2 O CH2CH3 Diethyl Ether F H C C O C H F F F Enflurane NonFlammable Anaesthetics Harmless 59 60 15 H Aldehydes and Ketones R C O R R H3C Propanone (ACETONE) H H 3C H3C H C O C O H Methanal (formaldehyde) H C O H3C CH3CH2 δ− O C δ+ (butter flavour) O H C C H N R H N R H O H C O Propanal CH3 Butadione O RNH 2 C C CH3CH2CH2CH2 Ethanal (acetaldehyde) H3C Carvone Butanone Carbonyls readily undergo Nucleophilic Attack al – aldehydes, one - ketones H H C H2 Most Reactive Group – π− electrons + polarisation C O C O H3C CH3 (spearmint flavour) Resonance Structures H CH3 Ketone Aldehyde O O O O O C O Pentanal Imine N H N R R - H2O 61 62 Highly Polar; Low molecular weight acids show Appreciable Solubility in Water. High Bpt – Extensive H-bonds to themselves and water Carboxylic acids and esters pKa = 4 - 5 , O C C O H + H2O O + H H2O + C NaOH HO Red ants O O H OH CH3 Methanoic acid H3O O C C OH O Na pKa = < Cl O OH 0.7 Cl O C C Cl 1.48 OH < C C H Cl C C H 2.86 O H < Cl Sodium Benzoate H OH OH O O HO H C C H 4.76 63 OH Propanoic acid OH Br 4-Bromo-2-ethylpentanoic acid O ( )n OH n = 1 = malonic acid n = 2 = succinic acid n = 3 = glutaric acid OH CH3CH2 Ethanoic acid O Ethanedioic acid (oxalic acid) Benzoic acid Cl O O O C O O O water = 14 HO2C CO2H Terephthalic acid CO2H CO2H Phthalic acid 64 16 Esterification – condensation reaction, where H2O is lost + CH3CH2 OH CH3 Acetic acid (ethanoic acid) OH CH3 O CH2 CH3 Ethyl acetate HCl or H2SO 4 H+(catalyst) O O + OH Ph 皂化 O O soap H3C OH Benzoic acid Ph H+(catalyst) O O O O CH3 Methyl benzoate O Ethyl propanoate Saponification is commonly used to refer to the reaction of a metallic alkali (base) with a fat or oil to form soap. Saponifiable substances are those that can be converted into soap. H O Methyl formate O vinyl acetate Alcohol part appears first in the name 65 66 How does soap work? (1) Ester molecules cannot H-bond to each other, because they do not have an –OH. (2) Consequently, B.pt is much lower than that of alcohols and acids of comparable mass (3) H-bonding to water is possible - low mw esters are soluble in water (4) Solubility rapidly decreases with carbon chain length. H O H Boiling points Hexane = 69 ºC Diethyl ether = 56 ºC Ethanol = 78 ºC Ethanoic acid = 118 ºC Ethyl acetate = 77 ºC 67 O R O H O H R 68 17 Omega 3 Fish Oil Fatty Acids In chemistry, especially biochemistry, a fatty (脂肪 ) acid is a carboxylic acid often with a long unbranched aliphatic tail (chain), which is either saturated or unsaturated Omega-3s are not just "good fats". They are truly essential for health and vitality The major process for transforming animal and vegetable oils and fats into oleochemicals(人造奶 油) is hydrolysis, the splitting of natural triglycerides into crude glycerine(甘油 ) and cruded mixed fatty acids, under the influence of water, temperature and pressure. Essential Fatty Acids lift depression, elevate mood, reduce stress and hyperactivity, increase focus, concentration and learning. Healthy Skin & Coat Omega Fatty Acids is an advanced veterinarian formulated Essential Fatty Acid (EFA) formula for dogs. It contains the ratios of all natural Omega Fatty Acids 3, 6 and 9 needed to assist in reducing shedding and improving your dogs overall skin, coat and vascular health. 69 Amines 70 Lewis bases where Et = CH2CH3 H2NEt ethylamine primary NH2 H2N HNEt2 NEt3 diethylamine triethylamine secondary tertiary Putrescine (found in decaying meat) Amphetamine (dangerous stimulant) where Me = CH3 H2NMe methylamine primary HNMe2 dimethylamine secondary NH2 NMe3 trimethylamine tertiary N H Piperidine 71 N Isopropylamine Triethylamine NH2 72 18 Amides – not acids or bases Cocaine R' C N R' R N R O O caffeine C Features of a Peptide Bond; 1. Usually inert 2. Planar to allow delocalisation 3. Restricted Rotation about the amide bond 4. Rotation of Groups (R and R’) attached to the amide bond is relatively free 73 R H2N C COOH H AMINO ACIDS O C H NH2 formamide H 3C O C Chirality – The Handedness of molecules Many organic compounds can exist as mirror-image twins A pair of mirror-image twins is called a pair of enantiomers, whose molecules are nonsuperimposable mirror images of one another O H NH 2 NH2 acetamide 74 Involves a tetrahedral sp3 atom CH3 benzamide C CH2 CH3 OH Chiral Centre O C CH3 HO H C H 2N NH2 urea All are high melting point solids, only benzamide not soluble in water CH3 H C OH 2-Butanol Interchanging any two groups at a chiral centre (stereocentre) that CH2 CH2 bears four different groups CH3 CH3 converts one enantiomer into another 75 76 19 CH3 H C OH CH3 CH3 HO C H CH3 2-Propanol O If any of the groups attached to the tetrahedral atom are the same, the centre is achiral. H models of the molecule and its mirror image and then determine whether they are superimposable H S-(+)-Carvone Principle component of Caraway seed oil and responsible for the characteristic odour The ultimate way to test for molecular chirality is to construct O Screwdriver is achiral Screws are chiral Golf club is chiral Gloves are chiral R-(-)-Carvone Principle component of Spearmint oil and responsible for the characteristic odour Spearmint 【植】綠薄荷 Receptor Sites in the Nose are Chiral Caraway (用於調味或製藥的 )葛縷子籽 77 78 Optical isomers Nobel Prize 2001 Enantiomers differ only in optical properties – rotate plane-polarized light in opposite directions. Professor William Knowles Professor Ryoji Noyori Professor K. Barry Sharpless For synthesis of optically active compounds – asymmetric synthesis 79 80 20