Organic Chemistry: Chirality

Revise Organic Chemistry: Chirality for Chemistry 4 (IAL) WCH14 (A2 Level) — revision notes and instant AI marking.

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Edexcel IAL Chemistry · Organic Chemistry

Chirality

Some molecules have a "handedness" — they come as two non-identical mirror-image twins, and telling those twins apart (and knowing when a reaction scrambles them) is the whole game of this chapter.

Quick Summary

Chiral centre = a carbon bonded to 4 different groups → no plane of symmetry.
One chiral centre → 2 enantiomers (non-superimposable mirror images).
Enantiomers have identical chemical/physical properties, except: (1) how they rotate plane-polarised light, and (2) how they interact with other chiral things (smell, drug action).
A racemic mixture = 50:50 mix of both enantiomers → optically inactive (rotations cancel).
Number of stereoisomers = 2ⁿ where n = number of chiral centres.
S_N1 mechanism → flat carbocation intermediate → nucleophile attacks from either face → racemic mixture produced.
S_N2 mechanism → nucleophile attacks from the back, opposite the leaving group → inversion of configuration (like an umbrella flipping inside out) → enantiopure reactant gives enantiopure product.

1. Chirality & Optical Isomers

What actually makes a carbon "chiral"?

Picture your two hands. They look identical at a glance — same fingers, same shape — but if you try to place your left hand exactly on top of your right hand (palms facing the same way), they never line up. Your thumbs point the wrong way. That's the essence of chirality (from the Greek word for "hand"): an object that is a mirror image of itself, but the mirror image can't be rotated or flipped to match the original.

In organic chemistry, this happens at a chiral carbon (also called a chiral centre or asymmetric carbon) — a carbon atom bonded to four completely different atoms or groups. Because all four groups are different, there's no way to find a "mirror plane" running through the molecule that splits it into two identical halves — hence "asymmetric."

H ←── Group 4 | H₃C ── C ── OH ←── Group 2 | Br ←── Group 1 Groups attached to central C: H, OH, Br, CH₃ → all 4 different → this carbon IS a chiral centre

Analogy that actually helps

Think of the chiral carbon as a claw machine with four different prizes attached — a teddy bear, a car, a ball, a book. However you rotate that claw in space, its mirror image (viewed in a mirror) will never line up with the original, because the prizes are all different and arranged in a specific 3D order around the centre.

Enantiomers: the mirror-image twins

A molecule with one chiral centre exists as exactly two stereoisomers called enantiomers. They are:

Mirror images of each other (like your left and right hand)
Non-superimposable — no amount of rotating one in 3D space will make it match the other
Otherwise built from exactly the same atoms, in the same bonding pattern — same molecular formula, same connectivity

2. Racemates & Optical Activity

Same molecule, different personality

Here's the strange and important part: enantiomers are built from identical atoms connected in an identical way, so almost every physical and chemical property is exactly the same — same melting point, same boiling point, same density, same reactivity with non-chiral (achiral) reagents. If you weren't paying attention, you might think they were literally the same substance.

two things