Question

Enantiomers must have (A) Chiral centre (B) Same rate of reaction with chiral reagent (C) Same direction of angle of rotation with PPL (D) Same boiling point

Short answer

Enantiomers must have a chiral centre, thus, option (A) is true. They don't always react at the same rate with chiral reagents, thus option (B) is false. They rotate plane-polarized light in opposite directions, thus option (C) is false. Enantiomers have identical physical properties such as boiling point due to identical chemical composition, thus option (D) is true.

Step-by-step solution

Option A: Chiral Centre

Enantiomers must have a chiral centre. A chiral centre is an atom (usually carbon) that is connected to four different substituents. This characteristic allows enantiomers to exist as non-superimposable mirror images. Therefore, option A is true.

Option B: Same rate of reaction with chiral reagent

Enantiomers do not always have the same rate of reaction with chiral reagents. This is because the different spatial orientations of enantiomers can affect how they react with chiral reagents. As a result, enantiomers could have either different or similar rates of reaction with chiral reagents. Therefore, option B is false.

Option C: Same direction of angle of rotation with PPL

Enantiomers have opposite directions of angle of rotation with plane-polarized light (PPL). This property is called optical activity. One enantiomer will rotate the plane of polarized light to the left (levo, or counterclockwise rotation), while its mirror image will rotate the plane of polarized light to the right (dextro, or clockwise rotation). Therefore, option C is false.

Option D: Same boiling point

Enantiomers have the same physical properties, such as boiling point, because they are made up of the same atoms and overall chemical composition. The only differences between enantiomers are their spatial arrangements (stereochemistry) and how they interact with other chiral molecules. Therefore, option D is true.

Key concepts

Chiral Centre

A chiral centre is fundamental to the structure of enantiomers. It typically refers to a carbon atom bonded to four distinct groups, creating a molecule without an internal plane of symmetry. Picture a tetrahedron with each corner distinctly marked; this geometric shape models the spatial arrangement at a chiral centre. A molecule can have one or multiple chiral centres, each contributing to the potential complexity of the molecule's 3D structure.

Understanding the concept of a chiral centre elucidates why enantiomers are non-superimposable mirror images of each other. Each chiral centre gives rise to two configurations (R or S), which are like left and right hands—similar yet opposite. This aspect of spatial arrangement explains the unique behaviors and reactions of enantiomers, as they interact differently with other chiral substances, including biological molecules such as proteins and enzymes.

Optical Activity

Optical activity is a mesmerizing property unique to chiral molecules. It refers to the ability of a substance to rotate the plane of plane-polarized light (PPL). A polarimeter, which measures this rotation, can determine the direction and degree of rotation, providing insight into the enantiomeric composition of the sample. Enantiomers exhibit optical activity in opposite directions: one enantiomer (the 'left-handed' one) will rotate light counterclockwise, designated as (-), while the other (the 'right-handed' one) will rotate light clockwise, designated as (+).

This characteristic is pivotal in fields such as pharmacology, where the optical activity of a drug can affect its efficacy and safety. For example, one enantiomer may be biologically active, while its mirror image might be less effective or even harmful, showing the real-world implications of this molecular phenomenon.

Physical Properties of Enantiomers

Enantiomers, despite their differing spatial configurations, share almost identical physical properties. Boiling points, melting points, densities, and refractive indices remain the same because these properties mainly depend on the molecular weight and overall connectivity of atoms within the molecule. The interesting takeaway here is that although enantiomers interact differently with polarized light and chiral environments, their responses to physical influences such as temperature and pressure are nearly indistinguishable.

It's this semblance in physical properties that often makes separation of enantiomers challenging in a lab. However, knowing that they behave identically in a non-chiral environment is useful in chemical synthesis and analysis. When designers of drugs or other materials require a specific enantiomer, they must employ special techniques, such as chiral chromatography, to isolate the desired stereochemistry.