Core Concepts
In this article, we will discuss line structures and their applications in organic chemistry.
Topics Covered in Other Articles
Introduction to Line Structures
In chemistry, there are multiple ways to represent the composition and/or structure of a compound including: Lewis Dot structures, condensed formulas, and Newman projections. Which representation a chemist chooses to work with depends on the information they are trying to convey. In organic chemistry, a popular choice is line structures (sometimes called skeletal structures). In a line structure, carbon-carbon bonds are represented by a single line and hydrogen atoms are completely omitted, allowing organic chemists to quickly draw structures with long chains. Line structures also help in quickly getting a sense of a molecule’s geometry and reactivity. Knowing how to both draw and interpret line structures not only saves time, but is helpful in understanding drawn organic compounds.
Drawing and Interpreting Line Structures
Line structures are structures in which carbon-carbon bonds are represented by a single line and carbon-hydrogen bonds are not drawn. In the following example, we have the progression of a simple 5 carbon chain from a Lewis dot structure into a line structure. The original Lewis structure, depicts every carbon, hydrogen, and bond between them. In the next figure, each carbon atom is taken out of the structure while the bond lines are connected to each other. The carbon atoms are still here, just not drawn for simplicity. Finally, the hydrogen atoms and their bonds are omitted as well. Leaving the last, most simplified structure below.
In organic chemistry, it is quite common to see long carbon chains, and drawing each of them out can be quite tedious. The line structure above not only is much quicker to draw, but also much quicker to interpret from a viewer’s perspective. This becomes especially useful with larger, more complicated molecules. As a general rule of thumb, each “peak” of the line structure indicates a CH₂ group, while each “end” indicates a CH₃ group. Note that even without the atoms written out, the shape of the line structure follows that of the original Lewis structure.
Heteroatoms
In more complex molecules containing other elements besides carbon and hydrogen, the atom or group is simply added to the structure, using their elemental symbol. For example, in 1-bromobutane, a bromine atom is simply added to the final carbon indicating their bond to each other. A detailed walk-through is shown below. Note that on non-carbon atoms, there are never any implied hydrogen atoms.
Notice that the lone pairs are omitted in the final structure. When drawing reaction mechanisms, it is much clearer to draw the lone pairs on an atom, especially if they play an important role in the reaction.
Functional Groups
In other cases, entire groups may be added onto the carbon chain. It is important to keep in mind the addition of a group or element changes the number of hydrogen atoms bonded to the carbon. In the following example, a methyl group is present on the 3rd carbon, therefore only one hydrogen can be bonded here. Of course, this can change if the carbon atom carries a charge.
These distinctions are critical in ensuring line structures are being correctly interpreted.
3-Dimensional Geometry in Line Structures
Line structures are also useful in quickly seeing a molecule’s geometric orientation in space. Depending on the complexity of your molecule, a 2-dimensional depiction may not be enough. In line structures this is done with dashed and solid wedges. Wedges are drawn in place of the single line representing a given bond. In the example below, the dashed wedge represents the bond of a methyl group going into the plane (away from you) while the solid wedge represents the bond of a methyl group coming out of the plane (toward you). The remaining bonds of the line structure represent molecules that are in line with the plane of the paper.
Wedge notation is also useful in efficiently identifying and representing enantiomers. In the following example, two 2-butanol enantiomers are shown. We can immediately tell that these two molecules are different. Had these molecules been represented as a traditional, 2-D line structure, one would think they are exactly the same!
Double and Triple Bonds
In instances of double/triple bonds in your molecule, add an additional line or two between molecules bonded in that fashion on your line structure. In the following example, an alkene is shown with a double line representing the double bond. Since carbons are omitted, we are left with a simple line structure, and an extra line indicating a double bond.
Had this been a triple bond, we would see an additional line like in the following alkyne. The carbons participating in the triple bond cannot bond to any extra hydrogen atoms. Also, note that the molecular geometry in the line structure follows typical molecular geometry rules. As triple bonds (alkynes) have linear geometry, the line structure follows suit.
In the next example, a carbon-oxygen double bond exists. In this case, we combine the principles we’ve discussed above and use an extra line to indicate a double bond, while including an O atom to indicate a non carbon-carbon bond.
Ring Structures
Line diagrams are not only reserved for linear carbon chains. In the following example a phenol ring is depicted in line structure format. All of the above rules apply. Each carbon-carbon bond is drawn only now in ring form. Note the placements of the double bonds correspond to the placements of the double bonds in the original structure. Once again, Hydrogens coming off the carbon chain are omitted.
Notice how the hydrogen on the oxygen atom was kept in the final structure. As said above, hydrogen atoms are always shown when they are bonded to heteroatoms.
Chair Conformations
With our new knowledge on line structures, wedges, and rings, we can now apply them to other concepts in organic chemistry! One example is in chair conformations. When converting from cyclohexane to a chair (or vise versa), wedges indicate whether functional groups on the chair should be drawn up or down. As shown below, a solid wedge on the ring structure indicates an upward group while a dashed wedge indicates downward. (Note that this is regardless of whether the group is axial or equatorial).
