Module 1: Lesson 3 The Structure of Water

Water is the medium for life Life evolved in water due to the protection it provides from UV light. Organisms typically contain 70-90% water. Chemical   reactions occur in aqueous   environments. Water is a critical determinant of the structure and function of proteins, nucleic acids, and membranes. The octet rule dictates that there are 4 electron pairs around an oxygen atom in water. These electrons are in four sp3 orbitals. Two of these pairs covalently link   two hydrogen atoms to a central oxygen atom. The two remaining pairs   are nonbonding (lone pairs). Water geometry is a distorted tetrahedron. The electronegativity of the oxygen atom induces a net dipole moment. Because of the dipole moment, water can serve as both a hydrogen bond donor and acceptor.   Hydrogen Bonds Hydrogen bonds are strong dipole-dipole or charge-dipole interactions that arise between a covalently bound hydrogen and a lone pair of electrons.   Typically involve 2 electronegative atoms, frequently nitrogen and oxygen. Hydrogen bonds are strongest when the bonded molecules allow for linear bonding patterns.   3 atoms involved are in a line. Water can serve as both an H donor and an H acceptor. Hydrogen bonding in water is cooperative. Cooperative effect - Increases donor strength of a hydrogen bond if it concurrently acts as an acceptor for a 2nd hydrogen bond. Up to 4 H-bonds per water molecule give water its: * High boiling point, high melting point and large surface tension * H-bonds between neighboring molecules are weak (20 kJ/mol) relative to the H-O covalent bonds (420 kJ/mol) Water as a Solvent Universal Good solvent for charged and polar substances * amino acids and peptides * Small alcohols; carbs Poor Solvent for non polar substances * non polar gases * aromatic environments * aliphatic chains of relating to or being an organic compound (such as an alkane) having an open chain structure