Describe the differences between the alkanol and alkanoic acid functional groups in carbon compounds
Alkanols such as methanol ( ) contain the hydroxyl functional group-OH, resulting in high melting and boiling points.
Alkanoic acids such as methanoic acid, also known as formic acid ( ), contain the carboxylic acid functional group ( ), resulting in even higher melting and boiling points due to the presence of stronger intermolecular forces. This will be explained further in the subsequent dotpoints.
Remember- Alkanols have a -OH functional group, while alkanoic acids have a functional group.
Explain the difference in melting point and boiling point caused by straight- chained alkanoic acid and straight-chained primary alkanol structures
Keep in mind that differences in melting and boiling points arise from differences in intermolecular bonding. Dispersion forces arising from polar molecules and hydrogen bonding are examples of intermolecular bonding. Intramolecular forces that bind the atoms that make up the molecule don’t affect the melting or boiling point, only the reactivity.
As a quick recap, intermolecular forces are the bonds between molecules, whereas intramolecular forces are the bonds within molecules.
Alkanols have high melting and boiling points due to the polarity and potential for hydrogen bonding present within the molecules.
Similarly, the polarity of an alkanoic acid is one reason why the melting and boiling points of an alkanoic acid are high, a property increased due to the greater molar mass of alkanoic acids relative to alkanols. When combined with the potential for two occurrences of hydrogen bonding to occur on the chain (and thus much strong intermolecular forces, driving up the melting and boiling point), this means that the melting and boiling points of alkanoic acids are higher than that of alkanols.
Remember- Alkanoic acids have higher melting and boiling points because the chain increases intermolecular hydrogen bonding.
Identify esterification as the reaction between an acid and an alkanol and describe, using equations, examples of esterification
As with most identify dotpoints, this one is fairly straightforward. Simply remember the definition of esterification, and be prepared to use at least one equation to back your definition up. As a note, the formation of ethyl ethanoate, as shown below, is an easy example because it’s balanced to begin with.
Esterification is the reaction between an acid and an alkanol, through which an ester is formed. It is a reversible reaction, and the products are the ester and water. Concentrated sulfuric acid is used as a catalyst.
For example, the formation of ethyl ethanoate from ethanol and ethanoic acid is as follows:
Remember- Esterification reacts an alkanol and an alkanoic acid to form an ester and water, in a reversible reaction.
Identify the IUPAC nomenclature for describing the esters produced by reac- tions of straight-chained alkanoic acids from C1 to C8 and straight-chained primary alkanols from C1 to C8
For naming the esters produced by straight-chained alkanoic acids, the alkyl alkanoate produced follows a very conventional nomenclature. By now you should know the prefix for C1 to C8 (Meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-). All that remains is identifying which part of an ‘alkyl alkanoate’ is derived from the alkanol, and which part is derived from the alkanoic acid.
The ‘alkyl’ portion is named by the alkanol reacted with the alkanoic acid, which names the ‘alkanoate’ portion. For example, if propanol is reacted with ethanoic acid, the resulting ester is propyl ethanoate (Which has a pear-like smell). Similarly, butanol and butanoic acid react to form butyl butyrate (Pineapple-like scent).
Remember- Alkanol (1) and alkanoic acid (2) react to form alkyl (1) alkanoate (2).
Describe the purpose of using acid in esterification for catalysis
Concentrated sulfuric acid is the most common catalyst used during esterification. This is primarily due to its nature as a strong dehydrating agent, and its subsequent effect upon the reaction. Be sure to explain how the forward reaction is increased through Le Chatelier’s principle rather than just stating that it is.
In the process of esterification, concentrated sulfuric acid is used as a catalyst. Acting as a strong dehydrating agent, concentrated sulfuric acid can rapidly remove any water produced during the reaction.
According to Le Chatelier’s principle, a system will work to minimise any disturbance to its equilibrium. As such, if water is being removed, then the equilibrium will shift to favour the forward reaction in order to replace the lost water, increasing the amount of ester produced. As such, concentrated sulfuric acid can be used to increase the yield of ester.
Remember- Acid is used during esterification not only to lower the activation energy required for the reaction to take place, but also to increase the output of products.
Explain the need for refluxing during esterification
Refluxing involves the heating of reactants in a flask, from which vapours rise through a cooling condenser. This causes the reactants to condense and fall back into the flask, preventing them from escaping.
According to collision theory, more heat leads to more collisions between molecules, leading to a faster rate of reaction. Normally higher temperatures would also lead to faster evaporation, meaning unacceptable amounts of reactants would be lost to the environment. Refluxing keeps reactants constrained to the flask, where they can continue to react. This means higher temperatures and thus faster reactions can be achieved in a viable manner.
In addition, refluxing also acts as a safety mechanism since the vapours from esterification are flammable.
Remember- Refluxing allows for a higher temperature, and thus reaction rate to be achieved while increasing safety given the volatility of the compounds used.
Identify data, plan, select equipment and perform a first-hand investigation to prepare an ester using reflux
This experiment can be time-consuming if excessive amounts of reactants are used. Use only a moderate quantities of reactant and boiling chips, and ensure the mixture is properly refluxed. Excessive reactants or boiling chips will only extend the time taken for the reaction to begin.
- Place 10-20ml of ethanol and an equal amount of the ethanoic acid into a 50ml
- Add 1ml of concentrated sulfuric acid into the mixture of reactants, and then add a moderate amount of boiling
- Gently clamp the flask and refluxing apparatus onto a retort stand, dipping the flask into a water
- Run a hose through the refluxing apparatus, ensuring water is entering through the bottom and exiting through the top. This ensures that water is always present in the cooling jacket, even if the water supply
- Heat the water bath using either a Bunsen burner or a hot plate, and reflux the mixture until two discernible layers
- Add 50ml water into the remaining mixture, and pour into a separating funnel. Remove the lower The remaining mixture should be a relatively pure ester. Sodium bicarbonate can be used to neutralise any excess acid if desired.
Outline some examples of the occurrence, production and uses of esters
Remembering one example of an ester may be useful. However, don’t waste your effort remembering more than two. In addition to the ones listed below, two more were listed previously. These include propyl ethanoate as pear, and butyl butyrate as pineapple.
Esters can be found naturally in fruits and artificially in flavourings and fragrances such as perfumes and colognes. As esters are cheaper to produce artificially than to extract from natural sources, they are often used to replace natural flavouring and fragrances. Examples of scents and flavours include ethyl butanoate as peach, octyl ethanoate as orange, and methyl butanoate as apple.
Process information from secondary sources to identify and describe the uses of esters as flavours and perfumes in processed foods and cosmetics
This dotpoint simply requires you to remember the applications of a few esters. Learning two or three should suffice.
- Ethyl ethanoate is used industrially as a solvent, with its most common use being a nail polish remover.
- Other esters are also used as flavours or perfumes, given that they are an equally effective, yet cheaper alternative to natural scents and Examples treated previously are listed again below: