What are Aldehydes Ketones and Carboxylic Acids?
Aldehyde Ketone and Carboxylic Acids are the carbonyl compounds containing a double bond or carbon-oxygen. These are very important organic compounds in the field of organic chemistry, and they also have many industrial applications. The common carbonyl group presence in the two classes of compounds makes them display the same chemical properties. However, aldehydes are more reactive compared to ketones due to the presence of free hydrogen atoms.
Organic compounds that contain a carbon-oxygen double bond is called the carboxyl group, which is one of the essential functional groups in organic chemistry. At the same time, the carbonyl group is also one of the important groups present in the living system compounds.
What are Aldehydes?
Aldehydes are organic compounds that contain the functional group -CHO.
These carbonyl compounds contain the central carbonyl-carbon, which is single bonded to the R group (any of the alkyl group) and a hydrogen atom and doubly bonded to oxygen.
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Here, R stands for the aryl or alkyl group.
Preparation of Aldehydes
The acid chlorides are reduced to aldehydes with hydrogen molecules in the palladium catalyst, which is spread on barium sulfate.
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This reaction is called Rosenmund reduction, and it is mostly used in the preparation of aromatic aldehydes. But, the same reaction cannot be used for the preparation of formaldehyde and ketones.
Properties of Aldehydes
Aldehydes' structure represents an sp2 hybridized central carbon that is connected double to oxygen and has a single bond with hydrogen.
Small aldehydes are the one that are quite soluble in water.
Acetaldehyde and formaldehyde are great examples of this. Also, industrially, these two aldehydes are quite important.
In general, aldehydes tend to undergo either polymerization or oligomerization.
The carbonyl center of the aldehyde contains an electron-withdrawing nature. Thus, the aldehyde group is considered somewhat polar.
What are Ketones?
Ketones are organic compounds with the functional group C=O and the structure R-(C=O)-R'.
These are the carbonyl compounds that have carbon-containing substituents on both sides of the double bond of the carbon-oxygen. The ketone group's carbonyl carbon is of sp2 hybridized, and the structure of ketones is a trigonal planar, which is centered around the carbonyl carbon. The bond angles of this structure fall at approximately 120°. Because the carbon-oxygen bond makes the carbonyl group polar (the oxygen is more electron-withdrawing to that of carbon), ketones tend to be electrophilic at the carbon atom and nucleophilic at the oxygen atom.
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Industrially, ketones are mass-produced for their use as pharmaceuticals, solvents, and s precursors for polymers. Some of the important ketones are methyl ethyl ketone (also known as butanone), acetone, and cyclohexanone.
Preparation of Ketones
Acid chlorides on reaction with dialkyl cadmium form ketones. Dialkyl cadmium themselves are prepared from the Grignard reagents.
2R-Mg-X + CdCl2 → R2Cd + 2 Mg(X)Cl
2RCOCl + R2Cd → 2R-CO-R + CdCl2
This method is useful in a manner; the mixed ketones are prepared very conveniently.
Properties of Ketones
Ketones always are polar in nature because of the presence of a polar carbonyl group. Thus, they have higher boiling points compared to the non-polar compounds.
It cannot form alcohols of any intermolecular hydrogen bond-like since no hydrogen is attached to an oxygen atom.
Ketones contain the large dipole moments to that of ethers or alcohols because of pi electrons' shifting.
Ketones also react with hydrogen cyanide to produce cyanohydrins. Normally, the reaction is carried out in the presence of a base, which acts as a catalyst. The reaction proceeds slowly in the absence of a base.
The majority of the ketones form bisulphite addition products when added to sodium bisulfite.
What is Carboxylic Acid?
They are the organic compounds, containing a (C=O)OH group, which is attached to an R group (here, R refers to the molecule's remaining part).
Commonly, the COOH group is called a carboxyl group. Generally, the carboxylic acids can be expressed via the formula R-COOH and have a polar nature. They can also participate in hydrogen bonding due to their hydrogen bond donating nature of the O-H bond and the hydrogen bond accepting nature of the C=O group. Generally, these have higher boiling points compared to water and tend to form stable dimers.
Carboxylic acids play an essential role in producing food additives, pharmaceuticals, polymers, and solvents. Adipic acid, acetic acid, and citric acid are some carboxylic acids that are useful extremely industrially.
Preparation of Carboxylic Acids
Primary alcohols are oxidized readily to the carboxylic acids with the common oxidizing agents such as potassium permanganate in the alkaline media or neutral, acidic, or potassium dichromate chromium trioxide in the acidic media.
RCH2OH → RCOOH
CH3(CH2)8CH2OH → CH3(CH3)3COOH
Properties of Carboxylic Acids
Carboxylic acids are polar compounds and can enter extensively into the hydrogen bonding.
The aromatic carboxylic acids are practically insoluble in cold water, whereas all the carboxylic acids are soluble in the organic solvents like ether, alcohol, benzene, and more.
Carboxylic acids are the most acidic among the organic acids, but they are less acidic than mineral acids, namely sulphuric acid and nitric acid.
FAQs on Aldehydes Ketones and Carboxylic Acids
1. What are aldehydes, ketones, and carboxylic acids, and how are they structurally different?
Aldehydes, ketones, and carboxylic acids are all organic compounds containing a carbonyl group (C=O). The key difference lies in the atoms attached to this group:
- Aldehydes have a carbonyl group bonded to at least one hydrogen atom and an alkyl or aryl group (R-CHO).
- Ketones have a carbonyl group bonded to two alkyl or aryl groups (R-CO-R').
- Carboxylic acids have a carbonyl group bonded to a hydroxyl group (-OH) and an alkyl or aryl group (R-COOH).
2. What are some common examples and uses of aldehydes in daily life and industry?
Aldehydes have several important applications based on their reactivity. Key examples include:
- Formaldehyde (Methanal): Used as a preservative for biological specimens (as formalin), in the manufacturing of resins like Bakelite, and as a disinfectant.
- Acetaldehyde (Ethanal): Used in the production of acetic acid, perfumes, and flavouring agents.
- Benzaldehyde: Known for its almond-like scent, it is widely used in the perfume, dye, and food industries.
3. How are ketones generally prepared, and what are their primary industrial applications?
A common method for preparing ketones is the oxidation of secondary alcohols. They can also be prepared by reacting acid chlorides with dialkyl cadmium. Industrially, ketones are valued for their properties as solvents and chemical intermediates. For example:
- Acetone (Propanone): A widely used solvent for paints, nail polish remover, and plastics. It is also a precursor for synthesising polymers like perspex.
- Cyclohexanone: An essential intermediate in the production of nylon.
4. What is the Rosenmund reduction, and why is it specific for preparing aldehydes?
The Rosenmund reduction is a chemical reaction used to prepare aldehydes by reducing acid chlorides (R-COCl) with hydrogen gas. The reaction uses a palladium catalyst that is deliberately 'poisoned' with barium sulphate (Pd/BaSO₄). This poisoning is crucial because it deactivates the catalyst just enough to stop the reduction at the aldehyde stage, preventing it from being further reduced to a primary alcohol. This specificity makes it a valuable method for aldehyde synthesis.
5. How is the nomenclature of carboxylic acids determined according to IUPAC rules?
According to the IUPAC system, the name of a carboxylic acid is derived from its parent alkane by replacing the final '-e' with the suffix '-oic acid'. The carbon atom of the carboxyl group (-COOH) is always numbered as carbon 1. For example, a three-carbon chain with a carboxyl group is named Propanoic acid. If other substituents are present, their position is indicated by number, as in 3-Chlorobutanoic acid.
6. Why are aldehydes generally more reactive than ketones towards nucleophilic addition reactions?
Aldehydes are more reactive than ketones towards nucleophiles for two main reasons:
- Electronic Factors: Ketones have two electron-donating alkyl groups attached to the carbonyl carbon. These groups reduce the positive charge (electrophilicity) on the carbon, making it less attractive to nucleophiles. Aldehydes only have one such group, making their carbonyl carbon more electrophilic.
- Steric Factors: The carbonyl carbon in an aldehyde is less crowded as it is attached to a small hydrogen atom. In contrast, the two bulkier alkyl groups in a ketone create more steric hindrance, making it physically more difficult for a nucleophile to approach and attack the carbonyl carbon.
7. How does the carbonyl group influence the boiling points of aldehydes and ketones?
The carbonyl group (C=O) is highly polar due to the higher electronegativity of oxygen compared to carbon. This polarity creates strong dipole-dipole attractions between molecules. As a result, aldehydes and ketones have significantly higher boiling points than nonpolar hydrocarbons of similar molecular mass. However, their boiling points are lower than those of corresponding alcohols because, unlike alcohols, they cannot form intermolecular hydrogen bonds with each other.
8. Compare the acidity of carboxylic acids with phenols and alcohols. What makes carboxylic acids much stronger acids?
The order of acidity is Carboxylic Acids > Phenols > Alcohols. The primary reason for the high acidity of carboxylic acids is the stability of the conjugate base (carboxylate ion, RCOO⁻) formed after donating a proton. The carboxylate ion is highly stabilised by resonance, where the negative charge is delocalised over two equally electronegative oxygen atoms. The phenoxide ion (from phenol) is less stable as its resonance structures involve delocalising the charge onto the less electronegative carbon atoms of the ring. The alkoxide ion (from alcohol) has no resonance stabilisation, making alcohols the weakest acids of the three.
9. What is the key difference between the Aldol and Cannizzaro reactions, and which type of aldehyde undergoes each?
The fundamental difference lies in the presence or absence of an α-hydrogen (a hydrogen atom on the carbon adjacent to the carbonyl group).
- Aldol Reaction: This reaction is characteristic of aldehydes and ketones that possess at least one α-hydrogen. It occurs in the presence of a dilute base (like NaOH) and involves the formation of a β-hydroxy aldehyde or ketone (an aldol).
- Cannizzaro Reaction: This reaction is specific to aldehydes that do not have an α-hydrogen (e.g., formaldehyde, benzaldehyde). In the presence of a concentrated base, these aldehydes undergo a self-oxidation and reduction (disproportionation) reaction to yield a molecule of alcohol and a molecule of the salt of a carboxylic acid.
10. How can you chemically distinguish between an aldehyde and a ketone using common laboratory tests?
Aldehydes and ketones can be distinguished because aldehydes are easily oxidised, while ketones are not. Two common tests are:
- Tollens' Test (Silver Mirror Test): When an aldehyde is warmed with Tollens' reagent (ammoniacal silver nitrate), the aldehyde is oxidised to a carboxylate ion, and the Ag⁺ ions are reduced to metallic silver, which forms a shiny silver mirror on the inner surface of the test tube. Ketones do not react.
- Fehling's Test: When an aliphatic aldehyde is heated with Fehling's solution, it gives a red-brown precipitate of copper(I) oxide. Ketones and aromatic aldehydes generally do not give a positive test.
