Chemistry · General Principles Of Organic Chemistry · NEET
No. Both are types of stereoisomers (same bonding, different 3D arrangement), but the mirror-image test separates them. If the two structures are non-superimposable mirror images, they are enantiomers. If they are stereoisomers but NOT mirror images, they are diastereomers. So enantiomers are one special pair, and everything else that is a stereoisomer but not a mirror image is a diastereomer.
Draw both molecules and ask two questions in order. Question 1: Are they mirror images of each other? If NO, they are diastereomers. If YES, ask Question 2: Can one be placed exactly on top of the other (superimposable)? If they cannot be superimposed, they are enantiomers. If they CAN be superimposed, they are the same molecule (this is the meso case).
Cis and trans (geometrical) isomers are diastereomers. Cis-2-butene and trans-2-butene are stereoisomers, but they are not mirror images of each other, so by definition they are diastereomers. This is why cis and trans forms have different melting points and boiling points, just like other diastereomers do. The next concept, cis-trans isomerism, covers this in detail.
Yes, mostly. Enantiomers have identical melting point, boiling point, density and solubility in ordinary solvents. They differ in only two things: they rotate plane-polarised light by equal angles but in opposite directions (one is + / dextro, the other is - / laevo), and they react differently with other chiral molecules. Diastereomers, by contrast, have DIFFERENT physical properties and can be separated by ordinary methods like distillation or crystallisation.
A meso compound has chiral centres but also has an internal plane of symmetry, so its mirror image can be superimposed on the original. That makes it the same molecule, not a pair. A meso compound is therefore optically inactive (the internal halves cancel each other, called internal compensation). Example: meso-tartaric acid. A meso form is a diastereomer of the (+) and (-) forms, not an enantiomer of them.
The maximum number of stereoisomers is 2 raised to the power n, where n is the number of chiral (asymmetric) carbon centres. So 1 chiral centre gives 2 (a pair of enantiomers), 2 chiral centres give up to 4, and 3 give up to 8. The count can be LESS than 2^n when the molecule has symmetry that creates a meso form (as in tartaric acid, which has 2 centres but only 3 stereoisomers).
The incorrect statement regarding chirality is:
How many products (including stereoisomers) are expected from the monochlorination of 2-methylbutane, (CH3)2CH-CH2-CH3?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Not necessarily. Diastereomers include cis-trans (geometrical) isomers, which need not be optically active at all, and also meso forms, which are optically inactive despite having chiral centres. Optical activity depends on whether the molecule is chiral, not on whether it is a diastereomer.
No. A racemic mixture is a 1:1 mix of two enantiomers, and it shows zero optical rotation because the two rotations cancel. Diastereomers are a relationship between two different, separable compounds. A racemic mixture is about equal amounts of the SAME enantiomeric pair.
No, these are separate branches. Structural (constitutional) isomers differ in the order of bonding of atoms. Stereoisomers have the SAME bonding order but differ in 3D arrangement. Enantiomers and diastereomers are both under the stereoisomer branch.
Because superimposability is the single test that decides chirality. A molecule that is non-superimposable on its mirror image is chiral and has an enantiomer. If it IS superimposable, it is achiral (or meso) and optically inactive. Many one-mark NEET statements hinge on this exact word.
