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Topic 20 - Organic chemistry

Topic 20 - Organic chemistry. Introduction- functional groups and naming Nucleophilic substitution reactions Elimination reactions Condensation reactions Reaction pathways Stereoisomerism. 20.1 Ester. Functional group: -COOC- Condensation reaction or esterification

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Topic 20 - Organic chemistry

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  1. Topic 20- Organic chemistry • Introduction- functional groups and naming • Nucleophilic substitution reactions • Elimination reactions • Condensation reactions • Reaction pathways • Stereoisomerism

  2. 20.1 Ester • Functional group: -COOC- • Condensation reaction or esterification • Many fruit-smelling esters • The bond in triglycerides (lipids) • Alcohol + carboxylic acid ester + water • CH3-OH + HOOC-CH3 CH3OOC-CH3 + H2O (The underlined forms water)

  3. Esterification Butanoicacid Methanol Name: • The alcohol part: Stem + yl: Methyl- • The acid part: the salt name; -buthanoate => Methyl-buthanoate

  4. Which names?

  5. Which names? Ethylethanoatepropylethanoate 2-butylethanoate penthylpropanoate

  6. Amine • Relatives to ammonia => weak bases • Functional group –NH2 • H-bonds => higher bp’s, smaller ones are water-soluble etc. • Name: stem + suffix: -ylamine(or prefix amino-) • Methylamine CH3-NH2 • Ethylamine CH3-CH2-NH2

  7. Amide • Functional group: -CONH • Name: stem + suffix: -anamide • Methanamid H-CONH2 • EthanamidCH3-CONH2 • Peptidebond in proteins

  8. Nitrile • Functional group: -CN • Former name: cyanides • Name: stem + suffix: nitrile • Metanenitrile: HCN • Ethanenitrile: CH3-CN

  9. Nucleophiles and electrophiles- oftenneeded in organicreactions • Nucleophile- nucleuslover • Has freeelectronpair and whole or part negative charge • The larger the negative charge - the better the nucleophile • Eg: C=C, H2O, -OH, -CN, NH3 • Electrophile-electronlover • Has wholeor part positive charge • The larger the positive charge - the better the Electrophile • Eg: C=O, H+, C-Cl,

  10. 20.2 Nucleophilic substitution reactions Nu: + R-X  Nu-R + X: It’s important to know the difference of these because they will undergo different forms of nucleophilic substitution reactions

  11. SN2 or SN1? SN2 SN1 Monomolecular = one species in the rate determining step. Rate = k [org] Tertiary halogenalkanes Heterolytic fission of substrate rate determining step Formation of inermediate carbocation Racemix mixture formed • Bimolecular = two species in the rate determining step. Rate = k [org]*[Nu] • Primary halogenoalkanes • Steric hindrance • One-step with transition state • Inversion of configuration

  12. Factors affecting the rate • The halogen in the halogenoalkaneis important for the reaction rate: C-I > C-Br > C-Cl > C-F (cf. bond strength) • The stronger the nucleophile, the faster the SN2-reaction CN- > OH- > NH3 > H2O

  13. SN2 substitution reaction with ammonia • H3N: + R-X R-NH2 + HX • Forming an amine

  14. SN2 substitution reaction with CN- as nucleophile R-X + CN- R-CN+ X-Forming a nitrile The carbon chain will be prolonged with one carbon atom

  15. Reactions with nitriles • Nitrile + H2Ni catalystAmine CH3CH2-CN + H2/Ni  CH3CH2-NH2 • Nitrile + Acidic water Carboxylic acid CH3CH2-CN + H+/H2O  CH3CH2-COOH

  16. 20.3 Elimination reactions • CH3CH2-Br + OH-CH2=CH2 + H2O + Br- Condition: Hot and concentrated sodium hydroxide solution in ethanol. Reflux. • Opposite to addition reaction • 2 kinds; E1 and E2 • If the conditions are different: A substitution reaction CH3CH2-Br + OH- CH3CH2-OH + Br- Will happen in warm dilute solution of sodium hydroxide, the hydroxide ion will work as a nucleophile.

  17. The E1 mechanism (similar to SN1) • Involves heterolytic fission of C-X bond • Tertiary halogenalkane • Via a carbocation intermediate

  18. The E2 mechanism (similar to SN2) • Concerted bimolecular reaction • Primary halogenalkane • Via transition state

  19. Dehydration reaction of alcohol to form alkene • E1 or E2? • Excess of H2SO4, ~170oC

  20. 20.4 Condensation reaction • 2 Organic molecules  1 Organic molecule + water • Acid catalysed • Esterification, etherification • Common in biochemistry, formation of polysaccharides, proteins, nucleic acids

  21. Esterification Name: R-yl R’-oate Esters: flavouring agents (food, perfume), solvents, plastics (polyesters) H2O

  22. Amide condensation AmineCarboxylicacidAmideWater H2O

  23. PolycondensationsPlastics • Polyesters: polyethylene terphtalate (PET) • Polyamides;nylone

  24. Benzene-1,4-dicarboxylic acid + Ethane-1,2-diol Cancondensewith a diol Cancondensewith a dicarboxylicacid Monomers withtwofunctionalgroupsarerequired

  25. Polyamides HOOC-R-COOH + H2N-R’-NH2 H2N-R’-NH-OC-R-COOH Di-amine amide bond HOOC-(CH2)4-COOH + H2N-(CH2)6-NH2  HOOC-(CH2)4-CONH-(CH2)6-NH2 + water hexanedioicacid + 1,6-diaminohexane  Nylon

  26. Reaction pathways 1. Elimination reaction. Hot, concentrated and reflux 2. Substitution reaction. SN1 or SN2 3. Substitution reaction. SN1 or SN2. (Can be substituted up 4 times to a quarternaryammonum salt) 4. Condensation reaction. Acid catalyst (or alkaline catalyst, but more common when hydrolysis of ester). Equilibrium reaction. 5. Condensation reaction. Acid catalyst (or alkaline catalyst). Equilibrium reaction. 6. Nitrile to amin: Reduction with H2 and Ni-catalyst

  27. Stereoisomerism • Different location in space of atoms or groups

  28. Structural isomers • Chain: e.g. n-butane : methylpropane • Positional:e.g. 1-propanol : 2-propanol • Functional groups: Ethanoic acid, CH3-COOH : Methyl methanoate HCOOCH3 Propanal, CH3-CH2-CHO : Propanone, CH3-CO-CH3

  29. Geometric • Double bond = p and s bonds. No free rotation around a double cis-2-buten trans-2-buten • cis-1,2-dichloroethane: bp = 60.3oC • trans-1,2-dichloroethane: b.p. = 47.5oC

  30. cis-but-2-ene-1,4.dioic trans-but-2-ene-1,4.dioic m.p= 286oC m.p.= 130oC with decomposition. Strong H-bond between molecules in trans. Strong H-bond in the molecule in cis.

  31. Cyclic compoundscan also give geometric isomers • Cis and trans

  32. Optical mirror plane enantiomers to each other

  33. Stereo isomers Different location in space. Geometric cis-2-buten trans-2-buten Optical mirror plane enantiomers to each other

  34. A carbon with 4 different substituents is said to have an asymmetric centre or to be chiral. Chiral can also mean the whole molecule. Enantiomers have the same physical properties except that they rotate the plane of polarised light in different directions. Very important which form of the enantiomer in biology and medicine.

  35. Polarimeter • Light: Electromagnetic radiation. Normal light oscillating in all directions. • Plane-polarized light: When normal light is sends throuhg a polarizing filter only waves in the same plan can pass. If two polarizing filters places 90o to each other the light will be compleataly blocked. • Polarimeter:Measure how much the light will rotate when pass through a solution with optically avtive substance. A sample holder between two filter

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