Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
2-Norbornyl cation wikipedia , lookup
Ring-closing metathesis wikipedia , lookup
Woodward–Hoffmann rules wikipedia , lookup
Ene reaction wikipedia , lookup
Hydroformylation wikipedia , lookup
Organosulfur compounds wikipedia , lookup
Homoaromaticity wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Petasis reaction wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Name the homologous series C4H10 C4H8 In groups discuss everything you know about these 2 homologous series. What type of hydrocarbons do they belong to? What about C6H12? What structures are possible with this molecular formula? What type of hydrocarbons do these belong to? What about C6H6? STRUCTURE OF BENZENE Primary analysis revealed benzene had... an a a empirical formula of CH molecular mass of 78 formula of C6H6 and and STRUCTURE OF BENZENE Primary analysis revealed benzene had... an a a empirical formula of CH molecular mass of 78 formula of C6H6 Kekulé and suggested that benzene was... PLANAR CYCLIC and HAD ALTERNATING DOUBLE AND SINGLE BONDS STRUCTURE OF BENZENE HOWEVER... • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed • all six C—C bond lengths were similar; C=C bonds are shorter than C-C • the ring was thermodynamically more stable than expected STRUCTURE OF BENZENE HOWEVER... • it did not readily undergo electrophilic addition - no true C=C bond • only one 1,2 disubstituted product existed • all six C—C bond lengths were similar; C=C bonds are shorter than C-C • the ring was thermodynamically more stable than expected To explain the above, it was suggested that the structure oscillated between the two Kekulé forms but was represented by neither of them. It was a RESONANCE HYBRID. Benzene • To be able to describe and explain the structure of benzene • To know specific reactions associated with benzene and other common substituted aromatic compounds C6H6 = benzene = an aromatic hydrocarbon = an arene X-ray diffraction and evidence for benzene’s structure Two structures are used to represent benzene: The KEKULÉ structure; 1865-1872 Cyclohexa-1,3,5-triene Bond lengths would be 0.154nm and 0.134nm alternating The modern DELOCALISED structure – 1930s X-ray diffraction data has shown that the carbon atoms in benzene are at the corners of a REGULAR HEXAGON. These data have also shown all C-C bonds to be 0.139nm; therefore an intermediate between a single and double bond Thermochemical data and stability of benzene When unsaturated hydrocarbons are reduced to the corresponding saturated compound, energy is released. The amount of heat liberated per mole (enthalpy of hydrogenation) can be measured. When cyclohexene (one C=C bond) is reduced to cyclohexane, 120kJ of energy is released per mole. C6H10(l) + H2(g) C6H12(l) If benzene contained three separate C=C bonds how much energy would it release per mole when reduced to cyclohexane Theoretical - 360 kJ mol-1 (3 x -120) Theoretically, if benzene contained three separate C=C bonds it would release 360kJ per mole when reduced to cyclohexane C6H6(l) + 3H2(g) C6H12(l) Benzene releases only 208kJ per mole when reduced, putting it lower down the energy scale 2 - 120 kJ mol-1 3 Experimental - 208 kJ mol-1 Thermochemical data and stability of benzene Benzene is 152kJ per mole more stable than expected. This value is known as the RESONANCE ENERGY. Theoretical - 360 kJ mol-1 (3 x -120) 2 - 120 kJ mol-1 MORE STABLE THAN EXPECTED by 152 kJ mol-1 3 Experimental - 208 kJ mol-1 Consider the electron configuration of carbon The electronic configuration of a carbon atom is 1s22s22p2 2p 2 2s 1 If you provide a bit of energy you can promote (lift) one of the s electrons into a p orbital. The configuration is now 1s22s12p3 1s 2p 2 2s 1 Why would this be favourable? 1s Hybridisation of orbitals The four orbitals (1 x s and 3 x p) combine (HYBRIDISE) to give 4 new orbitals. All four orbitals are energetically equivalent. 2s22p2 2s12p3 4 x sp3 HYBRIDISE sp3 HYBRIDISATION Hybridisation of orbitals Alternatively, only 3 orbitals (1 x s and 2 x p) combine (HYBRIDISE) to give 3 new orbitals. All 3 orbitals are energetically equivalent. The remaining 2p orbital is unchanged. 2s22p2 2s12p3 3 x sp2 2p HYBRIDISE sp2 HYBRIDISATION 2s22p2 Hybridisation of orbitals 1 3 2s 2p HYBRIDISE 3 x sp2 sp2 HYBRIDISATION 2s22p2 2s12p3 HYBRIDISE sp3 HYBRIDISATION 4 x sp3 2p Alkanes vs Alkenes In ALKANES, the 4 sp3 orbitals repel each other into a tetrahedral arrangement. In ALKENES, the 3 sp2 orbitals repel each other into a planar arrangement and the 2p orbital lies at right angles to them Alkenes Covalent bonds are formed by overlap of orbitals. The resulting bond is called a SIGMA (δ) bond. An sp2 orbital from each carbon overlaps to form a single C-C bond. Alkenes The two 2p orbitals also overlap. This forms a second bond; it is known as a PI (π) bond. For maximum overlap and hence the strongest bond, the 2p orbitals are in line. This gives rise to the planar arrangement around C=C bonds and a shorter bond than in the corresponding alkane (0.134nm). Ethene 2 sp2 orbitals overlap to form a sigma bond between the 2 carbon atoms s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds 2 2p orbitals overlap to form a pi bond between the 2 carbon atoms the resulting shape is planar with bond angles of 120º Structure of benzene – a delocalised structure Theory: Instead of three localised (in one position) double bonds, the six p (p) electrons making up those bonds are delocalised around the ring by overlapping the p orbitals. There would be no double bonds and all bond lengths would be equal. It would also give a planar structure. 6 single bonds one way to overlap adjacent p orbitals another possibility delocalised pi orbital system This final structure fits with the reactivity of benzene: 1. Very stable 2. Resistant to electrophilic addition, typically seen in alkenes. 3. Undergoes electrophilic substitution, which does not affect the delocalised pi orbital system. Benzene X-ray studies show that, • A Benzene molecule is a flat (planar) molecule. All carbon and hydrogen atoms lie in the same plane. • It has a regular hexagon structure with all six carbon atoms lying at the corners. Each carbon atom is bonded to three other atoms. • All carbon-carbon bond lengths are equal at 139 pm. • All CC angles (or CH angles) are equal at 120°. IR spectra show that benzene does not have the typical strong absorptions of CH bonds in CH2 and CH3 groups in the wavenumber range 2962-2853 cm-1 , nor the C=C absorption of an alkene, like oct-1-ene, just below 1700 cm-1. Instead, and unlike alkanes and alkenes, benzene has strong absorptions at about 3050 cm-1and 750 cm-1. All this provides further evidence that benzene does not have normal C-C or C=C bonds in its structure. Naming arenes Be careful, this can be confusing! How do you name benzene derivatives with more than one substituent? • Names use either phenyl (C6H5- notation) or the benzene notation. • General rule is that when a hydroxy (OH) or amine (NH2) group is substituted into the ring, they use the PHENYL notation – Phenol and phenylamine respectively. • In majority of other cases, compounds named as substituted products (derivatives) of benzene, e.g. nitrobenzene (C6H5-NO2) or methylbenzene (C6H5-CH3) 1,3,5-trichlorobenzene 1-bromo-4-chlorobenzene phenylamine Benzene – Key reactions Benzene reacts mainly via electrophilic substitution. The electron density of the benzene ring attracts electrophiles. Reactions are NOT addition. Instead, electrophiles substitute into the ring, maintaining the delocalised structure and stability of the ring. N.B. Benzene can be drawn as either: BUT remember, benzene IS NOT made up of alternating single and double bonds! Reactions of benzene (derivatives) The reactions of arenes • The simplest and most important arene is benzene. Unfortunately, benzene is toxic and mildly carcinogenic, so it cannot be used except in research and certain industrial processes. Fortunately, the reactions of benzene are also given by many of its derivatives and in the experiments below you will be using methoxybenzene • Besides studying the reactions of arenes you will also be comparing arenes with alkanes and alkenes by repeating the methoxybenzene tests with cyclohexane and cyclohexene. • Wear goggles for the whole of this class practical and remember to keep bottles of methoxybenzene, cyclohexane and cyclohexene well away from any flames. A Combustion • Working in a fume cupboard, put 3 drops of methoxybenzene on a small pea-sized ball of mineral wool in a crucible. Set fire to the methoxybenzene using a lighted splint. • Repeat the test with cyclohexane and then with cyclohexene. 1. Describe and compare the flames from each compound. 2. Write an equation for the combustion of methoxybenzene bearing in mind what you have observed. B Bromination • Carefully, add 5 drops of methoxybenzene to 1 cm3 of 2% bromine in an inert solvent. Dip a glass rod in concentrated ammonia and bring this near the mouth of the test tube to see if fumes of hydrogen bromide have been produced. • Repeat the test with cyclohexane and then with cyclohexene. Describe what happens with each compound. 1. Which of the compounds a) reacted with the bromine b) reacted to produce hydrogen bromide? 2. Which of the compounds have taken part in a) substitution reactions b) addition reactions? (Hint: Would HBr be produced in an addition reaction?) Write possible equations for the reactions which have occurred. (Remember that the reactions of methoxybenzene will involve the benzene ring.) C Nitration • Carefully add 1 cm3 of concentrated nitric acid to 1 cm3 of water. Then, add 5 drops of methoxybenzene and warm the mixture in a water bath. Describe what you observe. 1. When methoxybenzene reacts with nitric acid, one of the products is methoxynitrobenzene, CH3O-C6H4-NO2. Write a possible equation for the reaction. 2. In this class practical, like many others with organic chemicals, you have used toxic, corrosive and highly flammable materials. What steps have been taken in this practical to use them safely and successfully? Key reactions of benzene Nitration: Reagents conc. nitric acid and conc. sulphuric acid (catalyst) Conditions heat under reflux at 55°C Equation C6H6 + HNO3 Halogenation: Reagents chlorine and a halogen carrier (catalyst) Conditions reflux in the presence of a halogen carrier (Fe, FeCl3, AlCl3). C6H5NO2 + H2O nitrobenzene Chlorine is non polar so is not a good electrophile. The halogen carrier is required to polarise the halogen Equation C6H6 + Cl2 C6H5Cl + HCl Key reactions of benzene Sulfonation Reagents ‘fuming’ sulfuric acid (mixture of conc H2SO4 and dissolved SO3 – conc H2SO4 at r.t. does not react in this way) Conditions room temperature [H+] Equation C6H6 + SO3 Hydrogenation Reagents H2 in presence of Ni catalyst Conditions Heat at 200ºC Equation C6H6 + 3H2 C6H5SO3H C6H12 (benzene sulfonic acid) Key reactions of benzene Friedel-Crafts: Alkylation Overview Alkylation involves substituting an alkyl (methyl, ethyl) group – hard to limit substitutions (alkyl groups activate ring) Reagents Halogenoalkane (RX) and anhydrous aluminium chloride AlCl3 Conditions reflux Electrophile a carbocation R+ (e.g. CH3+) Equation C6H6 + C2H5Cl C6H5C2H5 + HCl Industrial Alkylation Industrial Alkenes are used instead of haloalkanes but an acid must be present Phenylethane, C6H5C2H5 is made by this method Reagents ethene, anhydrous AlCl3 , conc. HCl Electrophile C2H5+ (an ethyl carbonium ion) Equation C6H6 + C2H4 Mechanism the HCl reacts with the alkene to generate a carbonium ion electrophilic substitution then takes place as the C2H5+ attacks the ring Use ethyl benzene is dehydrogenated to produce phenylethene (styrene); this is used to make poly(phenylethene) - also known as polystyrene C6H5C2H5 (ethyl benzene) Key reactions of benzene Friedel-Crafts: Acylation Overview Acylation involves substituting an acyl (methanoyl, ethanoyl) group. Electron withdrawing carbonyl ensures monosubstitution Reagents Acyl chloride (RCOX) and anhydrous aluminium chloride AlCl3 Conditions Reflux 50°C Electrophile Equation RC+= O ( e.g. CH3C+O ) C6H6 + CH3COCl C6H5COCH3 + HCl Mechanisms of substitution Describe the mechanism of the electrophilic substitution reactions of benzene in: • Nitration • Friedel-Crafts reactions • Halogenation – including the formation of the electrophile Electrophilic substitution - steps 1. Formation of the electrophile e.g. NO2+ 2. Electrons from ring form intermediate Delocalisation remains over 5 carbons 3. H+ removed from intermediate reforming ring of delocalised electrons 4. Reform catalyst Formation of the electrophile • Nitration HNO3 + H2SO4 H2NO3+ + HSO4- H2O + NO2+ + HSO4- • Alkylation CH3CH2Cl + AlCl3 CH3CH2+ + AlCl4• Acylation CH3COCl + AlCl3 CH3CO+ + AlCl4• Halogenation Br2 + FeBr3 Br+ + FeBr4- Phenol melting point (°C) boiling point (°C) C6H5OH 40 - 43 182 C6H5CH3 -95.0 111 Increased acidity: delocalisation of charge Decrease acidity: high electronegativity of oxygen Very weak acid Reactions of phenol Reactivity of phenol • Increased electron density • Easier to donate electrons Amines • Methylamine: CH3NH2 • Dimethylamine: (CH3)2NH • Trimethylamine: (CH3)3N Reactions of amines i characteristic smell Fishy! ii miscibility with water Short chain alkylamines – soluble Long chain or arylamines – insoluble Increasing solubility: hydrogen bonding and reaction with water Decreasing solubility: Non-polar organic chains and rings Aryl amines less alkaline than alkyl amines – why? iii formation of salts with acid Form ionic salts that are more soluble than the original amine R-NH2 + HCl RNH3+ + Cl- Hydrated metal ions 2+ H O H H H H O O Cu H H H Hexaaquacopper (II) ion H H O O O Covalent bond H H Dative bond Hydrated metal ions H H O H Cu Dative bond H H H O O-H O Covalent bond H H O O-H (s) An ammine complex ion 2+ H O H NH3 H3N Cu NH3 H3N H O H Tetraamminediaquacopper (II) ion Reactions of amines iv complex ion formation with copper(II) ions Lone pair of electrons on nitrogen forms dative bonds to metal cation v treatment with ethanoyl chloride and halogenoalkanes Amines act as nucleophiles Reactions of amines Cause of formation of secondary, tertiary and quaternary amines Preparation of phenylamine 1. Nitration of benzene 2. Reduction of nitrobenzene N.B. LiAlH4 is not a suitable reagent Preparation of phenylamine Formation of diazonium ions NaNO2/HCl (forms HNO2 in situ) • RNH2 R-N+≡N Cl• Alkyl diazonium ions decompose on formation to form R+ + N2 (g) • R+ joins with nearest nucleophile to form e.g. ROH or RCl • Benzenediazonium ions are stable below 10oC • Why is this reaction done at 5oC? Reactions of benzenediazonium ion • If allowed to warm above 10oC: – forms benzene cation and nitrogen gas – cation reacts to form e.g. phenol or chlorobenzene • Reaction with phenol or phenylamine class compounds – azo dyes – N.B. In fact the phenol has formed phenoxide in the solution of sodium hydroxide Azo dyes • Predict the structure of the azo dye formed between benzenediazonium and napthalene-2-ol Homework Synthetic techniques Tuesday: organic preparation practical assessment Produce a hand illustrated description of each of these techniques: • refluxing • purification by washing, eg with water and sodium carbonate solution • solvent extraction • recrystallization • drying • distillation • steam distillation • melting temperature determination • boiling temperature determination Review questions 1-3 p 206 Amines and amides • Why is the amine basic when the amide isn’t? What would happen if… • 1,6-diaminohexane came into contact with hexan1,6-dioyl chloride? • Draw the products • What class of reaction is this? Polymers • Addition – – – – Alkenes Double bonds open Initiator starts reaction e.g. peroxide Polystyrene Addition polymers POLYTHENE Addition polymers POLYTETRAFLUOROETHENE (PTFE – Teflon) Addition polymers POLYPROPENE Addition polymers POLYCHLOROETHENE (PVC) Polymers • Condensation – Diol and dioyl chloride/dioic acid or diamine and dioyl chloride/dioic acid – Small molecule eliminated Condensation polymerisation Formation of nylon Simplified condensation polymerisation H2N--NH2 HOOC--COOH H2N--NH2 HOOC--COOH -HN--NHOC--COHN--NHOC--COH-O-H H-O-H H-O-H chloroethene tetrafluoroethene propene Poly(propene) Poly(tetrafluoroethene) Poly(chloroethene) Properties of polymers Nylon (polyamide) • Soften above their melting temperatures, Tm, thermoplastics • The amorphous regions contribute elasticity and the crystalline regions contribute strength and rigidity. • The planar amide (-CO-NH-) groups are very polar, so nylon forms multiple hydrogen bonds among adjacent strands. Because the nylon backbone is so regular and symmetrical nylons often have high crystallinity and make excellent fibres. • Parallel strands can participate in extended, unbroken, multi-chain β-pleated sheets, a strong and tough supermolecular structure similar to that found in natural silk. • When dry, polyamide is a good electrical insulator. However, polyamide is hygroscopic. Nylon is less absorbent than wool or cotton. Properties of polymers Poly(ethenol) (PVA, Polyvinyl alcohol ) • Water soluble polymer • It is odourless and nontoxic. • It has high tensile strength and flexibility, as well as high oxygen and aroma barrier properties. • However these properties are dependent on humidity. The water, which acts as a plasticiser, will then reduce its tensile strength. • It is fully degradable and dissolves quickly • Used as a water-soluble film useful for packaging. An example is the envelope containing laundry detergent in "liqui-tabs". Amino acids i acidity and basicity and the formation of zwitterions ii separation and identification by chromatography iii effect of aqueous solutions on plane polarised monochromatic light iv formation of peptide groups in proteins by condensation polymerization v reaction with ninhydrin. Proteins Structure • Where R can be a variety of groups e.g. -H glycine -CH3 alanine -CH2CH2COOH glutamic acid -CH2OH serine -CH2SH cysteine -CH2CH2CH2CH2NH2 lysine • Proteins are pH sensitive – predict the structure of an amino acid at pH1, 7 and 12 • In neutral solutions the acid group (-COOH) loses its hydrogen ion becoming negative and the amine group gains a hydrogen ion to become positive: • H3N+CH2COOZwitter ion Isoelectric point • The charges cancel so it would not be attracted towards either a positive or negative electrode (iso = same) • In acidic conditions (pH < 7 e.g. fruit or vinegar) hydrogen ions are added to the –COO• H3N+CH2COOH • This would be attracted towards the cathode (negative electrode) • In alkaline conditions (pH > 7 e.g. baking soda) hydrogen ions are taken from the positive amine group • H2NCH2COO• This would be attracted towards the anode (positive electrode) • The acidic (e.g. glutamic acid and aspartic acid) and basic (e.g. ornithine, arginine, lysine and histidine) side chains are also affected • Different proteins have their isoelectric points at different pHs depending on the combination of acidic and basic side groups • pH effects proteins because removing charges from side chains removes the electrostatic attractions between different sections and the tertiary structure unravels (denatures) e.g. souring milk – pH 4.6 the protein separates as curd The two enantiomers of alanine, D-Alanine and L-Alanine Of the standard α-amino acids, all but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amino acids are found in some proteins produced by enzyme posttranslational modifications Polymerisation - condensation Using chromatography Chromatography involves the separation and identification of compounds using a stationary phase (solid) and a mobile phase (liquid or gas). Identifying amino acids - ninhydrin and Rf values