Ethane, Ethylene, and Acetylene

Let s now use the principles of hybridization to determine the type of bonds in ethane, ethylene, and acetylene. 

According to the Lewis structure for ethane, CH3CH3, each carbon atom is singly bonded to four other atoms. As a result  

All of the bonds in ethane are o bonds. The C-H bonds are formed from the overlap of one of the three sp hybrid orbitals on each carbon atom with the orbital on hydrogen. The C-C bond is formed fi m.the ovalap h ljiid irbit oji acLcarbon atom. 

Each C-H bond Is formed by overlap of an sp hybrid on C with a Is orbital on H. 

Finally, a model of ethane shows that rotation can occur around the central C-C CT bond. 

Sample Problem 1.8 What orbitals are used to form each bond in methanol, CH3OH  

To solve this problem, draw a valid Lewis stmcture and count groups around each atom. Then, use the rule to determine hybridization two groups = sp, three groups = sp, and four groups = sp . 

All C - H bonds are formed from C Hi. The C-0 bond is formed from CgpS-OjpS. The O - H bond is formed from Ojp3- Hi.  

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Practice working with your Learning By Modeling software Construct molecular models of ethane ethylene and acetylene and compare them with respect to their geometry bond angles and C—H and C—C bond distances... 

Defining ethane, ethylene and acetylene to have bond orders of 1, 2 and 3, the constant a-is found to have a value of approximately 0.3 A. For bond orders less than 1 (i.e. breaking and fonning single bonds) it appears that 0.6 A is a more appropriate proportionality constant. A Mulliken style measure of the bond strength between atoms A and B can be defined from the density matrix as (note that this involves the elements of the product of the D and S matrices). 

Belay N, L Daniels (1987) Production of ethane, ethylene and acetylene from halogenated hydrocarbons by methanogenic bacteria. Appl Environ Microbiol 53 1604-1610. 

The calculated deprotonation energies of ethane, ethylene and acetylene by SCF Hartree-Fock (FIF) and MP2 methods follow the expected order 456, 455 (basis... 

Note how we have resorted to another form of representation of the ethane, ethylene, and acetylene molecules here, representations that are probably familiar to you (see Section 1.1). These line drawings are simpler, much easier to draw, and clearly show how the atoms are bonded - we use a line to indicate the bonding molecular orbital. They do not show the difference between a and rr bonds, however. We also introduce here the way in which we can represent the tetrahedral array of bonds around carbon in a two-dimensional drawing. This is to use wedges and dots for bonds instead of lines. By convention, the wedge means the bond is coming towards you, out of the plane of the paper. The dotted bond means it is going away from you, behind the plane of the paper. We shall discuss stereochemical representations in more detail later (see Section 3.1). 

In comparing the activation energies for reactions of hydroxyl with ethane, ethylene, and acetylene it may be concluded that a similar reaction occurs in all three cases, namely, H atom abstraction. The activation energy increases from ethane to acetylene depending upon the C-H bond energy. 

G.J.H. Vannes, A. Vos, Single-crystal structures and electron density distributions of ethane, ethylene and acetylene. I. Single-crystal X-ray structure determinations of two modifications of ethane. Acta Cryst. B 34, 1947-1956 (1978)... 

See text for notations and explanations. The barriers for the gas-phase ethane, ethylene, and acetylene are also added. All energies in kilocalories/mole. 

Schnabel, P., and Irion, M. F., Reactions of Fe( (3 acetylene—reaction paths and rate constants. Ber. Bunsenges. Phys. Chem. 96,1101 (1992). 

Come and co-workers [182] have described a linear notation which, at least for acyclic compounds, is very close to the notation familiar to a chemist. For example, the molecules of ethane, ethylene and acetylene are denoted by CH3/CH3, CH2//CH2 and CH///CH, respectively. The neopentane molecule and neopentyl free radical are denoted by C(CH3)4 and C(CH3)3/CH2., respectively. The main rules of this notation are as follows. 

When a 0.20 mol mixture of ethane, ethylene and acetylene is passed through ammoniacal CU2CI2 solution, 7.6 grams of precipitate is formed. The remaining mixture is passed through bromine solution and 8 grams of bromine is used up. How many moles of ethane were there in the original mixture ... 

Defining ethane, ethylene and acetylene to have bond orders of 1, 2 and 3, the constant is found to have a value of approximately 0.3 A. For bond orders less than 1 (i.e. 

Bond Lengths and Bond Strengths for Ethane, Ethylene, and Acetylene... 

The conjugate bases formed by removing a proton from ethane, ethylene, and acetylene are each carbanions—species with a negative charge on carbon. 

The value of the charge density at a bond critical point can be used to define a bond order (Bader et al. 1983 Cremer and Kraka 1984). The molecular graphs for ethane, ethylene, and acetylene are shown in Fig. 2.8. In each case the unique pair of trajectories associated with a single (3, — 1) critical point is found to link the carbon nuclei to one another. Multiple bonds do not appear as such in the topology of the charge density. Instead, one finds that the extent of charge accumulation between the nuclei increases with the assumed number of electron pair bonds and this increase is faithfully monitored by the value of p at the bond critical point, a value labelled p, . For carbon-carbon bonds, one can define a bond order n in terms of the values of Ph using a relationship of the form... 

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