Jumat, 09 Oktober 2009

Lewis Structures

Now that you've seen how valence bond theory works, it's time to learn the language that organic chemists use to describe the structures of molecules in which atoms are connected by covalent bonds. That language is pictorial, and, like other languages, it contains a number of "dialects". In order to speak like a "native", you have to understand their dialects. Those of primary concern are

  • Lewis electron dot structures
  • Lewis structures
  • Bond line notation
  • Condensed formulas
  • Mixed metaphors

Figure 1 presents three alternative pictures of methane, starting with the depiction we saw during our discussion of valence bond theory. Representation B in Figure 1 labels the nuclei of the atoms with their atomic symbols. Drawing C shows just the atomic symbols and the shared pairs of electrons that constitute the four C-H covalent bonds. Drawing C is a Lewis electron dot structure for methane.

Figure 1

Alternative Representations of Methane

Lewis Electron Dot Structures

Figure 2 animates the rules for drawing a Lewis electron dot structure using C2H6as an example. Click within the figure to view the animation.

Figure 2

Drawing Lewis Electron Dot Structures

Drawing electron dot structures always involves trial and error, even when you follow the rules. Figure 3 illustrates one pitfall in trying to draw a valid Lewis electron dot structure for C2H4.

Figure 3

You're Going to Have to Do This More Than Once

The procedures outlined in Figures 2 and 3 have dealt with very simple molecules. Things become more complicated as the number of atoms that we have to connect becomes larger. This is because there will be more than one acceptable way to join the atoms. In other words, there will be more than one acceptable molecular structure for a given molecular formula. Compounds that have the same molecular formula but different atomic connections are called constitutional isomers. There are two constitutional isomers for the molecular formula C4H10. Figure 4 demonstrates drawing Lewis electron dot structures for these compounds.

Figure 4

Constitutional Isomers of C4H10

The name of the compound on the left in Figure 4 is butane. It is a colorless liquid with a boiling point of 0oC. It's what burns when you flick your BIC. Butane is a linear molecule; you can trace a path from the first to the fourth carbon atom without lifting your pencil from the paper.

The compound on the right in Figure 4 goes by different names. It is commonly called isobutane. On formal occassions it goes by its proper name 2-methylpropane. It is a colorless liquid with a boiling point of -12oC. It has a branched structure; you can't draw a line through all four carbon atoms without lifting your pencil off the paper.
Lewis Structures

Drawing each bond in a molecule as two dots gets old very fast. To save time chemists usually depict a bond as a line drawn from one atomic symbol to another. Such representations are called Lewis structures rather than Lewis electron dot structures. Figure 5 shows Lewis structures for each of the molecules presented in Figures 1-4.

Figure 5

Lewis Structures: Saving Some Time

When a molecule contains atoms with one or more lone pairs of electrons, the non-bonding pairs are normally shown as two dots.

Bond Line Notation

Generally organic chemists depict molecular structures using a convention known as bond line notation. According to this approach, a bond between two atoms is represented by a line. The ends of the line correspond to the positions of the bonded atoms. Only atoms other than carbon and hydrogen are shown explicitly. Thus the bond line notation for ethane is ___ . It is implied that there is a carbon atom at each end of the line. Furthermore, since the valence of carbon is four, it follows there are three hydrogen atoms connected to each carbon. Figure 6 illustrates the bond line notations for those compounds in Figure 5 that contain carbon-carbon bonds.

Figure 6

Talkin' Like an Organic Chemist

Figure 7 compares Lewis electron dot structures, Lewis structures, and bond line notations of several compounds that contain heteroatoms, i.e. atoms other than carbon and hydrogen. Note that hydrogen atoms attached to heteroatoms are shown explicitly, while hydrogen atoms bonded to carbons must be inferred.

Figure 7

Alternative Representations of Organic Structures

Condensed Formulas

Another convention that chemists use involves condensed formulas. Unlike bond line notation, where carbon and hydrogen atoms are inferred while bonds are shown explicitly, condensed formulas show all the atoms explicitly and the bonds between them are inferred. Thus, the condensed formulas for methane and ethane are CH4 and CH3CH3, respectively. The condensed formula for ethane makes it easy to forget that there is a bond between the two carbons. While an alternative formulation, H3CCH3, makes that bond more obvious, it is not commonly used. Condensed formulas for linear structures are straightforward. The formula for pentane, C5H12, a colorless liquid with a boiling point of 36oC, is CH3CH2CH2CH2CH3. Sometimes this is abbreviated CH3(CH2)3CH3.

When writing the formula for a branched molecule, the convention calls for enclosing the branching group(s) in parentheses. For example, the condensed formula for isobutane (2-methylpropane) is CH3CH(CH3)CH3. The carbon atom of the methyl group enclosed in parentheses is attached to the carbon atom bearing the single hydrogen. Alternatively, some chemists would write this as (CH3)2CHCH3. The condensed formula for neopentane (2,2-dimethylpropane), a colorless liquid that boils at 10oC, is CH3C(CH3)2CH3. Two equivalent formulations are (CH3)3CCH3 and (CH3)4C. Compare the notations for pentane and neopentane. Be certain you understand the differece between these two formulations.

You should practice drawing Lewis structures and bond line notations of condensed formulas until your brain automatically fills in the structural information hidden in such formulas.

Tips for Drawing Chemical Structures

* Remember that the valence of an atom is the difference between the number of electrons in its valence shell and the number of electrons in the corresponding filled shell. For the atoms we will deal with most often the valences are: H = 1, C = 4, N = 3, O = 2, halogen = 1.
* Univalent atoms must be terminal atoms.
* Multivalent atoms are normally interior atoms.
* Multivalent atoms may occupy terminal positions if they form multiple bonds to interior atoms.
* Don't forget to include "non-bonding" electron pairs. Nitrogen has 1, Oxygen 2, and the halogens have 3.
* The index of hydrogen deficiency will offer insight into the presence or absence of multiple bonds.
* For neutral molecules:

1. Carbon has 4 bonding options: 1. 4 single bonds 2. 2 single bonds and 1 double bond 3. 2 double bonds 4. 1 single bond and 1 triple bond
2. Nitrogen has 3 bonding options: 1. 3 single bonds 2. 1 single bond and a double bond 3. 1 triple bond
3. Oxygen has two bonding options: 1. 2 single bonds 2. 1 double bond

Table 3

Bonding Options for C, N, and O

Option 1
Option 2
Option 3
Option 4

Mixed Metaphors

Finally, you will frequently encounter structural formulas that contain a mixture of bond line notation and condensed formulas. For example, a recent article in the Journal of Natural Products reported the isolation of a compound known as glacin B from the leaves of the pond-apple tree. Extracts of the leaves of this tree, which is native to Florida, have been used in traditional medicines as an insecticide and a parasiticide. The structure of glacin B is shown in Figure 8. Note the various conventions that are used in this depiction of the molecule.

Figure 8

Putting It All Together: How Chemists Communicate

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