Organic Chemistry

Module 1. Hydrogen bonding and hydrophobic interactions

Lesson 1
CONCEPTS OF HYDROGEN BONDING

1.1 Introduction

• Covalent bond between two atoms - formed by sharing of electrons present in outer most valence shells of bonding atoms.
• Covalent bond is formed between two similar or dissimilar atoms.

• The shared electron pair is attracted equally by both the atoms
• Therefore, the pair of electrons lies exactly midway between the two bonded atoms.
• The bond is not polarized – non-polar covalent bond between the identical atoms.
• Example: molecules are H₂, F₂, O₂, N₂, etc.(Fig. 1.1 Homonuclear diatomic molecules)
  

• In case of covalent bond formed between two dissimilar atoms

• Generally, one of the atoms has a higher affinity for the shared/bonded electrons.
• Therefore, the shared electron pair is pulled closer to that atom (atom of high electronegativity elements → F, O, N). It is a polarized bond.

• Examples → HF, H₂O , NH₃ (Fig. 1.2)

• Such asymmetric distribution of bonded electrons leads development of partial charge on atoms of the molecule called polar covalent bond represented in
• The atoms of electronegative element (F,O, N) acquire partial negative charge.
• The hydrogen atom acquires partial positive charge denoted as δ+.
• The partial positive charge on hydrogen atom in such molecules will be attracted electro-statically by the partial negative charge on atom of the electronegative element in other molecule of such compound(Fig. 1.4 Intermolecular hydrogen bonding)
  

• The electrostatic attraction between hydrogen atom of one molecule and electronegative atom of another molecule (generally of the same substance) is known as hydrogen bond/bonding.
• Hydrogen bond is represented by a dotted line (…..).
• It is a weak secondary bond with low bond energy and purely electrostatic in nature.
• It acts as a bridge between two electronegative atoms of the molecules via hydrogen atom.
• Organic compounds such as given below also form hydrogen bond

Alcohols : R-OH
Phenols : Ar-OH
Carboxylic acids : R-COOH
Amines : R-NH₂ (Primary) and R₂-NH (Secondary)
Amides : R-CONH₂ (Fig. 1.5)

• Molecules of water and primary amines- have two hydrogen atoms- therefore involves three hydrogen bonding per molecule.
• Molecule of other compounds- have only one hydrogen atom- therefore involves two hydrogen bonding per molecule.
• Amongst the examples given above
  

• In carboxylic acids, the hydrogen bonding is limited to the association of two molecules only.
• In other compounds, the hydrogen bonding may extend to several molecules- association of several molecules.

• Hydrogen bonding may occur between molecules of different substances also – a common example is formation of hydrogen bond between molecule of water and molecule of compounds referred above (alcohol, amine, phenol etc.).(Fig. 1.6)

• Even compounds like lower aldehydes and ketones, which do not form hydrogen bond between their own molecules, they do form this bond with molecule of water
• Therefore low molecular weight aldehydes (formaldehyde and acetaldehyde) and ketone (acetone) are soluble in water because of hydrogen bonding.(Fig. 1.7)

The extent of hydrogen bonding in alcohols increases as R/OH ratio increases so higher alcohol are insoluble in water and waxy solids