IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2HL only~13 min read
Enantiomers
Hold your hands up in front of you. They have the same parts joined in the same order, they are perfect mirror images, and yet you cannot lay one on top of the other so that everything matches. Molecules can do the same thing, and when they do, the two versions can smell different, taste different, or work as a medicine while their mirror image does nothing at all.
📘 What you need to know
A chiral carbon (or chiral centre) is a carbon with four different atoms or groups attached to it.
That carbon is asymmetric — the molecule has no plane of symmetry through it.
A molecule with one chiral centre exists as a pair of enantiomers: non-superimposable mirror images.
Enantiomers are drawn using stereochemical formulas, with wedges and hashes.
Enantiomers have identical physical properties except one: they rotate plane-polarised light in opposite directions. This is called being optically active.
A racemic mixture is a 50:50 mix of both enantiomers. It is optically inactive, because the two rotations cancel.
Diastereomers have more than one chiral centre and are not mirror images, so they have different physical and chemical properties.
Enantiomers behave differently in chiral environments — which is why they matter so much in drugs and in smell.
Finding the chiral centre
The test is short: look at every carbon in turn and count the four things attached to it. If all four are different, that carbon is chiral. If any two are the same, it is not.
The two molecules differ by a single CH2, which is enough to break the symmetry. Butan-2-ol exists as two enantiomers; propan-2-ol exists as only one compound, however you draw it.
Quick screening trick. A chiral carbon can never be a CH2, a CH3, or part of a C=C or C≡C. It must have four single bonds and only one hydrogen at most. So scan for carbons with exactly one H and three different heavier groups — that is where a chiral centre will be.
Drawing the two enantiomers
Because chirality is about three-dimensional shape, you cannot show it with a flat formula. You need the wedges and hashes from the formulas page: a bold wedge for a bond coming towards you, a hashed wedge for one going away.
Both structures have the same four groups on the same central carbon. What differs is the order they are arranged in around it, and no amount of turning the page will make them match.
🧩 Drawing a pair of enantiomers
Put the chiral carbon in the middle of your space, with room on all four sides.
Draw two plain lines going up and out, one to the left and one to the right. These bonds lie in the plane of the paper.
Draw a bold wedge down and out for the bond coming towards you, wide end away from the carbon.
Draw a hashed wedge down the other side for the bond going behind the paper, hashes getting longer as they go.
Label the four groups, then draw the mirror image by swapping left for right. Check the two drawings are not the same thing rotated.
The commonest mistake here is drawing the “mirror image” by swapping just two groups at the top and leaving the wedges alone. That gives you the same molecule again. Reflect the whole drawing, wedges included — if the wedge was on the left, it goes on the right.
What is actually different about them
This is the strange part. Enantiomers have the same bonds, the same bond lengths, the same relative molecular mass. So they have the same melting point, the same boiling point, the same density and the same solubility. There is exactly one physical property that separates them.
The one difference
Enantiomers rotate plane-polarised light by the same angle in opposite directions
Ordinary light vibrates in every plane at once. Push it through a polariser and only one plane survives — that is plane-polarised light. Send it through a solution of a single enantiomer and the plane comes out rotated. One enantiomer turns it clockwise, the other turns it anticlockwise by the same number of degrees.
The angle depends on the tube length and the concentration as well as on the substance, so a polarimeter reading is only meaningful when those are fixed. The clockwise enantiomer is labelled (+) and the anticlockwise one (−).
Racemic mixtures
A racemic mixture (or racemate) contains equal amounts of the two enantiomers. Each molecule still rotates the light, but for every molecule turning it clockwise there is one turning it anticlockwise, so the effects cancel exactly and the mixture is optically inactive. A polarimeter reads zero.
This matters commercially. Ordinary laboratory reactions have no way of preferring one mirror image over the other, so making a chiral compound from non-chiral starting materials almost always gives a racemate. Separating the two is slow and expensive, so many drugs are sold as racemic mixtures even though only one enantiomer does the work. A compound containing only one enantiomer is called enantiopure.
Why enantiomers behave differently in the body. Physical properties are identical, but biology is not symmetrical. Enzymes and receptors are themselves chiral, so they fit one enantiomer and not the other — exactly like a left hand fitting a left glove. That is why one enantiomer of carvone smells of spearmint and the other smells of caraway seeds, from the same molecular formula.
Diastereomers: more than one chiral centre
If a molecule has two chiral centres, each one can be arranged two ways, so there are up to four stereoisomers rather than two. Only some of those pairs are mirror images.
Enantiomers are pairs where every chiral centre is reversed. They are mirror images and share all physical properties except optical rotation.
Diastereomers are pairs where some but not all centres are reversed. They are not mirror images, so they have genuinely different melting points, boiling points, solubilities and chemical reactivity.
Because diastereomers have different physical properties, you can separate them by ordinary methods like distillation or crystallisation. Enantiomers you cannot — that is the whole difficulty of making an enantiopure drug.
Worked examples
WORKED EXAMPLE
Identify the chiral carbon in 2-chlorobutane, CH3CHClCH2CH3, and explain why the other carbons are not chiral.
Step 1: number the carbons and take each in turnC1 is a CH3, so it has three identical hydrogens. Not chiral.Step 2: check carbon 2attached to: CH3, Cl, H, CH2CH3Four different groups. This is the chiral centre.Step 3: check the remaining carbonsC3 is a CH2, so it has two identical hydrogens. C4 is a CH3. Neither can be chiral.carbon 2 onlyOne chiral centre means exactly two stereoisomers, and they are enantiomers of each other.
WORKED EXAMPLE
State whether each of these molecules is chiral: propan-1-ol, butan-2-ol, and 2-methylpropan-2-ol.
Step 1: propan-1-ol, CH3CH2CH2OHThe carbon bearing the OH is a CH2, so it already has two identical hydrogens.not chiralStep 2: butan-2-ol, CH3CH(OH)CH2CH3Carbon 2 carries OH, H, CH3 and CH2CH3 — four different groups.chiralStep 3: 2-methylpropan-2-ol, (CH3)3COHCarbon 2 carries OH and three methyl groups. Three of the four are identical.not chiralonly butan-2-ol is chiralNotice the pattern: the chiral one is the secondary alcohol. A chiral centre needs exactly one hydrogen, which is what “secondary” gives you.
WORKED EXAMPLE
A student measures the optical rotation of three samples of the same chiral compound and gets +14°, −14° and 0°. Explain each result.
Step 1: what a non-zero reading meansA single enantiomer rotates the plane of polarised light by a fixed angle in a fixed direction.+14° is one pure enantiomer, rotating clockwiseStep 2: interpret the equal and opposite readingThe same magnitude with the opposite sign is the signature of the mirror image.−14° is the other pure enantiomer, rotating anticlockwiseStep 3: interpret zeroEqual numbers of each enantiomer means the two rotations cancel exactly.0° is a racemic mixture, 50:50, optically inactivetwo pure enantiomers and one racemateZero rotation does not prove the sample is achiral. It could be a racemate, and those are two very different situations.
Enantiomers and diastereomers compared
Feature
Enantiomers
Diastereomers
Number of chiral centres
one or more, all reversed
two or more, only some reversed
Mirror images?
yes
no
Melting and boiling points
identical
different
Solubility
identical
different
Optical activity
equal and opposite rotation
not usually optically active
Can you separate them easily?
no, properties are the same
yes, by normal physical methods
💡 Exam tip
Look for a carbon with exactly one hydrogen. That is the fastest way to find a chiral centre, because four different groups leaves room for only one H.
Mark the chiral centre with an asterisk when a question asks you to identify it. It is the standard notation and leaves no doubt.
Reflect the whole drawing, wedges included, when drawing the second enantiomer. Swapping only the flat groups gives you the same molecule.
Say “identical physical properties except optical rotation”. That single clause is worth a mark on its own.
A racemate is optically inactive, not achiral. Every molecule in it is still chiral.
Explain biological differences through chiral receptors, not through shape alone. Enzymes are chiral, so they can tell the two apart.
⚠ Common mix-up
Thinking four different atoms are needed. They can be groups. In butan-2-ol two of them are CH3 and CH2CH3, both carbon, but they are different groups.
Missing that a CH2 can never be chiral. Two hydrogens on the same carbon rules it out immediately.
Drawing two structures that are the same molecule rotated. If you can turn one to match the other, they are not enantiomers.
Claiming enantiomers have different melting points. They do not. That is diastereomers.
Saying a racemate is not chiral. It is a mixture of two chiral compounds; the rotation cancels, the chirality does not vanish.
Confusing cis-trans with optical isomerism. Both are configurational, but cis-trans needs a blocked rotation and optical isomerism needs a chiral centre.
Assuming a molecule with two chiral centres has only two stereoisomers. Two centres give up to four.
Up next: Mass Spectrometry Fragmentation Patterns — that is the end of isomerism. From here on we work the other way round: given an unknown compound, how do you use instruments to work out what it is?
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