Optical isomerism

Chirality is the property of an object — or a molecule — of being non-superimposable on its own mirror image, exactly like a left hand and a right hand. A molecule with this property is chiral; its two mirror-image forms are called enantiomers. A molecule that can be superimposed on its mirror image (the two are identical) is achiral. The most common source of chirality at this level is a chiral carbon (also called an asymmetric carbon or a stereocentre): a carbon atom bonded to four different groups. When all four substituents on a carbon differ, its mirror image cannot be rotated to match the original — the two spatial arrangements are genuinely distinct structures. This is a purely geometric consequence of the carbon's tetrahedral, $sp^3$ bonding: four different groups pinned to the corners of a tetrahedron admit exactly two non-interconvertible arrangements, related as object and mirror image. Two ideas keep this test precise. First, "different" means chemically different, not merely drawn differently — a branch that looks distinct on paper can still be an identical group once you trace it out fully, so every one of the four attached groups must be compared carefully, not eyeballed. Second, optical isomerism is a different phenomenon from geometrical isomerism even though both are stereoisomerism: geometrical (cis/trans) isomerism comes from restricted rotation about a double bond or a ring, while optical isomerism comes from the three-dimensional tetrahedral arrangement around a chiral centre. One is about a locked bond; the other is about a locked shape in space.

Key qualitative facts about enantiomers:

• They have identical connectivity — the same atoms bonded in the same order — so they share ordinary physical properties such as melting point, boiling point and density, and they react identically toward ordinary (achiral) reagents.
• They differ only in how they interact with plane-polarised light: one enantiomer rotates the plane of polarisation in one direction, and its mirror-image partner rotates it by an equal amount in the opposite direction. This behaviour is why they are called "optical" isomers, and it is measured with an instrument called a polarimeter. (The precise numerical treatment — specific rotation, in degrees per unit path length and concentration — is not required at this level.)
• A 1:1 mixture of the two enantiomers, a racemic mixture, shows no net rotation at all, because the equal-and-opposite rotations exactly cancel.
• Optical isomerism is one of two branches of stereoisomerism at this level; the other, geometrical isomerism, arises from a different physical cause (a locked double bond), so the two must never be conflated even though both preserve the same connectivity.

The reasoning behind chirality is entirely geometric, and it is worth walking through once so the "four different groups" test feels like a consequence rather than a rule to memorise. A carbon atom with four single bonds is $sp^3$-hybridised, and its four bonds point toward the corners of a regular tetrahedron centred on the carbon. If three or more of those four groups are identical, the molecule has an internal mirror plane or an axis of symmetry running through it — you can always find a way to rotate the mirror-image structure so it lands exactly back on the original. Superimposition succeeds, and no optical isomerism arises. The moment all four groups become distinct, that internal symmetry vanishes. There is now no rotation of the molecule that can make its mirror image coincide with the original arrangement, because swapping any two groups (which is effectively what forming the mirror image does at a tetrahedral centre) always produces a spatially different structure. The two arrangements — original and mirror image — are related the way a left glove is related to a right glove: same parts, same connections, permanently different handedness. This is exactly the same geometric fact that makes a hand chiral, just realised in a tetrahedral atom instead of a macroscopic shape. This also explains why the optical behaviour is so selective: since the two enantiomers differ only in the three-dimensional arrangement around the chiral centre and not in which atoms are bonded to which, every ordinary chemical and physical interaction — melting, boiling, reacting with a symmetric reagent — treats them identically. Plane-polarised light is different because polarised light itself has a handedness (a preferred plane of oscillation), so it is the one probe sensitive enough to tell the two mirror-image arrangements apart, rotating its plane one way for one enantiomer and the opposite way for the other.

Question. Two molecules are given: 2-chlorobutane, $\text{CH}_3-\text{CHCl}-\text{CH}_2-\text{CH}_3$, and propan-2-ol, $\text{CH}_3-\text{CH(OH)}-\text{CH}_3$. For each, decide whether it contains a chiral carbon, and hence whether it can show optical isomerism.

Step 1. Locate the candidate carbon in each molecule. In 2-chlorobutane, $C_2$ (the carbon bearing $\text{Cl}$) is attached to the most varied set of neighbours, so it is the one to test. In propan-2-ol, $C_2$ (bearing $-\text{OH}$) is the analogous candidate.

Step 2. List all four groups on $C_2$ of 2-chlorobutane: $\text{Cl}$, $\text{H}$, $\text{CH}_3$ (from $C_1$), and $\text{CH}_2\text{CH}_3$ (the ethyl group from $C_3$–$C_4$). Chlorine, hydrogen, methyl and ethyl are four genuinely distinct groups — none repeats.

Step 3. List all four groups on $C_2$ of propan-2-ol: $\text{OH}$, $\text{H}$, $\text{CH}_3$ (from $C_1$), and $\text{CH}_3$ (from $C_3$). Here two of the four groups are identical $-\text{CH}_3$ units.

Step 4. Apply the four-different-groups test and conclude. 2-chlorobutane's $C_2$ carries four different substituents, so it is a chiral carbon: 2-chlorobutane exists as a pair of enantiomers. Propan-2-ol's $C_2$ carries two identical methyl groups, so it fails the test: propan-2-ol has no chiral carbon and shows no optical isomerism.

2-chlorobutane is chiral (one pair of enantiomers); propan-2-ol is achiral.

Competitive stretch

The same logic extends immediately to butan-2-ol, $\text{CH}_3-\text{CH(OH)}-\text{CH}_2-\text{CH}_3$: its $C_2$ carries $\text{OH}$, $\text{H}$, $\text{CH}_3$ and $\text{CH}_2\text{CH}_3$ — four distinct groups — so butan-2-ol, unlike propan-2-ol, is chiral. Notice the pattern: replacing one of propan-2-ol's two identical methyl groups with a longer, different chain is enough to turn an achiral molecule into a chiral one, because it is the repeat of a group, not its length, that destroys chirality.

• Judging a group as "different" by how it looks drawn rather than tracing its actual atoms — two branches that look dissimilar on paper can still be chemically identical, and two that look similar can differ.
• Confusing optical isomerism with geometrical isomerism: optical isomerism comes from a tetrahedral chiral centre with four different groups, not from restricted rotation about a double bond.
• Assuming enantiomers differ in ordinary physical properties like boiling point or density — at this qualitative level they are identical in these respects and differ only in their interaction with plane-polarised light.
• Attempting to assign "R" or "S" labels or calculate specific rotation values — this level of detail is explicitly beyond ISC Class 11 and reserved for later study; the syllabus asks only for conceptual awareness of chirality and enantiomers.

Ask the tutor: Want to practice spotting chiral carbons in a few more molecules, like 3-chloropentane or 2-bromobutane?