Hydrogen Bond Definition
A hydrogen bond is an attractive dipole-dipole interaction between a partially positive charged hydrogen atom in one molecule and a partially negative charged atom in the same or different molecule. As the name suggests, a hydrogen bond always involves a hydrogen atom, but the other atom can be any more electronegative element. Most hydrogen bonds form between hydrogen (H) and oxygen (O), fluorine (F), or nitrogen (N).
Hydrogen bonding seems counterintuitive, because it involves atoms that already participate in chemical bonds. What you need to understand is that being in a bond doesn’t change the electronic properties of the atoms. Bonds don’t cancel out their attraction to other atoms. For hydrogen bonding to occur, two conditions must be met:
- The electronegative atom must be small. The smaller the size of the atom, the greater its electrostatic attraction. So, fluorine is better at forming hydrogen bonds than iodine.
- The hydrogen atom must be bonded to a highly electronegative atom. The greater the electronegativity, the stronger the polarization. So, hydrogen bonded to oxygen is more able to form a hydrogen bond than hydrogen bonded to carbon.
Hydrogen Bond Strength
As chemical bonds go, hydrogen bonds are not very strong. The bond energy ranges between 1 and 40 kcal/mol. They are weaker than covalent bonds (which are, in turn, weaker than ionic bonds). A hydrogen bond is about 5% the strength of the covalent O-H bond. Hydrogen bonds are stronger than van der Waals forces.
Types of Hydrogen Bonds
The two types of hydrogen bonds are intramolecular hydrogen bonds and intermolecular hydrogen bonds.
- Intramolecular hydrogen bonds – Intramolecular hydrogen bonds occur within a single molecule. This happens when two functional groups in a molecule are arranged so they can attract each other. An example occurs in salicylic acid. The alcohol (-OH) group on the ring attracts the carboxylic acid group (the double-bonded oxygen). Intermolecular hydrogen bonding also occurs between DNA base pairs.
- Intermolecular hydrogen bonds – Intermolecular hydrogen bonds occur between atoms of two different molecules. This occurs when one molecule contains a partially positive hydrogen atom and the other molecule contains a partially negative atom. This type of bonding occurs between water molecules. It also occurs between water and alcohols and aldehyde.
Examples of Hydrogen Bonds
Both inorganic and organic molecules participate in hydrogen bonds. Here are some examples:
- Hydrofluoric acid (HF): Hydrofluoric acid forms what is called a symmetric hydrogen bond, where the proton is spaced halfway between two identical atoms. A symmetric hydrogen bond is stronger than the regular hydrogen bond. It’s comparable to the strength of a covalent bond.
- Ammonia (NH3): Intermolecular hydrogen bonds form between hydrogen of one molecule and nitrogen of another. In the case of ammonia, the bond that forms is very weak because each nitrogen has one lone electron pair. This type of hydrogen bonding with nitrogen also occurs in methylamine.
- Acetylacetone (C5H8O2): Intramolecular hydrogen bonding occurs between hydrogen and oxygen.
- DNA: Hydrogen bonds form between base pairs. This gives DNA its double helix shape and makes replication of the strands possible, as they “unzip” along the hydrogen bonds.
- Nylon: Hydrogen bonds are found between the repeating units of the polymer.
- Proteins: Intramolecular hydrogen bonds result in protein folding, which helps the molecule maintain stability and assume a functional configuration.
- Polymers: Polymers that contain carbonyl or amide groups form hydrogen bonds. Examples include urea and polyurethane and the natural polymer cellulose. Hydrogen bonding in these molecules increases their tensile strength and melting point.
- Alcohol: Ethanol and other alcohols contain hydrogen bonds between hydrogen and oxygen.
- Chloroform (CHCl3): Hydrogen bonding occurs between hydrogen of one molecule and chlorine of another molecule.
Importance of Hydrogen Bonding
Hydrogen bonding is critical for life on Earth. Hydrogen bonds between water molecules help maintain a stable temperature near large bodies of water, allow humans to cool themselves via perspiration, and cause ice to float. The bonds are critical for biomolecules, such as DNA, cellulose, and proteins. Hydrogen bonds are key to drug design.
Interesting Effects of Hydrogen Bonds
Hydrogen bonding results in some interesting and unusual effects.
- Melting and Boiling Point – Usually, substances of similar molecular weights have similar melting and boiling points. But, alcohols have much higher boiling points than ethers of comparable molecular weight. The hydrogen bonding in alcohol increases the boiling point because extra energy is required to break the hydrogen bonds and permit boiling.
- Volatility – Molecules that experience hydrogen bonding have higher boiling points, so they are less volatile.
- Solubility – Hydrogen bonding explains why alcohols are soluble in water, but alkanes are not. Intermolecular hydrogen bonding in alcohols lets them form hydrogen bonds with water, too. Nonpolar alkanes can’t form these bonds. However, increasing the length of the carbon chain in alcohols decreases their solubility because the chain gets in the way of hydrogen bond formation.
- Viscosity and Surface Tension – Hydrogen bonding reduces an affected molecule’s ability to flow, so it has higher viscosity and surface tension.
- Lower Density of Ice Than Water – Hydrogen bonding produces a cage-like structure in ice. In contrast, liquid water is not as closely packed. So, ice has a lower density than water and floats.
- Phase Change Anomalies – Hydrogen bonding causes some compounds to be liquid at a certain temperature, then solid as temperature increases, and then liquid past another temperature.
- Deliquescence – Sodium hydroxide (NaOH) displays deliquescence partly because the OH– reacts with moisture in air to form a hydrogen-bonded species. A similar process occurs with some other molecules.
- Self-Healing Polymers – Smart rubber and other self-healing polymers use hydrogen bonds to “heal” when torn.
Heavy Water Hydrogen Bonds
Hydrogen bonds with heavy water (where the isotope of hydrogen is deuterium) are even stronger than those with normal water (where the isotope of hydrogen is tritium). Hydrogen bonds involving tritiated water are even stronger.
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