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2.10 Reactions of alcohols

c. describe the following chemistry of alcohols: i. combustion ii. reaction with sodium iii. substitution reactions to form halogenoalkanes, including reaction with PCl 5 and its use as a qualitative test for the presence of the –OH group

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2.10 Reactions of alcohols

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  1. c. describe the following chemistry of alcohols: • i. combustion • ii. reaction with sodium • iii. substitution reactions to form halogenoalkanes, including reaction with PCl5 and its use as a qualitative test for the presence of the –OH group • iv. oxidation using potassium dichromate (VI) in dilute sulfuric acid on primary alcohols to produce aldehydes and carboxylic acids and on secondary alcohols to produce ketones 2.10 Reactions of alcohols Connector Draw the following functional groups as displayed formulae: primary alcohol secondary alcohol aldehyde ketone carboxylic acid Crowe2009

  2. Physical properties of alcohols • Alcohols - boiling points relatively high compared with corresponding alkanes (e.g. ethanol, 78oC ethane -88.5oC). • Smaller alcohols miscible with water, unlike the alkanes. • Suggest the reason for these. • Hydrogen bonding between the molecules.

  3. Combustion of alcohols Alcohols are highly flammable and may be used as fuels. Write a balanced equation for the complete combustion of ethanol. C2H5OH (l) + 3O2(g) 2CO2 (g) + 3H2O (g) HOMEWORK - Bioethanol How is it produced? How is it used? Discuss the advantages and disadvantages of its use as a fuel.

  4. Reactions of alcohols 1. acidified potassium dichromate(VI) • primary alcohols give aldehydes K2Cr2O7/H+ H CH3CH2CH2OH CH3CH2C=O [-2H] propanal • What type of reaction is this? • Excess acidified K2Cr2O7 oxidises the aldehyde to the corresponding carboxylic acid: OH CH3CH2C=O propanoic acid • Acidified KMnO4 is too strong an oxidising agent to produce propanal, propanoic acid is obtained. HEAT

  5. Reactions of alcohols 1. acidified potassium dichromate(VI)(continued) • Secondary alcohols give ketones: K2Cr2O7/H+ (CH3)2CHOH (CH3)2C=O [-2H] propanone • Tertiary alcohols do not react with K2Cr2O7/H+ HEAT

  6. Reactions of alcohols 2. Breaking the O-H bond i) reaction with sodium sodium + ethanol  sodium ethoxide + hydrogen write a balanced symbol equation 2Na + 2C2H5OH  2NaOC2H5 + H2 ii) esterification H+ propan-1-ol + ethanoic acid  propyl + water ethanoate Write structural formulae of the reactants and products.

  7. Reactions of alcohols 3. Nucleophilic substitution (breaking the C-O bond)What is a nucleophile? What is substitution? R-OH + X- R-X + OH- R-OH + PCl5 RCl + POCl3 + HCl * ( * Test for –OH ) What would be observed? What further tests would you carry out? Rapid exothermic reaction producing misty fumes of HCl (g), which turn moist blue litmus paper red.

  8. Reactions of alcohols 3. Nucleophilic substitution (breaking the C-O bond) (continued) R-OH + X- R-X + OH- conc.H2SO4 R-OH + Br- R-Br + OH- reflux conc.H2SO4 R-OH + NaBr  R-Br + H2O reflux (NaBr or KBr and conc. H2SO4 heat under reflux)* *A metal halide and concentrated sulphuric acid should not be used when making a bromoalkane or an iodoalkane, because sulphuric acid is an oxidising reagent and unwanted side reactions occur.

  9. Reactions of alcohols 3. Nucleophilic substitution (breaking the C-O bond) (continued) R-OH + X- R-X + OH- red phosphorus R-OH + I- R-I + OH- reflux red phosphorus 3R-OH + PI3 3R-I + H3PO3 reflux (Red phosphorus and bromine or iodine, heat under reflux.)

  10. Reactions of alcohols 4. Dehydration reactions heat/conc. H2SO4 alcohol alkene -H2O Write the symbol equations for the dehydration reactions of, butan-1-ol and butan-2-ol. Name the three products.

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