Atomic orbitals, sp hybrid

The combination of one 2 5 atomic orbital and two 2p atomic orbitals forms three equivalent sp hybrid orbitals (Figure 1.18). Because they are derived from three atomic orbitals, sp hybrid orbitals always occur in sets of three. The third 2p atomic orbital (remember 2p 2py, and 2pP) is not involved in hybridization and consists of two lobes lying perpendicular to the plane of the hybrid orbitals [Figure 1.18(c)]. 

The combination of one 2s atomic orbital and one 2p atomic orbital forms two equivalent sp hybrid orbitals. Because they are derived from two atomic orbitals, sp hybrid orbitals always occur in sets of two (Figure 1.21). 

The mathematical combination of one 2s atomic orbital wave function and two 2p atomic orbital wave functions forms three equivalent sp hybrid orbital wave functions. Because they are derived from three atomic orbitals, sp hybrid orbitals always occur in sets of three. As with sp orbitals, each sp hybrid orbital (three-dimensional plot of (f/) consists of two lobes, one larger than the other. The axes of the three sp hybrid orbitals lie in a plane and are directed toward the corners of an equilateral triangle the angle between sp hybrid orbitals is 120°. The third 2p atomic orbital (remember 2p, 2p 2p) is not involved in hybridization (its wave function is not mathematically combined with the other three) and remains as two lobes lying perpendicular to the plane of the sp hybrid orbitals. Figure 1.14 shows three equivalent sp orbitals along with the remaining unhybridized 2p atomic orbital. Each sp orbital has 33% s-character and 67% p-character (one 2s orbital, two 2p orbitals). 

For example, in formaldehyde, H2CO, one forms sp hybrids on the C atom on the O atom, either sp hybrids (with one p orbital "reserved" for use in forming the n and 7i orbitals and another p orbital to be used as a non-bonding orbital lying in the plane of the molecule) or sp hybrids (with the remaining p orbital reserved for the n and 7i orbitals) can be used. The H atoms use their 1 s orbitals since hybridization is not feasible for them. The C atom clearly uses its sp2 hybrids to form two CH and one CO a bondingantibonding orbital pairs. 

As another example, the 2s and 2p orbitals on the two N atoms of N2 can be formed into pairs of sp hybrids on each N atom plus a pair of p atomic orbitals on each N atom. The sp hybrids directed... 

CaveU and Chapman made the interesting observation that a difference exists between the orbital involved in the quatemization of aromatic nitrogen heterocycles and aromatic amines, which appears not to have been considered by later workers. The lone pair which exists in an sp orbital of the aniline nitrogen must conjugate, as shown by so many properties, with the aromatic ring and on protonation or quatemization sp hybridization occurs with a presumed loss of mesomerism, whereas in pyridine the nitrogen atom remains sp hybridized in the base whether it is protonated or quaternized. Similarly, in a saturated compound, the nitrogen atom is sp hybridized in the base and salt forms. 

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.

Let s look at an example. In ammonia (NH3), the nitrogen atom is sp hybridized, so all four orbitals arrange in a tetrahedral structure, just as we would expect. But only three of the orbitals in this arrangement are responsible for bonds. So, if we look just at the atoms that are connected, we do not see a tetrahedron. Rather, we see a trigonal pyramidal arrangement ... 

The left-most C atom (in the structure drawn below) is sp3 hybridized, and the C-H bonds to that C atom are between the sp3 orbitals on C and the Is orbital on H. The other two C atoms are sp hybridized. The right-hand C-H bond is between the sp orbital on C and the Is orbital on H. The c a C triple bond is composed of one sigma bond formed by overlap of sp orbitals, one from each C atom, and two pi bonds, each formed by the overlap of two 2p orbitals, one from each C atom (that is a 2py—2py overlap and a 2pz—2pz overlap). 

Hybridisation is the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals. In the case of a carbon atom, the one 2s orbital can mix with the three 2p orbitals to form four hybrid orbitals known as sp hybrid orbitals. 

When we consider double bonds to oxygen, as in carbonyl groups (C=0) or to nitrogen, as in imine functions (C=N), we find that experimental data are best accommodated by the premise that these atoms are sp hybridized (Figure 2.22). This effectively follows the pattern for carbon-carbon double bonds (see Section 2.6.2). The double bond is again a combination of a ct bond plus a jt bond resulting from overlap of p atomic orbitals. The carbonyl... 

D) BF3 is a trigonal-planar molecule because electrons can be found in only three places in the valence shell of the boron atom. As a result, the boron atom is sp hybridized, which leaves an empty 2p orbital on the boron atom. BF3 can therefore act as an electron-pair acceptor, or Fewis acid. It can use the empty 2p orbital to pick up a pair of nonbonding electrons from a Fewis base to form a covalent bond. BF3 therefore reacts with Lewis bases such as NH3 to form acid-base complexes in which all of the atoms have a filled shell of valence electrons. 

In five-membered heterocycles, formally derived from benzene by the replacement of a CH=CH unit by a heteroatom, aromaticity is achieved by sharing four p-electrons, one from each ring carbon, with two electrons from the heteroatom. Thus in pyrrole, where the heteroatom is N, all the ring atoms are sp hybridized, and one sp orbital on each is bonded to hydrogen. To complete the six 7i-electron system the non-hybridized p-orbital of N contributes two electrons (Box 1.9). It follows that the nitrogen atom of pyrrole no longer possesses a lone pair of electrons, and the compound cannot function as a base without losing its aromatic character. 

In cases where there are no electronically driven distortions, the orbital description provides no better account of the chemistry than the bond valence model. Rather it tends to make an essentially simple situation more complex. For example, consider the phosphate and nitrate anions, and NOJ. In orbital models the P atom is described as sp hybridized and the N atom as sp hybridized, but these descriptions are just representations of the spherical and cylindrical harmonics appropriate to the observed geometries. They provide no explanation for why P is four but not three coordinate, or why N is three but not four coordinate. The bond valence account given in Chapter 6 is simpler, more physical, and more predictive. The orbital description is merely a rather complicated way of saying that the ions obey the principle of maximum symmetry but implying that the constraints are related in some unspecified way to the properties of one-electron orbitals rather than to the ionic sizes. 

See Fig. 2-12. The C and N each have one cr bond and one unshared pair of electrons, and therefore each needs two sp hybrid HO s. On each atom one sp hybrid orbital forms a cr bond while the other has the unshared pair. Each atom has a p, AO and a p. AO. The two p, orbitals overlap to form a ir, bond in the xy -plane the two p, orbitals overlap to form a n, bond in the Arz-plane. Thus, two ir bonds at right angles to each other and a a bond exist between the C and N atoms. 

The self-association and/or complex formation of organozinc componnds involves considerable rehybridization of the zinc valence orbitals. When only one coordinate bond is formed, the zinc atom becomes sp -hybridized and the resnlting complex is planar or nearly so with bond angles around the zinc of abont 120°. The zinc centre then still has one unoccupied valence orbital and remains coordinatively nnsatnrated. Three-coordinate zinc, however, is relatively rare and only occurs when steric crowding around the zinc prevents the approach of a fourth ligand. 

FIGURE 3.27 The pattern of bonding in ethyne (acetylene). The carbon atoms are sp hybridized, and the two remaining p-orbitals on each C atom form two Tt-bonds. The resulting pattern is very similar to that for nitrogen (Fig. 3.15), but two C—H groups replace the N atoms. 

Atoms with two charge clouds undergo hybridization by combination of one atomic s orbital with one p orbital, resulting in two sp hybrid orbitals that are oriented 180° from each other. Since only one p orbital is involved when an atom undergoes sp hybridization, the other two p orbitals are unchanged and are oriented at 90° angles to the sp hybrids, as shown in Figure 7.10. 

Both carbon atoms have octets, but each carbon is bonded to just two other atoms, requiring two sigma bonds. There are no lone pairs. Each carbon atom is sp hybridized and linear (180° bond angles). The sp hybrid orbitals are generated from the s orbital and the px orbital (the p orbital directed along the line joining the nuclei). The py orbitals and the pz orbitals are unhybridized. 

In allene, the central carbon atom is sp hybridized and linear (Section 2-4), and the two outer carbon atoms are sp2 hybridized and trigonal. We might imagine that the whole molecule lies in a plane, but this is not correct. The central sp hybrid carbon atom must use different p orbitals to form the pi bonds with the two outer carbon atoms. The two unhybridized p orbitals on the sp hybrid carbon atom are perpendicular, so the two pi bonds must also be perpendicular. Figure 5-19 shows the bonding and three-dimensional structure of allene. Allene itself is achiral. If you make a model of its mirror image, you will find it identical with the original molecule. If we add some substituents to allene, however, the molecule may be chiral. 

Ethyne (acetylene) has a C-C triple bond. Each carbon bonds to only two other atoms to form a linear CH skeleton. Only the carbon 2s and 2px have the right symmetry to bind to only two atoms at once so we can hybridize these to form two sp hybrids on each carbon atom leaving the 2py and 2pz to form Jt MOs with the 2p orbitals on the other carbon atom. These sp hybrids have 50% each s and p character and form a linear carbon skeleton. 

Ketene looks pretty unlikely It is CH2=C=0 with two tt bonds (C=C and C=0) to the same carbon atom. The orbitals for these jt bonds must be orthogonal because the central carbon atom is sp hybridized with two linear o bonds and two p orbitals at right angles both to the o bonds and to each other. Can such a molecule exist When acetone vapour is heated to very high temperatures (700-750 °C) methane is given off and ketene is supposed to be the other product. What is isolated is a ketene dimer (C4H4O2) and even the structure of this is in doubt as two reasonable structures can be written. 

When an atom is sp hybridized, one s orbital combines with one p orbital. This allows an atom like carbon to make two double bonds or one single and... 

In the benzene molecule, each of the six carbon atoms undergoes sp hybridization. Benzene has a planar shape and the carbon - carbon bond lengths are 139 pm. The six carbon ring is formed by the overlap of the sp orbitals of the carbon atoms. The resulting p orbitals then overlap to form t bonds. 

The shape of simple carbanions as determined on the basis of a number of experiments is found to be pyramidal, similar to that of amines. The central carbon atom is sp hybridized with the unshared pair occupying one apex of the tetrahedron (if the electron pair is viewed as a substituent). These species invert rapidly at room temperature, passing through a higher energy planar form in which the electron pair occupies a p-orbital. However, when the carbanion is stabilized by delocalization, it assumes sp hybridization for effective resonance. 

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