Alcohols/Alkanols
The alkanols (like ethanol and propanol) all contain the hydroxyl functional group (-OH), and thus, undergo a number of similar reactions.
Reactions of the Alkanols
Alkanols + Reactive Metals
Alkanols of lower molecular mass (eg methanol, ethanol and propanol) react with highly reactive metals (eg sodium, lithium and calcium) to produce hydrogen gas and salts of the alkanols, called alkoxides.
alkanol + reactive metal → H2 + alkoxide
For example, look at the reaction between ethanol and sodium metal:
2Na(s) + 2CH3CH2OH(l) → 2CH3CH2ONa(s) + H2(g)
This reaction is similar to the reaction which occurs between sodium and water, as both ethanol and water technically have the same hydroxyl group.
Alkanols + Oxidising Agents
Alkanols will react with powerful oxidizing agents (eg acidified potassium dichromate (VI) and acidified potassium manganate (VII) to form organic acids. These reactions involve an intermediate alkanal (or aldehyde) being formed. For example, take the oxidation of ethanol to ethanoic acid:
CH3CH2OH(l) + [O] → H2O + CH3CHO(l)
ethanol water + ethanal
CH3CHO(l) + [O] → CH3COOH(aq)
ethanal ethanoic acid
Note: Ethanal is also called acetaldehyde, and ethanoic acid is commonly called acetic acid.
Potassium dichromate (VI) and potassium manganate (VII) can be used interchangeably for this reaction. Throughout the reaction, if potassium manganate (VII) is used, the mixture will turn from purple to colourless. If potassium dichromate (VI) is used (as above) the colour of the mixture will change from orange to green.
Dehydration of Alkanols to Alkenes
In a previous post, we discussed the hydration of alkenes to form alkanols. This is the exact opposite, where instead, a molecule of water is removed from one molecule of the alkanol to produce an alkene.
The alkanols will react with a dehydrating agent (such as concentrated sulfuric acid, phosphoric acid, anhydrous aluminium oxide or anhydrous zinc chloride) to form an alkene through dehydration. The water molecule is lost across two adjacent carbon atoms in the chain, causing the formation of a double bond between those two carbon atoms.
In the reaction above, the ethanol is heated with concentrated sulfuric acid at a temperature of 170 degrees Celsius to be dehydrated. Dehydration can also be achieved by passing ethanol vapour (or the vapour of the alkanol being dehydrated) over activated alumina (Al2O3) heated to 450 degrees Celsius.
Condensation Reactions (Conversion of Alkanols to Esters)
Alkanols react with alkanoic acids to form an ester and water (we will discuss esters and alkanoic acids in later posts).
The mixture of the alkanoic acid, alkanol and a few drops of concentrated sulphuric acid should be heated in a water bath for a few minutes. This mixture should then be poured into a concentrated solution of sodium chloride.
During the reaction, a water molecule is lost between the functional group of the alkanol and the alkanoic acid, forming an ester. The dehydrating agent (sulphuric acid) allows for the removal of the water molecule.
CH3COOH + C2H5OH ⇋ CH3COOC2H5 + H2O
The new functional group, highlighted in red (COO) is the carboxylate group, is the functional group of the ester. The new carbon to oxygen bond is the ester linkage.
The name of the ester is found by combining the names of the alkanol and the alkanoic acid.
methanol + ethanoic acid → methyl ethanoate
As you can see, the alkyl part comes from the alkanol and the '-oate' part comes from the alkanoic acid.
Although the name has the alkanol first, the molecular and structural formulae are usually written with the alkanoic acid part first.
Combustion of Alkanols
Alkanols' combustion reactions are highly exothermic (they release lots of heat), which is why they are used as fuels. For example:
2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(g) + heat
The reaction above is a complete combustion. In an insufficient supply of oxygen, the products may include carbon monoxide, carbon and hydrogen- similar to the combustion of hydrocarbons.
Uses of Alkanols
Ethanol is a useful compound due to its several helpful properties. Comparing it to water, another polar covalent compound, ethanol has a higher molar mass, heat of combustion, lower melting point and lower boiling point. As a result, ethanol is useful as a solvent, fuel, germicide and antifreeze.
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