How to tell if protons are homotopic, enantiotopic, or diastereotopic

What we have said so far explains to you why homotopic and enantiotopic groups always appear identical in the NMR spectrum, but diastereotopic protons may not. Now we will give a quick guide to determining what sort of pair you are dealing with in a given molecule. The key is to draw your molecule twice. In each drawing (or model if you prefer) replace one of the Hs (we’ll assume we’re looking at protons, but the argument works for other groups too—Me groups, for example) with an imaginary group ‘G’. Write down the first structure you get, with stereochemistry shown. Next, write down the structure you get by replacing the other H with the group G. Now the more difficult bit: identify the stereochemical relationship between the two molecules you have drawn.

• If they are identical molecules, the protons are homotopic.

• If they are enantiomers, the protons are enantiotopic.

• If they are diastereoisomers, the protons are diastereotopic. This is really just a simpler way of doing what we did with black and green above, but it is easy to do for any molecule. Take the first of our examples, and replace each H in turn by G. These two molecules are identical because just turning one over gives the other: the protons are homotopic.

Now for the next example. The two molecules are not identical: to make one into the other you need to reflectin the plane of the paper, so they are enantiomers, and the Hs are enantiotopic. There is another term we must introduce you to in relation to this molecule, which will become useful in the next chapter, and that is ‘prochiral’. The molecule we started with here was not chiral—it had a plane of symmetry. But by changing just one of the Hs to a different group we have made it chiral. Molecules that are achiral but can become chiral through one simple change are called prochiral. Now we will choose one of the three pairs of Hs in the cyclohexanone example. The starting molecule is, of course, now chiral, and the two molecules we get when we replace each H by G are now diastereoisomers: one has G and OMe anti, the other syn, and the pairs of hydro gens are diastereotopic. The same is true for the other CH₂ groups. Furthermore, the methyl groups attached to the ring will be diastereotopic too, and we expect them to appear as two 3H singlets.

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مجلة تسليم تدعو الباحثين والأكاديميين للمشاركة في أعدادها القادمة

المجمع العلمي يواصل إقامة محاضراته الأسبوعية في مقام الإمام المهدي (عجل الله فرجه)

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المزيد

الآخبار الصحية

دراسة علمية تكشف مفاجأة حول الأسباب الحقيقية للسمنة

دراسة: الأطباء يتجاهلون "سببا شائعا" لارتفاع ضغط الدم

نظامك الغذائي غني بالبروتين؟.. احذر هذه الآثار الجانبية

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المزيد

الآخبار التكنلوجية

ISOFIX: معيار الأمان الذهبي لمقاعد الأطفال في السيارات

هل المحرك الأكبر دائمًا الأفضل للسيارة؟

اختبارات ناجحة لمحرك "VK-800" الروسي وتطويره لطائرة بايكال الخفيفة

رئيس إنفيديا: الذكاء الاصطناعي الصيني "محفز للتقدم العالمي"

المزيد

مواضيع ذات صلة

Diastereotopic protons in acyclic compounds

Spotting diastereotopic protons in the NMR spectrum

The size of the geminal coupling constant

Geminal (2J) coupling

Heteroatoms in rings have axial and equatorial lone pairs

Four-membered rings

Ring size and NMR

Baldwin’s rules and ring opening

Combatting ΔS‡—the Thorpe–Ingold effect

Thermodynamic control over ring size

Making heterocycles: ring-closing reactions

Related effects in other types of compounds