Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number of chiral centres present in threonine is

Step-by-Step Solution

Answer

Threonine has 2 chiral centres.

Step 1: Understand the Structure of Threonine

Threonine is an amino acid with a chemical structure comprising a carbon backbone, an amino group (-NH2), a carboxyl group (-COOH), and a side chain. Its molecular formula is C4H9NO3.

Step 2: Identify and Sketch the Structure

Sketch the structure of threonine. Notably, threonine has a central carbon (known as the alpha carbon) that is bonded to four different groups: a hydrogen atom, an amino group, a carboxyl group, and a side chain containing a hydroxyl group bound to another carbon.

Step 3: Identify Chiral Centres

A chiral centre is a carbon atom that has four different substituents. In threonine, the alpha carbon is chiral because it is connected to a hydrogen, an amino group, a carboxyl group, and a side chain with a hydroxyl group. Additionally, the carbon in the side chain bonded directly to the hydroxyl group is also chiral because it is attached to a hydrogen, hydroxyl group, and two different carbon chains.

Step 4: Count the Number of Chiral Centres

After identifying the chiral centres, we found two chiral carbons in threonine: the alpha carbon and the carbon in the side chain. Thus, threonine has two chiral centres.

Key Concepts

Threonine structure

Threonine is one of the standard amino acids in the body, and it plays a vital role in protein synthesis. It's known for having a structure that consists of a central carbon, termed the alpha carbon, to which four different groups are attached:

• An amino group (-NH₂), which is essential for forming peptide bonds with other amino acids.
• A carboxyl group (-COOH), which is one of the defining features of an amino acid.
• A unique side chain characterized by a carbon-oxygen bond, including a hydroxyl group, giving threonine its specific properties.
• A hydrogen atom (H), completing the tetravalency of the carbon atom.

This structure with its specific side chain determines the chemical nature of threonine and contributes to its role in forming proteins. Understanding its molecular formula, C₄H₉NO₃, we see how its atoms are organized to allow it to interact and bond with other molecules, a critical characteristic for its function in biological systems.

Chiral centres identification

A chiral centre in a molecule is a carbon atom that is bonded to four different groups. Identifying these centres is crucial in organic chemistry, especially in amino acids like threonine, as they determine the molecule's optical activity. In threonine, we identify two significant chiral centres:

• The alpha carbon, which is linked to the amino group, carboxyl group, hydrogen, and the unique side chain.
• The second chiral centre is located in the side chain itself, specifically at the carbon bonded to the hydroxyl group. This carbon links to a hydrogen atom and two different carbon chains, confirming its status as a chiral centre.

By correctly identifying these chiral centres, we establish that threonine exhibits chirality, meaning it can exist in more than one form with distinct spatial arrangements, which can significantly affect biochemical reactions.

Amino acids chirality

Chirality in amino acids is a fundamental concept in understanding how proteins function. Amino acids can have one or more chiral centres that allow them to exist as stereoisomers. These different forms can have distinct biological activities and interact differently with other chiral molecules.

In threonine, the presence of two chiral centres means it has multiple stereoisomers, making it unique among amino acids. More specifically, it can form four stereoisomers: two pairs of enantiomers, which are non-superimposable mirror images.

• Stereochemistry in amino acids is essential because it influences how these molecules are recognized and utilized by enzymes and other proteins in the body.
• Each stereoisomer of an amino acid like threonine can have different biological properties and activities.

Understanding the chirality of amino acids aids in the study of biochemical pathways and the development of pharmaceuticals, since the specific arrangement of atoms influences how a substance can act therapeutically.