Alcohols, Carboxylic acids and Esters

Alcohols , carboxylic acids and esters

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Alcohols

Alcohols have the functional group -OH. They are a homologaus series of organic compounds. The first 3 are methanol, ethanol and propanol , all 3 are highly flammable. Ethanol is most commonly used in alcoholic drinks

The alcohols methanol, ethanol and propanol all have the following properties:

  1. They are colourless liquids that dissolve in water to form a neutral solution(pH7).

  2. They react with sodium to produce hydrogen and a salt. For example:

    ethanol + sodium → hydrogen + sodium ethoxide

    This reaction is similar but less vigorous to the reaction of water with sodium. This is due to the similarity in structure between water and the –OH group inalcohols.

  3. They burn in the air, releasing energy and producing carbon dioxide andwater.

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Alcohols

Displayed formula for methanol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/018_bitesize_gcse_tschemistry_alcoholscarboxylicacidsandesters_methanol_table.gif)Displayed formula for ethanol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/019_bitesize_gcse_tschemistry_alcoholscarboxylicacidsandesters_ethanol_table.gif)Displayed formula for propanol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/020_bitesize_gcse_tschemistry_alcoholscarboxylicacidsandesters_propanol_table.gif)

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Carboxylic acids

They have the function of -COOH, they are weak acids becasue they are only partly ionised hydrogen. They are a homoloagus series of organic compounds.

Ethanoic acid from ethanol

Vinegar is an aqueous solution containing ethanoic acid. Ethanoic acid is formed from the mild oxidation of the ethanol (which is an alcohol). This can be achieved through:

  • The addition of chemical oxidising agents - such as acidified potassium dichromate.

  • The action of microbes in aerobic conditions (in the presence of oxygen). This happens on a small scale when a bottle of wine is left open and exposed to air. On a commercial scale, it is achieved in a fermenter using acetic acid bacteria

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Carboxylic acids

Carboxylic acidNumber of C atomsStructural formulaDisplayed formula Methanoic acid 1 HCOOH Displayed formula for methanoic acid (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/021_bitesize_gcse_tschemistry_alcoholscarboxylicacidsandesters_methanoicacid_table.gif) Ethanoic acid 2 CH3COOH Displayed formula for ethanoic acid (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/022_bitesize_gcse_tschemistry_alcoholscarboxylicacidsandesters_ethanoicacid_table.gif) Propanoic acid 3 CH3CH2COOH Displayed formula for propanoic acid (http://www.bbc.co.uk/schools/gcsebitesize/science/images/triple_science/023_bitesize_gcse_tschemistry_alcoholscarboxylicacidsandesters_propanoicacid_table.gif)

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Carboxylic acids

Ionisation of weak acids – Higher tier

Strong acid, such as hydrochloric acid, ionise fully in water:

HCl(aq) → H+(aq) + Cl–(aq)

Their aqueous solutions have a high concentration of hydrogen ions, H+. This gives them a low pH.

Carboxylic acids are weak acids. They do not completely ionise when they are dissolved in water. Instead only some of their molecules ionise to form H+ ions:

CH3COOH(aq) Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) CH3COO–(aq) + H+(aq)

This means that an aqueous solution of a weak acid will have a higher pH compared to the same concentration of an aqueous solution of a strong acid

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Esters

Esters are a group of organic compounds with the function COO-

  • They are volitile
  • They have distinctly fruity smells
  • They can be used as food flavourings or in perfumes
  • Name of alcoholName of carboxylic acidName of ester Ethanol Propanoic acid Ethyl propanoate Butanol Methanoic acid Butyl methanoate Pentanol Ethanoic acid Pentyl ethanoate
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Esters

Ethyl ethanoate is the ester made from ethanol and ethanoic acid. Sulfuric acid is added to act as a catalyst in the reaction.

ethanol + ethanoic acid Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) ethyl ethanoate + water

CH3CH2OH(aq) + CH3COOH(aq) Equilibrium symbol (http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_equilibrium.gif) CH3CH2OOCCH3(aq) + H2O(l)

The distinctive smell of ethyl ethanoate (which is like modelling glue) can be detected as the reaction proceeds. Excess ethanoic acid in the reaction mixture isneutralised with sodium hydrogencarbonate, then a few drops of the mixture added to water so that the smell can be detected more effectively.

The first part of an ester’s name comes from the alcohol - it ends with the letters 'yl'. The second part of its name comes from the carboxylic acid - it ends with the letters 'oate'.

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