17.2    Reactions of  Alcohols

Summary

• Summary of reactions that alcohols undergo:
  1. Combustion
  2. Dehydration to give alkenes
  3. Reaction with sodium
  4. Substitution to give halogenoalkanes
  5. Oxidation
  6. Esterification

 

Combustion

• Reagent     : Oxygen supply
  Condition : Heat
  Product    : Carbon dioxide(and carbon monoxide) and water
• Alcohols undergo complete combustion under excess oxygen to give carbon dioxide and Example:C2H5OH + 3O2 → 2CO2 + 3H2O

 

• When oxygen is limited, carbon monoxide and carbon soot might be formed

 

Dehydration

• Reagent/  : Excess concentrated sulfuric acid, H2SO4 or hot Catalyst                    aluminium oxide, Al2O3
  Condition : Heat at about 170 °C
  Product                   : Alkenes
• When ethanol is heated with concentrated sulfuric acid, the alcohol is dehydrated, a water molecule is removed from the Ethene is produced.
• During dehydration, the -OH group gets removed together with a hydrogen atom from the next-door carbon atom.

• 1. Alternatively, dehydration can be carried out by passing ethene through hot aluminium oxide. The ethene gas produced is collected over water
  2. At the end of the reaction, the apparatus is left to cool with the delivery tube out of water. This is done so that the cold water would not be sucked back into the hot delivery tube, cracking it eventually.

• For more complicated alcohols, be careful of the possibility of more than one product being formed and also the possibility of geometric isomers in  the alkenes. This is due to a different hydrogen being removed from the alcohol. An example is butan-2-ol.

 

Reaction with sodium metal, Na

• Reagent     : Sodium metal, Na
  Condition  :  Room temperature
  Product    : Alkoxides and hydrogen gas
• Acids react with metals to produce hydrogen gas and a salt. Alcohols are very weak acids, so they undergo this reaction as well

 

• When sodium metal is added to ethanol, the sodium metal sinks and bubbles of hydrogen gas are released. The salt formed is sodium ethoxide, a white solid
  2CH3CH2OH + 2Na → 2CH3H2O⁻Na⁺ + H2

 

• This reaction can be used as a test for hydroxy, -OH group. Bubbles of hydrogen gas is released if a hydroxy group is present

 

• Remember, in order to produce a hydrogen gas molecule, H2, two hydrogens from two molecules of alcohols must be used

 

Substitution to give halogenoalkanes

• The -OH group of the alcohol can be substituted by a halogen to produce a halogenoalkane. The halogen can be obtained from three sources:
  1. Hydrogen halide, HX ; where X = Cl/Br/I
  2. Phosphorus halide, PCl5 or PBr3 or PI3
  3. Thionyl chloride(or sulfur dichloride oxide), SOCl2

• With hydrogen halide, HX:
  1. The alcohol is refluxed with sodium halide, NaX and concentrated sulfuric acid, H2SO4 to produce the hydrogen halide, HX.
     NaX + H2SO4 → HX + NaHSO4 ; where X = Cl/Br
     For  iodine, phosphoric(V) acid, H3PO4 is used instead of  sulfuric   acid.
  2. The hydrogen halide, HX is the reacted with the alcohol. Take ethanol as an example:
     CH3CH2OH + HX → CH3CH2X + H2O    ; where X = Cl/Br/I

• With phosphorus halide, PCl5/PCl3/PBr3/PI3:
  1. Alcohols react with phosphorus(V) chloride, to produce a halogenoalkane, white fumes of hydrogen chloride gas is also observed. Take ethanol as an example:
     CH3CH2OH + PCl5 → CH3CH2Cl + POCl3 +  HCl
  2. This can be used as a test for the hydroxy, -OH group. White fumes of hydrogen chloride gas is observed if hydroxy group is present
  3. For phosphorus(III) halides, a halogenoalkane is also obtained. The phosphorus halides are prepared in situ by mixing red phosphorus and the halogen.
     3CH3CH2OH + PX3 → 3CH3CH2X + H3PO4     ; where X = Cl/Br/I

• With thionyl chloride, SOCl2:
  1. Alcohols react with thionyl chloride to produce a Sulfur dioxide and hydrogen chloride are are given off as well.
     CH3CH2OH + SOCl2 → CH3CH2X + SO2  + HCl       ; where X = Cl/Br/I
  2. This reaction is useful in obtaining pure halogenoalkane. This is because the other two products are gases, and they can be separated from the mixture easily

 

Oxidation

• Reagent     : Acidifted potassium dichromate(VI), K2Cr2O7    or acidifted potassium  manganate(VI), KMnO4
  Condition : Heat under  reflux
  Product    : Primary alcohol    – Aldehydes and carboxylic acids
  Secondary alcohol – Ketones
  Tertiary alcohol    – Will not be oxidised

 

• Different class of alcohols(primary, secondary or tertiary) will behave differently during Therefore, it is a very useful test to distinguish them.
• 1. For primary alcohols, if the alcohol used is in excess and the product formed is distilled off as soon as possible, aldehydes are formed. Take ethanol as an example, ethanal is formed
     CH3CH2OH + [O] → CH3CHO + H2O
  2. If the alcohol used is limited and heated under reflux, the primary alcohol will be oxidised to aldehyde then to carboxylic acid. Take ethanol as an example, from the aldehyde formed, ethanoic acid is formed.
     CH3CHO + [O] → CH3COOH…or the full equation from ethanol to ethanoic acid: CH3CH2OH + 2[O] → CH3COOH +  H2O

• For secondary alcohols, the secondary alcohol will be oxidised to a ketone. Take propan-2-ol as an example, propanone is formed
  CH3CH(OH)CH3 + [O] → CH3COCH3 +  H2O

• For tertiary alcohols, they will not be oxidised. This is because there is no hydrogen atom from the carbon atom holding the -OH group can be removed

• For observation:
  1. If K2Cr2O7 is used as oxidising agent: colour of solution changes from orange to green.
  2. If KMnO4 is used as oxidising agent : colour of solution changes from purple to colourless.

• Summary:

 

Esterification

• Reagent       : Carboxylic acid
  Conditions : Heat under reflux with concentrated sulfuric acid, H2SO4 as catalyst
  Product        : Esters

 

• Esters are derivatives of carboxylic In an ester, the hydrogen from the -COOH group of carboxylic acid is replaced by an alkyl group. The alkyl group came from the alcohol. Some common esters and their naming:

 

• Note that the name of an ester is ‘alcohol + carboxylic acid’.
• For example, to make methyl butanoate, methanol and butanoic acid are used. Both of them are heated under reflux with the presence of catalyst. Esters can be detected from a sweet-smelling odour.
  CH3OH + CH3CH2CH2COOH ⇌ CH3CH2CH2COOCH3 + H2ONote:

• To find out more about aldehydes and ketones, refer Chapter 18
• To find out more about carboxylic acids and esters, refer Chapter 19