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Organic Chemistry : Alcohol

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Organic Chemistry : Alcohol

  1. 1. 5.0 Alcohol ~ Organic compound with at least one hydroxyl group (–OH) which act as functioning group. Alcohol has the general formula of CnH2n+1OH or sometimes CnH2n+2O. The naming of alcohol end with ~ol. 5.1 Nomenclature of alcohol (naming & classifying alcohol) The way of naming alcohol is similar to the way of naming alkene [1] Find the longest carbon chain with –OH in it, and name accordingly [2] Identify the alkyl group that attached towards the “parent” chain and name the alkyl [3] Give the prefic of di- ; tri- or tetra based on how many similar alkyl attached toward it [4] Give the numbering of alkyl based on the carbon number based on alcohol
  2. 2. CH3CH2CH(CH3)CH2OH C(CH3)3CH(OH)CH2CH3 C(CH3)3CH2C(OH)(CH3)2 4-methylpentan-2-ol 3,3-dimethylbutan-2-ol 3-ethyl-2,4-dimethylpentan-2-ol 3-methylpentan-2-ol 3,4-dimethylheptan-4-ol 5-ethyl- 2,5-dimethylheptan-2-ol 4-methylpentan-1-ol 4-ethyl-2-methylhexan-2-ol 5-ethyl-2-methylheptan- 3-ol 2-methylbutan-1-ol 2,2-dimethylpentan-3-ol 2,4,4-trimethylpentan-2-ol
  3. 3. Isomerism in alcohol. Alcohol may exhibit structural isomerism and in some case, optical isomerism. For example, butanol, C4H9OH, may have 5 different isomers
  4. 4. Practice : write out all the possible isomers for pentanol, C5H11OH
  5. 5. 5.2 Physical properties of alcohol (A) Boiling point of alcohol Similar to other organic compounds, the boiling point of alcohol increased with number of carbon Similar to other organic compounds, the boiling point ……………. with the number of carbon as the weak Van Der Waals forces increase with ………………………… of the compound. Though, the hydrogen bonding are weaker when goes down to homologous series as the polarity of molecules ……………… as the number of carbon increase. Similar too, to other organic compound, alcohol with more branch has lower boiling point than a straight-chain molecule Alcohol CH3OH C2H5OH C3H7OH C4H9OH C5H11OH C6H13OH C7H15OH C8H17OH Boiling point oC 46 78 90 115 135 152 169 190 Boiling point trend BOILING POINT INCREASE DOWN HOMOLOGUS SERIES increase molecular mass decrease
  6. 6. Straight chain molecules have higher boiling point compare to branched chain because straight chain molecule has a ………… surface area than a branched chain molecule. The more branches attached to the parent chain, the ……………. the surface area ; …………… the forces of attraction between molecules ; ………… the boiling point. Molecules Butan-1-ol Butan-2-ol 2-methylpropan-2-ol Boiling point (0C) 117 99 82 larger smaller weaker lower
  7. 7. The number of the hydroxyl group in an organic compound is also one of the major factor which contribute to its boiling point The boiling point of the alcohol increase with the number of –OH. This is a result caused by more ………………….. bond formed between –OH of the molecules. So the more the –OH ; stronger the hydrogen bond ; higher the boiling point. Molecules Butan-1-ol Butan-1,2-diol Butan-1,2,3-triol Boiling point (0C) 117 208 274 hydrogen
  8. 8. Compare to alkane and haloalkane, alcohol has a higher boiling point Alcohol has the highest boiling point compare to other organic compound because it forms strong ……..…………. bond between the molecules. Fluoroethane has a higher boiling point than propane as fluoroethane is a ……………………. molecules and so, the weak ………………………… forces are stronger than propane since propane is a …………………..…. molecule. Compound Ethanol (C2H5OH) Propane (C3H8) Fluoroethane (C2H5F) Relative molecular mass 46 44 48 Boiling point (oC) 78 – 4.2 7 hydrogen polar Van Der Waals’ Non-polar
  9. 9. B)Solubility of alcohol in water Hydrogen bonding occur between alcohol molecules because of the presence of ……………. group. This bring 2 important consequences toward hydrogen where It cause the boiling points of alcohol higher than those in alkanes and haloalkanes It cause lower alcohol (methanol and ethanol) to be completely miscible with water. hydroxy
  10. 10. Solubility decrease with the increase of number of carbon in alcohol. Butan- 1-ol and pentan-1-ol are slightly miscible with water and the rest become more and more insoluble. This is due to the non-polar properties of alkyl which attached to the –OH, directly influence the efficiency of hydrogen bond, causing the poplar bonding to be more obvious than hydrogen bonding. (dipole-dipole interaction between R- and R- are more obvious) Ethanol is a good solvent for both polar and non polar solute because it contain non polar (………….) group and a polar group (……………) in it. As a result, ethanol is used as solvent in many industries alkyl hydroxyl
  11. 11. (C) Acidity of alcohol Alcohols are generally a weak acid. Table below shows the pKa value of some alcohols and water As shown in the table above, alkyl-alcohol is weaker than phenyl-alcohol. This is a result of the different effect of the group that attached to hydroxyl group –OH. Alkyl is an electron …………..…….. group whereas phenyl is an electron …………..……… group. Compounds Methanol (CH3OH) Ethanol (C2H5OH) Propan-1-ol (C3H7OH) Phenol (C6H5OH) p-methylphenol CH3-C6H5OH Water pKa 15.5 16.0 18.0 10.0 11.0 14.0 donating withdrawing
  12. 12. Alcohol Explanation δ+ δ− CH3CH2–O–H Ethanol Ethanol dissociate in water according to the equation CH3CH2–OH + H2O CH3CH2–O- + H3O+ Alkyl group, which is an electron donating group, donate electron to O and caused the electron density of O in R–OH increase. As a result, O is more readily to accept proton, which makes the equilibrium favours to left. Phenol Phenol dissociate in water according to the equation C6H5–OH + H2O C6H5–O- + H3O+ The phenyl group is an electron-withdrawing group, which withdrawn the electron density from partially negative charge, δ−, from O making O less readily to accept proton. As a result, O is more readily to donate proton which makes equilibrium favour more to right.
  13. 13. 5.3 Chemical properties of alcohols 5.3.1 Preparation of alcohol in industries. Alcohol can be prepared by a few methods in industries / laboratory 1. Fermentation 2. Hydration of alkene (see Chapter 2) 3. Hydrolysis of haloalkane (see Chapter 4) 4. Grignard reagent (see Chapter 4) Name of reaction Reagent used and condition Equation Fermentation of glucose Zymase enzyme C6H12O6 2 CH3CH2OH + 2 CO2 Glucose ethanol carbon dioxide
  14. 14. Name of reaction Reagent used and condition Equation Hydration of alkene Steam (H2O) --------- Phosphoric acid, (H3PO4 ) At 300oC ; 60 atm @ Concentrated H- 2SO4 at 800C. Hydrolysis of haloalkane NaOH (aq) under reflux 1-chloropropane sodium propan-1-ol Hydroxide Reaction of Grignard reagent Aldehyde / ketone with H2SO4 CH3CH2MgBr + CH3CH=O CH3CH2CH(OH)CH3  → + OH3
  15. 15. 5.4 Chemical reaction of alcohol Aliphatic alcohol undergoes 2 types of reaction which involve R–O–H where : ⇒ Fission of O – H ⇒ Fission of C – O • Formation of alkoxide • Formation of ester • Oxidation of alcohol • Dehydration of alcohol • Reaction with hydrogen halide • Reaction with phosphorous halide (PX5) or thionyl chloride, SOCl2 Name of reaction Reagent used and condition Equation Formation of alkoxide Sodium (Na) 2 CH3CH2O–H + 2 Na 2 CH3CH2O–Na+ + H2 (g) Ethanol sodium sodium ethoxide hydrogen Esterification Carboxylic acid (R–COOH) catalysed by conc. sulphuric acid (H2SO4) Ethanol propanoic acid ethyl propanoate water
  16. 16. Name of reaction Reagent used and condition Equation Oxidation of alcohol Acidified KMnO4 or acidified K2Cr2O7 + heat propan-1-ol propanal propanoic acid propan-2-ol propanone Dehydration (removal of water) from alcohol Excess conc. H2SO4 at 1800C or Alumina (Al2O3) at 350oC Halogenation of alcohol Phosphorous pentachloride (PCl5) CH3CH2CH2OH + PCl5 CH3CH2CH2Cl + POCl3 + HCl
  17. 17. (1) Reaction with sodium metal When sodium is added to alcohol, a white solid (sodium alkoxide) formed and effervescences occur and hydrogen is released. Example : 2 CH3CH2CH2OH + 2 Na 2 CH3CH2CH2O-Na+ + H2 Propan-1-ol sodium propoxide Sodium alkoxide formed dissolve readily in water to form back alcohol + sodium hydroxide The reaction is slower than when sodium reacts with water. Reactivity decrease with the class increase 30 alcohol < 20 alcohol < 10 alcohol (less reactive) Sodium hydroxide (NaOH) cannot react with aliphatic alcohol. a) CH3CH(OH)CH3 + Na b) C(CH3)2(OH)CH2CH3 + K c) CH3C(CH3)(OH)CH2CH3 + Na CH3CH(O–Na+)CH3 + ½ H2 C(CH3)2(O–K+)CH2CH3 + ½ H2 CH3C(CH3)(O–Na+)CH2CH3 + ½ H2
  18. 18. (2) Esterification : Formation of ester When excess alcohol (R–OH) react with carboxylic acid (R”COOH) and catalysed by a few drops of concentrated sulphuric acid and heat under reflux. R–OH + R”COOH R”COO–R + H2O Alcohol Carboxylic acid Ester Water CH3CH2OH Ethanol CH3COOH Ethanoic acid CH3CH2CH2OH Propan-1-ol CH3CH2CH2COOH Butanoic acid CH3CH2CH2CH2OH Butan-1-ol CH3CH2COOH Propanoic acid CH3CH2OH Ethanol CH3CH2COOH Propanoic acid CH3COOCH2CH3 Ethyl ethanoate H2O CH3CH2CH2COOCH2CH2CH3 propyl butanoate H2O CH3CH2COOCH2CH2CH2CH3 butyl propanoate H2O CH3CH2COOCH2CH3 ethyl propanoate H2O
  19. 19. Esterification can also be achieved by replacing carboxylic acid with alkanoyl chloride Example : ethanol ethanoyl chloride ethyl ethanoate hydrogen chloride Note that in the reaction above, no acidic medium is required. Compare to carboxylic acid, alkanoyl chloride is more reactive than carboxylic acid. Also, the reaction produces a white fume of hydrogen chloride as side product.
  20. 20. (3) Oxidation of alcohol Using strong oxidising agent such as acidified potassium dichromate [K2Cr2O7 / H+], alcohol can be oxidise to form carbonyl compound and even to carboxylic acid. Using different categories of alcohol, different type of carbonyl compounds are formed. Class Example Reaction 10 alcohol (methanol) CH3OH 10 alcohol CH3CH2CH2OH propan-1-ol 20 alcohol CH3CH(OH)CH3 propan-2-ol 30 alcohol CH3C(CH3)(OH)CH3 2-methylpropan-2-ol No reaction
  21. 21. Note the following changes occur in the oxidation of alcohol Oxidation of primary (10) alcohol will yield an aldehyde while oxidation of secondary (20) alcohol will yield a ketone. Aldehyde formed from 10 alcohol can be further oxidised to form carboxylic acid. For the case of methanal, further oxidation of methanal will yield carbon dioxide and water. Tertiary (30) alcohol is not oxidised when react with strong oxidising agent as it does not have H attached to the C–OH. The differences in behaviour of alcohols toward oxidising agents may be used to distinguish between 10 alcohol, 20 alcohol and 30 alcohol. So, this is consider a basic test to distinguish between the class of alcohol used.
  22. 22. Alcohol Product Alcohol Product CH3CH2CH2COOH CH3CH2COCH2CH3 CH3CH2COCH3 C(CH3)3COOH No reaction CH(CH3)2COCH3
  23. 23. In industries, oxidation of alcohol is carried under catalytic dehydrogenation, where hydrogen is removed from the alcohol, forming aldehyde, ketone and even an alkene. Note that, unlike oxidation using acidified potassium manganate (VII), here, the side product is hydrogen gas. Furthermore, aldehyde and ketone formed are not further oxidised. Class Example Reaction 10 alcohol CH3CH2CH2OH propan-1-ol 20 alcohol CH3CH(OH)CH3 propan-2-ol 30 alcohol CH3C(CH3)(OH)CH3 2-methylpropan-2-ol
  24. 24. (4) Dehydration of alcohol Dehydration of alcohol is an elimination reaction where water is removed from organic compound. Dehydration of alcohol can be carried out under these conditions : Heating mixture of excess concentrated acid such as H2SO4 at 1800C Passing alcohol vapour over aluminium oxide (Al2O3) as catalyst at 3000C. Dehydrating 1o alcohol will yield only 1 product whereas dehydrating 2o alcohol will yield 2 products.
  25. 25. Class Example Result 10 alcohol CH3CH2CH2OH propan-1-ol Propan-1-ol propene 20 alcohol CH3CH2CH(OH)CH3 butan-2-ol 30 alcohol CH3CH2C(CH3)(OH)CH3 2-methylbutan-2-ol
  26. 26. The major/minor products of the alkene formed followed Saytzeff’s Rule where alkene containing the greater alkyl is predominant. (H atom from a lesser C–H is preferably to be eliminated) However, if excess alcohol react with concentrated H2SO4, ether is given off. 2 CH3CH2CH2OH CH3CH2CH2–O–CH2CH2CH3 + H2O Propan-1-ol dipropyl ether Same result is given off by using aluminium oxide (Al2O3). The ease of dehydration increase in order from 10 alcohol < 20 alcohol < 30 alcohol. Example : Write out the possible products for dehydration of these alcohols 1. CH3CH2CH(OH)CH2CH3 2. CH(CH3)2CH(OH)CH3 3. C(CH3)3OH CH3CH2CH=CHCH3 CH(CH3)2CH=CH2 + C(CH3)2=CHCH3 C(CH3)2=CH2
  27. 27. (5) Halogenation of alcohol – formation of haloalkane As introduced in the earlier chapter, haloalkane can be prepared by adding conc. hydrochloric acid (HCl) with the aid of zinc chloride, ZnCl2 to alkene. This mixture is called as Lucas reagent. Halogenation can also be carried out using halogen rich compound, such as phosphorous (V) pentachloride (PCl5) or thionyl chloride (SOCl2). CH3CH2CH2OH + HCl (conc) propan-1-ol CH3CH2CH(OH)CH3 + PCl5 butan-2-ol CH3C(CH3)(OH)CH3 + SOCl2 2-methylpropan-2-ol CH3CH2CH2Cl + H2O CH3CH2CH(Cl)CH3 + HCl + POCl3 C(Cl)(CH3)3 + HCl + SO2
  28. 28. To prepare a bromoalkane, reagent used is concentrated hydrobromic acid, HBr, catalysed by concentrated sulphuric acid CH3CH2CH2OH + HBr (conc) Propan-1-ol CH3CH2CH2Br + H2O
  29. 29. 5.4 Phenol and Aromatic alcohol 5.4.1 Manufacturing of phenol There are 3 methods of making phenol. # The cumene process # The hydrolysis of chlorobenzene / diazonium salt # Alkali fusion with sodium benzenesulphonate (1) Synthesising phenol – cumene reaction Step 1 : Formation of cumene using benzene ring and propene. Benzene propene cumene Step 2 : Oxidation of cumene.
  30. 30. Step 3 : Decomposition by sulphuric acid : Migration of phenyl group cumene hydroperoxide phenol propanone (2) Hydrolysis of chlorobenzene : Dow Process Phenol has been process using Dow process widely in chemical industries. It generally involve 2 steps. Step 1 : Hydrolysis of chlorobenzene by NaOH to form phenoxide salt. chlorobenzene sodium phenoxide Step 2 : Distillation of phenoxide salt mixed with hydrochloric acid. Sodium phenoxide phenol
  31. 31. (3) Hydrolysis of diazonium salt In laboratory, phenol is prepared by hydrolysis of diazonium salt. Effervescence occur and a colourless gas is given out, along with the white fume of hydrogen chloride benzenediazonium chloride water phenol The formation of azo will be discussed extensively when we are in amine later part
  32. 32. 5.5 Chemical reaction of phenol The –OH act as ring activating groups when attached to benzene. As a result, it activates the rings and cause benzene to be more reactive. Consequently, phenol is more reactive toward electrophilic substitution than benzene. (1) Halogenation of phenol When bromine water is added to phenol at room temperature, brown colour of bromine water decolourised and formed a white precipitate of 2,4,6- tribromophenol. (2) Nitration of phenol When concentrated nitric acid (HNO3) is used to react with phenol, it formed a yellow crystalline solid of 2,4,6-trinitrophenol. This yellow crystal is used in dyeing industries and make explosive
  33. 33. (3) Reaction with iron (III) chloride, FeCl3 When a few drops of iron (III) chloride solution is added to phenol, a violet- blue colouration produced. A methylphenol produce blue colour.
  34. 34. 5.5 Chemical Test to distinguish between alcohols Differentiate Chemical test Observation Methanol (CH3OH) and other alcohol Acidified potassium manganate (VII) Positive test : Methanol Purple colour of potassium manganate is decolourised. Effervescence (Bubbling) occurs. Gas released turn lime water chalky Equation : CH3OH H2C=O CO2 + H2O Ethanol (C2H5OH) and other alcohol Iodoform test Positive test : Ethanol Add NaOH then iodine and heated gently. Pale yellow crystal of triiodomethane is formed. Equation : CH3CH2OH + 4I2 + 6 CHI3 + 5 I- + HCOO- Alkan-2-ol (R-CHCH3) OH and other alcohol Iodoform test Positive test : alkan-2-ol Add NaOH then iodine and heated gently. Pale yellow crystal of triiodomethane is formed. Equation : R–CH(OH)CH3 + 4 I2 + 6 CHI3 (s) + RCOO– + 5 I– + 5 H2O → ]O[→ ]O[
  35. 35. Phenol with other alkylalcohol Bromine water Positive test : Phenol Add bromine water directly to phenol. The brown colour of bromine water is bleached instantly and a white precipitate is formed. Equation : refer above Iron (III) chloride Positive test : Phenol Add iron (III) chloride solution to phenol and a violet-blue solution formed instantly.
  36. 36. CH3CH2CH2CH2CH2OH + HBr CH3CH2CH2CH2CH2Br + H2O Mol of 1-bromopentane formed = 15.0 / 5(12) + 11(1) + 80 = 0.09934 mol Since only 60% ; mol of 1-bromopentane should be formed = 0.09934 x 100 / 60 mol = 0.1657 mol Mass of pentan-1-ol required = 0.1657 x [ (5(12) + 12(1) + 16) ] = 14.6 g (3.s.f. with unit) Excess concentrated H2SO4 under reflux/ Al2O3 heated strongly CH3CH2CH2CH2CH2OH CH3CH2CH2CH=CH2 + H2O
  37. 37. KMnO4 / H+ or K2Cr2O7 / H+ under reflux CH3CH2CH2CH2CH2OH + KMnO4 / H+ CH3CH2CH2CH2COOH + H2O CH3COOH catalysed by conc. H2SO4 under reflux / CH3COCl CH3CH2CH2CH2CH2OH + CH3COOH CH3CH2CH2CH2OCOCH3 + H2O
  38. 38. 100 Type of reaction : elimination reaction
  39. 39. Reagent : PCl5 Observation : White fume released by leaf alcohol while the other does not Equation : CH3CH2CH=CHCH2CH2OH + PCl5 CH3CH2CH=CHCH2CH2Cl + POCl3 + HCl
  40. 40. C10H20O 156 alkene alcohol
  41. 41. Citronellol : optical isomerism Geraniol : geometrical isomerism Observation : brown colour of aqueous bromine decolourised Explanation : due to the presence of unsaturated C=C Equation :
  42. 42. chlorine gas Electrophilic aromatic substitution neutralisation Bromine water white precipitate is formed brown colour of bromine remain unchanged
  43. 43. Carbon attached with –OH, that was surrounded by 1 carbon secondary primary tertiary
  44. 44. orange green Isomer 3
  45. 45. 5
  46. 46. (i) sodium metal (ii) Br2 (aq) (iii) NaOH(aq) (iv) CH3COCl
  47. 47. (v) hot acidified K2Cr2O7 (vi) PCl5
  48. 48. B (pentan-2-ol) B (pentan-2-ol) A yellow precipitate is formed