9.4 Alcohols Flashcards
Alcohol
All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions.
The general formula of an alcohol is CnH2n+1OH.
Alcohols are colourless liquids that dissolve in water to form neutral solutions.
In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’.
Examples of alcohol
Methanol, ethanol, propanol and butanol.
Carboxylic acids
Carboxylic acids is the name given to compounds containing the functional group carboxyl, -COOH.
The naming of a carboxylic acid follows the pattern alkane + oic acid.
Examples of carboxylic acids
Methanoic acid, ethanoic acid, propanoic acid, butanoic acid.
Making ethanoic acid
Alcohols undergo oxidation to produce carboxylic acids when treated with oxidising agents.
When ethanol is heated with acidified potassium dichromate solution the ethanol oxidises to ethanoic acid.
The equation for the reaction is:
CH3CH2OH + [O] → CH3COOH + H2O
The oxidising agent is represented by the symbol for oxygen in square brackets.
The reaction is slow so the mixture is heated to its boiling point for about an hour; to avoid the substances evaporating a condenser is placed above the reaction flask that prevents volatile liquids from escaping.
During the reaction the potassium dichromate turns from orange to green.
Predicting products
Organic molecules that belong to the same homologous series react in the same way, so the products of those reactions can be predicted.
Homologous series are families or groups of organic compounds that have similar features and chemical properties due to them having the same functional group.
All members of a homologous series have:
The same general formula.
The difference in the molecular formula between one member and the next is CH2.
Gradation in their physical properties.
Same functional group.
Similar chemical properties.
The chemistry of homologous series is therefore determined by the functional group.
We can use this to predict how other molecules in a homologous series will react.
Although the homologous series allows us to predict what the reaction products should be, it tells us nothing about the rate or extent of the reaction.
For example, as the chain length increases in alcohols the combustion or oxidation reactions may be slower or incomplete as the carbon chain influences the reactivity of the functional group.
Fermentation
Ethanol (C2H5OH) is one of the most important alcohols.
It is used as fuel (for vehicles in some countries) and as a solvent.
It is the type of alcohol found in alcoholic drinks such as wine and beer.
It can be produced by fermentation where sugar or starch is dissolved in water and yeast is added.
The mixture is then fermented between 15 and 35 °C with the absence of oxygen for a few days.
Yeast contains enzymes that break down sugar to alcohol.
If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured.
The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide.
Fractional distillation of ethanol
Fermentation produces a dilute solution of ethanol which needs to be separated from the reaction mixture.
This is done using fractional distillation.
The mixture is heated to 78 ºC which is the boiling point of ethanol but below that of water (100 ºC).
The ethanol evaporates and its vapours pass through a condenser, where they cool and condense, forming liquid ethanol.
The water and any other impurities remain behind in the reaction flask.
When the temperature starts to increase to 100 ºC heating should be stopped. The water and ethanol have now been separated.
Practical 7 (Heat of Combustion of Alcohols)
Aim: To investigate the heat of combustion of four alcohols.
Procedure - Using a measuring cylinder, place 100 cm3 of water into a copper can.
Record the initial temperature of the water and the mass of the empty burner.
Fill the burner with the test alcohol and record its new mass.
Place the burner under the copper can, light the wick and place the lid on.
Stir the water constantly with the thermometer (calorimeter lids allow for this) and continue heating until the temperature rises by 25 ºC.
Immediately extinguish the flame and measure and record the mass of the spirit burner.
Repeat procedure for other alcohols, making sure the variables are kept the same.
