Problem 105 - A Complete Resource Book in Chemistry for JEE Main

Question

2,4 -dinitrofluorobenzene is used in peptide chemistry because (a) it makes the amino acids insoluble in water and thus making them readily crystallizable (b) it undergoes electrophilic substitution (c) it undergoes nucleophilic substitution (d) it has a characteristic colour

Step-by-Step Solution

Answer

2,4-dinitrofluorobenzene primarily undergoes nucleophilic substitution (option c).

Step 1: Understand the Role of 2,4-dinitrofluorobenzene

2,4-dinitrofluorobenzene (often called Sanger's reagent) is primarily used to react with the amino group of amino acids or peptides. This involved reaction is a type of substitution, which allows scientists to tag the terminal amino acid in a peptide chain.

Step 2: Identify the Type of Substitution Reaction

2,4-dinitrofluorobenzene reacts with amino acids through a substitution mechanism. Due to the presence of the electron-withdrawing nitro groups, the fluorine is a good leaving group, facilitating a nucleophilic substitution reaction.

Step 3: Evaluate the Options

Examine the options given: (a) crystallization of amino acids, (b) electrophilic substitution, (c) nucleophilic substitution, (d) characteristic color. Based on the role of Sanger's reagent, we determine it is primarily involved in nucleophilic substitution (option c).

Step 4: Confirm and Conclude

The main application of 2,4-dinitrofluorobenzene in peptide chemistry is to react with the amino group of the terminal amino acid. This confirms that it mainly undergoes nucleophilic substitution. Option (c) is the correct answer.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a key reaction in organic chemistry, especially within peptide chemistry, where it involves the exchange of a leaving group by a nucleophile. To break this down, a nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond. In the context of Sanger's reagent, the nucleophile is often the amino group of an amino acid or peptide.

This reaction happens because certain conditions make the leaving group easy to replace. In 2,4-dinitrofluorobenzene, the nitro groups are electron-withdrawing, which weakens the carbon-fluorine bond. This makes fluorine a favorable leaving group.

When the amino group of an amino acid acts as the nucleophile, it attacks the carbon attached to the leaving group (fluorine), resulting in the formation of a stable bond and the departure of the fluorine atom. In essence, this process helps tag an amino acid, which is a crucial step in identifying the terminal amino acid residues in peptide chains.

Sanger's Reagent

Sanger's reagent, more formally known as 2,4-dinitrofluorobenzene, is a chemical used primarily in biochemistry for the analysis of proteins and peptides. Named after Frederick Sanger, who pioneered its use, this reagent is particularly useful for determining the amino acid sequence of peptides.

• It chemically modifies the free amino terminus of peptides, making it recognizable.
• The subsequent breakdown of the peptide under analysis makes it easier to identify each terminal amino acid through chromatographic techniques.

This process not only allows scientists to identify leading amino acid residues but also provides insight into the overall amino acid composition. Sanger's work, which was foundational in peptide sequencing, relied heavily on this reagent, emphasizing its importance in biochemical research. It remains a vital tool in laboratories engaged in peptide chemistry.

Amino Acids

Amino acids are the building blocks of proteins and play crucial roles in various biological processes. They are organic compounds composed of an amino group \(-NH_2\), a carboxyl group \(-COOH\), and a variable side chain specific to each amino acid.

• There are 20 standard amino acids, each with unique properties and functions.
• The sequence in which amino acids link together determines a protein's structure and function.

In peptide chemistry, understanding amino acids is fundamental in studying enzyme functions, understanding protein folding, and analyzing peptide sequences.

During nucleophilic substitution, the amino group in amino acids serves as a nucleophile. This makes amino acids particularly interesting in reactions involving Sanger's reagent, where identifying the terminal amino acid is crucial for analysis. Changing this amino group aids in confirming peptide structures, further underscoring how essential amino acids are in biochemical processes.