alcohols Flashcards
(10 cards)
Alcohols intro.
Alcohols are very useful substances with many uses in industry, as solvents and as a fuel.
Alcohols contain the -OH functional group.
Many common alcohols are liquids at room temperature. There are hydrogen bonds between molecules due to the O-H bond within each molecule.
Alcohols are classed as primary, secondary or tertiary - this depends on the number of R (C based) groups on the C that has the OH functional group.
Making ethanol by fermentation.
Ethanol is the most common alcohol.
It is mainly made by the anaerobic respiration of yeast.
Glucose is used as the fuel source for the yeast - the glucose comes from a source of carbohydrates (e.g. sugar cane, sugar beet, rice and maize) .
The glucose is mixed with yeast in water at about 35°C in the absence of air.
C6H12O6 —> 2CH3CH2OH + 2CO2 (yeast, 35°C, aqueous, no air)
This reaction produces a mixture of water, ethanol and dead yeast (the ethanol formed kills the yeast). The dead yeast is filtered off and the ethanol separated by fractional distillation.
Making alcohols by hydration of ethane.
Alkenes react with steam in the presence of strong acids (usually concentrated phosphoric or sulphuric).
The acid acts as a catalyst.
The reaction is reversible and conditions are changed either to make the alcohol from the alkene or vice versa. For example, the use of high pressure pushes the equilibrium to the right to make more alcohol. The reactions are both catalysed by conc. phosphoric/sulfuric acid.
This reaction has an electrophilic addition mechanism.
The alkene reacts with H+ to form a carbocation. The main product is produced via the most stable carbocation intermediate.
Dehydration.
Alcohols that contain an H atom on the C atom adjacent to the C atom with the OH group are dehydrated when reacted with hot, concentrated H2SO4/H3PO4 at 180°C.
This reaction is reversible - alcohols can also be made by the hydration of alkenes. The conditions are adjusted to favour the forwards or backwards reaction depending on whether it is being used to make the alkene or alcohol.
The reaction to make alkenes from alcohols is dehydration because it involves the loss of water.
The reaction has an elimination reaction.
A mixture of alkenes may be produced as the H could come from any C atom adjacent to the one with the OH group.
If an alkene has stereoisomers, then both stereoisomers will be formed.
Bioethanol.
Ethanol made from carbohydrates is common fuel. For example, 10% of every litre of petrol for cars is actually bioethanol.
Bioethanol is renewable as it is made from raw materials that can be replaced (crops and yeast).
Bioethanol is carbon neutral in theory as there are no net CO2 emissions to the atmosphere from its use.
Photosynthesis in crops takes in 6CO2 - 6CO2 + 6H2O —> C6H12O6 + 6O2
Fermenting to make ethanol release 2CO2 - C6H12O6 —> 2C2H5OH + 2CO2
Burning ethanol releases 4CO2 - 2C2H5OH + 6O2 —> 4CO2 + 6H2O
However, in the process of going from crop to using the fuel, there are other processes to consider including fuel for machinery and transport meaning that it is not actually 100% carbon neutral.
There are also growing concerns that too much agricultural land is being used for growing crops to make biofuels rather than to grow food.
Oxidation (general).
When alcohols are burned, they form carbon dioxide and water.
However, many alcohols undergo partial oxidation with a mild oxidation (e.g. K2Cr2O7 in sulphuric acid).
The sulphuric acid acts as a proton donor as the reaction needs H+.
Each of these oxidations involves the breaking of a C-H bond on the C of the functional group. If there is no C-H bond then there is no oxidation under these mild conditions as C-C bonds do not break so easily.
In each reaction, the organic compound is warmed with a mixture of potassium dichromate (VI) and sulphuric acid.
The dichromate (VI) ion in acid acts as a mild oxidising agent, and in the process is itself reduced from Cr+6 to Cr+3 with a colour change from orange to green.
CrO72- + 14H+ + 6e- —> 2Cr3+ + 7H2O
1° alcohol —> aldehyde —> carboxylic acid
2° alcohol —> ketone
3° alcohol
Reflux
The reactions goes right from 1° alcohol through to carboxylic acids or 2° alcohols to ketones if the mixture is heated under reflux.
In reflux, the reaction is done in a flask with a vertical condenser. This means that as chemicals boil they cool, condense and fall back into the reaction mixture. This allows reactions to be done around the bps of the substance.
Simple distillation.
If you want to make an aldehyde from a 1° alcohol and stop it reacting further to make the carboxylic acid, then the aldehyde needs to be removed from the reaction mixture before it can react further.
Aldehydes have lower bps than the 1° alcohols they are made from. This is because the strongest intermolecular force in aldehydes are permanent dipole dipole forces and not the stronger hydrogen bonds between alcohol molecules.
Instead of heating the reaction mixture under reflux, the reaction can be set up to distil off the aldehyde as soon as it is made.
Testing for aldehydes - Tollens’ reagent.
positive test - colourless —> silver mirror
note - Contains Ag(NH3)2 + which itself is reduced to Ag (i.e. from Ag+1 to Ag0)
Testing for aldehydes - Fehling’s solution
positive test - blue solution —> brick-red ppt
note - contains Cu2+ which itself is reduced to Cu2O (i.e. from Cu2+ to Cu+1)
