chemistry

Prochirality

Prochirality is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Prochirality rather than just read about it. In short: In stereochemistry, prochiral molecules are those that can be converted from achiral to chiral in a single step, such as changing one atom. An achiral species which can be converted to a chiral in two steps is called proprochiral.

Key takeaways

Prochirality belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Prochirality to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Prochirality from memory before moving on to harder problems.

Reference excerpt

In stereochemistry, prochiral molecules are those that can be converted from achiral to chiral in a single step, such as changing one atom. An achiral species which can be converted to a chiral in two steps is called proprochiral. A molecule having only one plane of symmetry, or an inversion point and no plane of symmetry, is prochiral if it is possible to change one of the two sides or to destroy the symmetry in another way. But a molecule with more symmetry, such as ethane, may require two substitutions to become chiral, and is thus proprochiral. Methane requires three substitutions to become chiral. If two identical substituents are attached to an sp3-hybridized atom, the descriptors pro-R and pro-S are used to distinguish between the two. Promoting the pro-R substituent to higher priority than the other identical substituent results in an R chirality center at the original sp3-hybridized atom, and analogously for the pro-S substituent. A trigonal planar sp2-hybridized atom can be converted to a chiral center when a substituent is added to the re or si (from Latin rectus 'right' and sinister 'left') face of the molecule. A face is labeled re if, when looking at that face, the substituents at the trigonal atom are arranged in increasing Cahn-Ingold-Prelog priority order (1 to 2 to 3) in a clockwise order, and si if the priorities increase in anti-clockwise order; note that the designation of the resulting chiral center as S or R depends on the priority of the incoming group. The concept of prochirality is necessary for understanding some aspects of enzyme stereospecificity. Alexander Ogston pointed out that when a symmetrical molecule is placed in an asymmetric environment, such as the surface of an enzyme, supposedly identically placed groups become distinguishable. In this way he showed that earlier exclusion of non-chiral citrate as a possible intermediate in the tricarboxylate cycle was mistaken. Another biochemical example of prochirality is glycerol. It is achiral, but when it is phosphorylated (at carbon number 3 in stereospecific numbering) the molecule becomes the chiral glycerol 3-phosphate, also called L-α-glycerophosphoric acid. A triacylglycerol having the same fatty acid at carbon 1 and carbon 3 is achiral, but when one of these two is released by hydrolysis, the resulting diacylglyerol is chiral.

References

Illustrations

Worked examples

Example 1 — a first encounter with Prochirality

Start with the simplest possible case. Write down what Prochirality claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Prochirality before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Prochirality ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Prochirality

In research: Prochirality appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Prochirality in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Prochirality is common in secondary-school and first-year university syllabi. It links to neighbouring topics Stereochemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Prochirality outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Prochirality in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Prochirality means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Prochirality out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Prochirality in simple terms?

Why does Prochirality matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Prochirality?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Prochirality.