AN INTRODUCTION TO THE CHEMISTRY OF ALCOHOLS

THE CHEMISTRY OF ALCOHOLS CONTENTS Structure of alcohols Nomenclature Isomerism Physical properties

THE CHEMISTRY OF ALCOHOLS Before you start it would be helpful to… Recall the definition of a covalent bond Recall the difference types of physical bonding Be able to balance simple equations Be able to write out structures for simple organic molecules Understand the IUPAC nomenclature rules for simple organic compounds Recall the chemical properties of alkanes and alkenes

CLASSIFICATION OF ALCOHOLS Aliphatic • general formula CnH2n+1OH - provided there are no rings • the OH replaces an H in a basic hydrocarbon skeleton

CLASSIFICATION OF ALCOHOLS Aliphatic • general formula CnH2n+1OH - provided there are no rings • the OH replaces an H in a basic hydrocarbon skeleton PRIMARY 1° SECONDARY 2° TERTIARY 3°

Alcohols are named according to standard IUPAC rules NAMING ALCOHOLS Alcohols are named according to standard IUPAC rules • select the longest chain of C atoms containing the O-H group; • remove the e and add ol after the basic name • number the chain starting from the end nearer the O-H group • the number is placed after the an and before the ol ... e.g butan-2-ol • as in alkanes, prefix with alkyl substituents • side chain positions are based on the number allocated to the O-H group e.g. CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3 is called 5-methylhexan-2-ol

STRUCTURAL ISOMERISM IN ALCOHOLS Different structures are possible due to... A Different positions for the OH group and B Branching of the carbon chain butan-1-ol butan-2-ol 2-methylpropan-2-ol 2-methylpropan-1-ol

BOILING POINTS OF ALCOHOLS Increases with molecular size due to increased van der Waals’ forces. Alcohols have higher boiling points than similar molecular mass alkanes This is due to the added presence of inter-molecular hydrogen bonding. More energy is required to separate the molecules. Mr bp / °C propane C3H8 44 -42 just van der Waals’ forces ethanol C2H5OH 46 +78 van der Waals’ forces + hydrogen bonding

BOILING POINTS OF ALCOHOLS Increases with molecular size due to increased van der Waals’ forces. Alcohols have higher boiling points than similar molecular mass alkanes This is due to the added presence of inter-molecular hydrogen bonding. More energy is required to separate the molecules. Mr bp / °C propane C3H8 44 -42 just van der Waals’ forces ethanol C2H5OH 46 +78 van der Waals’ forces + hydrogen bonding Boiling point is higher for “straight” chain isomers. bp / °C butan-1-ol CH3CH2CH2CH2OH 118 butan-2-ol CH3CH2CH(OH)CH3 100 2-methylpropan-2-ol (CH3)3COH 83 Greater branching = lower inter-molecular forces

BOILING POINTS OF ALCOHOLS Increases with molecular size due to increased van der Waals’ forces. Alcohols have higher boiling points than similar molecular mass alkanes This is due to the added presence of inter-molecular hydrogen bonding. More energy is required to separate the molecules. Mr bp / °C propane C3H8 44 -42 just van der Waals’ forces ethanol C2H5OH 46 +78 van der Waals’ forces + hydrogen bonding Boiling point is higher for “straight” chain isomers. bp / °C butan-1-ol CH3CH2CH2CH2OH 118 butan-2-ol CH3CH2CH(OH)CH3 100 2-methylpropan-2-ol (CH3)3COH 83 Greater branching = lower inter-molecular forces

SOLVENT PROPERTIES OF ALCOHOLS Solubility Low molecular mass alcohols are miscible with water Due to hydrogen bonding between the two molecules Heavier alcohols are less miscible Solvent properties Alcohols are themselves very good solvents They dissolve a large number of organic molecules