Hybrid orbitals carbon radical

The vinyl H2C=CH radical can be produced by cleavage of a C-H bond in ethene, and has been studied in the gas phase. The unpaired electron clearly occupies a carbon sp hybrid orbital, to lapse into the language of descriptive organic chemistry, but there are regions of space where the, 6-spin electrons have... 

The CHO radical is a o-radical and the main qualitative feature of the comparison between Tables 2 and 3 is the tendency for the a GHOs to be less contracted in CHO than is found in CH20. In particular, the sp2 hybrid on carbon which nominally contains the unpaired electron is considerably expanded (exponent 1.5923 compared to 1.8660 in CH20). The hydrogen orbital and the aoc orbital are also noticeably expanded while the lone pairs on oxygen are largely unaffected. 

For the vinyl radical, the hyperfine coupling for the a-carbon is 107.6 G, which would suggest 10% s character in the hybrid orbital. The vinyl radical clearly has the odd electron in a c-type orbital because the (3 protons of the vinyl radical have distinct hyperfine couplings with a = 37 G for the c/i-H, and a = 65 G for the trans-M. The cyclopropyl radical also is a c radical with an a- C a value of 98 G, whereas the cyclohexyl radical, which has a nearly planar radical center, has an a- C a value of 41 G. 

Carbon radicals are classified as primary (1°), secondary (2°), or tertiary (3°) by the number of R groups bonded to the carbon with the unpaired electron. A carbon radical is sp hybridized and trigonal planar, like sp hybridized carbocations. The unhybridized p orbital contains the unpaired electron and extends above and below the trigonal planar carbon. 

We saw earlier (Sec. 2.21) that the methyl radical may not be quite flat that hybridization of carbon may be intermediate between sp- and sp For the allyl radical, on the other hand— and for many other free radicals—flatness is clearly required to permit the overlap of p orbitals that leads to stabilization of the radical. 

The ff-t5Tpe ethoxycarbonyl radical is on the contrary less nucleophilic than the acetyl radical (Table 29) in this Ccise the unpaired electron occupies a hybrid orbital and the incipient positive charge in the transition state cannot be stabilized by the lone-pair electron of the alkoxy group, as with the alkoxyalkyl radical, so that only the inductive effect is working and a clean reduction of nucleophilicity is observed. The remarkable fact is therefore that the same substituent, an a-alkoxy group, produces opposite polar effects depending on the electronic configuration of the carbon-centered radical. 

Carbon radicals have an unpaired electron in a nonbonding orbital. The possible hybridization schemes are shown below. 

The carbon atom in the methyl radical is also sp hybridized. The methyl radical differs by one unpaired electron from the methyl cation. That electron is in the p orbital. Notice the similarity in the ball-and-stick models of the methyl cation and the methyl radical. The potential maps, however, are quite different because of the additional electron in the methyl radical. 

All single bonds are a bonds. All double bonds are composed of one a bond and one tt bond. All triple bonds are composed of one a bond and two rr bonds. The easiest way to determine the hybridization of a carbon, oxygen, or nitrogen atom is to look at the number of tt bonds it forms If it forms no tt bonds, it is sp hybridized if it forms one TT bond, it is sp hybridized if it forms two tt bonds, it is sp hybridized. The exceptions are carbocations and carbon radicals, which are sp hybridized— not because they form a TT bond, but because they have an empty or half-filled p orbital (Section 1.10). 

A carbon radical is a trivalent species containing a single electron in a p orbital. A carbanion is viewed as a tetrahedral species containing a pair of electrons in an orbital (1). We have viewed a carbocation (carbenium ion) as an sp hybridized, trigonal planar carbon with an empty p orbital (2). A radical, which contains one electron in an orbital, can be tetrahedral, planar, or in between, with properties of both a carbanion and a carbocation. As shown in 3, a reasonable in between structure is a flattened tetrahedron (the actual structure of radicals will be discussed below). In terms of its reactivity, radical 3 could be considered electron rich or electron poor. In most of its reactions, the electron-deficient characterization is the most useful for predicting products. 

A free radical is a short-lived intermediate. It is a species possessing an unpaired electron due to deficiency of one electron and usually results from homolytic cleavage of a covalent bond or addition of radical to a multiple bond. A typical carbon radical is sp hybridized with the unpaired electron in the perpendicular unhybridized p-orbital. 

Experimental evidence indicates that the geometric structure of most alkyl radicals is trigonal planar at the carbon having the unpaired electron. This structure can be accommodated by an r/> -hybridized central carbon. In an alkyl radical, the p orbital contains the unpaired electron (Fig. 10.2). 

Because of the radical-stabilizing influence of this electron delocalization, it is reasonable to expect that the BDE of an allylic C—H is significantly weaker than that of a primary C—H. In fact, based on bond dissociation enthalpies, we conclude that an allyl radical is even more stable than a 3° alkyl radical. Note that because of the larger amount of s character in its carbon sp hybrid orbital, a vinylic C—H bond is stronger (has a larger bond dissociation enthalpy) than any sp C—bond and is never abstracted in homolytic reactions. 

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