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Bonding: Intermolecular Forces. Covalent Bonds Covalent bonding leads to two types of molecules . Giant Structures Eg . Diamond, sand Strong covalent bonds between atoms create a large network resulting in a high melting and boiling point. Simple Molecular Structures
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Covalent BondsCovalent bonding leads to two types of molecules Giant Structures Eg. Diamond, sand Strong covalent bonds between atoms create a large network resulting in a high melting and boiling point Simple Molecular Structures Eg. Methane, hydrogen chloride Few covalent bonds per molecule. Exist as gases or low boiling liquids therefore have low melting and boiling points.
How we know intermolecular forces exist Gases have large distances between molecules, short range forces between molecules are insignificant Cool and Compress Distances between molecules decrease and the forces between become significant. Gases liquefy (G->L) Cool and Compress Distances are further reduced. Forces become more significant. Liquid solidifies. (L->S)
Hydrogen Bonding Hδ+-Fδ---------------Hδ+-Fδ- • Strongest of the intermolecular forces • Affects the physical properties of the compounds
Effect of H bonding on Organic compounds- case 2 • Propanone is miscible in water but it has no H bonding between propanone molecules • This is because its O atoms can bond to water molecules.
Effect of H bonding on Organic compounds- case 3 • p- nitro phenol or 4- nitro phenol is having intermolecular H bonds , stronger forces of interaction between molecules and so its BP is 279°C • o- nitro phenol or 2- nitro phenol has intramolecular H bonds, weaker forces of interaction between molecules and so has lower BP of 216°C
Biological importance of H bonding • DNA pairing occurs due to H bonds. • Secondary structure of proteins (α helix andβ pleated proteins) • α helix-is an example of INTRA MOLECULAR H Bonding
Hydrogen bonds in DNA Base Pairs 2 H bonds between Adenine and Thymine 3 H bonds between Cytosine and Guanine
