Hydrogen bonds can occur in both organic and inorganic molecules.
They are not chemical bonds (such as a covalent bond) but instead a strong dipole-dipole attraction.
Hydrogen bonds occur between a hydrogen atom that is chemically bonded to a highly electronegative atom or group and another molecule with with a highly electronegative atom at has a lone pair of electrons.
Electronegative Atoms and Hydrogen Bonds
Oxygen (O), Nitrogen (N) and Fluorine (F) are all very electronegative atoms and are the main atoms that form hydrogen bonds. They each bond to Hydrogen (H) with a strong covalent bond.
Each of these atoms can form at least one strong covalent bond with Hydrogen (H). When bonded with hydrogen each atom has at least one lone pair of electrons. For example Oxygen bonds with two hydrogen atoms and is left with two lone pairs of electrons.
The large difference in electronegativities between these atoms and hydrogen results in the bonds (O-H, N-H and F-H) being polar. This means the hydrogen atoms in the bonds have a large positive partial charge and the other atom (O, N or F) has a large negative partial charge.
When the molecules come together the partially positive hydrogen atoms on one molecule are strongly attracted to the lone pair of electrons on another molecule.
This strong attraction is a Hydrogen Bond. They’re stronger than van der Waals’ forces, but weaker than ionic and covalent bonds.
Hydrogen Bonding In Water
Hydrogen bonds occur in water and so we’ll use water as an example to illustrate what a hydrogen bond is.
As mentioned above, the high electronegativity of oxygen gives the hydrogen atoms in water molecules a partially positive charge. The oxygen atoms in water molecules have two lone pairs of electrons. This is shown in the diagram below with the partial charges denoted with a delta sign (𝛿).
When two water molecules come close enough a hydrogen bond forms between the hydrogen atom on one molecule and one of the lone pairs of electrons on the oxygen atom of the other molecule. Hydrogen bonds are usually denoted with a dashed line, as shown in the diagram below.
Each water molecule has two hydrogen atoms and two lone pairs of electrons. This means that each water molecule can form up to four hydrogen bonds with other water molecules.
The hydrogen bonding in water is strong enough to give it a relatively high boiling point and some other special properties.
Hydrogen Bonding in Organic Molecules
Organic molecules such as proteins, alcohols and carboxylic acids can form hydrogen bonds. They increase the boiling points and can give molecules their shape.
DNA has a double helix structure composed of two strands. These strands are held together by hydrogen bonds between the bases. The hydrogen atoms are bonded to a nitrogen atom on one base and form a hydrogen bond with a lone pair of electrons on either a nitrogen or oxygen atom on the opposite base.
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