Lone pair electrons, carbon atom reactivity

Carbon dioxide is a linear molecule with equivalent C O distances of 1.16 A (Vol pin and Kolomnikov, 1973). The bond strength in C02 is measured to be D= 127.1 kcal mol 1, relatively weak compared with the CO bond in carbon monoxide (D = 258.2 kcal mol ) (Bard et al., 1985 Latimer, 1952 Weast, 1978). Resonance structures of the C02 molecule as illustrated in Figure 3.2 show that its chemical reactivity is associated either with the presence of carbon oxygen double bonds and lone-paired electrons on the oxygen atoms or with the electrophilic carbon atom. The quantitative mole-... 

The anomeric carbon atom, C-l, in pyranose and furanose sugars is unique in that it is the only carbon atom in these molecules which is bonded to two atoms which are more electronegative. These more-electronegative atoms necessarily have lone-pair electrons. It is the electronic structure which arises from the electronegativity differences and the presence of these lone-pair electrons that gives rise to the special reactivity and structural properties of the anomeric carbon center. 

The carbonyl compounds discussed in Chapter 17 have a lone pair on an atom attached to the carbonyl group that can be shared with the carbonyl carbon by resonance, which makes the carbonyl carbon less electron deficient. We have seen that the reactivity of these carbonyl compounds is related to the basicity of Y (Section 17.5). The weaker the basicity of Y , the more reactive is the carbonyl group because weak bases are less able to donate electrons by resonance to the carbonyl carbon and are better able to withdraw electrons inductively from the carbonyl carbon. 

Carbon monoxide is a colorless, odorless, flammable, almost insoluble, very toxic gas that condenses to a colorless liquid at — 90°C. It is not very reactive, largely because its bond enthalpy (1074 kj-mol-1) is higher than that of any other molecule. However, it is a Lewis base, and the lone pair on the carbon atom forms covalent bonds with J-block atoms and ions. Carbon monoxide is also a Lewis acid, because its empty antibonding Tr-orbitals can accept electron density from a... 

The reactivity of carbenes is strongly influenced by the electronic properties of their substituents. If an atom with a lone pair (e.g. O, N, or S) is directly bound to the carbene carbon atom, the electronic deficit at the carbene will be compensated to some extent by electron delocalization, resulting in stabilization of the reactive species. If both substituents are capable of donating electrons into the empty p orbital of the carbene, isolable carbenes, as e.g. diaminocarbenes (Section 2.1.6), can result. The second way in which carbenes can be stabilized consists in complexation. The shape of the molecular orbitals of carbenes enable them to act towards transition metals as a-donors and 71-acceptors. The chemical properties of the resulting complexes will also depend on the electronic properties of the metallic fragment to which the carbene is bound. Particularly relevant for the reactivity of carbene complexes are the ability of the metal to accept a-electrons from the carbene, and its capacity for back-donation into the empty p orbital of the carbene. 

Alkyl halides (RX) are good substrates for substitution reactions. The nucleophile (Nu ) displaces the leaving group (X ) from the carbon atom by using its electron parr or lone pair to form a new a bond to the carbon atom. Two different mechanisms for nucleophilic substitution are SnI and 8 2 mechanisms. In fact, the preference between S l and 8 2 mechanisms depends on the structure of the alkyl halide, the reactivity and structure of the nucleophile, the concentration of the nucleophile and the solvent in which reaction is carried out. 

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

The lone pair of electrons of 2.44 is delocalised on to the carbonyl group as shown, increasing the electron density at the aldehydic carbon atom. This renders it less reactive to nucleophilic attack. 

A lone pair on the hydroxide oxygen forms a new bond to an a proton. Simultaneously, the C-H bond breaks. Both electrons of that bond end up on the carbon atom and give it a lone pair of electrons and a negative charge (a carbanion). However, carbanions are generally very reactive, unstable species that are not easily formed. Therefore, some form of stabilisation is involved here. 

We take this symbolism to mean that the carbon atom is sharing two electrons with each hydrogen atom and has one unshared electron. This intermediate has no charge, but is very reactive. It can, for example, couple or combine with another radical, pairing both lone electrons and forming a new single bond making the molecule ethane ... 

The phenyl radical is considered to be one of the most reactive hydrocarbon radicals though the reasons for this high reactivity have not been clear. Three different electronic configurations have been proposed for it. In the first, the electron remains in the s -orbital of the carbon atom at which bond scission has occurred (i.e. it is a or-type radical). In the second, an electron from the ir-system can pair with the unpaired electron to give a lone pair in the p -orbital and leave 5 electrons in the 6-centre 7r-system. In the third, the carbon atom at which scission has occurred becomes divalent and does not participate in the 7r-system this leaves a radical with 5 electrons in a 5-centre w-system. 

The electron-withdrawing effect of the halogen, coupled with that of the carbonyl oxygen, leads to a very electron-deficient carbon, and this is not effectively counteracted by the lone pairs on halogens such as chlorine. Consequently, the carbonyl carbon atom is very sensitive to nucleophilic addition to form a tetrahedral intermediate. The collapse of the tetrahedral intermediate with the expulsion of the halide ion, which is a good leaving group, enhances the reactivity of the acyl halides (Scheme 3.64a). The direct fission of the acyl halide C-X bond leads to the formation of an electrophilic acylium ion (Scheme 3.64b). 

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