Optical Activity and Chiral Centres in Synthesis

Optical Activity and Chiral Centres in Synthesis

Chirality in Organic Synthesis

The formation and manipulation of chiral centres is one of the most important considerations in modern organic synthesis. Many biologically active molecules — drugs, amino acids, sugars — are chiral, and often only one enantiomer has the desired biological effect.

Recap: Chirality

A molecule is chiral if it contains a carbon (or other atom) bonded to four different groups. The two non-superimposable mirror images are called enantiomers. They have identical physical properties but rotate plane-polarised light in opposite directions and interact differently with other chiral molecules (such as enzymes and receptors).

Racemic Mixtures from Achiral Starting Materials

When a reaction creates a new chiral centre from a flat (planar) intermediate or starting material, the nucleophile or reagent can attack from either face with equal probability. This produces a racemic mixture (50:50 mixture of enantiomers, showing no net optical rotation).

Example 1: Nucleophilic addition of HCN to ethanal

CH₃CHO + HCN → CH₃CH(OH)CN

The carbonyl carbon is sp² (planar, trigonal). The CN⁻ nucleophile can attack from above or below the plane:

• Attack from above → R-enantiomer
• Attack from below → S-enantiomer
• Both are equally likely → racemic mixture

Example 2: Reduction of butanone

CH₃COCH₂CH₃ + NaBH₄ → CH₃CH(OH)CH₂CH₃

The carbonyl group is again planar. The H⁻ (hydride) can approach from either face → racemic butan-2-ol.

SN1 Reactions and Racemisation

In an SN1 mechanism, the leaving group departs first to form a planar carbocation. The nucleophile can then attack from either side:

• If the starting material was a single enantiomer, the product is a racemic mixture (complete racemisation, or at least significant racemisation)
• This is evidence for the SN1 mechanism — the intermediate must be planar for equal attack from both sides

Example: Hydrolysis of (R)-2-bromobutane via SN1:

(R)-CH₃CHBrCH₂CH₃ → [CH₃C⁺HCH₂CH₃] → racemic CH₃CH(OH)CH₂CH₃

The carbocation is trigonal planar (sp²) → attack from both faces → both R and S products.

SN2 Reactions and Inversion

In an SN2 mechanism, the nucleophile attacks from the back (the side opposite the leaving group) in a single concerted step. This causes complete inversion of configuration at the chiral centre (Walden inversion):

(R)-substrate → (S)-product (or vice versa)

The configuration at the chiral centre is flipped, like an umbrella turning inside out.

This is how SN2 can be distinguished from SN1 experimentally — SN2 gives inversion, SN1 gives racemisation.

Asymmetric Synthesis

Producing a single enantiomer is called asymmetric synthesis (or enantioselective synthesis). Methods include:

1. Chiral catalysts

A catalyst that is itself chiral can favour the formation of one enantiomer over the other. The catalyst creates an asymmetric environment around the reaction site, making one face of the planar intermediate more accessible.

Example: The Sharpless epoxidation uses a chiral titanium catalyst to convert allylic alcohols to epoxides with high enantiomeric excess (>90% of one enantiomer).

Industrial example: The synthesis of L-DOPA (a drug for Parkinson's disease) uses a chiral rhodium phosphine catalyst in an asymmetric hydrogenation. This was developed by Knowles and Noyori (Nobel Prize 2001).

2. Enzymes

Enzymes are biological catalysts that are inherently chiral (made of L-amino acids). They catalyse reactions with very high enantioselectivity because their active site is a specific 3D shape that accommodates only one enantiomer of the substrate or produces only one enantiomer of the product.

Example: Lipase enzymes can selectively hydrolyse one enantiomer of an ester while leaving the other intact — this is a kinetic resolution.

3. Chiral pool synthesis

Starting from naturally occurring chiral molecules (amino acids, sugars, terpenes) that are already enantiopure, and building the target molecule while preserving the existing chirality.

4. Resolution of racemates

If a racemic mixture has been produced, the enantiomers can be separated:

• Diastereomeric salt formation: React the racemate with a single enantiomer of a chiral acid or base. The two resulting salts are diastereomers (not enantiomers), which have different physical properties (solubility, melting point) and can be separated by fractional crystallisation. Then regenerate the free enantiomers.

• Chiral chromatography: Use a chiral stationary phase in HPLC. Each enantiomer interacts differently with the chiral stationary phase and elutes at a different retention time.

Enantiomeric Excess (ee)

The enantiomeric excess measures how much one enantiomer predominates over the other:

ee (%) = (|[R] − [S]| / ([R] + [S])) × 100

• ee = 0% → racemic mixture
• ee = 100% → pure single enantiomer

Alternatively, ee can be determined from optical rotation:

ee (%) = (observed rotation / rotation of pure enantiomer) × 100

Worked Example

A synthesis produces a mixture of 85% R-enantiomer and 15% S-enantiomer. Calculate the enantiomeric excess.

ee = (85 − 15) / (85 + 15) × 100 = 70/100 × 100 = 70%

Why Single-Enantiomer Drugs Matter

Thalidomide is the most notorious example of why chirality matters in drug design:

• The (R)-enantiomer was an effective sedative
• The (S)-enantiomer was a teratogen (caused birth defects)
• The drug was prescribed as a racemate in the 1950s/60s
• Even if a pure enantiomer had been given, it racemises in vivo (converts between R and S in the body)

This case led to strict regulatory requirements for testing each enantiomer of a chiral drug separately.

Other examples:

• Ibuprofen: Only the S-enantiomer is the active anti-inflammatory; the R-enantiomer is inactive but slowly converts to S in the body
• Adrenaline (epinephrine): The R-enantiomer is 20× more potent than the S-enantiomer
• Naproxen: The S-enantiomer is the desired anti-inflammatory; the R-enantiomer causes liver damage

Industrial Importance

Single-enantiomer drugs are a multi-billion pound industry. Chiral synthesis and resolution techniques allow pharmaceutical companies to:

• Reduce the dosage (only the active enantiomer is given)
• Reduce side effects (the inactive or harmful enantiomer is excluded)
• Obtain new patents on single-enantiomer versions of existing drugs ("chiral switches")

Exam Tips

• When a reaction creates a chiral centre from a planar intermediate, always state that a racemic mixture forms and explain why (equal probability of attack from both sides)
• For SN1 → racemic mixture; for SN2 → inversion of configuration — know these as stereochemical outcomes
• If asked about the importance of chirality in drug design, reference thalidomide and explain that enantiomers can have different biological effects
• Enantiomeric excess calculations are straightforward — use the formula and show your working
• When discussing asymmetric synthesis, name a specific method (chiral catalyst, enzyme, or chiral pool) rather than giving a vague answer