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Hydroxy Compounds

Hydroxy Compounds

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Hydroxy Compounds

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  1. Chapter 33 Hydroxy Compounds 33.1Introduction 33.2Nomenclature of Hydroxy Compounds 33.3Physical Properties of Hydroxy Compounds 33.4Acidic Properties of Hydroxy Compounds 33.5Preparation of Hydroxy Compounds 33.6Reactions of Alcohols 33.7Reactions of Phenol 33.8Uses of Alcohols

  2. 33.1 Introduction (SB p.206) Hydroxy compounds: Organic compounds with the hydroxy group(s) (–OH) attached to an alkyl group or aromatic ring • 1. Alcohols • Compounds containing one or more –OH attached to an alkyl group • General formula for alcohols with –OH group: CnH2n+1OHe.g.

  3. 33.1 Introduction (SB p.206) Primary alcohol Secondary alcohol Tertiary alcohol • Depending on the number of alkyl groups attached to the carbon which is bonded to the hydroxy group • Alcohols can be classified as:

  4. 33.1 Introduction (SB p.207) • Phenols • Compounds that have a – OH group directly attached to a benzene ring • General formula for phenols: Ar – OHe.g.

  5. 33.2 Nomenclature of Hydroxy Compounds (SB p.207) Alcohols 1. Select the longest possible straight chain to which the hydroxyl group is directly attached. Change the name of the alkane correspending to this chain by dropping the final ‘-e’ and adding the suffix ‘-ol’.

  6. 2. Number the longest possible straight chain in such a way so as to give the carbon atom bearing the hydroxyl group the lower number. 3. Designate the position of the hydroxyl group by using this number, and also the positions of other substituents by using the numbers corresponding to their positions along the carbon chain.

  7. 33.2 Nomenclature of Hydroxy Compounds (SB p.207) Examples:

  8. 33.2 Nomenclature of Hydroxy Compounds (SB p.208) Phenols • Phenol will be the parent name when a benzene ring containing a –OH group • When substituents are present, the –OH group is assumed to be in position 1, and numbers are assigned to the substituents according to their positions in the benzene ring • e.g.

  9. 33.2 Nomenclature of Hydroxy Compounds (SB p.208) Solution: (a) (i) (ii) A secondary alcohol (b) (i) (ii) A primary alcohol (c) (i) (ii) A secondary alcohol Example 33-1 For each of the following hydroxy compounds: (i) Draw the complete structural formula. (ii) Classify them as primary, secondary or tertiary alcohols. (a) Hexan-3-ol (b) 3-Methylbutan-1-ol (c) 2-Methylcyclopentanol Answer

  10. 33.2 Nomenclature of Hydroxy Compounds (SB p.209) (a) Check Point 33-1 (a) Draw the structural formulae of all isomers of alcohols having the molecular formula C4H10O. Give their IUPAC names. Answer

  11. 33.2 Nomenclature of Hydroxy Compounds (SB p.209) (b) 4-Bromophenol Check Point 33-1 (b) Draw the structural formulae of three isomeric bromophenols with the molecular formula C6H4BrOH. Give their IUPAC names. Answer

  12. 33.3 Physical Properties of Hydroxy Compounds (SB p.209)

  13. 33.3 Physical Properties of Hydroxy Compounds (SB p.210) Boiling Point and Melting Point Intermolecular hydrogen bonds between alcohol molecules is stronger than van der Waals’ forces between alkane molecules  alcohols have higher b.p. and m.p.

  14. 33.3 Physical Properties of Hydroxy Compounds (SB p.210) Branching of the carbon chain reduces the surface area in contact with other molecules  Reduction in the extent of intermolecular hydrogen bonds that can be formed between neighbouring molecules

  15. 33.3 Physical Properties of Hydroxy Compounds (SB p.211) Density • Simple alcohols are less dense than water at 20°C • Phenols are slightly denser than water at 20°C • The densities of alcohols increase with increasing relative molecular masses

  16. 33.3 Physical Properties of Hydroxy Compounds (SB p.211) Solubility • Alcohols with a relatively short carbon chain (e.g. methanol, ethanol, propan-1-ol and propan-2-ol) are completely miscible with water • The solubility decreases as the hydrocarbon chain increasese.g. the solubility of butan-2-ol is 8 g per 100 g of water

  17. 33.2 Nomenclature of Hydroxy Compounds (SB p.209) (a) Check Point 33-2 (a) Arrange the following compounds in order of increasing boiling points: Answer

  18. 33.2 Nomenclature of Hydroxy Compounds (SB p.209) (b) (i) CH3CH2OH (ii) (iii) Check Point 33-2 (b) Which member in each of the following pairs is more soluble in water? (i) CH3CH2OH or CH3CH2CH2CH2OH (ii) (iii) Answer

  19. 33.4 Acidic Properties of Hydroxy Compounds (SB p.212) • Alcohols are neutral • Alcohols show acidic properties when reacting with strong bases (e.g. sodium metal) • The acidic property depends on the stability of the alkoxide ion formed which is partly influenced by the structure of the carbon chain

  20. 33.4 Acidic Properties of Hydroxy Compounds (SB p.213)

  21. 33.4 Acidic Properties of Hydroxy Compounds (SB p.213) • 3° alkoxide ions are the least stable • ∵ 3 electron-releasing alkyl groups release electrons to the negatively charged oxygen atom • 3° alcohol is the least acidic Phenols have smaller pKa stronger acids than alcohols

  22. 33.4 Acidic Properties of Hydroxy Compounds (SB p.214) • Phenols are more acidic than alcohols • ∵ phenoxide ion is more stable than alkoxide ion due to resonance stability (4 resonance structures can be drawn)

  23. 33.4 Acidic Properties of Hydroxy Compounds (SB p.214) The negative charge disperses over the entire benzene ring and oxygen atom by extended delocalized  electron cloud phenoxide ion is stabilized

  24. 33.4 Acidic Properties of Hydroxy Compounds (SB p.214) Simple tests to distinguish alcohols, phenols, carboxylic acids Sodium hydroxide Sodium hydrogencarbonate Alcohols Phenols Carboxylic acids No reaction React to give salts and water React to give salts and water No reaction No reaction CO2 evolved Phenol is acidic enough to react with NaOH but not Na2CO3 The order of acidity of some organic compounds and water:

  25. 33.4 Acidic Properties of Hydroxy Compounds (SB p.215) Solution: The acidity of the compounds increases in the order: Example 33-2 Arrange the following compounds in order of increasing acidity. Answer

  26. 33.4 Acidic Properties of Hydroxy Compounds (SB p.215) (a) CF3CH2OH is more acidic, because –CF3 is a strong electron-withdrawing group. It exerts a negative inductive effect on the conjugate base of CF3CH2OH (i.e. CF3CH2O–) and thus stabilizes the CF3CH2O– ion. Check Point 33-3 Which of the following compounds is more acidic? Explain briefly. (a) CF3CH2COH and CH3CH2OH Answer

  27. 33.4 Acidic Properties of Hydroxy Compounds (SB p.215) (b) is more acidic. The reason is that the conjugate base is stabilized by the negative inductive and resonance effects of the –NO2 group. Check Point 33-3 Which of the following compounds is more acidic? Explain briefly. (b) Answer

  28. 33.5 Preparation of Hydroxy Compounds (SB p.216) amylase 60°C 2(C6H10O5)n + nH2O  nC12H22O11 C12H22O11 + H2O  2C6H12O6 C6H12O6 2CH3CH2OH + 2CO2 The concentration of ethanol can be increased by fractional distillation maltase 15°C zymase 15°C Preparation of Alcohols Fermentation of Carbohydrates

  29. 33.5 Preparation of Hydroxy Compounds (SB p.216) Alkaline Hydrolysis of Haloalkanes • Nucleophilic substitution reaction of haloalkanes whereby 1°, 2° and 3° alcohols can be prepared • The equilibrium lies towards the right∵ OH– is a better nucleophile than halide ion

  30. 33.5 Preparation of Hydroxy Compounds (SB p.216) Reduction of Aldehydes and ketones • Reduction of aldehydes and ketones respectively by powerful reducing agents (e.g. LiAlH4 in dry ether) produces 1° and 2° alcohols

  31. 33.5 Preparation of Hydroxy Compounds (SB p.217) Preparation of Phenol Industrial Process • Alkaline hydrolysis of chlorobenzene under severe conditions of high temperature and pressure produces phenol

  32. 33.5 Preparation of Hydroxy Compounds (SB p.217) Laboratory Process • By refluxing benzene and conc. sulphuric(VI) acid for 1 day to form benzenesulphonic acid and then subject to alkaline hydrolysis

  33. 33.5 Preparation of Hydroxy Compounds (SB p.217) • By hydrolysis of benzenediazonium salt

  34. 33.5 Preparation of Hydroxy Compounds (SB p.216) Example 33-3 Comment on the feasibility of the following preparation of hydroxy compounds in the school laboratory. (a) KOH CH3CH2CH2Cl  CH3CH2CH2OH (b) Solution: (a) The preparation of the alcohol is feasible in this way. It is a nucleophilic substitution reaction of haloalkanes, and the nucleophile is the hydroxide ion, OH–. (b) The preparation of phenol is not feasible in this way. It is because halobenzenes do not undergo nucleophilic substitution reactions unless under severe reaction conditions. Answer

  35. 33.6 Reactions of Alcohols (SB p.218) Reactions of alcohols can be classified into two main types:1. Reactions involving the cleavage of the C – O bond: 2. Reactions involving the cleavage of the O – H bond:

  36. 33.6 Reactions of Alcohols (SB p.218) Reaction Involving Cleavage of the C – O bond Formation of Haloalkanes Reaction with Hydrogen Halides • Alcohols react with hydrogen halides by nucleophilic substitutions • The order of reactivity of hydrogen halides: • HI > HBr > HCl • The order of reactivity of alcohols: 3° > 2° > 1° < methyl

  37. 33.6 Reactions of Alcohols (SB p.219) Reactions of Alcohols with Hydrogen Chloride • Secondary and tertiary alcohols • 2° and 3° alcohols react by SN1 mechanism • e.g.

  38. 33.6 Reactions of Alcohols (SB p.219) Step 1: The alcohol is protonated Step 2: The protonated alcohol dissociates to give a carbocation and water

  39. 33.6 Reactions of Alcohols (SB p.219) Step 3: The carbocation reacts with a nucleophile to give the product

  40. 33.6 Reactions of Alcohols (SB p.219) 2. Primary alcohols and methanol • 1° alcohol and methanol react by SN2 mechanism • Step 1: The alcohol is protonated

  41. 33.6 Reactions of Alcohols (SB p.220) Step 2: The halide ion displaces a molecule of water from the carbon to give the product

  42. 33.6 Reactions of Alcohols (SB p.220) Lucas Test • Lucas reagent: Solution of ZnCl2 in conc. HCl • Used to to differentiate between simple primary, secondary and tertiary alcohols • When an alcohol is treated with Lucas reagent, the corresponding chloroalkane is formed.

  43. 33.6 Reactions of Alcohols (SB p.220) • 1°, 2°, 3° alcohols react with Lucas reagent at different rates

  44. 33.6 Reactions of Alcohols (SB p.221) • The order of the reactivity of alcohols towards Lucas reagent: 3° alcohol > 2° alcohol > 1° alcohol • The reactions are believed to take place via the SN1 mechanism which involves the formation of carbocation in the rate-determining step

  45. 33.6 Reactions of Alcohols (SB p.221) The relative stability of carbocations:  tertiary carbocations are formed rapidly from tertiary alcohols primary carbocations are difficult to form, so primary alcohols react very slowly

  46. 33.6 Reactions of Alcohols (SB p.221) Reactions of Alcohols with Hydrogen Bromide • Alcohols react with hydrogen bromide to give bromoalkanes • Hydrogen bromide is generated in situ by adding excess conc. H2SO4 to NaBr • Iodoalkanes cannot be prepared by reacting HI with alcohols as iodide will be oxidized by conc. H2SO4

  47. 33.6 Reactions of Alcohols (SB p.222) Laboratory set-up for the reaction of an alcohol with PBr3 Reaction with Phosphorus Halides

  48. 33.6 Reactions of Alcohols (SB p.222) • PI3 is used in iodination of alcohols • 3R – OH + PI3 3R – I + H3PO3 • Chloroalkanes are formed readily from the reaction of alcohols with PCl5 R – OH + PCl5 R – Cl + POCl3 + HCl • SOCl2 converts 1° and 2° alcohols to chloroalkanes R – OH + SOCl2 R – Cl + SO2 + HCl (1° or 2°)

  49. 33.6 Reactions of Alcohols (SB p.222) Dehydration of Alcohols Formation of Alkenes • Alkenes are formed when alcohols are heated with strong acids • Elimination reactions are favoured at high temperatures

  50. 33.6 Reactions of Alcohols (SB p.223) Laboratory set-up for dehydration of an alcohol Dehydration occurs when passing alcohol vapour over aluminium oxide at 350°C