Edexcel A Level Chemistry:复习笔记3.5.2 Reactions of Alcohols

Reactions of Alcohols

 

Combustion of alcohols

  • Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water

alcohol + oxygen → carbon dioxide + water

3.4-Hydroxy-Compounds-Combustion-of-Alcohols

Complete combustion of alcohols to produce carbon dioxide and water

Conversions to halogenoalkanes

  • These reactions involve replacing the hydroxyl group in an alcohol molecule with a halogen atom(known as halogenation)
  • Different methods are required for each halogen

Chlorination

  • Phosphorus(V) chloride is added to the alcohol resulting in a vigorous reaction at room temperature
    • This means the mixture doesn't need heating
  • This reaction also produces two inorganic products: phosphoryl chloride and hydrogen chloride

CH3CH2CH2OH + PCl5 → CH3CH2CH2Cl + POCl3 + HCl

  • Chlorination of tertiary alcohols can be carried out in a different way by mixing (shaking) with hydrochloric acid at room temperature
  • An example equation for the reaction of 2-methyl propan-2-ol is:

(CH3)3COH + HCl → (CH3)3CCl + H2O

  • This reaction does not work for primary and secondary alcohols

Bromination

  • This reaction is carried out using a warmed mixture of potassium bromide and 50% concentrated sulfuric acid with the reacting alcohol
  • More concentrated sulfuric acid would oxidise bromide ions to bromine resulting in different products
  • The reaction can be written as two equations as the inorganic reactants first react together to form hydrogen bromide and potassium sulfate

2KBr + H2SO4  → K2SO4 +2HBr

  • The resulting hydrogen bromide then reacts with the alcohol, for example the reaction with butan-1-ol would be as follows:

CH3CH2CH2CH2OH + HBr → CH3CH2CH2CH2Br + H2O

Iodination

  • This reaction is carried out using a mixture of red phosphorus and iodine with the alcohol whilst heating under reflux
  • Similar to bromination, the reaction can be written as two equations as the inorganic reactants first react to form phosphorus(III) iodide

2P + 3I2 → 2PI3

  • The reaction for the iodination of ethanol would be:

3C2H5OH + PI3 → 3C2H5I + H3PO3

  • This reaction results in the formation of phosphoric acid as shown above

Dehydration to Alkenes

  • Dehydration is done by heating the alcohol with concentrated phosphoric acid
  • The reaction is similar to the elimination reaction of a halogenoalkene
    • The OH group and hydrogen of adjacent carbons are removed forming a C=C bond
  • The equation for the dehydration of ethanol would be

CH3CH2OH → CH2=CH2 + H2O

  • Phosphoric acid does not appear in the equation as the water formed dilutes the concentrated phosphoric acid

Oxidation of alcohols

  • Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids
  • Secondary alcohols can be oxidised to form ketones only
  • Tertiary alcohols do not undergo oxidation
  • The oxidising agents of alcohols include acidified K2Cr2O7
  • Acidified potassium dichromate(VI), K2Cr2O7, is an orange oxidising agent
    • Acidified means that that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid)
    • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced
    • This reduction requires hydrogen (H+) ions which are provided by the acidic medium
      • When alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
  • The primary alcohol is added to the oxidising agent and warmed
  • The aldehyde product has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms
  • If the aldehyde is not distilled off, further refluxing with excess oxidising agent will oxidise it to a carboxylic acid
  • Since ketones cannot be further oxidised, the ketone product does not need to be distilled off straight away after it has been formed

oxidation

Oxidation Stages of Primary Alcohols

3.4-Hydroxy-Compounds-Oxidation-of-Secondary-Alcohols

Oxidation of propan-2-ol by acidified K2Cr2O7 to form a ketone

  • The presence of an aldehyde group (-CHO) in an unknown compound can be determined by the oxidising agents Fehling’s and Tollens’ reagents

Fehling’s solution

  • Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent
  • When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu2+ ions are reduced to Cu+ ions
    • In the alkaline conditions, the carboxylic acid formed will be neutralised to a carboxylate ion (the -COOH will lose a proton to become -COO- )
    • The carboxylate ion (-COO-) will form a salt with a positively charged metal ion such as sodium (-COO-Na+)
  • The clear blue colour of the solution turns opaque red due to the formation of a copper(I) oxide precipitate
  • Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution

3.5-Carbonyl-Compounds-Fehlings-Solution

The copper(II) ions in Fehling’s solution are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to copper(I) ions in the Cu2O precipitate

Tollens’ reagent

  • Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution
    • Tollen’s reagent is also called ammoniacal silver nitrate solution
  • When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag+ ions are reduced to Ag atoms
    • In the alkaline conditions, the carboxylic acid will become a carboxylate ion and form a salt
  • The Ag atoms form a silver ‘mirror’ on the inside of the tube
  • Ketones cannot be oxidised and therefore give a negative test when warmed with Tollens’ reagent

3.5-Carbonyl-Compounds-Tollens-Reagent

The Ag+ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms

Different practical techniques

  • Because of the easier oxidation of aldehydes compared to alcohols, two different techniques are used
    • Heating under reflux
    • Distillation with addition

Heating under reflux

  • This technique is used when we want full oxidation
    • Producing a carboxylic acid for a primary alcohol
    • Producing a ketone for a secondary alcohol

10.2.10-Heating-under-Reflux-1

Apparatus set up for heating under reflux

  • This set up means any products of oxidation remain in the reaction mixture
  • Products which boil off condense in the vertical condenser then return to the heating flask

Distillation with addition

  • This technique is used when we do not want to complete oxidation
    • To obtain an aldehyde rather than carboxylic acid for primary alcoholdistillation-with-addition

Apparatus set up for distillation with addition

 

  • Only the oxidising agent is heated whilst the alcohol is slowly added
  • When the aldehyde is formed it immediately distils off as it has a much lower boiling point than the alcohol used to make it
  • The aldehyde is then collected in the reciever

 

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