Theo Nash
There are several ways to distinguish between an alcohol and a carboxylic acid. One method is the sodium bicarbonate test, where the evolution of CO2 with carboxylic acid produces brisk effervescence. Another way is to test their reaction with carbonates and hydrogen carbonates; carboxylic acid will react to produce CO2 gas, turning lime water milky, whereas alcohol will not react.
| Characteristics | Values |
|---|---|
| Sodium bicarbonate test | Carboxylic acid reacts with sodium bicarbonate to form sodium acetate and carbon dioxide, which is indicated by brisk effervescence. Alcohols do not respond to this test. |
| Reaction with alkali metal | Carboxylic acids have a high tendency to donate a proton and are very reactive. Alcohols are mildly acidic and have a tendency to donate protons but are not as reactive. |
| Iodoform test | This test can be used to distinguish between the two substances. |
| Litmus test | Blue litmus paper turns red for carboxylic acids and remains blue for alcohols. |
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What You'll Learn
Boiling point
The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. At this point, the vapour pressure of the liquid is equal to the atmospheric pressure exerted on it. Boiling points are influenced by the strength of the intermolecular forces present in the substance.
Alcohols have higher boiling points than alkanes with similar molecular weights. For example, ethanol (boiling point: 78 °C) and propane (boiling point: -42 °C) have similar molecular weights, but ethanol's boiling point is much higher. This is because alcohols can form hydrogen bonds, which are stronger than the van der Waals dispersion forces that are the only intermolecular forces present in alkanes. As a result, it takes more energy to separate alcohol molecules than alkane molecules, leading to a higher boiling point for alcohols.
The boiling point of an alcohol also depends on the number of carbon atoms it contains. As the number of carbon atoms increases, so do the van der Waals dispersion forces and dipole-dipole interactions, leading to higher boiling points. This is because the molecules get longer and have more electrons, increasing the size of the temporary dipoles formed.
Additionally, the boiling point of an alcohol can be influenced by the presence of hydroxyl groups (-OH). The more "exposed" the hydroxyl group is, the more it can interact with other OH groups, leading to a higher boiling point.
In summary, the boiling point of an alcohol can be distinguished experimentally by comparing it to the boiling points of other substances, particularly alkanes with similar molecular weights. The presence of hydrogen bonding, the number of carbon atoms, and the interaction of hydroxyl groups will influence the boiling point of an alcohol.
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Solubility
One such test involves the use of bromine water. When a few drops of an unknown substance are added to bromine water, the solution's colour change can indicate whether the substance is an alcohol or a carboxylic acid. Alcohols do not typically react with bromine water, so the solution will remain brownish-orange. On the other hand, carboxylic acids react with bromine to form other compounds, causing the solution to lose its colour and become light orange or even colourless.
Another test involves the use of blue litmus paper. If a sample of the unknown substance is applied to blue litmus paper and it turns red, the substance is a carboxylic acid. If there is no change in colour, it is an alcohol.
The Ceric Ammonium Nitrate test is also a reagent test for alcohols. When a solution of an organic compound is mixed with ceric ammonium nitrate, the appearance of a red colour indicates the presence of an alcohol due to the -OH group.
Additionally, the sodium bicarbonate test can be used to distinguish between alcohols and carboxylic acids based on their chemical reactivity. If a sample of the unknown compound is mixed with sodium bicarbonate (also known as sodium hydrogen carbonate), the reaction will indicate the nature of the substance. If the mixture exhibits bubbling or effervescence due to the release of carbon dioxide gas, it is a carboxylic acid. Alcohols, on the other hand, do not react with sodium bicarbonate in the same way, so there will be no effervescence.
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Reactivity
Alcohols and carboxylic acids can be distinguished experimentally by considering their reactivity. Carboxylic acids, being a stronger acid, have a high tendency to donate a proton and are very reactive. Alcohols, on the other hand, are only mildly acidic and less reactive. They do possess the tendency to donate protons, but to a lesser extent.
The sodium bicarbonate test is often used to distinguish between the two. In this test, the compound is reacted with sodium bicarbonate. If the compound is a carboxylic acid, it will react to form sodium acetate and carbon dioxide, which will be observed as a brisk effervescence. Alcohols do not respond to this test, allowing for an experimental distinction to be made.
Another possible method to distinguish between alcohols and carboxylic acids is through their reaction with alkali metals. Carboxylic acids will react with alkali metals, whereas alcohols will not. This difference in reactivity can be used as a distinguishing factor.
Additionally, the iodoform test can be used as a means of identification. This test specifically identifies the presence of methyl ketones, which are characteristic of certain carboxylic acids. Alcohols do not contain methyl ketones and hence will not react in this test.
These experimental methods allow for the distinction between alcohols and carboxylic acids based on their differing reactivities.
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Combustion
The combustion of alcohols and carboxylic acids can be used to distinguish between the two compounds. Alcohols are organic compounds that contain one or more hydroxyl (-OH) functional groups attached to a saturated carbon atom. On the other hand, carboxylic acids have a functional group (-COOH) attached to an alkyl group.
The reactivity of the compounds differs significantly. Carboxylic acids have a high tendency to donate a proton and are very reactive. When carboxylic acids react with sodium bicarbonate, they produce sodium acetate and a brisk effervescence of carbon dioxide. This reaction can be represented as:
\ RCOOH+NaHCO_{3}\to RCOONa+H_{2}O+CO_{2}\uparrow \>
The evolution of carbon dioxide turns lime water milky, indicating the presence of carboxylic acid. This sodium bicarbonate test is considered the best method to experimentally distinguish between carboxylic acids and alcohols.
In contrast, alcohols are mildly acidic and less reactive. They possess the tendency to donate protons, but to a lesser extent than carboxylic acids. Importantly, alcohols do not react with carbonates and hydrogen carbonates, which can be used as a distinguishing factor.
Another test to distinguish between the two compounds is the Ceric Ammonium Nitrate test, which is specific for alcohols. Alcohols give a characteristic red color with ceric ammonium nitrate due to the presence of the -OH group. However, this color disappears if the reaction mixture is left for some time or if excess ceric ammonium nitrate is added.
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Chemical tests
Several chemical tests can be used to distinguish between an alcohol and a carboxylic acid. Here are some methods:
Sodium Bicarbonate Test
The sodium bicarbonate test is a widely recognised method to differentiate between the two compounds. In this test, sodium bicarbonate (NaHCO3) reacts with carboxylic acid to produce sodium acetate, water, and carbon dioxide gas, which causes a brisk effervescence. This reaction can be represented by the equation:
\RCOOH+NaHCO_3 \to RCOONa+H_2O+CO_2 \uparrow
On the other hand, when sodium bicarbonate is mixed with an alcohol, there is no evolution of carbon dioxide gas, and no observable reaction occurs. This distinct difference in reactivity helps identify whether the substance is a carboxylic acid or an alcohol.
Reaction with Carbonates and Hydrogen Carbonates
Another distinguishing factor between alcohols and carboxylic acids is their reaction with carbonates and hydrogen carbonates. Carboxylic acids react with these compounds to produce carbon dioxide gas, which can turn limewater milky. The reaction with carbonate can be represented as:
\CO_2 + Ca(OH)_2 \to CaCO_3 \downarrow + H_2O
Alcohols, however, do not react with carbonates or hydrogen carbonates, and there is no observable change in the solution.
Iodoform Test and Reaction with Alkali Metal
The iodoform test and the reaction with alkali metal are also mentioned as potential methods to distinguish between alcohols and carboxylic acids. However, the specific procedures and observations of these tests were not readily available.
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