Key difference - intermolecular vs. intramolecular hydrogen bonding
Molecules are formed when atoms of either the same or different elements come together to share electrons and form covalent bonds . There are two types of forces of attraction that hold the covalent molecules together. These are called intermolecular forces and intramolecular forces . Intermolecular forces are the forces of attraction that occur between two molecules, while intramolecular forces occur within the molecule itself. Hydrogen bonds are special types of bonds that are formed in molecules formed by a hydrogen atom that shares electrons with a strongly electronegative atom. Hydrogen bonds can occur as both intermolecular and intramolecular forces. The main difference between intermolecular and intramolecular hydrogen bonds is that an intermolecular hydrogen bond occurs between two adjacent molecules , while an intramolecular hydrogen bond occurs within the molecule itself.
It is important to understand the function of these two forces separately in order to understand how they hold a molecule or a covalent bond together.
This article explains
1. What is hydrogen bonding? 2. What is intermolecular hydrogen bonding? - Definition, characteristics and properties, examples 3. What is an intramolecular hydrogen bond? - Definition, characteristics and properties, examples 4. What is the difference between intermolecular and intramolecular hydrogen bonds?
What is hydrogen bonding?
When hydrogen, which is moderately electronegative, becomes covalently bonded to a strongly electronegative atom, the pair of electrons they share becomes more biased towards the strongly electronegative atom. Examples of such atoms are N, O and F. There must be a hydrogen acceptor and a hydrogen donor for a hydrogen bond to be formed. The hydrogen donor is the strongly electronegative atom in the molecule and the hydrogen acceptor is the strongly electronegative hydrogen atom in the neighboring molecule and should have a lone pair of electrons.
Hydrogen bonds can occur either between two molecules or within the molecule. These two types are known as intermolecular hydrogen bonds and intramolecular hydrogen bonds, respectively.
What is intermolecular hydrogen bonding?
Intermolecular hydrogen bonds can occur between identical or dissimilar molecules. The position of the acceptor atom should be properly aligned so that it can interact with the donor.
Let's look at a water molecule to clearly understand the scenario.
The electron pair that is shared between H and O atoms is more strongly attracted to the oxygen atom. Therefore, the O atoms are slightly negatively charged compared to the H atom. O atom is represented as δ and H atom is represented as δ +. When a second water molecule approaches the former, an electrostatic bond is formed between the δ-O atom of one water molecule and the δ + H atom of the other. The oxygen atoms in the molecules act as donor (B) and acceptor (A), with one oxygen atom donating hydrogen to the other.
Due to the hydrogen bond, water has very special properties. It is a good solvent and has a high boiling point and high surface tension . In addition, ice has a lower density than water at 4 ° C. Therefore, ice floats on liquid water and protects the underlying aquatic life in winter. Because of these properties in water, it is called a universal solvent and plays an important role in sustaining life on earth.
What is intramolecular hydrogen bonding?
If a hydrogen bond occurs within two functional groups of the same molecule, one speaks of an intramolecular hydrogen bond. This occurs when the hydrogen donor and the acceptor are both within the same molecule.
In the O-nitrophenol molecule, the O atom in the -OH group is more electronegative than the H and thus δ-. The H atom, on the other hand, is δ +. Therefore, the O atom of the -OH group acts as an H donor, while the O atom of the nitro group acts as an H acceptor.
Difference between intermolecular and intramolecular hydrogen bonds
Intermolecular hydrogen bonds: Intermolecular hydrogen bonds occur between two neighboring molecules.
Intramolecular hydrogen bonds: Intramolecular hydrogen bonds occur within the molecule itself.
Intermolecular hydrogen bonds: Intermolecular hydrogen bonds have high melting and boiling points and a low vapor pressure.
Intramolecular hydrogen bonds: Intramolecular hydrogen bonds have low melting and boiling points and a high vapor pressure.
Intermolecular hydrogen bond: the stability is comparatively high.
Intramolecular hydrogen bond: the stability is comparatively low.
Intermolecular Hydrogen Bonding: Water, methyl alcohol, ethyl alcohol, and sugar are examples of intermolecular hydrogen bonding.
Intramolecular hydrogen bonds: O-nitrophenol and salicylic acid are examples of intramolecular hydrogen bonds.
Summary - Intermolecular vs. Intramolecular Hydrogen Bonding
Compounds with intermolecular hydrogen bonds are more stable than compounds with intramolecular hydrogen bonds. Intermolecular hydrogen bonds are responsible for connecting one molecule to another and holding it together. In contrast, when intramolecular hydrogen bonds occur, molecules are less available to interact with each other and the molecules are less prone to sticking together. This leads to a decrease in the boiling point and melting point. In addition, molecules with intramolecular hydrogen bonds are more volatile and have a comparatively higher vapor pressure.
Compounds with intermolecular hydrogen bonds are easily soluble in compounds of a similar nature, whereas compounds with intramolecular hydrogen bonds do not dissolve easily.
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“O-Nitrophenol Hydrogen Bridge” by NEUROtiker - Own work (Public Domain) via Commons Wikimedia
"210 Hydrogen Bonds Between Water Molecules-01" from OpenStax College - Anatomy & Physiology, Connexions website . (CC BY 3.0) via Commons Wikimedia