Question

For a compound to be chiral, it must have (A) Chiral centre (B) Axis of symmetry (C) Centre of Inversion (D) Optical activity

Short answer

The correct option is (A) Chiral centre. A compound is considered chiral if it has a chiral centre, usually a carbon atom bonded to four different groups, resulting in non-superimposable mirror images and optical activity.

Step-by-step solution

Option A: Chiral Centre

Chiral compounds have non-superimposable mirror images. A chiral centre, also known as a stereocenter, is an atom in a molecule, most commonly a carbon atom, bonded to four different groups. The presence of a chiral centre can give rise to optical activity, which is a characteristic of chiral compounds.

Option B: Axis of Symmetry

Compounds that have an axis of symmetry are not chiral, because if a molecule can be superimposed on its mirror image along the axis of symmetry, then they are not chiral. The presence of an axis of symmetry would not make a molecule chiral; instead, it is an indication of achirality.

Option C: Centre of Inversion

A compound with a centre of inversion is also not chiral. The centre of inversion is a point at which all the atoms in a molecule can be inverted through to occupy the same positions as their corresponding atoms in the mirror image of the molecule. If a molecule has a centre of inversion, it can be superimposed on its mirror image, which means that it is not chiral.

Option D: Optical Activity

Optical activity is a property of chiral compounds and is demonstrated by their ability to rotate the plane of plane-polarized light. While optical activity is observed in chiral compounds, it is a consequence rather than a requirement for a compound to be chiral. After analyzing all the options, we can see that the presence of a "chiral centre" directly relates to a compound being chiral. Therefore:

Answer

The correct option is (A) Chiral centre.

Key concepts

Chiral Centre

A chiral centre, often referred to as a stereocenter, is a fundamental concept in understanding chirality in chemistry. Imagine a central atom, usually carbon, connected to four different groups. This is the basic structure of a chiral centre. You might wonder why this arrangement is so special. The answer lies in its capacity to create two non-superimposable mirror images, a hallmark of chirality.

Here's a simple analogy: picture your left and right hands. Although they look alike, you cannot superimpose one onto the other to match exactly. The same is true for molecules with a chiral centre. Such a molecule cannot be aligned perfectly with its mirror image.

This distinct arrangement makes the molecule unique and directly contributes to its chirality. However, keep in mind, not all molecules with multiple different groups are chiral. There are specific conditions where these molecules still show symmetry, rendering them achiral. Nonetheless, the presence of a chiral centre is a strong indicator and often directly correlates to chirality in a compound.

• Chiral centres are most commonly carbon atoms.
• They must be bonded to four distinct groups.
• Chirality leads to non-superimposable mirror images.

Optical Activity

Optical activity is a fascinating property that some molecules exhibit, closely linked to chirality. When light is polarized, it's usually split into two waves, one of which oscillates in a plane. An optically active compound can rotate this plane of polarized light either to the left or the right.

This optical rotation is often measured with a device called a polarimeter, providing insights into the chiral nature of a compound. However, it’s critical to grasp that while optical activity is indicative of chiral compounds, its absence doesn't necessarily mean the molecule isn't chiral.

Optical activity depends on the presence of a molecule's distinct spatial arrangement. The actual rotation's degree can be influenced by factors like:

• The nature of the compound itself.
• The concentration of the solution.
• The length of the light path through the sample.

Thus, optical activity is an important tool in identifying and studying chiral substances, acting as a window into the asymmetric nature of these fascinating molecular structures.

Stereocenter

The concept of a stereocenter expands on the idea of chirality through chiral centres. A stereocenter is any atom in a molecule that leads to stereoisomers when two groups are exchanged. While all chiral centres are stereocenters, not every stereocenter results in chirality.

In simpler terms, consider a molecule with several potential points of symmetry or asymmetry. Each of these points is essentially a stereocenter. However, it's the specific arrangement of these stereocenters relative to each other that determines whether the molecule is truly chiral.

• Stereocenters are key to understanding the 3D arrangement of a molecule.
• Two groups swapping at a stereocenter create different stereoisomers.
• Stereocenters include but are not limited to chiral centre carbons.

Recognizing stereocenters in complex molecules helps chemists understand and predict the behavior of these substances in different environments and reactions. Identifying these locations within a compound lays the groundwork for further exploring its chemical properties and potential uses.