Carboxylic Acids Test - 1

Explanation-

Bond Length

• Bond length is a measure of the distance between the nuclei of two chemically bonded atoms in a molecule.
• It is approximately equal to the sum of the covalent radii of the two bonded atoms.
• For covalent bonds, the bond length is inversely proportional to the bond order – higher bond orders result in stronger bonds, which are accompanied by stronger forces of attraction holding the atoms together.
• Short bonds are a consequence of these strong forces of attraction.
• Bond length increases in the following order: triple bond < double bond < single bond.

Given data and Analysis-

A resonant structure can be formed as shown below.

Here at x and y resonance is occurring.

So the bond length of x and y is the same.

Z is a single bond. So x = y < z .

Explanation-

• A decarboxylation reaction is a chemical reaction in which carbon dioxide is removed from any compound. While carboxylation is the addition of carbon dioxide to any compound. Decarboxylation reaction is possible only if there is a carboxyl group in the compound.
• Sodium benzoate is a chemical that is used to preserve beverages. The maximum limit of usage of sodium benzoate is 0.1 of the weight of the product. A little amount of sodium benzoate is naturally found in some fruits like apples, prunes, etc. Soda-lime is used in respiration. It eliminates carbon dioxide and prevents carbon dioxide poisoning. It is used as a drying agent. It absorbs carbon dioxide.
• When sodium benzoate is heated with soda lime, it undergoes decarboxylation. The carboxyl group in sodium benzoate is converted to sodium carbonate. It also forms benzene. The chemical reaction is given below:

C₆H₅COONa + NaOH \(\xrightarrow{CaO \: (\Delta)} \)  CaOC₆H₆ + Na₂CO₃

So option 1 is the correct answer.

Important Points

• Benzoic acid can directly produce benzene. This process is called pyrolysis. The term pyro means heat or high temperature and lysis means a break or decomposition.
• The decarboxylation of sodium benzoate in the presence of soda lime occurs at lower temperatures.
• Benzoic acid can also produce benzene by a decarboxylation reaction.
• This reaction is called the Duma reaction.

Explanation-

Hell Volhard Zelinsky Reaction-

• Hell Volhard Zelinsky Reaction Mechanism is quite different from other halogenation reactions as it takes place in the absence of a halogen carrier. The reaction is used for the halogenation of carboxylic acids at the alpha carbon. This reaction is named after the chemists – Carl Magnus Von Hell, Jacob Volhard, and Nikolay Zelinsky.
• The reaction is initiated by the addition of phosphorus tribromide (catalytic amount) and the further addition of one molar equivalent of diatomic bromine.
• Carboxylic acids having α−H atom undergo the Hell-Volhard-Zelinsky reaction.

Alpha carbon and Alpha Hydrogen-

• The alpha carbon (α-C) in organic molecules refers to the first carbon atom that attaches to a functional group, such as a carbonyl.
• An α carbon is the one that is attached directly to the functional group. and the hydrogen attached to the α carbon is called α hydrogen.

Of the given options, option 3  has α hydrogen present in it so the Hell-Volhard-Zelinsky reaction can take place.

Explanation:

pKa value-

• The negative logarithmic of K_a is denoted by pK_a.
• The pK_a value determines whether an acid is strong or weak.
• The dissociation of acid in an aqueous solution is indicated by pK_a.
• If an acid’s pK_a is high, it is a weak acid, and if it is low, it is a strong acid.
• The concentration of acid, conjugate base, and H⁺ affects the pK_a.
• Carbolic acid is the weakest acid among the options given above, so it will have the maximum pKa value.

Explanation:

Boiling point-

• A liquid‘s boiling point is the temperature at which its vapor pressure is equal to that of the gas above it. The normal boiling point of a liquid is the temperature at which one atmosphere (760 torrs) is equal to its vapor pressure.
• The boiling point depends on intermolecular hydrogen bonding.
• Thus alcohols tend to have a higher boiling point than ketones because they have an OH group, unlike carbonyls.
• Carboxylic acids have higher boiling points than aldehydes, ketones, and given alcohol of comparable molecular mass.
• This is because of the formation of stronger intermolecular H-bonding than alcohol.

So the correct order of the boiling point is III < II < I

Explanation-

Zwitter ion-

• “A zwitterion is a molecule that has both positive and negative regions of charge.” 
• In the solid-state, amino acids exist as dipolar ions called zwitterions.
• While discussing whether a substance is zwitterionic or not, the pH range in which the information is required must be specified (because a sufficiently alkaline solution will change the zwitterion to an anion, and a sufficiently acid solution will change it to a cation)
• The best-known examples of Zwitterions are amino acids. They have a group of amines (basic) and a group of carboxyls (acidic).

Given options and analysis-

• Glycine is known as α- amino acid and it exists in zwitterion form in an aqueous medium.
• Similarly Anthranilic acid and Sulphanilci acid both can form Zwitter ion.
• Picric acid is an organic compound. 2, 4, 6 – trinitrophenol is the IUPAC name of picric acid. It is an acid phenol and an explosive compound also. 

It does not form a Zwitter ion.

Explanation-

Carbolic acid-

• Phenol is also known as carbolic acid. It is an aromatic organic compound with the molecular formula C₆H₅OH. It is a white crystalline solid that is volatile.

It does not contain - COOH group.

Picric acid-

• Picric acid is an organic compound. 2, 4, 6 – trinitrophenol is the IUPAC name of picric acid. It is an acid phenol and an explosive compound also. 

It does not contain - COOH group.

Anthranilic acid-

• Anthranilic acid is an aromatic acid with the formula C₆H₄(NH₂)(COOH) and has a sweetish taste. The molecule consists of a benzene ring, ortho-substituted with a carboxylic acid and an amine. 

It contains a  - COOH group.

Sulphanilic acid-

• Sulfanilic acid (4 - amino benzene sulfonic acid) is an organic compound with the formula H₃NC₆H₄SO₃. It is an off-white solid. It is a zwitterion, which explains its high melting point.

It does not contain - COOH group.

Explanation-

Valeric acid-

• Valeric acid or pentanoic acid is a straight-chain alkyl carboxylic acid with the chemical formula CH₃(CH₂)₃COOH. Like other low-molecular-weight carboxylic acids, it has an unpleasant odor. It is found in the perennial flowering plant Valeriana officinalis, from which it gets its name.
• Its primary use is in the synthesis of its esters. Salts and esters of valeric acid are known as valerates or pentanoates.
• Valeric acid is also known as pentanoic acid. 
• Valeric acid is used as an intermediate in the manufacture of flavors and perfumes, ester-type lubricants, plasticizers, and vinyl stabilizers.
• IUPAC name of valeric acid is pentanoic acid.
• Valeric acid is a straight-chain saturated fatty acid containing five carbon atoms. It has a role as a plant metabolite. It is a short-chain fatty acid and a straight-chain saturated fatty acid. It is the conjugate acid of a valerate.

The chemical structure of valeric acid is

.

Explanation-

Unsaturated acids-

• Unsaturated acids have one or more carbon-carbon double bonds. The term unsaturated indicates that fewer than the maximum possible number of hydrogen atoms are bonded to each carbon in the molecule.
• α-Hydroxy acids on heating undergo intermolecular dehydration and form lactides.
• β-Hydroxy acid undergoes intramolecular dehydration and gives unsaturated acid on heating.

From the given options Option 2 is β-Hydroxy acid so it will give unsaturated acid on heating.

Explanation:

• NO₂ on benzoic acid shows two effects -m and -I.
• On meta-position, the mesomeric (-m) effect has no significance, so on the ortho position nitro group shows -m,-I effect, on meta-position -I effect and on para position -m, -I effect.
• Ortho-nitro benzoic acid shows an ortho effect means NO₂ is present at the ortho position which is closer to COOH and is an electron-withdrawing group and will pull electrons towards itself, O^- has less charge so it will easily release H⁺ hence it increases its acidity. The Ortho effect forces the carbonyl group to change its polarity due to which resonance decreases and is less stable which increases acidity.
• Also para-Nitro benzoic acid has increased acidic strength as NO₂ at the para position will withdraw electrons but will be less than that of ortho nitro benzoic acid. 
• whereas meta benzoic acid shows only -I effect hence its acid strength is less than that of o-nitro and p-nitro.

So the order will be

• O-Nitro < P-Nitro < M-Nitro.