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Structure of Aldehyde: Structure, Physical Properties and Uses of Aldehydes, Practice Problems & Frequently Asked Questions(FAQs)

Structure of Aldehyde: Structure, Physical Properties and Uses of Aldehydes, Practice Problems & Frequently Asked Questions(FAQs)

Aldehydes play significant roles in humans and other living species.

Examples of aldehydes include carbohydrates (including starch, cellulose, and sugars), which are based on compounds with a ketone or aldehyde group in addition to the hydroxyl groups; ; and retinal, which is an aldehyde that, upon combining with a protein (or opsin) in the eye's retina to form rhodopsin, which is primary compound involved in the vision process.

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When rhodopsin is exposed to light, it undergoes cis-trans isomerization in the retina. The resulting change in molecular geometry may be responsible for the generation of a nerve impulse, which is then transmitted to the brain and perceived as a visual signal.

Table of content:

  • Structure of Aldehyde
  • Physical properties of Aldehydes
  • Uses of Aldehydes
  • Practice problems
  • Frequently asked questions(FAQs)

Structure of Aldehyde:

A hydrogen atom is connected to one end of the carbonyl group (>C=O) in aldehydes. The carbonyl group's other end can be connected to a second hydrogen atom or a hydrocarbon group like an alkyl or aryl group. Aldehydes have the general structural formula (R-CHO), where R denotes the alkyl or aryl group. The aldehyde group's carbonyl carbon has an sp2 hybrid orbital. The aldehyde has a triangular planar structure. This structure's bond angle is around 120o. Aldehydes are nucleophilic at the oxygen atom and electrophilic at the carbon atom of carbonyl because carbon-oxygen interactions polarise the carbonyl group(oxygen is more electron-withdrawing than carbon).

Physical Properties of aldehyde:

Boiling point: The boiling points of aldehydes often increase with increasing molecular weight. The strength of the intermolecular force determines the boiling point.

  • Van der Waals Dispersion Forces: As the length and number of atoms of a molecule increase, so do the forces of attraction between them. The boiling point of aldehydes increases as the number of carbon atoms increases.
  • Van der Waals Dipole Dipole attractions: Aldehydes are inherently polar due to the existence of carbon-oxygen double bonds. A permanent dipole and a nearby molecule are attracted to each other. This is why aldehydes have greater boiling points than similar-sized hydrocarbons.

Solubility:

In water, aldehydes(up to four carbon atoms) are miscible. Although aldehydes cannot make hydrogen bonds with one another, they can form hydrogen bonds with water molecules. A hydrogen bond is formed when one of the slightly positive hydrogen atoms in a water molecule attracts one of the lone pairs on the oxygen atom of an aldehyde.

The presence of a hydrogen bond link between the polar carbonyl group and water molecules is responsible for aldehyde solubility in water.

However, the solubility of aldehyde in water diminishes significantly as the length of the alkyl chain (carbon chain) rises. The greasy alkyl chain begins to interfere with water solubility. They break the very strong hydrogen bonds between water molecules without replacing them with anything, making the process less profitable in terms of energy, and therefore decreasing solubility.

As a result, the higher members (those with more than four carbon atoms) are insoluble in water. Aldehydes, on the other hand, are soluble in organic solvents (like dissolves like), such as benzene, ether, chloroform, and alcohol.

Bonding:

The carbonyl (>C=O) functional group is made up of a carbon atom that is double-bonded to an oxygen atom. Oxygen is more electronegative than carbon, and as a result of this difference in electronegativity, oxygen has a strong tendency to pull electrons in a carbon-oxygen bond towards itself. This causes the formation of dipoles with a slight positive charge on the carbon atom and a slight negative charge on the oxygen atom. As a result, the carbon-oxygen double bond becomes extremely polar.

A nucleophile can target the slightly positive carbon atom in the carbonyl group, while electrophiles attack the slightly negative oxygen atom.

Uses of aldehydes:

Aldehydes are important intermediates in the manufacture of plastics, plasticizers, solvents, and dyes. They're used in the textile, food, rubber, plastic, leather, chemical, and healthcare industries, among others. Perfumes and essences are made from aromatic aldehydes and higher aliphatic aldehydes.

The aldehyde is primarily utilised in the production of acetic acid, but it is also used to produce ethyl acetate, peracetic acid, pyridine derivatives, perfumes, dyes, polymers, and synthetic flavouring agents. Because of its solvent and antibacterial qualities, formaldehyde has a wide range of applications. It is used in the production of polymers.

Formaldehyde is a strong disinfectant, germicide, fungicide, and used as a preservative Benzaldehyde is employed in organic synthesis, primarily in the production of rubber promoters and as a food flavouring ingredient. It is employed in the production of perfumes, flavourings, plasticizers, and gasoline additives, as well as in the synthesis of amino acids.

Practice problems:

Q.1. Aldehydes undergoes

(A) Nucleophilic addition reactions
(B) Electrophilic substitution reactions
(C) Nucleophilic substitution reactions
(D) Electrophilic addition reactions

Answer: (A)

Solution: Aldehydes contain a polar -C=O group. The entering nucleophile attacks the sp2 hybridised carbon, breaking the double bond and converting it to sp3 resulting in the formation of a tetrahedral alkoxide intermediate. This goes through another fast step to form an addition product.

Q.2. What is the common name of Ethanal?

(A) Acetaldehyde
(B) Formaldehyde
(C) Propionaldehyde
(D) Butyraldehyde

Answer: (A)

Solution: The formula of ethanal is CH3CHO and its IUPAC name is Ethanal and its common name is Acetaldehyde. The following table shows the common names and IUPAC names of some aliphatic aldehydes;

Formula

Common name

IUPAC name

HCHO

Formaldehyde

Methanal

CH3CHO

Acetaldehyde

Ethanal

CH3CH2CHO

Propionaldehyde

Propanal

CH3CH2CH2CHO

Butyraldehyde

Butanal

Q.3. Which of the following has the lowest water solubility?

(A) Propanone
(B) Methanal
(C) Ethanal
(D) Pentanal

Answer: (D)

Solution:Because of the capacity of the polar carbonyl group to make hydrogen bonds with water molecules, the lower aldehydes and ketones are miscible in water in all amounts. However, as the length of the alkyl chain rises, solubility rapidly diminishes.

Q.4. Predict the boiling point of propanal if the boiling points of methoxyethane and propanol are 281 K and 370 K, respectively.

(A) 370 K
(B) 281 K
(C) 283 K
(D) 322 K

Answer: (D)

Solution: Aldehydes have a higher boiling point than non-polar hydrocarbons with same molecular weights. However, because there is no intermolecular hydrogen bonding, their boiling temperatures are lower than those of equivalent alcohols.

Frequently asked questions(FAQs):

1. Why do only aldehydes react with Fehling's reagents?
Answer:
The reaction between aldehyde and Fehling’s reagent is an oxidation reaction. Although aldehydes are easily oxidised, ketones are not. Because aldehydes have the aldehydic-H which can be oxidised under mild oxidising agents like Fehling’s reagent but ketones do not have the aldehydic-H.

2. Why do aldehydes oxidise so easily?
Answer:
Aldehydes have a proton attached to the carbonyl carbon that can be abstracted, making them easy to oxidise into carboxylic acids. Ketones are often inert to these oxidation conditions due to the lack of that hydrogen.

3. Why are aldehydes and ketones water soluble?
Answer:
Aldehydes and ketones cannot form hydrogen bonds with one another, but they can form hydrogen bonds with water molecules, which is why aldehydes and ketones are so soluble in water. This is also due to dipole-dipole interactions and dispersion forces.

4. Aromatic aldehydes are less reactive in nucleophilic addition reactions than aliphatic aldehydes. justify?
Answer:
Generally nucleophilic addition reactions are given by aldehydes, and in case of aromatic aldehydes the electrophilicity of the carbonyl carbon is less, due to the +R effect of the benzene ring. But no such effect is observed in the case of aliphatic aldehyde.

Hence, because of the + R effect, aromatic aldehydes are less reactive in nucleophilic addition reactions than aliphatic aldehydes.

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