Organic Chemistry · Stereochemistry at Tetrahedral Centers

Prochirality

8 min read

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

A molecule is when it is achiral but can be converted into a chiral molecule in a single step — usually by replacing one of two identical groups at a tetrahedral carbon, or by adding a group to one face of a trigonal (planar) carbon. Prochirality is the hidden stereochemistry of symmetric-looking molecules: two groups that look identical to us are not truly equivalent when the molecule meets a chiral environment, such as an enzyme active site.

The simplest way to spot a is a tetrahedral carbon bearing two identical groups (pattern a, a, b, c). Ethanol (CH₃CH₂OH) is the classic case: the CH₂ carbon carries two H atoms, a CH₃ group, and an OH group. Replace one H with deuterium (D) and the product CH₃CHDOH is chiral. One change makes the molecule chiral — that is prochirality.

Why this matters

Enzyme stereospecificity: Enzymes are chiral, so they distinguish and faces that achiral reagents treat as identical — e.g., aconitase reacts with only one "arm" of prochiral citric acid.
Asymmetric synthesis: Chiral reagents, catalysts, or enzymes attack one face or replace one group selectively, turning prochiral starting materials into single enantiomers.
Drug metabolism: Many drugs are metabolized at prochiral centers; chiral metabolic enzymes can turn an achiral drug into a chiral metabolite whose enantiomers differ in activity or toxicity.
Exams: "Are these groups homotopic, enantiotopic, or diastereotopic?" and "Which face is Re, which is Si?" are classic stereochemistry questions.

The college version

Core Concepts

Prochirality centers

A prochirality center is a tetrahedral atom bonded to two identical groups plus two different groups (pattern a, a, b, c). If the identical groups were different, the carbon would be a stereocenter; because they are identical, the molecule is achiral — but only barely. Examples:

Ethanol (CH₃CH₂OH): the CH₂ carbon is bonded to H, H, CH₃, OH.
Acetaldehyde (CH₃CHO): the trigonal carbonyl carbon bears O, CH₃, and H — a prochiral center with two .

Homotopic, enantiotopic, and diastereotopic groups

Classify identical groups by an imaginary replacement experiment: swap one group at a time for D and compare the two products:

Homotopic: replacing either group gives the identical molecule. Example: the three H atoms of the CH₃ group in ethanol — replace any one with D and you get the same CH₂DCH₂OH (no stereocenter is created). are indistinguishable even by enzymes.
Enantiotopic: replacing one group gives one enantiomer, the other gives the opposite. Example: the two H atoms on the CH₂ carbon of ethanol — replacement gives (R)- or (S)-CH₃CHDOH. Achiral reagents see these as equal; chiral reagents can select one.
Diastereotopic: replacement gives diastereomers, which happens when the molecule already has a stereocenter elsewhere. Example: the two H atoms on the CH₂ group adjacent to the stereocenter in (S)-2-butanol. are chemically different even to achiral reagents — different NMR shifts, different rates.

Pro-R and pro-S

The two identical groups at a prochirality center are labeled pro-R and pro-S. Imagine replacing one identical group with a higher-priority group (D for H, CH₂CH₃ for CH₃): if the resulting stereocenter is R, the original group was pro-R; if S, pro-S. Fischer-projection shortcut: with the two identical groups on horizontal bonds, replacing the left group gives R (pro-R) and the right gives S (pro-S).

Enantiotopic faces: Re and Si

A trigonal carbon bearing three different substituents has two faces, Re and Si. Assign priorities by the Cahn–Ingold–Prelog rules and view a face: if priorities 1 → 2 → 3 trace clockwise, the face is Re (rectus, right); counterclockwise is Si (sinister, left). The faces are enantiotopic: addition to one face gives one enantiomer, addition to the other gives the opposite — provided the addition creates a stereocenter (four different groups on the new tetrahedral carbon). For acetaldehyde, addition of HCN gives the chiral cyanohydrin CH₃CH(OH)CN, so Re- and Si-face attack give enantiomeric products; an achiral reagent makes a racemic mixture, while a chiral catalyst can deliver one enantiomer.

Prochirality in biological chemistry

Citric acid, the first intermediate of the citric acid cycle, is prochiral: its central carbon bears two identical CH₂COOH arms. Chemists long assumed the arms were equivalent, until Alexander Ogston (1948) argued that a chiral enzyme active site could distinguish them. Aconitase indeed reacts with only one arm, converting citrate to isocitrate with complete selectivity. The same principle explains why NADH delivers its hydride to only one face of pyruvate (giving enantiopure L-lactate) and why so many metabolic steps are stereospecific.

Common Confusions

Do Not Confuse	With	Difference
Prochiral	Chiral	Prochiral molecules are achiral; they merely can become chiral in one step
Homotopic groups	Enantiotopic groups	Homotopic replacement gives the same molecule; enantiotopic gives enantiomers
Enantiotopic groups	Diastereotopic groups	Enantiotopic need a chiral agent to be distinguished; diastereotopic differ even for achiral reagents
Pro-R group	R configuration	Pro-R is the label of an identical group; replacing it creates an R stereocenter
Re face	Si face	Re = clockwise priority sequence viewing the face; Si = counterclockwise (viewing the other side flips them)
Faces of acetaldehyde	Faces that always give chiral products	Faces are enantiotopic, but with H⁻ the product (ethanol) is achiral; enantiomeric products appear only when addition creates a stereocenter (e.g., HCN)
"Identical groups are always equivalent"	Equivalence in chiral environments	True for achiral environments; false in a chiral environment (enzyme), which can distinguish enantiotopic groups and faces

Eli explains

The same idea, in plain words

Explain it like I’m 10

A prochiral molecule is like identical twins with "LEFT" and "RIGHT" written on their shirts in invisible ink. To us they are the same, but a special magnifying glass (an enzyme) can tell them apart. Give one twin a hat — change one group — and suddenly the molecule has a left hand and a right hand. One small change makes it chiral.

Worked example

Example 1: Classifying the hydrogens of ethanol

Ethanol, CH₃CH₂OH, has a prochirality center at the CH₂ carbon (H, H, CH₃, OH).

Step 1 — CH₃ hydrogens: replace any one with D → CH₂DCH₂OH, the same achiral molecule each time → homotopic.

Step 2 — CH₂ hydrogens: replace one with D → (R)-CH₃CHDOH; the other → (S)-CH₃CHDOH → enantiotopic.

Step 3 — check for diastereotopic groups: ethanol has no other stereocenter, so none are diastereotopic. Method: (1) list sets of identical groups; (2) replace each member in turn; (3) compare products — same molecule, enantiomers, or diastereomers.

Example 2: Assigning pro-R and pro-S in ethanol

Draw ethanol in a Fischer projection with the two H atoms of the CH₂ group on horizontal bonds.

Left H is pro-R: replace it with D and assign the new stereocenter. Priorities: OH (1) > CH₃ (2) > D (3) > H (4). With H on a horizontal bond (toward the viewer), the observed 1 → 2 → 3 rotation is inverted, giving R.
Right H is pro-S: the same replacement on the right gives S.

Rule: with two identical H's on horizontal bonds, the left H is pro-R and the right H is pro-S.

Example 3: Re and Si faces of acetaldehyde

Acetaldehyde, CH₃CHO, has a trigonal carbonyl carbon bonded to O (priority 1), CH₃ (2), and H (3). Viewed from one face, 1 → 2 → 3 trace clockwise → Re face; from the other, counterclockwise → Si face. The faces are enantiotopic. Addition of HCN gives the cyanohydrin CH₃CH(OH)CN: attack on the Re face gives one enantiomer and attack on the Si face gives the other, so an achiral reagent yields a racemic mixture. A chiral catalyst can force attack on a single face and deliver one pure enantiomer — the logic of modern asymmetric synthesis.

Key takeaways

Prochiral = achiral now, chiral after one change.
Prochirality center: tetrahedral carbon with pattern a, a, b, c.
Replacement test: same molecule = homotopic; enantiomers = enantiotopic; diastereomers = diastereotopic.
Pro-R/pro-S: replace the group with a higher-priority group; R result → pro-R, S result → pro-S.
Fischer shortcut: identical groups on horizontal bonds — left group is pro-R, right is pro-S (for two H's).
Trigonal carbon with three different groups: Re face = clockwise priority sequence; Si = counterclockwise.
Enantiotopic faces give enantiomeric products only when addition creates a stereocenter (HCN on acetaldehyde, not H⁻).
Enzymes are chiral environments: they distinguish enantiotopic groups and faces (citric acid, pyruvate, NADH).
Diastereotopic groups are distinguishable even by achiral reagents (NMR, rates).

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

What substituent pattern identifies a tetrahedral prochirality center?
Show answer
A tetrahedral carbon bearing two identical groups and two different ones (pattern a, a, b, c).
The two H atoms on the CH₂ of ethanol are homotopic, enantiotopic, or diastereotopic? Explain the test.
Show answer
Enantiotopic — replacing one H with D gives (R)-CH₃CHDOH and replacing the other gives (S)-CH₃CHDOH, a pair of enantiomers. (The CH₃ hydrogens, by contrast, are homotopic.)
In a Fischer projection with two H's on horizontal bonds, which H is pro-R?
Show answer
The left H is pro-R (replacing it with a higher-priority group gives an R stereocenter).
How do you decide whether a face of a trigonal carbon is Re or Si?
Show answer
Assign Cahn–Ingold–Prelog priorities to the three substituents, view the face, and check the 1 → 2 → 3 sequence: clockwise = Re, counterclockwise = Si.
Why can aconitase distinguish the two identical arms of citric acid even though citric acid is achiral?
Show answer
The active site is a chiral environment; enantiotopic groups are distinguishable by chiral reagents even though achiral ones see them as identical (Ogston, 1948).
Give one reason prochirality matters for drug metabolism.
Show answer
Chiral metabolic enzymes can convert an achiral (prochiral) drug into chiral metabolites whose enantiomers differ in activity or toxicity.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Prochiral: An achiral molecule that can become chiral in one step
Prochirality center: Tetrahedral carbon with two identical groups (a, a, b, c)
Homotopic groups: Identical groups whose replacement gives the same molecule
Enantiotopic groups: Identical groups whose replacement gives enantiomers
Diastereotopic groups: Identical groups whose replacement gives diastereomers
Pro-R / Pro-S: Labels for the two identical groups at a prochirality center
Re face / Si face: The two faces of a trigonal carbon with three different groups
Enantiotopic faces: Faces whose reaction gives enantiomeric products

Sources & references

openstax.org — Organic Chemistry

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.