Hydrogen bond is an important concept taught in Introductory Organic Chemistry. It is a type of intermolecular attractive force that is stronger than the Van der Waals force. This force affects several physical, chemical, and biological properties of shape, nature, and reactivity.
To represent a hydrogen bond, dash or dotted lines (....) are used. These lines indicate that a hydrogen atom is sandwiched between two electronegative atoms.
The first common mistake a student can make is assuming any electronegative atom can form a Hydrogen bond. In the chapter on Hydrogen Bonding, I have explicitly mentioned that only highly electronegative atoms, such as Nitrogen (N), Oxygen (O), and Fluorine (F), can form Hydrogen bonds. However, there are special circumstances where exceptions are possible. This means that several functional groups containing Oxygen, Nitrogen, or Halogen atoms, such as acids, amides, aldehydes, haloalkanes, and amines, can be explored for Hydrogen bond-forming interactions.
Here, let us use an aldehyde, such as acetaldehyde (CH3CHO), and an alcohol, like ethanol (CH3CH2OH), as an example to learn how to draw a Hydrogen bond correctly.
1) The first step – Drawing the molecular structures.
2) Second step – Identifying H-bond donors and acceptors
In the chapter "Hydrogen Bonding" from the Organic Chemistry Fundamentals course, I have mentioned the characteristics to look for in donors and acceptors of the Hydrogen bond.
In our example, the Hydrogen atom covalently bonded to the electronegative Oxygen atom acts as a Hydrogen bond donor, while the lone pair on the Oxygen atom serves as the Hydrogen bond acceptor.
The second common mistake that a student can make is assigning the Hydrogen atom of the acetaldehyde as a Hydrogen bond donor. It is crucial to note that a Hydrogen atom must be covalently bonded to the O, N, or F atom to serve as a Hydrogen bond donor.
3) Last step – Bond angle of the Hydrogen bond
After correctly identifying the H-bond donors and acceptors, the next step is to draw the Hydrogen bond between two molecules.
There are several ways to accomplish this, which I will outline below. However, only one of these representations is correct (marked in green).
We learnt in the chapter how important it is to correctly draw the bond angle of the Hydrogen bond, as the bonding angle has a significant impact on the strength of Hydrogen bonds.
The ideal bond angle is 180o , which occurs when the Hydrogen donor atom and the lone pair of the acceptor atom are in a straight line. When the angle decreases, the strength of the H-bond also decreases. No Hydrogen bonds are formed when the angle is 90o or less. Therefore, the third common mistake is drawing the Hydrogen bonds at an angle less than 180o.
In this article, I have outlined three ways in which students can make mistakes when drawing an H-bond.
Hydrogen bonding is a crucial force that maintains the shape of our DNA and helps bind enzymes to their receptors. By learning and applying this concept correctly, we can give Hydrogen bonding the recognition it deserves.
Related Article –
Donor and Acceptors atoms in a Hydrogen bond
Why are Hydrogen bonds stronger than dipole-dipole interactions?
Related Chapter –
Topic – Hydrogen Bonding, Chapter - Intermolecular Forces, Organic Chemistry Fundamentals Course