Properties of Alcohols (OCR A Level Chemistry)

Intermolecular Forces 

• Alcohols are compounds that contain at least one hydroxy (-OH) group
• The general formula of alcohols is C_nH_2n+1OH
• Alcohols can be prepared by a wide range of chemical reactions

Hydrogen bonding

• Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen
• In an alcohol, there are O-H bonds present in the structure
• Therefore hydrogen bonds set up between the slightly positive hydrogen atoms (δ+ H) and lone pairs on oxygens in other molecules
• The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms
• In alkanes, the only intermolecular forces are temporary induced dipole-dipole forces
• Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules
• Therefore, the boiling point of alkanes is lower than the boiling point of the respective alcohols
  • For example, the boiling point of propane is -42 ^oC and the boiling point of propanol is 97 ^oC

Temporary induced dipole-dipole forces

• As with all molecules, there are also temporary induced dipole-dipole forces between alcohols
• The boiling points increase as the number of carbon atoms in the chains increases. It takes more energy to overcome the temporary induced dipole-dipole forces and so the boiling points rise
• This is because the dipole is caused by the changing position of the electron cloud, so the more electrons there are, the larger the temporary induced dipole-dipole forces will be

Solubility

• The small alcohols are completely soluble in water
• If they are mixed with water, there will be one solution
• However, solubility falls as the length of the hydrocarbon chain in the alcohol increases
• The hydrocarbon chains are forcing their way between water molecules and so breaking hydrogen bonds between those water molecules
• The -OH end of the alcohol molecules can form new hydrogen bonds with water molecules, but the hydrocarbon can not form hydrogen bonds

• Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to one other carbon atom (or alkyl group). 
• Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups)
• Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)

Classifying primary, secondary and tertiary alcohols and alcohols with more than one alcohol group