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Hydrogen Bonding

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Molecular Forces: Electrostatic forces relating to or involving molecules.

They are of two types:

[1]. Intermolecular forces (IMFs):

These are forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions).

Intermolecular forces are also referred to as noncovalent interactions or nonbonded interactions. These intermolecular forces, collectively called van der Waals forces, are all electrical in nature (involving electrons).

They are weak compared to the intramolecular forces, the forces which keep a molecule together.

[2]. Intramolecular force:

An intramolecular force is any force that holds together the atoms making up a molecule or compound. They contain all types of chemical bond.

They are stronger than intermolecular forces, which are present between atoms or molecules that are not actually bonded.

Introduction

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Hydrogen bonding

Definition: “Hydrogen bonding is a non-covalent force of electrostatic attraction between a partially positively charged hydrogen atom attached to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) and another nearby electronegative atom with a partial negative charge (due to polarity) or full negative charge (due to lone pair of electrons).”

Hydrogen bonding typically occurs when a hydrogen atom bonded to O, N, or F, is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule.

A hydrogen bond is a weak type of force that forms a special type of dipole-dipole attraction; it is not a true chemical bond. It is represented by dotted lines as shown below:

These bonds are generally stronger than ordinary dipole-dipole and dispersion forces, but weaker than true covalent and ionic bonds.

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Explanation:

Due to the difference in the electronegativity between hydrogen and the other electronegative atom, the bonding electron pair is drawn towards the electronegative atom and so the bond connecting them becomes polar.

The hydrogen atom acquires a positive charge while the electronegative atom bears the negative charge.
Hydrogen bonding results from the electrostatic interaction between the positively charged hydrogen atom and the negatively charged electronegative atom (like F, O or N) . The second electronegative atom may be a part of the same molecule or it may belong to a different molecule.

Example: Water molecules can hydrogen bond to each other. When they do, a H-atom covalently bonded to O-atom in one water molecule is attracted to a lone pair of electrons on the O-atom in another water molecule.

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Examples of Molecules having Hydrogen Bonding

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Anatomy of a Hydrogen Bond:

In order for a hydrogen bond to occur there must be both a hydrogen donor and an acceptor present.
The strongest hydrogen bonding is formed between a strong donor (like F-H and O-H in acid) and a strong acceptor.

The hydrogen donor in a hydrogen bond is the atom to which the hydrogen atom participating in the hydrogen bond is covalently bonded, and is usually a strongly electronegative atom such as N, O, or F.

The hydrogen bond donor must have a sufficiently large δ⁺ charge caused by bonding to a highly electronegative element (O, N, or F; or in uncommon cases by strong electron-withdrawing inductive effects).

The hydrogen acceptor is the neighboring electronegative ion or molecule, and must posses a lone electron pair and sufficiently high electron density in order to form a hydrogen bond.

General hydrogen bond structure.

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Types of Hydrogen Bonds:

Hydrogen bonds can occur within one single molecule, between two like molecules, or between two unlike molecules. They are of two major types.

[1]. Intramolecular Hydrogen Bonds:

Intramolecular hydrogen bonds are those which occur within different parts of a single molecule. In order for this to happen, both a hydrogen donor an acceptor must be present within one molecule, and they must be within close proximity of each other in the molecule. For example, dotted lines shown below in ortho nitrophenol, ortho nitrobenzoic acid and salicyldehyde (O-hydroxy benzaldehyde) etc.

[2]. Intermolecular Hydrogen Bonds: Intermolecular hydrogen bonds occur between separate molecules. They can occur between any number of like or unlike molecules as long as hydrogen donors and acceptors are present and in positions in which they can interact. For example, intermolecular hydrogen bonds can occur between NH₃ molecules alone, between H₂O molecules alone, or between NH₃ and H₂O molecules.

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Conditions for Hydrogen Bonding:

The necessary conditions for the formation of hydrogen bonding are:

1. High electronegativity of atom bonded to hydrogen:

Hydrogen bonding cannot occur without significant electronegativity differences between hydrogen and the atom it is bonded to.

The molecule must contain an atom of high electronegativity such as F, O or N bonded to hydrogen atom by a covalent bond. The examples are HF, H₂O and NH₃.

Highly electronegative atoms like O, N and F will make the attached H-atom to acquire a partial positive charge needed to hydrogen bond with the lone electron pair in another molecule.

Electronegativity: It is the tendency of an atom or molecule to draw electrons towards itself.

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2. Small size of Electronegative atom:

The electronegative atom attached to the H-atom by a covalent bond should be quite small.

Smaller the size of the electronegative atom, greater will be the attraction for bonded electron pair and greater will be the polarity between H and electronegative atom.

This results in the formation of stronger hydrogen bond.

Example:

Although N and Cl both have almost same electronegativity i.e. 3.0, NH₃ shows hydrogen bonding while HCl doesn’t show. It is because Cl (chlorine) is bigger in size than N (nitrogen) and despite its electronegativity, the size of the atom is such that its electron density is too low to form hydrogen bonds.

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Difference between H-bond & Covalent Bond

H-bond:

It is formed between a hydrogen and a highly electronegative atom such as F, O, N.
It involves dipole-dipole attractive interactions.
Hydrogen bond is very weak as compared to a covalent bond.
The strength of hydrogen bond is intermediate between the weak van der waal’s forces and the strong covalent bonds.
Bond dissociation energy for H-bond is only 12.6 - 41.8 kj/mol.

Covalent bond:

It involves mutual sharing of electrons between two atoms.
It is formed between two atoms which may be out of the same element or of different elements.
The bond strength of this bond is sufficiently high.
Thus, the bond dissociation energy of a covalent bond is 209 - 418 Kj/mol.

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Applications/Consequences of H-bond

[1]. High melting & boiling points: The compound having H-bond show abnormally high melting and boiling point. It is due to the fact that some extra energy is needed to break these hydrogen bonds.

Examples:
Hydrogen bonding explains why water, ammonia, and hydrogen fluoride all have far higher boiling points than methane (bp -161.6°C), even though all four compounds have similar molecular weights.

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Q No. 1: Explain why the order of boiling point is H₂O > HF > NH₃

Explanation:

H₂O can form 2 hydrogen bond per molecule
NH₃ and HF can form only 1 hydrogen bond per molecule
The boiling point of H₂O is higher than NH₃ and HF

Q No.2: Explain why the order of boiling point is HF > NH₃.

Explanation: – Comparing N and F, F is more electronegative – The intermolecular hydrogen bond formed between HF molecules is stronger than NH₃ – HF has a higher boiling point than NH₃

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Hydrogen bonding is important in many chemical processes.

[2]. H-Bonding and Water: Life depends on hydrogen bonds in water.

High Specific Heat = water resists changes in temp. helps regulate cells.

Surface tension is due to hydrogen bonds between water molecules

The polarity of water causes it to be cohesive and adhesive.

Cohesion: Water molecules stick to other water molecules by hydrogen bonding

Adhesion: Water molecules stick to other polar molecules by hydrogen bonding

One of the most important consequences of hydrogen bonding is that it causes water to be a liquid rather than a gas at 25°C.

Calculations indicate that in the absence of hydrogen bonding, water would have a boiling point near -80°C and would not exist as a liquid unless the temperatures were lower than that temperature.
If this has been the case, it is highly unlikely that life could have developed on the planet Earth.

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Explanation of lower density of ice than water :

When water freezes, water molecules form a crystalline structure maintained by hydrogen bonding. Solid water, or ice, is less dense than liquid water.

Ice is less dense than water because the orientation of hydrogen bonds causes molecules to push farther apart, which lowers the density.

Actually molecules arrange themselves in a rigid tetrahedral structure due to which cage like spaces remain in their bonding and so lowers the density.

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[3]. Surface tension: Compared to most other liquids, water also has a high surface tension.

The water strider takes advantage of the fact that the water surface acts like an elastic film that resists deformation when a small weight is placed on it. This is all due to the surface tension of the water produced by hydrogen bonding.

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[4]. H-Bonding and Solubility:

The organic compounds which can form Hydrogen bonds with molecules of water are soluble.

Examples:

Hydrogen bonding is responsible for water's unique solvent capabilities.

Methanol and water are miscible in all proportions; so are the mixtures of ethanol and water and mixtures of both propyl alcohols and water.

A factor in understanding their solubility is that the molecules are capable of forming strong hydrogen bonds to each other. E.g., of ethanol and water is as follows:

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[5]. Hydrogen bonding in organic molecules containing nitrogen:

Hydrogen bonding also occurs in organic molecules containing N-H groups - in the same sort of way that it occurs in ammonia. Examples range from simple molecules like CH₃NH₂(methylamine) to large molecules like proteins and DNA.

Hydrogen bonds are also responsible for determining the three-dimensional structure of folded proteins including enzymes and antibodies. The hydrogen bonds help the proteins and nucleic acids form and maintain specific shapes.

Intramolecular hydrogen bonding is partly responsible for the formation and maintainance of secondary, tertiary, and quaternary structures of proteins and nucleic acids.

Hydrogen bonds hold the base pairs of double-stranded DNA together. E.g. Thymine hydrogen bonds with adenine. Cytosine hydrogen bonds with guanine.

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[6]. H-Bonding and Viscosity: Those substances which are capable of forming hydrogen bonds tend to have a higher viscosity than those that do not.

Substances which have the possibility for multiple hydrogen bonds exhibit even higher viscosities. Example: Viscosity of ethylene glycol is more than ethanol and that glycerin is even more viscous than ethanol and ethylene glycol .

[7]. H-Bonding and Chelation: Although individual hydrogen bonds are not very strong, a series of hydrogen bonds is very secure and provides more stabilizing effect to the molecule or compound.

When one molecule hydrogen bonds through two or more sites with another molecule, a ring structure known as a chelate is formed.

Chelating compounds are useful for removing or mobilizing molecules and atoms such as metals. Example: EDTA (ethylenediaminetetraacetic acid).

Note: A chelate is a chemical compound composed of a metal ion and a chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. In other words, a chelating agent is a multidentate ligand.

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[8]. H-Bonding in Alcohols: An alcohol is an organic molecule containing a carbon atom attached to a hydroxyl group (-OH) group.

Any molecule which has a hydrogen atom attached directly to an oxygen or a nitrogen is capable of hydrogen bonding.

Example:
Hydrogen bonding accounts for the fact that ethyl alcohol has a much higher boiling point (+78.5°C) than dimethyl ether (-24.9°C) even though the two compounds have the same molecular weight.

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[10]. H-Bonding in Cellulose: Cellulose is an organic compound found in trees, cotton, and dried hemp, and can be used to make paper, textiles, rayon, and cellophane.

Hydrogen bonds are important to maintaining the shape and form of the cellulose compound.

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[11]. H-Bonding in Wool: Wool being a protein fiber, is held together by hydrogen bonds, causing wool to recoil when stretched. However, washing at high temperatures can permanently break the hydrogen bonds and a garment may permanently lose its shape.

Wool is the hair of certain mammals. Most wool comes from sheep and goats, camels, etc. and special rabbits. Wool is a natural material.

[12]. H-Bonding in Drug-Receptor Binding:

Because of the presence of amino acid residues in receptors containing both the hydrogen bond donor and acceptor functionalities, hydrogen bonding plays a crucial role in binding ligands like drug molecules possessing similar functionality and aids in the expression or suppression of the activity/property/toxicity.

[13]. H-Bonding and Protein Folding:

Protein structure is partially determined by hydrogen bonding. Hydrogen bonds can occur between a hydrogen on an amine and an electronegative element, such as oxygen on another residue. As a protein folds into place, a series of hydrogen bond "zips" the molecule together, holding it in a specific three-dimensional form that gives the protein its particular function.

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[14]. H-Bonding and Antibodies:

Antibodies are folded protein structures that precisely target and fit a specific antigen.
Once the antibody is produced and attains its three-dimensional shape (aided by hydrogen bonding), the antibody will conform like a key in a lock to its specific antigen.
The antibody will lock onto the antigen through a series of interactions including hydrogen bonds.
The human body has the capacity to produce over ten billion different types of antibodies in an immunity reaction.

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