Carbenes can be divided into two types

We made two important observations regarding the structure of car benes that we will now return to and seek an explanation for. Firstly, we said that the X-ray crystal structure of the stable, crystalline carbene on the left shows that the bond angle at the carbene C is 102° and, secondly, we said that many carbenes can be observed by ESR—in other words, they have unpaired electrons. Spectroscopic investigations of a number of carbenes of differing structures have shown that they fall broadly into two groups: (1) those (which you will learn to call ‘triplets’) that ESR spectroscopy demonstrates have unpaired electrons and whose bond angles are 130–150° and (2) those (like the stable crystalline carbene above, and which you will learn to call ‘singlets’) that have bond angles of 100–110° but cannot be observed by ESR. Many carbenes, like CH2 itself, can be found in either group, although one may be more common.

All these observations can be accounted for by considering the electronic structure of a carbene. Carbenes have two-coordinate carbon atoms: you might therefore expect them to have a linear (diagonal) structure—like that of an alkyne—with an sp hybridized carbon atom.

Such a linear carbene would have six electrons to distribute amongst two σ orbitals and two (higher-energy) p orbitals. The two electrons in the degenerate p orbitals would remain unpaired because of electron repulsion in the same way as in molecular oxygen •O–O•.

Yet few carbenes are linear: most are bent, with bond angles between 100° and 150°, suggest ing a trigonal (sp2) hybridization state. An sp2 hybridized carbene would have three (lower energy) sp2 orbitals and one (high-energy) p orbital in which to distribute its six electrons. There are two ways of doing this. Either all of the electrons can be paired, with each pair occupying one of the sp2 orbitals, or two of the electrons can remain unpaired, with one electron in each of the p orbitals and one of the sp2 orbitals.

These two possibilities explain our two observed classes of carbene, and the two possible arrangements of electrons (spin states) are termed triplet and singlet. The orbitals are the same in both cases but in triplet carbenes we have one electron in each of two molecular orbitals and in singlet carbenes both electrons go into the sp2 orbital.

● Singlet and triplet carbenes Triplet carbenes have two unpaired electrons, one in each of an sp and a p orbital, while singlet carbenes have a pair of electrons in a non-bonding sp2 orbital and have an empty p orbital.

The existence of the two spin states explains the different behaviour of triplet and singlet carbenes towards ESR spectroscopy; the orbital occupancy also explains the smaller bond angle in singlet carbenes, which have an electron-repelling lone pair in an sp2 orbital.

we saw that the substituents on the carbene affect which of the two classes (which we now call singlet and triplet) it falls into. Why? All carbenes have the potential to exist in either the singlet or the triplet state, so what we mean when we say that a carbene such as: CH2 is a ‘triplet carbene’ is that the triplet state for this carbene is lower in energy than the singlet state. The opposite is true for :CCl2. Most type of carbenes are more stable as triplets because the energy to be gained by bringing the electron in the p orbital down into the sp2 orbital is insufficient to overcome the repulsion that exists between two electrons in a single orbital. For most triplet carbenes the singlet spin state that would arise by pairing up the two electrons lies only about 40 kJ mol−1 above the ground (triplet) state: in other words, 40 kJ mol−1 is required to pair up the two electrons. Carbenes that have singlet ground states (such as :CCl2) all have electron-rich substituents carrying lone pairs adjacent to the carbene centre. These lone pairs can interact with the p orbital of the carbene to produce a new, lower-energy orbital which the two electrons occupy. This stabilization of the lone pair provides the incentive that the electron in the p orbital needs to pair up in the sp2 orbital.

This interaction corresponds to the point we made above about adjacent lone pairs stabilizing carbenes via the delocalization shown in the margin, As these arrows suggest, carbenes that have heavily electron-donating substituents are less electrophilic than other carbenes: indeed, diamino carbenes can be quite nucleophilic. The division of carbenes into two types explains their structure. It also helps to explain some of their reactions, especially those that have a stereochemical implication. We will spend the rest of this chapter discussing how carbenes react

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تحمي من أمراض عديدة.. 6 فوائد لتناول البطاطا الحلوة

المزيد

الآخبار التكنلوجية

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

المعلمون في خطر.. خاصية جديدة في "تشات جي بي تي" تشرح الدروس

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مدينة غارقة.. هل يخفي قاع البحر الكاريبي سرّ حضارة مفقودة؟

المزيد

مواضيع ذات صلة

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Insertion into C–H bondsInsertion into C–H bonds

Making cyclopropanes

Carbenes react with alkenes to give cyclopropanes

The structure of carbenes depends on how they are made

اكتشاف العناصر

العناصر

Diazomethane makes methyl esters from carboxylic acids

Intramolecular radical reactions are more efficient than intermolecular ones

Alkyl radicals from boranes and oxygen