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A2 Chemistry Unit 1 1 The Structure of Benzene Hydrogenation of Benzene: Kekulé’s Equilibrium Model of Benzene Kekulé’s structure failed to explain benzene’s low chemical reactivity. Cyclohexene enthalpy change of hydrogenation = 120kJmol⁻₁ . Therefore benzene must have an enthalpy change of hydrogenation should be -360kJmol⁻₁ . (3x cyclohexene, 3x C=C) If C=C bonds were present, benzene should react similarly to alkenes. The enthalpy change is actually -208kJmol⁻₁ . Each C=C bond would be expected to decolourise bromine water. The real structure of benzene is more stable than Kekulé’s structure. Benzene does NOT take part in electrophilic addition reactions as expected from the C=C bonds. This energy is known as the resonance energy of benzene. C-C single bonds and C=C bonds have different bond lengths. Kathleen Lonsdale found that all of the carbon bonds were the same length 0.139nm. (between the lengths of CC and C=C). 2 The Delocalised Model of Benzene The Delocalised Model has the following features: o o Cyclic hydrocarbon – 6 C molecules and 6 H molecules. Arranged in a planar hexagonal ring where each C is bonded to 2 other C atoms and 1 H atoms. o o The shape is a trigonal planar with a bond angle of 120°. Each C atom has 4 outer shell electrons. 3 of these e¯ bond to 2 other C atoms and 1 H atom. The bonds in this plane are called sigma bonds. The 4th outer shell e⁻ in a 2p orbital above and below the plane of the carbon atoms. o The e⁻ in the p orbital overlap creating a ring of electron density above and below the plane of carbons. o The pi-bonds spread over all 6 carbons and the ring is said to be delocalised. Under normal conditions, benzene does not: o o o Decolourise bromine water React with strong acids such as HCl React with halogens such as bromine, chlorine or iodine. Addition reactions will disrupt the delocalisation of the ring structure. Instead, Benzene takes part in substitution reactions – a hydrogen (H) is replaced with another group. The organic product retains the delocalised structure. 3 Reactions of Benzene Nitration of Benzene Benzene’s high electron density attracts electrophiles. To preserve the ring’s stability, benzene takes part in ELECTROPHILIC SUBSTITUION reactions. C₆H₆ + HNO₃ C₆H₅NO₂ + H₂O HNO₃ + H₂SO₄ Formation of NO₂⁺: NO₂⁺ + HSO₄⁻ + H₂O The H⁺ reacts with the HSO₄⁻ to reform H₂SO₄. This is acting as a catalyst. H ⁺ + HSO₄⁻ H₂SO₄ Conditions: conc. HNO₃, conc. H₂SO₄, 50°C 4 Reactions of Benzene Halogenation of Benzene Benzene will react with halogens in the presence of a HALOGEN CARRIER. Halogen Carriers Include: FeCl₃ FeBr₃ AlCl₃ AlBr₃ Iron Metal Bromobenzene is used in the preparation of pharmaceuticals. Chlorobenzene is used as a solvent and in pesticides. Formation of Br⁺ (or Cl): Br₂ + FeBr₃ Br⁺ + FeBr₄⁻ Regeneration of Br⁺ (or Cl): H⁺ + FeBr₄⁻ FeBr₃ + HBr 5 Reactivity of Alkenes and Benzene Cyclohexene and Bromine Water: Benzene and Bromine: The pi-bond is localised – this gives cyclohexene HIGH ELECTRON DENSITY. Benzene has delocalised electrons spread over a ring structure. Alkenes have localised electrons. The pi-bond repels the electrons in the Br-Br bond inducing a dipole. The Br₂ molecule becomes polar. The electrons in the double bond attract to the Br+ causing the double bond to break. This forms a positive carbocation. The Br-Br bond breaks via heterolytic fission forming Br⁻. The Br⁻ is attracted to the intermediate carbocation forming a covalent bond. Benzene has LOWER ELECTRON DENSITY and CANNOT POLARISE Br₂. Benzene is therefore resistant to reactions with non-polar halogens. A halogen carrier is needed to generate a more powerful electrophile. The greater charge on Br₂ can attract the pi-electrons from benzene so the reaction can take place. Bromine +Benzene = orange Bromine + Benzene + Iron fillings = decolourised and white fumes of hydrogen bromide gas. 6 Phenols O⁻ Na⁺ Sodium Phenoxide + H₂O NaOH O⁻ Na⁺ Phenol OH Na + H₂ Sodium Phenoxide Solid at room temperature and pressure. Only a phenol when an OH group is directly linked to the ring. Slightly soluble in water as OH group can make hydrogen bonds with water. Benzene ring makes it less soluble than alcohols. 7 Bromination and Uses of Phenols OH Bromination of Phenol: Phenol Na Lone pair of electrons on the O group of phenol. This creates a HIGHER ELECTRON DENSITY. 3Br₂ (aq) This POLARISES the Br₂ which are more strongly attracted towards the ring structure. OH Br Br + 3HBr Br 2,4,6-tribromophenol Surfactants and Detergents Antiseptics and Disinfectants Pharmaceuticals Paints and Epoxy Resins 8 Carbonyl Compounds ᶞ⁻ ᶞ⁺ Ketone In the middle of the chain Aldehyde At the end of the chain Oxygen is more ELECTRONEGATIVE so electrons are more attracted to it than the carbon. 9 Reduction of Carbonyl Compounds Reducing Agent [H] = NaBH₄ (Sodium Borohydride) Water is the solvent NaBH₄ readily generates hydride ions. Ketone + [H] = Secondary Alcohol Aldehyde+ [H] = Primary Alcohol H H C O C H H H H :O H H O C C H H :H⁻ H H H OH C C H H H + OH⁻ 10 Oxidation of Carbonyl Compounds Oxidising Agent [O] = Acidified Potassium Dichromate Ions K₂Cr₂O₇/H⁺ Primary Alcohol Aldehyde Carboxylic Acid Heated under reflux before distillation. Secondary Alcohol Ketone NO COLOUR CHANGE 11 Chemical Tests on Carbonyl Compounds To detect the presence of a carbonyl compound: > 2,4-DNP A solution of 2,4-DNP in a mixture of methanol & sulphuric acid is known as BRADY’S REAGENT. Brady’s Reagent + Aldehyde/Ketone Yellow/Orange Precipitate. H H + H₂O = C C₂H₅ O= C C₂H₅ Difficult to distinguish between Heptan-2-one and Cyclohexanone as the b.ps are very similar. The 2,4-DNP derivatives have different m.ps thus allowing easy identification. Filter and recrystallise the 2,4-DNP derivative and record its melting point. This is compared against a database. 12 Chemical Tests on Carbonyl Compounds Aldehyde or Ketone? Tollens’ Reagent is a weak OXIDISING agent that distinguishes between aldehydes and ketones. Aldehyde + [O] Carboxylic Acid SILVER MIRROR FORMED Ketones are NOT OXIDISED by Tollens’ reagent. Making Tollens’ Reagent: NaOH + AgNO₃ until a brown precipitate of Silver Oxide is formed. Dilute NH₃ added until precp. dissolves. The colourless solution is aka Ammonical Silver Nitrate. 13 Carboxylic Acids Functional Group = COOH Solubility: Carboxylic Acid + Metal SALT + H₂(g) Carboxylic Acid + Base SALT + H₂O Carboxylic Acid + Carbonate SALT + CO₂ + H₂O As the carbon chain INCREASES, SOLUBILITY DECREASES. Molecules become more NON-POLAR. COOH’s are WEAK ACIDS and react with metals, bases and carbonates. Salts formed from COOHs aka CARBOXYLATES. Suffix ‘...oate’ First part of carboxylate comes from metal, base or carbonate. Carboxylic Acid Salt Formed Methanoic Acid Methanoate Ethanoic Acid Ethanoate Propanoic Acid Propanoate Butanoic Acid Butanoate 14 Esters COOHS + OHs produces an ESTER & H₂O Acid Catalyst: H₂SO₄ e.g. Ethanol + Propanoic Acid = Ethyl Propanoate + Water Ester Hydrolysis: An acid anhydride is formed by the removal of H₂O from 2 molecules of carboxylic acids. This produces an ESTER and a CARBOXYLIC ACID. This is the reverse of esterification. It ADDS WATER. Acid Hydrolysis = reflux + aqueous acid, reversible This process requires gentle heating. Esters from ACID ANHYDRIDES produce a GREATER YIELD. Alkaline Hydrolysis = reflux + aqueous alkali, makes sodium salt, nonreversible Esters are used in perfumes and as flavourings. 15 Fats and Oils Building Triglycerides Fats are used for: Insulation As an energy store To protect organs Fats and Oils are esters of a long chained carboxylic acid. Naming Fatty Acids: Unsaturated fats with MULTIPLE DOUBLE BONDS Double bond commonly between C9 and C10. Fats m/p ABOVE room temp. Oils m/p BELOW room temp. Triglycerides: Triglycerides are triesters of: Propane-1,2,3-triol (glycerol) 3 fatty acid molecules. Forming Triglycerides: Simple triglyceride derived from 2/3 of the SAME fatty acids. Natural/Mixed triglycerides derived from 2/3 DIFFERENT fatty acids. HDLs – carry cholesterol FROM ARTERIES back TO LIVER LDLs – carry cholesterol FROM LIVER TO TISSUES. Fatty acids can also be used to make BIODIESEL. 16 Amines Amines are derivatives of ammonia. Ammonia Primary Amine Secondary Amine Tertiary Amine Adrenaline, Amphetamine, Phenylephrine NH₃ RNH₂ R ₂NH R₃N Amines are WEAK BASES and ACCEPT protons. Each lone pair on the N atom accepts a H⁺ A DATIVE BOND forms between the lone pair of the N atom and the H⁺. Naming Amines Secondary Amines Alkylamine + Acid forms an ALKYLAMMONIUM SALT. e.g. Methylamine + Sulphuric Acid Methylammonium Sulphate N-methylpropylamine 17 Reactions of Amines Preparing Amines: Conditions: Excess NH₃ Solvent = Ethanal This is a NUCELOPHILIC SUBSTITUTION REACTION. CH₃CH₂CH₂Cl + NH₃ NH₃ + HCl CH₃CH ₂CH₂NH₂ + HCl NH₄⁺Cl⁻ Excess NH₃ is added so that it all reacts. Preparing Aromatic Amines: Nitroarenes are reduced using a mixture of TIN AND CONC. HCl Synthesis of Dyes from Phenylamines: 1) Diazotisation 2) Coupling Reaction + HNO₂ + 2HCl (<10°C) Formation of HNO₂: NaNO₂ +HCl HNO₂ +NaCl + H₂O 18 Br Bromobenzene Benzene Chlorobenzene Br₂/FeBr₃ Conc. HNO₃ Conc. H₂SO₄ 50°C NO₂ Sn/ Conc. Nitrobenzene HCl Reflux Cl Cl₂/AlCl₃ N NH₂ Phenylamine NaNO₂/ HCl (aq) <10°C N⁺ Cl⁻ Phenol, NaOH Benzenediazonium chloride N OH Azo Dye N 19 O⁻ Na⁺ Sodium Phenoxide + H₂O NaOH O⁻ Na⁺ OH Phenol Na + H₂ Sodium Phenoxide 3Br₂ (aq) OH Br Br + 3HBr 2,4,6-tribromophenol Br 20 Amino Acids Amino acids make PEPTIDES and PROTEINS. 20 different amino acids in body. They are α-amino acids and have a BASIC AMINE GROUP and an ACIDIC CARBOXYL GROUP. Amino Acid Isoelectric Point Glycine 5.97 Alanine 6.01 Leucine 5.98 Serine 5.68 Proline 6.48 Soluble in both acids and bases. R-group is usually –OH, -SH, -COOH or – NH₂ EXCEPT GLYCINE which has H as the R group (simplest). Carboxyl and Basic group can react to form a ZWITTERION (internal salt). H⁺ pH 1 OH⁻ pH 13 Carboxyl donates proton to basic group. There is no overall charge as they cancel out. The ISOELECTRIC POINT is the pH at which there is no net charge. The zwitterion exists in this pH. H Amino acid acts as a BASE and ACCEPTS a proton from the acid. + H₂O Amino acid acts as an ACID and DONATES a proton to the hydroxide ion. 21 Polypeptides and Proteins Amino acids join together to from PEPTIDES Amino acids join together in a CONDENSATION REACTION and so eliminate H₂O. Proteins are long polypeptides with more than 50 amino acids. Acid Hydrolysis of Polypeptides and Proteins: Heated under reflux 6 mol dm⁻₃ 24 hours Acid Solution Peptide is separated and both become positive ions. Alkaline Hydrolysis of Polypeptides and Proteins: Alkaline Solution Above 100°C. Peptide is separated into 2 original peptides and both become sodium salts. 22 Optical Isomerism STEREOISOMERS = same structural formula, DIFFERENT ARRANGEMENT in space. OPTICAL ISOMERS= stereoisomers that are NONSUPERIMPOSABLE mirror images. Optical isomers rotate the light CLOCKWISE & ANTICLOCKWISE. CHIRAL CARBON= Carbon attached to 4 DIFFERENT atoms A mixture of equal amounts of optical isomers is known as a RACEMIC mixture – the rotations cancel each other out. Optical isomers exist in all amino acids (expt. Glycine). Only 1 of the isomers is synthesised naturally & only 1 will react with an enzyme. 23 Chirality in Pharmaceutical Synthesis Advantage of single isomer: Risk from undesirable side effects reduced Drug doses reduced Separating optical isomers is difficult as they have the same properties. How to separate optical isomers: Use ENZYMES as biological catalysts Chiral Pool Synthesis Transition Element Complexes Ibuprofen has 2 optical isomers. It is sold as a mixture of both isomers. 24 Condensation Polymerisation Polyesters Ester Linkage To from an ester: A Carboxylic Acid An Alcohol (either on 1 or 2 molecules) -H lost from Alcohol, -OH lost from Carboxylic Acid Elimination of a by-product – usually H₂O. Ester Linkages e.g. Terylene and Poly(lactic) Acid Uses of Polyesters: oMachine-washable oMachine-dryable oResistant to stretching, shrinking and chemical attack oBurns easily 25 Polyamides Amide Linkage To form a polyamide: A Carboxylic Acid An Amine (can have 2 different monomers or just 1 with both functional groups.) Amide Linkage -OH lost from Carboxylic Acid, -H lost from Amine to form H₂O e.g. Nylon 6,6 and Kevlar Nylon 6,6: Used widely in Clothing Kevlar: -fire resistant -stronger than steel -fire fighter clothing and bullet-proof vests. 26 O = H _ O = Polyamide Nylon 6,6 + H – N – (CH₂)₆ - N – H _ H0 – C – (CH₂)₄ - C – OH H O = 1,6-diaminohexane H _ O = Hexane-1,4-dioic Acid + 2n-1 H₂O _ C – (CH₂)₄ - C – N – (CH₂)₆ - N H 27 Polyamides Nylon 6 O = _ H H O _ = H-N-(CH₂)₅-C-OH N-(CH₂)₅-C + n-1 H₂O 28 C _ = C H OH H N _ O N H C C H _ O N N _ O = H = HO O = Kevlar Polyamide H 29 C H H O Na⁺COO⁻ HO OH H H C C H H O O = C O C C O O = H = HO H = Terylene Polyester C C COO⁻Na⁺ OH 30 Addition and Condensation Polymerisation Addition polymers have 1 MONOMER and there is NO BY-PRODUCT. Addition polymers contain a DOUBLE BOND. Feature Addition Polymer Condensation Polymer Polyester Polyamide Functional Group C=C -COOH & -OH -COOH & NH₂ Monomer 1 1/2 1/2 Product Poly(alkene) Polyester + H₂O Polyamide + H₂O 31 Breaking down condensation polymers Hydrolysis of Polyesters NaOH/H₂O H⁺/H₂O 32 Breaking down condensation polymers Hydrolysis of Polyamides O O = = NaOH/H₂O Na⁺OOC⁻ – C – (CH₂)₄ - C –COO⁻ Na⁺ H⁺/H₂O ⁺ ⁺ 33 Degradable Plastics Degradable plastics break down into smaller fragments when exposed to HEAT, LIGHT OR MOISTURE. A biodegradable plastic breaks down COMPLETELY into CO₂ and H₂O Biodegradable polymers have bonds than undergo hydrolysis. Poly(lactic acid) – derived from corn starch Poly(glycolic acid) – isolated from sugar cane and unripe grapes. Photodegradable plastics are synthetic polymers designed to become weak and brittle when exposed to light for prolonged periods. 34 Separation by Chromatography Chromatography is used to separate components in a mixture. A mobile phase sweeps over a stationary phase. Different components have different AFFINITIES for the phases. The stationary phase slows the components down. They pass at different speeds thus separating the compound. ADSORPTION: The solid holds gas or liquid molecules ON THE SURFACE of a solid. THIN LAYER CHROMATOGRAPHY Stationary Phase: Solid Mobile Phase: Liquid GAS CHROMATOGRAPHY Stationary Phase: Solid/Liquid (silica) Mobile Phase: Gas A solid stationary phase separated by ADSORPTION. 35 Thin Layer Chromatography Rᶠ = distance moved by component distance moved by solvent front Limitations: Similar compounds have similar Rᶠ values. Difficult to find solvent that separates all of the compounds in a mixture. A chromatogram is a visible record showing the results of separation of a mixture. 36 Gas Chromatography Limitations: Stationary Phase: Thin layer solid/liquid coated on inside of capillary tube. Mobile Phase: Inert carrier gas e.g. helium Each component leaves the column at different times and is detected as it leaves the column. Retention time in gas chromatography is the time for a component to pass from the column inlet to the detector. The area under the peak is proportional to the amount of compound in a sample. Similar retention times Not all substances will be detected No reference for unknown compounds. Gas Chromatography Mass Spectrometry Components are separated by GC Detected by MS against a reference. GC-MS used in... Forensics Environmental Analysis Airport Security Space Probes 37 NMR Nuclear Magnetic Resonance Chemical Shift ᵟ is a scale that compares the frequency of an NMR adsorption with the frequency of the reference peak of TMS at 0ppm. TMS is added so that the spectrometer can be calibrated against the TMS reference peak. Low Resolution NMR Spectroscopy TMS is chemically UNREACTIVE and is removed from the sample after running the NMR. Solvent: D₂O Isotope of Hydrogen Produces no signal in spectrum as it has even number of nucleons. Other solvents: CDCl₃ (C peak usually removed from spectrum via evaporation) Won’t absorb radio waves High Resolution NMR Spectroscopy 38 NMR in Medicine An MRI scanner is like a large spectrometer in which the patient is the sample. MRI scanners detect SOFT TISSUE DAMAGE and in the diagnosis of tumours because they contain a high % of water. MRI scans are NON-IVASIVE so do NOT HARM the body’s cells. Disadvantages: Expensive, High training required 39 NMR Spectroscopy Description Triplet Quartet Singlet Chemical Shift/ppm 1.2 3.6 4.5 No. Of Protons Identity Coupling Pattern Example: Ethanol 3 2 1 R-CH₃ H₂-CO OH next to 2 H next to 3 H next to 0 H No. of peaks = No. of Chemical Environments Mass Spectrum = molecular ion (M) peak furthest right Identifies fragments m/z value Identity 15 CH₃⁺ 17 OH⁺ 28 C=0⁺ 29 C₂H₅⁺ CHO⁺ 31 CH₂OH⁺ 35 ³⁵Cl⁺ 37 ³⁷Cl⁺ 43 C₃H₇⁺ or CH₃CO⁺ 45 COOH⁺ 57 C₄H₉⁺ or C₂H₅CO⁺ 77 C₆H₅⁺ 79 ⁷⁹Br ⁺ 81 ⁸¹Br ⁺ 91 C₆H₅OH⁺ 40 An addition reaction is one Electrophilic Substitution is Electronegativity is a measure in which a reactant is a substitution reaction of the attraction of a bonded added to an unsaturated where an electrophile is an atom for the pair of electrons molecule to make a atom/s attracted to an in a covalent bond. saturated molecule. electron rich centre where it 1+1=2 accepts a pair of protons to form a new covalent bond. Delocalised electrons are shared between MORE THAN 2 atoms. A reaction mechanism is a series of steps that, together, make up an overall reaction. A redox reaction is one in which both reduction and oxidation take place. An electrophile is an atom/s attracted to an electron rich centre where it accepts a pair of protons to form a new covalent bond. A curly arrow shows the movement of an electron pair in the breaking or formation of a covalent bond. The stem is the longest carbon chain present in an organic molecule. In a substitution reaction, an atom/s is replaced with a different atom/s. The functional group is the part of an organic molecule responsible for its chemical reactions. Reflux is the continual boiling and condensing to make sure a reaction takes place without the contents drying. A suffix is the part of the name added after the stem. 41 An nucleophile is an A zwitterion is a dipolar ionic A condensation reaction is one atom/s attracted to an form of an amino acid that is in which 2 small molecules electron deficient centre formed by the donation of a react together to form a larger where it donates a pair H+ ion from the carboxyl molecule with the elimination of protons to form a new group to the amino group. of water. covalent bond. There is no overall charge. Esterification is the reaction of an alcohol with a carboxylic acid to form an ester and water. A peptide is a compound made of amino acids linked by peptide bonds. A chiral carbon is a carbon atom attached to 4 different atoms. Hydrolysis is a reaction with water or hydroxide ions that breaks a chemical compound into 2 compounds. Can be catalysed with an acid or alkali. High Density Lipoproteins (HDL) remove cholesterol from the arteries and carry it back to the LIVER for excretion or reuse. Sterioisomers are species with the same structural formula but with a different arrangement of atoms in space. Low Density Lipoproteins The isoelectric point is the pH (LDL) carry cholesterol value at which the amino and triglycerides from the acid exists as a zwitterion. LIVER TO THE TISSUES. Optical isomers (enantiomers) are sterioisomers that are non-superimposable mirror images of each other. 42 The repeat unit is the specific arrangement of atoms that occurs in the structure repeatedly. It is in brackets with letter ‘n’ outside of it. A biodegradable polymer is a polymer that breaks down completely into CO₂ and H₂O. A degradable polymer is a polymer that breaks down into smaller fragments when exposed to light, moisture or heat. Pharmalogical activity is the beneficial or adverse effects of a drug on living matter. A phase is a physically distinctive form of a substance (e.g. Solid, liquid or gaseous states of ordinary matter). Adsorption is the process by which a solid holds molecules of gas or liquid or solute as a thin film on the surface of a solid/liquid. The mobile phase is the phase that moves in chromatography. The stationary phase is the phase that does not move in chromatography. A chromatogram is a visible record showing the results of separation of a mixture. Rᶠ = distance moved by component distance moved by solvent front Retention time in gas chromatography is the time for a component to pass from the column inlet to the detector. Chemical Shift ᵟ is a scale that compares the frequency of an NMR adsorption with the frequency of the reference peak of TMS at 0ppm. 43