Hybridization and Bonding in Methane

One of the things that environmental scientists do is to keep track of important elements in the biosphere—in what form do these elements normally occur, to what are they transformed, and how are they returned to their normal state Careful studies have given clear, although complicated, pictures of the nitrogen cycle, the sulfur cycle, and the phosphorus cycle, for example. The carbon cycle begins and ends with atmospheric carbon dioxide. It can be represented in an abbreviated form as  

Virtually anywhere water contacts organic matter in the absence of air is a suitable place for methanoarchaea to thrive— at the bottom of ponds, bogs, and rice fields, for example. Marsh gas swampgas) is mostly methane. Methanoarchaea live inside termites and grass-eating animals. One source quotes 20 l /day as the methane output of a large cow. 

The scale on which methanoarchaea churn out methane, estimated to be lO -lO Ib/year, is enormous. About 10% of this amount makes its way into the atmosphere, but most of the rest simply ends up completing the carbon cycle. It exits the anaerobic environment where it was formed and enters the aerobic world where it is eventually converted to carbon dioxide by a variety of processes. 

When we consider sources of methane we have to add old methane, methane that was formed millions of years ago but became trapped beneath the Earth s surface, to the new methane just described. Firedamp, an explosion hazard to miners, occurs in layers of coal and is mostly methane. Petroleum deposits, formed by microbial decomposition of plant material under anaerobic conditions, are always accompanied by pockets of natural gas, which is mostly methane. 

An interesting thing happens when methane from biological processes leaks from sites under the deep-ocean 

We begin with the experimentally determined three-dimensional structure of a molecule, then propose bonding models that are consistent with the structure. We do not claim that the observed structure is a result of the bonding model. Indeed, there may be two or more equally satisfactory models. Structures are facts bonding models are theories that we use to try to understand the facts. 

All four sp orbitals are of equal energy. Therefore, according to Hund s rule (Section 1.1) the four valence electrons of carbon are distributed equally among them, making four half-filled orbitals available for bonding. 

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Introduction to Alkanes Methane, Ethane, and Propane 57 sp Hybridization and Bonding in Methane 58... 

Methane and the Biosphere 59 Bonding in Ethane 61 sp Hybridization and Bonding in Ethylene 61... 

Section 2.6 Bonding in methane is most often described by an orbital hybridization model, which is a modified for m of valence bond theory. Four equivalent sp hybrid orbitals of carbon are generated by mixing the 2s, 2p 2py, and 2p orbitals. Overlap of each half-filled sp hybrid orbital with a half-filled hydrogen I5 orbital gives a a bond. 

FIGURE 3.14 Each C H bond in methane is formed by the pairing of an electron in a hydrogen U-orbital and an electron in one of the four sp hybrid orbitals of carbon. Therefore, valence-bond theory predicts four equivalent cr-bonds in a tetrahedral arrangement, which is consistent with experimental results. 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

There is also a third type of reactive species that we shall discuss in detail in Chapter 9, namely radicals. Briefly, radicals are uncharged entities that carry an unpaired electron. A methyl radical CH3 results from the fission of a C-H bond in methane so that each atom retains one of the electrons. In the methyl radical, carbon is sp hybridized and forms three CT C-H bonds, whilst a single unpaired electron is held in a 2/ orbital oriented at right angles to the plane containing the ct bonds. The unpaired electron is always shown as a dot. The simplest of the radical species is the other fission product, a hydrogen atom. 

Alkanes have only -hybridized carbons. The conformation of alkanes is discussed in Chapter 3 (see Section 3.2.2). Methane (CH4) is a nonpolar molecule, and has four covalent carbon-hydrogen bonds. In methane, aU four C—H bonds have the same length (1.10 A), and all the bond angles (109.5°) are the same. Therefore, all four covalent bonds in methane are identical. Three different ways to represent a methane molecule are shown here. In a perspective formula, bonds in the plane of the paper are drawn as solid hues, bonds sticking out of the plane of the paper towards you are... 

More complicated hybrid orbitals are used in many other problems. For example, the bonding in methane is best described by combinations of one s orbital and three p orbitals, which make a set of four equivalent orbitals which point to the comers of a tetrahedron ... 

Linear response of hybridization to bond elongation. First the relation between hybridization and elongation of the C-H bond is considered. For this we need the mixed second order derivatives coupling the bond stretching with the hybridization ESVs. For every C-H bond in methane we can introduce diatomic coordinate frame with the t-axis directed along the bond and express the resonance integral related to this bond as ... 

The interaction of the Is orbital of a hydrogen atom with an sp3 hybrid on carbon leads to a crCH-bonding orbital or a (j CH-antibonding orbital (Fig. 1.17). Four of the bonding orbitals, with two electrons in each, point towards the corners of a regular tetrahedron, and give rise to the familiar picture for the bonds in methane shown in Fig. 1.18a. 

What then of the p orbitals that did not participate in the formation of sp2 hybrid orbitals These remain as p orbitals, one at each carbon, and each p orbital contains one electron. Together they form a bond, termed a n bond, between the carbon atoms. The mode of overlap is sideways on . This contrasts with the end-on overlap that results in formation of a bonds, and which in the present context involves C-H bonds in methane (Figure 1.2) and ethene, and the other bond between carbon and carbon in ethene. 

The correct answer is (D). According to electron configuration of carbon, the carbon atom shouldhave two electrons in the 2s orbital and one in each of the two 2p orbitals. The four carbon-hydrogen bonds in methane should be different because of the different orbitals, yet experimental evidence shows them to be the same. The solution is that one of the 2s electrons is raised to ap orbital, and the result is the fonnation of four identical hybridized sp orbitals. 

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