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ORGANIC CHEMISTRY

ORGANIC CHEMISTRY. AS Module 3. NAMING 1. Look for the longest carbon chain. This gives the base name for your molecule: 1 C = methan- 2 C = ethan- 3 C = propan- 4 C = butan- 5 C = pentan- 6 C = hexan-. E.g. Name me:. 3-methylhexane.

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ORGANIC CHEMISTRY

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  1. ORGANIC CHEMISTRY AS Module 3

  2. NAMING 1 • Look for the longest carbon chain. This gives the base name for your molecule: • 1 C = methan- 2 C = ethan- • 3 C = propan- 4 C = butan- • 5 C = pentan- 6 C = hexan-

  3. E.g. Name me: 3-methylhexane

  4. Functional group = an atom or group in the molecule that determines the chemical properties Recognise the reactive group in the molecule. When you know the functional group you can predict the reactions of the molecule E.g. CH3CH2-OH CH3CH2-Br CH3-CHO CH3-COOH CH2=CH2 CH3-CN FUNCTIONAL GROUPS

  5. -CHO and –CH2OH • Aldehydes have the -CHO grouping.E.g. propanal: • Alcohols have the -CH2OH groupingE.g. propan-2-ol

  6. NAMING 2 • Identify the functional groups/substituents and number the carbons in the chain, starting from one end, to keep the number of the functional group or substituent as low as possible. • Remember that a functional group gets priority for low numbering.

  7. E.g. Name me: 5-chloropentan-2-ol

  8. NAMING 3 • Substituents are named in front of the base name. • Remember di- = 2, tri- = 3 etc. if there is more than one of the particular substituent attached. • And remember to specify positions on the chain.

  9. E.g. Name me: 3,4,4-trichloropentanal

  10. STRUCTURAL = Same molecular formula, atoms(groups) bonded in different places: Chain Position Functional group STEREO = Same molecular formula and structure, atoms(groups) arranged differently in space: Geometrical (cis/trans) Optical (next year) ISOMERISMSTRUCTURAL & STEREO

  11. CHAIN ISOMERISM • Structural isomers with different carbon chains: • E.g. for C5H12:

  12. POSITION ISOMERISM • Structural isomers with different positions for the functional group: • E.g. for C3H7OH:

  13. FUNCTIONAL GROUP ISOMERS • Structural isomers with different functional groups: • E.g. for C4H8O:

  14. HOMOLOGOUS SERIES • Same: • Functional group • Chemical reactions • General formula& • Gradually changing physical properties

  15. ALKANES - SOURCE • From? Crude Oil • By? • 1. Fractional DistillationLearn fractions, order of B.Pts. & uses • 2. Cracking Be able to write an equationE.g. C14H30 can be cracked to give octane and ethene only • C14H30® C8H18 + 3C2H4

  16. THERMAL HIGH T + HIGH P ~800ºC FREE RADICAL PRODUCES MORE ALKENE MOLECULES FOR PETROCHEMICALS CATALYTIC LOWER T + CAT. ~450ºC ZEOLITE VIA CARBOCATION TO GIVE MORE SMALL ALKANES FOR PETROL 2 TYPES OF CRACKING

  17. ALKANES – PHYSICAL PROPERTIES • Symmetrical non-polar molecules • \ Intermolecular forces? • Weak Van der Waal’s • \ Low M.Pts. & B.Pts. compared to most covalent molecules of similar Mr • Also insoluble in water as they cannot form hydrogen bonds with water molecules

  18. ALKANES - REACTIONS • Saturated • Hydrocarbons • Unreactive except for 2 major reactions: • Combustion: E.g. butane • C4H10 + ?O2 4CO2 + 5H2O • Substitution by a halogen e.g. chlorine

  19. FREE RADICAL SUBSTITUTION 1

  20. FREE RADICAL SUBSTITUTION 2

  21. HALOALKANES • Polar molecules. Why? • So dipole-dipole forces and slightly higher M.Pts. etc. than the alkanes • Because of the bond polarity:d+C—Brd- • The carbon is attacked by nucleophiles (?)

  22. Nucleophilic Substitution 1 • Haloalkanes can be converted into: • Alcohols (NaOH(aq) + heat)CH3Br + OH-® CH3OH + Br- • Amines (XS conc. NH3(aq) + heat)CH3Br + 2NH3® CH3NH2 + NH4+ + Br- • Nitriles (KCN(ethanol) + heat)CH3Br + CN-® CH3CN + Br-

  23. NUCLEOPHILIC SUBSTITUTION 2 Note the nucleophilic attack by the CN- ion. The lone pair on the C attacks

  24. CURLY ARROWS • They show movement of an electron pair • They start on a lone pair or on a covalent bond • Remember to show clearly the molecule or ion produced after each stage of the mechanism. Don’t forget charges on ions

  25. LOOK AGAIN! The examiner is very strict about curly arrows in mechanismsNote: an extra C is added to the chain.

  26. Can be converted to carboxylic acids: Reflux with dilute acid (or alkali) E.g.CH3CN + 2H2O ®CH3COO- + NH4+ HYDROLYSIS Can be converted to amines: Heat in hydrogen with a Ni catalyst E.g.CH3CN + 2H2®CH3CH2NH2 REDUCTION NitrilesUseful Intermediates

  27. NUCLEOPHILIC SUBSTITUTION 3 • Ammonia as a nucleophile needs 2 stages:

  28. ELIMINATION FROM A HALOALKANE • Refluxing a haloalkane with KOH dissolved in ethanol produces an alkene. E.g: • CH3CH2CH2Br + OH- ®CH3CH=CH2 + H2O + Br- • Note: The change of solvent leads to a different reactionThe OH- acts as a base here (rather than as a nucleophile) as it picks up a proton.

  29. Elimination Mechanism

  30. ALCOHOLS • Homologous series? • Functional group –OH • Thus high M.Pts & B.Pts for typical covalent molecules, because? • Can form hydrogen bonds between molecules • Thus the smaller alcohols also mix with water.

  31. 3 TYPES OF ALCOHOL • Not Whiskey, Beer, and Wine! • Primary 1º • Secondary 2º • Tertiary 3º • According to the no. of Carbons attached to the Carbon with –OH attached to it:

  32. 1º2º3º 1º 2º & 3º Alcohols

  33. Reactions of Alcohols 1Oxidation • Oxidant of choice: • Acidified potassium dichromate • Colour change: • Orange to green when it oxidises something

  34. Oxidation of 1º Alcohols • 1º gives an aldehyde on heating and distilling off the product straight away: • CH3CH2OH + [O] ® CH3CHO + H2O • But refluxing the alcohol + oxidant gives the acid as the aldehyde is oxidised: • CH3CHO + [O] ® CH3COOH

  35. Oxidation of 2º Alcohols • Here the oxidant will only produce the ketone: • CH3CH(OH)CH3 + [O] ®CH3COCH3 + H2O • Note that the extent of oxidation depends on how many C—H bonds can be broken during the oxidation. The carbon chain does not break unless the oxidation is very vigorous i.e. combustion? • CH3CH2OH + ?O2® 2CO2 + 3H2O

  36. Note that there are no C—H bonds on the carbon attached to the hydroxy group. Therefore a tertiary alcohol will not be oxidised. (Non) Oxidation of 3º Alcohols

  37. Identifying Alcohols • The fact that the alcohols respond differently to oxidation gives us a simple sequence of tests to identify the type: • 1. Try oxidation of the alcoholIf it does not oxidise it is tertiary • 2. If it can be oxidised:Test the product of oxidation to see whether it is an aldehyde:

  38. Tests for aldehydes • Both Tollens & Fehlings can be used.Quote one accurately: • Tollens:Warming an aldehyde with Tollens causes the colourless soln. to give a silver mirror • Fehlings: Warming an aldehyde with Fehlings causes the blue soln. to give a red/brown ppt.

  39. Elimination from Alcohols • Heating an alcohol to 170ºC with conc. H2SO4 produces an alkene as a water molecule is eliminated. • The acid acts as a catalyst • CH3CH2CH(OH)CH3® H2O +mix of CH3CH2CH=CH2and CH3CH=CHCH3depending on which side of the C—OH the proton is removed from.

  40. Elimination Mechanism

  41. ALKENES • Homologous series? • Non-polar Hydrocarbons \ type of intermolecular forces? • Van der Waals\ low M.Pts. Etc. compared to alcohols and immiscible with water. • Exhibit a form of stereoisomerism called Geometrical since there is no free rotation about the double bond:

  42. Cis but-2-ene Trans but-2-ene Geometrical Isomerism

  43. Reactions of Alkenes • The C=C double bond is very reactive since it is a centre of electron density.One of the bonds is weaker than the other and this breaks open on reaction leaving the basic carbon chain intact. • Thus alkenes undergo addition reactions and are attacked by electrophiles i.e? • ELECTROPHILIC ADDITION

  44. Electrophilic Addition Reactions • Alkenes react with: • H—Br (or other hydrogen halides) • Br2 (a good test for alkenes as the brown colour of the bromine quickly fades to colourless) • Conc. H2SO4 (if the product is warmed with water an alcohol can be produced).

  45. Electrophilic Addition Mechanism 1

  46. Addition to Unsymmetrical Alkenes 1 • When an unsymmetrical molecule like H-Br is added to an unsymmetrical alkene like propene, two products are possible but only one is produced in any quantity: • CH3CH=CH2 + H-Br ® CH3CH(Br)CH3Very little of the 1-bromopropane is produced:

  47. Addition to Unsymmetrical Alkenes 2 • Reason? • The 2º carbocation produced on the way to+2-bromopropane: CH3CHCH3is more stable than the 1º carbocation produced on the way to 1-bromopropane: CH3CH2CH2+ • Order of stability of carbocations:3º > 2º > 1º

  48. Addition to Unsymmetrical Alkenes 3 • Remember to draw the carbocations when discussing stabilities • In order to write correct equations, if you are not asked for the mechanism, just remember that:The d+ part of the electrophile attaches to the carbon of the double bond which has most hydrogens(NOT an explanation!)

  49. Electrophilic Addition Mechanism 2

  50. Hydrogenation of Alkenes • Alkene + Hydrogen + Heat with Ni catalyst • Used to convert Unsaturated(?) vegetable oils into more saturated margarine. • The fewer the double bonds the harder the margarine. • E.g: R—CH=CH2 + H2® R—CH2-CH3

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