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

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 Stages of Primary 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

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

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

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 alcohol

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