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MD - MEDICINES BY DESIGN

Alcohols

Reactions for Synthesis

Drugs as Medicines

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Alcohol

Reactions of Alcohols

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Alcohol

Xenobiotics are substances which the body doesn't contain but which can effect it.  There are three types:

1. foods provide molecules which give us energy and keep our bodies maintained
2. drugs alter the biochemical processes in our bodies
3. poisons severely damage our biochemical processes and can cause death.

Ethanol is an alcohol which is a source of energy, affects our behaviour and in excess, can cause liver damage and even death.  Alcoholic drinks help people to relax by depressing the activity of the central nervous system.  It slows our reaction times and impairs our judgement.

Blood alcohol concentration (BAC) is closely related to the extent of the effects of alcoholic drinks.  Ethanol is sufficiently volatile to pass from the blood to the air in the lungs.  An equilibrium is set up:

K_c has a fixed value at a particular temperature.  Therefore a measurement of the ethanol concentration in the breath gives an indication of the BAC.  An early breathalyser is based on the reaction between potassium dichromate (VI) crystals which are orange.  They turn green when they oxidise ethanol to ethanal and ethanoic acid.

Roadside breath tests are not particularly reliable.  A more accurate technique is to use gas-liquid chromatography on either a blood sample or a urine sample.  A more recent technique is to use infra-red spectroscopy on a breath sample.

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Reactions of Alcohols

Alcohols contain the hydroxyl group -OH.  Alcohols are soluble in water because they are able to form H-bonds with water.  The solubility decreases as you go down the homologous series.

There are three types of alcohols:

• primary - OH group at the end of a hydr^oCarbon chain
• secondary - OH group in middle of chain, the carbon atom to which the OH group is attached also contains one H atom
• tertiary - OH group attached to a carbon atom which carries no H atoms.

The reactions of alcohols are summarised below:

1. Acidity.  Alcohols are very weak acids, so weak that they will react with neither sodium hydrogen carbonate nor sodium hydroxide.  They will, however, react with the strong base of pure sodium.
   
   

   2C2H5OH    +    2Na	2C2H5ONa	+    H2
   	sodium ethoxide

2. Esterification.  Alcohols react with carboxylic acids, in the presence of an acid catalyst, to form esters.
   
   

   C2H5OH    +    CH3CO2H	CH3CO2C2H5	+    H2O
   	ethyl ethanoate

3. Dehydration.  If ethanol vapour is passed over alumina (Al₂O₃) at 300^oC, ethene is produced.
   
   
   C₂H₅OH        C₂H₄    +    H₂O
4. Substitution by a Halogen.  Alcohols are converted to bromoalkanes by heating with hydrobromic acid.  This is generated in situ by reacting sodium bromide with concentrated sulphuric acid.
   
   

   C2H5OH    +    HBr	C2H5Br	+    H2O
   	bromoethane

5. Oxidation.  Primary alcohols are oxidised to aldehydes then carboxylic acids by heating with acidified potassium dichromate (VI) solution.
   
   

   C2H5OH	CH3CHO		CH3CO2H
   	ethanal		ethanoic acid

   
   
   To make the aldehyde, the oxidising agent is dripped slowly into the hot alcohol and the aldehyde distilled off as soon as it forms.
   
   To make the carboxylic acid the alcohol is heated under reflux with an excess of the oxidising agent to ensure the reaction goes to completion.
   
   With secondary alcohols, oxidation stops at the ketone.
   
   

   CH3CHOHCH3		CH3COCH3
   propan-2-ol		propanone

   
   
   Tertiary alcohols are not oxidised under these conditions.

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