Hybridization sp2 orbitals

As shown in Fig. 4.9 the hybrid sp2 orbitals all have basically the same shape. They are much more concentrated into regions that are in specific directions in space than are the simple atomic orbitals. For instance, simple atomic p orbitals have two regions in space, in opposite directions from the nucleus and located where the orbital would give a sizable electron density (Fig. 2.6). In ther/>2 orbitals, one of these regions has shrunk to a rather small size while the region in the oppo-... 

If one carbon 2s orbital combines with two carbon 2p orbitals, three hybrid sp2 orbitals are formed, and one p orbital remains unchanged. 

Similar behaviour is found with other atoms. Boron in the trivalent state has one s and two p electrons and hybridization leads to the formation of three equivalent hybrid sp2 orbitals lying in the same plane and with a valency angle of 120°. Experimental data6 for B(CH3)3 are in agreement with this prediction. Beryllium and mercury in the excited state necessary for bond formation, have one s and one p electron which form two hybrid sp bonds at an angle of 180° to each other. 

Hybridization to four 2sp3 orbitals occurs before bonding to four atoms. Three hybrid sp2 orbitals form if there is a double bond so the C atom is bonded to three atoms and one electron remains in the p orbital. Two hybrid sp orbitals occur if there is a triple bond or two double bonds. In this case, C is bonded to two atoms with two electrons remaining in p orbitals. 

The bonding pi orbital 71) follows regions separate from a line drawn between the two atoms in a bond. Two overlapping p orbitals will form n bonds to contain the additional shared electrons in molecules with double or triple bonds, n bonds prevent atoms from rotating about the central axis between them. The atomic orbitals that form the compound C2H4 are shown above to the left. Each H atom contains one electron in an s orbital and each C atom contains 4 valence electrons in three hybrid sp2 orbitals and one p orbital. The compound itself contains the molecular orbitals shown below to the left. There are five a bonds (white ovals) and one n bond (the shaded shapes). For additional examples of n bonds in carbon compounds, see Skill 6.1a. 

The O atom uses one of its sp or sp hybrids to form the CO a bond and antibond. When sp hybrids are used in conceptualizing the bonding, the other sp hybrid forms a lone pair orbital directed away from the CO bond axis one of the atomic p orbitals is involved in the CO n and 71 orbitals, while the other forms an in-plane non-bonding orbital. Alternatively, when sp hybrids are used, the two sp hybrids that do not interact with the C-atom sp2 orbital form the two non-bonding orbitals. Hence, the final picture of bonding, non-bonding, and antibonding orbitals does not depend on which hybrids one uses as intermediates. 

When we discussed sp3 hybrid orbitals in Section 1.6, we said that the four valence-shell atomic orbitals of carbon combine to form four equivalent sp3 hybrids. Imagine instead that the 2s orbital combines with only two of the three available 2p orbitals. Three sp2 hybrid orbitals result, and one 2p orbital remains unchanged- The three sp2 orbitals lie in a plane at angles of 120° to one another, with the remaining p orbital perpendicular to the sp2 plane, as shown in Figure 1.13. 

Figure 1.14 The structure of ethylene. Orbital overlap of two sp hybridized carbons forms a carbon-carbon double bond. One part of the double bond results from a (head-on) overlap of sp2 orbitals (green), and the other part results from (sideways) overlap of unhybridized p orbitals (red/blue). The ir bond has regions of electron density on either side of a line drawn between nuclei.

We saw in Chapter 1 that the carbon-carbon double bond can be described in two ways. In valence bond language (Section 1.8), the carbons are sp2-hybridized and have three equivalent hybrid orbitals that lie in a plane at angles of 120° to one another. The carbons form a cr bond by head-on overlap of sp2 orbitals and a tt bond by sideways overlap of unhybridized p orbitals oriented... 

What accounts for the stability of conjugated dienes According to valence bond theory (Sections 1.5 and 1.8), the stability is due to orbital hybridization. Typical C—C bonds like those in alkanes result from a overlap of 5p3 orbitals on both carbons. In a conjugated diene, however, the central C—C bond results from conjugated diene results in part from the greater amount of s character in the orbitals forming the C-C bond. 

Figure 16.19 An orbital picture and electrostatic potential map of benzyne. The benzyne carbons are sp2-hybridized, and the "third" bond results from weak overlap of two adjacent sp2 orbitals.

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp2 hybrid atomic orbitals disposed at 120° to each other in the same plane (plane trigonal hybridisation) are then employed. Finally, when carbon combines with two other atoms, e.g. in ethyne (acetylene) (p. 9) two sp1 hybrid atomic orbitals disposed at 180° to each other (idigonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

An oxygen atom can also form a double bond to carbon thus in propanone (acetone), Me2C=Q , the oxygen atom could use three sp2 hybrid orbitals one to form a a bond by overlap with an sp2 orbital of the carbon atom, and the other two to accommodate the two lone pairs of electrons. This leaves an unhybridised p orbital on both oxygen and carbon, and these can overlap with each other laterally (cf. C=C, p. 9) to form a n bond ... 

Both carbon atoms in ethene undergo sp2 hybridization. The C-H bonds involve overlap of sp1 carbon orbitals with Is orbitals of the H atoms. The carbon-carbon double bond involves the overlap of sp2 orbitals from each carbon to give the o bond and the side-on overlap of a p orbital from each carbon atom to give the n bond. 

Carbon atoms (1) and (4) use sp3 hybrid orbitals to form four sigma bonds, three by overlap with the hydrogen Is orbitals and one by overlap with an sp2 orbital from the central carbon (2). The two carbon atoms involved in the double bond undergo sp2 hybridization. They form C-H bonds by overlapping with Is orbitals of the H atoms. The C=C double bond is formed similarly to that described in (a). 

Carbon atoms (1) and (2) undergo sp2 hybridization, while carbon atoms (3), (4) and (5) undergo sp3 hybridization. The orbital overlaps are similar to those in 2-butene except carbon atoms (1) and (2) are involved in the double bond and carbon atom (2) is bonded to carbon atom (3) by sp2-sp3 overlap. The overlap of the hybrid orbitals with the Is orbitals of H atoms gives the C-H bonds. 

The central carbon is surrounded by three electron groups and is sp2 hybridized. The orbital diagrams for the un-hybridized atoms are ... 

Separation of covalently bonded atoms into QM and MM regions introduces an unsatisfied valence in the QM region this can be satisfied by several different methods. In the frozen-orbital approach a strictly localized hybrid sp2 bond orbital containing a single electron is used at the QM/MM junction [29]. Fro-... 

When a carbon atom undergoes sp2 hybridization, sp2 hybrid orbitals form n bonds, but the unhybridized p orbital forms a pi bond. 

The trigonal bond orbitals in the ten valence electron system as well as the two sets of trigonal lone pair orbitals in the 14 valence electron system are superpositions of it orbitals and o orbitals. The formation of such trigonally symmetric molecular orbitals from a-type and w-type molecular orbitals is entirely analogous in character to the formation of the three (sp2) hybrid atomic orbitals from one (s) and two ip) atomic orbitals which was discussed in the preceding section. This can be visualized by looking at the diatomic molecule... 

The n—7r stabilizing interaction which obtains in each conformation is listed above. Since the oxygen 2p lone pair AO is a better intrinsic donor orbital than the oxygen hybrid sp2 lone pair AO, we conclude that n—tt interactions favor the conformation in which all atoms are contained in the same plane. 

The O atom in furan has sp2 hybridization. One lone pair resides in the p orbital and is used in resonance the other resides in an sp2 orbital and is not used in resonance. 

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