What is a Bond-Line or Skeletal Formula?
Carbon and Hydrogen containing bonds are chemically inert; still, they form the backbone of many organic compounds. They provide a skeletal framework that can contain other heteroatoms or functional groups.
Such a carbon-hydrogen-containing chain of compounds together with other atoms or groups of atoms are conveniently represented using zig-zag lines, called the bond-line or skeletal formula.
In the bond-line structural representation, the carbon atom with its required number of hydrogen atoms is outside an observer's direct view. It is assumed to be present at the line ends and intersections. A line end (terminus) denotes a CH3 group, and an unsubstituted intersection denotes a CH2 group.
A single bond is shown by a single line (-), a double bond by two parallel lines (=), and a triple bond by three parallel lines (≡).
The zig-zag lines are always used for single and double bonds; however, triple bonds are drawn straight, considering the bond angle of 180o between two atoms.
Although carbon-hydrogen atoms at the terminus and intersections are not shown; however, other atoms offering significant chemical advantages such as oxygen, nitrogen, halogen, etc. or the other functional groups with their required number of hydrogen atoms are shown on the zig-zag line. For example, -NH2, -OH, -CO-, -O-, -COOH, etc.
Bond Angles in Bond-Line Formula- How to Draw
The skeletal formula considers the bond angles while drawing the structures. So, the substituents are introduced on the zig-zag chain, keeping in mind the bond angle.
The substituents face away from the intersection points avoiding crowding and keeping a minimum angle of 109o, the tetrahedral bond angle between two substituents in single bonded compounds, or a trigonal bond angle of 120o in double bonds.
Single bonded zig-zag chain- Introducing substituents
The connection between carbon atoms is shown as a zig-zag line in singly bonded carbon-hydrogen compounds.
Each carbon atom is tetrahedral in shape, with the hydrogen atoms hidden. A tetrahedral shape is when two atoms are in one plane; the third substituent atom is above (dark wedge facing the observer) and the fourth below the plane (shadowed wedge facing away from the observer). This orientation maintains the minimum angle of 109o, which avoids repulsive interaction between two bonds.
However, the two-dimensional zig-zag representation does not show any substituent containing bonds as wedges; instead, the bonds are shown as lines maintaining the tetrahedral bond angle.
A mono-substituent other than hydrogen is drawn away from the point of intersection as a line. The attached hydrogen is hidden. The di-substituent are drawn in two ways, A and B, that preserve the tetrahedral bond angle.
Double bonded zig-zag chain
The atoms containing the double bonds are planar, and the angle between the atoms is 120o, called the trigonal bond angle. At this bond angle, the substituents are spaced out equally, avoiding any crowding or angle constriction that can result in repulsive interactions.
More examples
Dash structural formula to bond-line or skeletal formula
1) 3-ethylpent-2-ene
2) 4-(methoxymethyl) cyclopent-2-en-1-ol
Condensed structural formula to bond line or skeletal formula examples
1) Condensed structural formula - ClCH2CH2CH(OCH3)CH3
Its bond-line representation is-
2) Condensed structural formula - H3CNHCH2COOH
Its bond-line representation is-