Orbital hybridization carbocations

Protonation of the alkyne is actually less favourable than protonation of an alkene, because the resulting vinyl cation is sp hybridized, having a bonds to just two substituents, a it bond, and a vacant p orbital. A vinyl cation is thus less stable than a comparable trigonal -hybridized carbocation, since ip-hybridization brings bonding electrons closer to carbon it thus becomes less tolerant of positive charge. Protonation, when it occurs, will be on... 

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. 

The orbitals of carbocations are generally sp" hybridized to that the three full orbitals are arranged in a trigonal planar geometry about the carbon nucleus. The remaining p orbital is empty and will readily accept a pair of electrons from another atom. Because of the symmetry of this geometric arrangement, nucleophilic attack is equally favorable above or below the plane fonned by the full orbitals. 

Resonance forms are classical structures used to describe a more complex system they do not actually exist. The species is more accurately described by a resonance hybrid which can be imagined as an average of the resonance forms. Resonance always stabilizes a system. Each atom in a resonance stabilized system has a p-orbital. Allylic carbocations are stabilized by delocalization of the positive charge. 

We discussed the planar shape of the methyl cation in Chapter 4 (p. 103), and the terf-butyl cation is similar in structure the electron-deficient central carbon atom has only six electrons, which it uses to form three a bonds, and therefore also carries an empty p orbital. Any carbocation will have a planar carbon atom with an empty p orbital. Think of it this way only filled orbitals contribute to the energy of a molecule, so if you have to have an unfilled orbital (which a carbocation always does) it is best to make that unfilled orbital as high in energy as possible to keep the filled orbitals low in energy, p orbitals are higher in energy than s orbitals (or hybrid sp, sp2, or sp3 orbitals for that matter) so the carbocation always keeps the p orbital empty. 

In a carbocation, the carbon bearing the positive charge is bonded to three other atoms and, as predicted by VSEPR, the three bonds about the cationic carbon are copla-nar and form bond angles of approximately 120°. According to the orbital hybridization model, the electron-deficient carbon of a carbocation uses sp hybrid orbitals to form a bonds to the three attached groups. The unhybridized 2p orbital lies perpendicular to the a bond framework and contains no electrons. Figure 6.3 shows a Lewis structure, a cartoon, and a calculated structure indicating the vacant 2p orbital for the fert-butyl cation. 

We will encounter several reactions in this chapter that proceed by a carbocation intermediate. We recall that the simplest carbocation, the methyl cation (CHj ), is a representative of the structure of carbocations. Its three bonding pairs of electrons are located in a trigonal planar arrangement in which the carbon atom is sp hybridized (Figure 6.2). The fourth orbital of carbon is an empty 2p orbital that is perpendicular to the plane of the three sp orbitals. Because carbocation is planar, attack of a nucleophile on the positively charged carbon to form a bond can occur with equal probability from the top or bottom face of the intermediate. Therefore, reactions in which a carbocation forms will not proceed by exclusive syn or anti addition. 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Carbocations contain a positively charged carbon with only three atoms or groups attached to it This carbon is sp hybridized and has a vacant 2p orbital... 

Figure 6.9 The structure of a carbocation. The trivalent carbon is sp -hybridized and has a vacant p orbital perpendicular to the plane of the carbon and three attached groups.

A great deal of evidence has shown that carbocations are planar. The divalent carbon is 5p2-hybridized, and the three substituents are oriented to the corners of an equilateral triangle, as indicated in Figure 6.9. Because there are only six valence electrons on carbon and all six are used in the three a bonds, the p orbital extending above and below the plane is unoccupied. 

The dichlorocarbene carbon atom is syj- -hybridized, with a vacant p orbital extending above and below the plane of the three atoms and with an unshared pair of elections occupying the third sp2 lobe. Note that this electronic description of dichlorocarbene is similar to that for a carbocation Section 6.9) with respect to both the sp2 hybridization of carbon and the vacant p orbital. Electrostatic potential maps further show this similarity (Figure 7.6). 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

Analyze the interaction between the vacant p orbital of a carbocation and an unshared electron pair on an adjacent nitrogen. Assume sp3 hybridization for nitrogen, and the conformation in which the unshared pair has maximum overlap with the carbon/) orbital. 

The capture of carbocations by alcohols involves a similar donation of a lone pair of electrons on oxygen to the vacant 2p atomic orbital of the sp2-hybridized, sextet carbocation. Note that charge must be conserved so the first formed product is a positively charged oxonium ion. 

Carbocations contain sp hybridized orbitals and thus have planar structures. S 1 mechanisms proceed via a carbocation intermediate, so a nucleophile attack is equally possible from either side of the plane. Therefore, a pure, optically active alkyl halide undergoing an S 1 substitution reaction will generate a racemic mixture as a product, as shown in Figure 3-6. 

Although the synthesis rules are routinely formulated in terms of hybrid orbitals, they are in fact purely empirical, and not entirely logical. On the one hand an electrophile, H+ is shown (2) to react with the pair of 7r-electrons of a C - C double bond to generate an intermediate carbocation. In the inverse process (3) a base (B ) is shown to attack through a C-H single bond without specifying the orbital that transfers the electron pair from C-H (double bond ) to C-C. This is an unnecessary complication as the empirical rules work well without the orbital baggage. 

Borane has the same structure as a carbocation. The boron is sjr hybridized, with trigonal planar geometry, and has an empty p orbital. Although neutral, it is electron deficient because there are only six electrons around the boron. It is a strong Lewis acid. An electron-deficient compound often employs unusual bonding to alleviate somewhat its instability. In the case of borane, two molecules combine to form one molecule of diborane ... 

A carbene is a reactive species having a carbon with only two bonds and an unshared pair of electrons. It is quite unstable and, like a carbocation, exists only as a transient intermediate in certain reactions. The simplest carbene, CH2, is called methylene. It is sp2 hybridized. The unshared pair of electrons occupies one of the. syr-hybridized AOs and the other two are used to form the bonds to the hydrogens. The remaining AO on the carbon is an unoccupied p orbital. 

---