Several chapters of this book have considered how to find out the structure of molecules. We have seen X-ray crystallography pictures, which reveal exactly where the atoms are in crystals; we have looked at IR spectroscopy, which gives us information about the bonds in the molecule, and at NMR spectroscopy, which gives us information about the atoms themselves and how they are joined up. Up to now, we have mainly been interested in determining which atoms are bonded to which other atoms and also the shapes of small localized groups of atoms. For example, a methyl group has three hydrogen atoms bonded to one carbon atom and the atoms around this carbon are located at the corners of a tetrahedron; a ketone consists of a carbon atom bonded to two other carbon atoms and doubly bonded to an oxygen atom, with all these atoms in the same plane. But, on a slightly larger scale, shape is not usually so well defi ned. Rotation is possible about single bonds and this rotation means that, while the localized arrangement of atoms stays the same (every saturated carbon atom is still always tetrahedral), the molecule as a whole can adopt a number of different shapes. Shown on the next page are several snapshot views of one molecule—it happens to be a pheromone used by pea moths to attract a mate. Although the structures look dissimilar, they differ from one another only by rotation about one or more single bonds. Whilst the overall shapes differ, the localized structure is still the same: tetrahedral sp³ carbons; trigonal planar sp² carbons. Notice another point too, which we will pick up on later: the arrangement about the double bond always remains the same because double bonds can’t rotate.

At room temperature in solution, all the single bonds in the molecule are constantly rotating—the chances that two molecules will have exactly the same shape at any one time are quite small. Yet, even though no two molecules have exactly the same shape at any one time, they are still all the same chemical compound—they have all the same atoms attached in the same way. We call the different shapes of molecules of the same compound different conformations.

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المزيد

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لكبح الجوع وخسارة الوزن.. "سر بسيط" في نظام جوكوفيتش ؟

هل يساعد الرجيم في علاج الأمراض العقلية؟

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المزيد

الآخبار التكنلوجية

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ذوبان الأنهار الجليدية يوقظ "عمالقة نائمة"!

9 يوليو يشهد أقصر يوم في التاريخ المسجل ؟!!

راتبه عشرات ملايين الدولارات.. زوكربرغ يخطف "عقل أبل"

المزيد

مواضيع ذات صلة

Synthons are idealized reagents

The E-selective Wittig reaction

The Z-selective Wittig reaction

The Peterson reaction is stereospecific

The one-step Julia olefination

The Julia olefination is stereoselective

Sulfoxide elimination—oxidation to enones

Addition of nucleophiles to alkynes

E-selective reduction of alkynes using sodium in liquid ammonia

Amine synthesis using functional group interconversions

Functional group interconversion

Multiple step syntheses: avoid chemoselectivity problems