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Organic Chemistry

Organic Chemistry. Introduction. Important definitions HOMOLOGOUS SERIES – a family of organic compounds which all fit the same general formula, neighbouring members differ from each other by CH 2 , have similar chemical properties and show trends in physical properties.

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Organic Chemistry

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  1. Organic Chemistry

  2. Introduction Important definitions HOMOLOGOUS SERIES – a family of organic compounds which all fit the same general formula, neighbouring members differ from each other by CH2, have similar chemical properties and show trends in physical properties. e.g. the alkanes all fit the general formula CnH2n+2, members of the family include methane (CH4), ethane (C2H6) and propane (C3H8).

  3. H H H H H H H C C C C C H H H H H EMPIRICAL FORMULA – the simplest ratio of moles of atoms of each element in a compound. MOLECULAR FORMULA – the actual number of moles of atoms of each element in a compound. STRUCTURAL FORMULA (aka displayed formula or graphical formula) – shows all of the atoms and the bonds between them. e.g. pentane (C5H12).

  4. A condensed structural formula can also be used which omits the bonds. So for pentane we can use: CH3CH2CH2CH2CH3 or CH3(CH2)3CH3 The formulas in the data book are called SKELETAL formulas. These must NOT be used in the exam. STRUCTURAL ISOMERS – compounds with the same molecular formula but a different arrangement of atoms. e.g. C4H10 represents

  5. H H H H H H C C C C H H H H butane and

  6. H H C H H H C C C H H H H H 2-methylpropane

  7. FUNCTIONAL GROUP – atom or group of atoms which gives an organic compound its characteristic chemical properties.

  8. C H Alkanes suffix – ane For example CH3CH3 ethane

  9. C C Alkenes suffix – ene For example ethene CH2=CH2

  10. C X Halogenoalkanes prefix halo - X = Cl, Br or I For example chloroethane CH3CH2Cl

  11. C OH Alcohols suffix – ol or prefix hydroxy- For example CH3CH2OH ethanol 2-hydroxypropanoic acid CH3CH(OH)COOH

  12. O C H Aldehydes suffix – al For example ethanal CH3CHO

  13. C O Ketones suffix –one or prefix oxo- For example propanone CH3COCH3 3 – oxobutanoic acid CH3COCH2COOH

  14. O C OH Carboxylic acid suffix – oic acid For example ethanoic acid CH3COOH

  15. C NH2 Amines suffix – amine or prefix amino - For example CH3NH2 methylamine aminoethanoic acid H2NCH2COOH

  16. O C OR Esters suffix – oate For example CH3COOC2H5 ethyl ethanoate

  17. H C H C C H Delocalised electrons C C H H C H Aromatic compounds Contain the BENZENE ring formula C6H6

  18. CH3CHCH2CH3 nitro group NO2 COOH 1 Br Cl 1 CH3 2 3 4 Cl Some examples (1-methylpropyl) benzene nitrobenzene 4-chloro-3-methylbenzenecarboxylic acid 1-bromo-2-chlorobenzene

  19. NH2 Sometimes the benzene ring is not regarded as the key part of the molecule. In these cases it is referred to as the PHENYL group. For example phenylamine

  20. CH3 O O CH3CHCH2C CH3CH2CH2C OH H O CH3CH2CCH2CH3 aldehyde butanal ketone pentan-3-one carboxylic acid 3-methylbutanoic acid

  21. alkene alcohol ketone ester carboxylic acid

  22. carboxylic acid ester aldehyde ether amine nitrile

  23. Physical Properties of Organic Molecules Alkanes worksheet What type of intermolecular force would you expect to find between alkanes, halogenoalkanes, aldehydes, ketones, alcohols and carboxylic acids? Use this information to deduce the relative boiling points of these homologous series and their solubility in water.

  24. The Alkanes The alkanes is an homologous series where all members fit the general formula CnH2n+2. They have trends in physical properties e.g. density and m.p. and b.p. all increase with Mr. They all undergo similar chemical reactions. Alkanes are SATURATED HYDROCARBONS. i.e. they contain only single C to C bonds and are made up of C and H atoms only.

  25. Alkanes are obtained from crude oil by fractional distillation. They are mainly used as fuels. The large Mr alkanes do not ignite easily so there is little demand for them as fuels so they are CRACKED to make smaller more useful alkanes and alkenes. Apart from combustion alkanes undergo few chemical reactions. This is for two main reasons:

  26. The bonds in alkanes are relatively strong. • The bonds have a relatively low polarity as the electronegativity of C and H is similar. • As a consequence alkanes can be used as lubricating oils, although they do degrade over time.

  27. Reactions of Alkanes: Combustion: Alkanes burn exothermically to produce carbon dioxide and water if there is a plentiful supply of oxygen. This is known as complete combustion. e.g. CH4 + 2O2 CO2 + 2H2O Write equations for the complete combustion of butane and octane.

  28. If there is a limited supply of oxygen incomplete combustion occurs and carbon monoxide or carbon are formed instead of carbon dioxide. e.g. CH4 + 1½O2 CO + 2H2O CH4 + O2 C + 2H2O What problems do the gases released on combustion of alkanes cause?

  29. Chlorination Methane does not react with chlorine in the dark but in the presence of ultraviolet light reacts to form hydrogen chloride. CH4 + Cl2 CH3Cl + HCl The mechanism for this reaction is known as free radical substitution. Substitution = replacement of an atom or group of atoms by a different atom or group of atoms.

  30. Cl Cl Free radical = species with an unpaired electron. Free radicals are formed by homolytic fission of bonds. In homolytic fission one electron from the shared pair goes to each atom. So

  31. Cl + Cl unpaired electron or Cl2 2Cl. Heterolytic fission of Cl – Cl would result in the formation of Cl+ and Cl-. There are three steps in the mechanism: initiation, propagation and termination

  32. Free radical substitution CH4 + Cl2 CH3Cl + HCl chlorination of methane i.e. homolytic breaking of covalent bonds Overall reaction equation Conditions ultra violet light (breaks weakest bond) excess methane to reduce further substitution

  33. Free radical substitution mechanism Cl2 Cl + Cl CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH3 + Cl CH3Cl CH3 + CH3 CH3CH3 UV Light initiation step two propagation steps termination step minor termination step Also get reverse of initiation step occurring as a termination step.

  34. Further free radical substitutions CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl ultra-violet light excess chlorine Overall reaction equations Conditions

  35. Write down two propagation steps to explain the formation of dichloromethane. Methane reacts in exactly the same way with bromine to form hydrogen bromide together with bromomethane, dibromomethane, tribromomethane and tetrabromomethane. Write down the mechanism for the reaction between chlorine and ethane to form chloroethane. Use the mechanism to explain why small amounts of butane are formed. How could the formation of further substitution products be minimised?

  36. The Alkenes All fit the general formula CnH2n. Are unsaturated hydrocarbons as they contain a C = C. Much more reactive than alkanes. Industrial importance of alkenes: • Making polymers (plastics) • Hydrogenation of vegetable oils to make margarine • Hydration of ethene to make ethanol.

  37. When naming alkenes have to include position of double bond, for example: CH3CH=CHCH3 is but - 2 - ene and CH3CH2CH=CH2 is but -1- ene Draw out and name all of the alkenes with the molecular formula C6H12. Alkenes undergo ADDITION reactions. Two substances combine to form one new substance. Unsaturated molecules are converted to saturated molecules.

  38. H H H H C = C H – C – C – H + H2 H H H H Reactions of Alkenes • Addition of hydrogen (hydrogenation) Alkenes react with hydrogen in the presence of a nickel catalyst at 180 °C to form an alkane. e.g. C2H4 + H2 C2H6 ethene ethane

  39. Most oils are esters of propane-1,2,3-triol (aka glycerol) with 3 long chain carboxylic acids (aka fatty acids). The esters are sometimes called triglycerides. Hydrogenation of these oils produces margarine. • The common fatty acids include • octadeca-9-enoic (oleic) acid – unsaturated acid found in most fats and olive oil • octadeca-9,12-dienoic (linoleic) acid – unsaturated acid found in many vegetable oils such as soyabean and corn oil

  40. CH2OOC(CH2)7CH=CH(CH2)7CH3 CHOOC(CH2)7CH=CH(CH2)7CH3 CH2OOC(CH2)7CH=CH(CH2)7CH3 Above is the triglyceride formed between propan-1,2,3-triol and oleic acid. Hydrogenation using a nickel catalyst and slight pressure removes some of the C=C. This enables the chains to pack together more closely which increases the van der Waals forces and hence m.p. so the oils are solidified forming margarine.

  41. H H H H C = C + Br2 H – C – C – H H H Br Br • Addition of halogens (halogenation) • Halogens react with alkenes at room temperature and pressure in a non-polar solvent to form a dihalogenoalkane. • e.g. C2H4 + Br2 C2H4Br2 ethene 1,2-dibromoethane

  42. Bromine water is used as a test for unsaturation. In the presence of an alkene, bromine water turns from red brown to colourless. Alkanes do not react with bromine water.

  43. C = C + Br2(aq) – C – C – Br Br Bromine Water Test For Alkenes colorless amber colorless

  44. H H C = C H H 3. Reaction with hydrogen halides (hydrohalogenation) Alkenes react with hydrogen halides (HCl, HBr etc.) to form halogenoalkanes. The reaction occurs at room temperature and pressure. e.g. H H + HBr H – C – C – H H Br C2H4 + HBr  CH3CH2Br ethene bromoethane

  45. H H H H C = C + H2SO4 H – C – C – H H H OSO3H H • Hydration (reaction with water) • This can be done in two ways: • a) Addition of concentrated sulphuric acid to form an alkyl hydrogensulphate. Water is then added to hydrolyse the product and produce an alcohol. The sulphuric acid is regenerated.

  46. H H H H H – C – C – H H – C – C – H + H2O H OH H OSO3H + H2SO4 C2H4 + H2SO4 CH3CH2OSO3H CH3CH2OSO3H + H2O C2H5OH + H2SO4 ethanol

  47. H H H H C = C + H2O H – C – C – H H H H OH b) Alkenes can also undergo direct hydration to form an alcohol. Ethene can be converted to ethanol by reaction with steam in the presence of a phosphoric(V) acid (H3PO4) catalyst at a pressure of 60 – 70 atm and a temperature of 300 °C. What advantages and disadvantages does this method have over production of ethanol by fermentation?

  48. 5. Addition Polymerisation The formation of polymers involves alkenes reacting with themselves to form a long chain molecule called a polymer. The individual molecules used to make the polymer are called monomers. Ethene is polymerised to form poly(ethene) nCH2 = CH2 CH2 CH2 n CH2 CH2 is the repeating unit n = about 100 to 10 000

  49. CH2=CHCH3 - CH2 – CH - CH3 - CH2 – CH - CH2=CHC6H5 C6H5

  50. poly(chloroethene) polyvinylchloride PVC CH2=CHCl - CH2 – CH - Cl CF2=CF2 - CF2 – CF2 - Poly (tetrafluoroethene) PTFE Non-stick coating (Teflon)

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