Lewis Structures of Molecules with Multiple Bonds

Lewis Structures of Molecules with Multiple Bonds 

OBJECTIVE To learn how to write Lewis structures for molecules with multiple bonds. 

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The injection of CO2 into the earth s crust is already being undertaken by various oil companies. Since 1996, the Norwegian oil company Statoil has separated more than 1 million tons of CO2 annually from natural gas and pumped it into a saltwater aquifer beneath the floor of the North Sea. In western Canada a group of oil companies has injected CO2 from a North Dakota synthetic fuels plant into oil fields in an effort to increase oil recovery. The oil companies expect to store 22 million tons of CO2 there and to produce 130 million barrels of oil over the next 20 years. 

Sequestration of CO2 has great potential as one method for decreasing the rate of global warming. Only time will tell whether it will work. 

Step 2 Form a bond between the carbon and each oxygen  

Step 3 Next, distribute the remaining electrons to achieve noble gas electron configurations on each atom. In this case twelve electrons (16 - 4) remain after the bonds are drawn. The distribution of these electrons is determined by a trial-and-error process. We have six pairs of electrons to distribute. Suppose we try three pairs on each oxygen to give 

Is this correct To answer this question we need to check two things  

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B. Lewis Structures of Molecules with Multiple Bonds... 

Lewis Structures for Molecules with Multiple Bonds Sometimes, you ll find that, after steps 1 to 4, there are not enough electrons for the central atom (or one of the central atoms) to attain an octet. This usually means that a multiple bond is present, and the following additional step is needed ... 

D. A conjugated molecule is a molecule with double bonds on adjacent atoms such as the molecule shown in A. Choice B and C give the definition of sigma and pi molecular orbitals. D is false because a resonance form is one of multiple equivalent Lewis structures, but these structures do not describe the actual state of the molecule. The anion will exist in a state between the two forms. 

Many of the Lewis structures in Chapter 9 and elsewhere in this book represent molecules that contain double bonds and triple bonds. From simple molecules such as ethylene and acetylene to complex biochemical compounds such as chlorophyll and plastoquinone, multiple bonds are abundant in chemistry. Double bonds and triple bonds can be described by extending the orbital overlap model of bonding. We begin with ethylene, a simple hydrocarbon with the formula C2 H4. 

Bond paths are observed between bonded atoms in a molecule and only between these atoms. They are usually consistent with the bonds as defined by the Lewis structure and by experiment. There are, however, differences. There is only a single bond path between atoms that are multiply bonded in a Lewis structure because the electron density is always a maximum along the internuclear axis even in a Lewis multiple bond. The value of pb does, however, increase with increasing Lewis bond order, as is shown by the values for ethane (0.249 au), ethene (0.356 au), and ethyne (0.427 au), which indicate, as expected, an increasing amount of electron density in the bonding region. 

Why is the complex OsHCl(CO)(P Pr3)2 stable, when it is unsaturated It has been argued that lone pairs on the alpha atom of a ligand M—X (M is a transition metal) can have a major influence on reactivity and structure. If M has empty orbitals of appropriate symmetry, X M tt donation creates an M—X multiple bond, with consequent transfer of electron density to M decreasing its Lewis acidity.23 The presence of a carbonyl ligand in OsHCl(CO)(P Pr3)2) increases the n-donor capacity of chloro by means of the push-pull effect making this molecule not a truly 16-valence electron species. 

In many molecules, the choice of which atoms are connected by multiple bonds is arbitrary. When several choices exist, all of them should be drawn. For example, as shown in Figure 3-1, three drawings (resonance structures) of C03 are needed to show the double bond in each of the three possible C — O positions. In fact, experimental evidence shows that all the C — O bonds are identical, with bond lengths (129 pm) between double-bond and single-bond distances (116 pm and 143 pm respectively) none of the drawings alone is adequate to describe the molecular structure, which is a combination of all three, not an equilibrium between them. This is called resonance to signify that there is more than one possible way in which the valence electrons can be placed in a Lewis structure. Note that in resonance structures, such as those shown for in Figure 3-1, the electrons are drawn in different places but the atomic nuclei remain in fixed positions. 

Begin with a single pair of dots between each pair of bonded atoms. If no arrangement of single bonds provides a Lewis structure whose atoms satisfy the octet rule, the molecule might have multiple bonds. 

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