4.2.1 Alcohols
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what functional group do alcohols contain
• hydroxyl OH
what is the general formula of an alcohol
• CnH2n+1 OH
what are some general differences in physical properties between alcohols and alkanes with the same number of carbon atoms
• alcohols are less volatile, higher mp/bp and greater water solubility
what is the reason for differences in physical properties between alcohols + alkanes with same no of C
• alkanes have non-polar bonds as electronegativity of H and C very similiar > alkane = non polar so very weak London forces • alcohol has polar O-H bond because of electronegativity difference > alcohol = polar so have weak LF + stronger hydrogen bonds between polar O-H
why do alcohols have high mp + are less volatile
• hydrogen bonds between OH require lots of energy to overcome
are alcohols soluble in water + why
• soluble in water > hydrogen bonds form between the polar OH group of alcohol and water molecules
what happens to solubility as the hydrocarbon chain in alcohol increases
• solubility decreases > influence of the OH polarity decreases and solubility of longer chains decreasw
why are alcohols very good solvents
• can dissolve both polar and non-polar molecules
what are the 3 different ways alcohols can be classified as
• primary • secondary • tertiary
alcohols can undergo combustion to produce what
• CO2 + H2O > releases heat
which types of alcohols can be oxidised
• primary + secondary
what can alcohols be oxidised by
• an oxidising agent
what can primary alcohols be oxidised to
• aldehyde • carboxylic acid
how can you make an aldehyde from an alcohol
• oxidation of primary alcohol • heat + distill with acidified potassium dichromate (oxidising agent) > aldehyde formed
what happens to dichromate ions in oxidation of alcohols
• reduced from orange to green
how do you prevent oxidation of aldehyde into carboxylic acid
• aldehyde removed by distillation as its formed > aldehyde has has lower boiling point than alcohol
why do aldehydes have lower bp than alcohols
• because they don’t have hydrogen bonds
how can you form a carboxylic acid from an alcohol
• primary alcohol heated strongly under reflux with excess of acidified potassium dichromate
why do you need excess oxidising agent for formation of carboxylic acid
• to ensure that all of the alcohol is oxidised
what does a secondary alcohol oxidise to form
• ketones
how can you form a ketone from an alcohol
• secondary alcohol oxidised under reflux with acidified potassium dichromate
what can tertiary alcohols oxidise to from
• nothing • tertiary alcohols do not undergo oxidation reactions
what is dehydration
• a reaction in which a water molecule is removed from a starting material
how do alcohols undergo dehydration + what forms
• alcohol heated under reflux with acid catalyst e.g. conc sulfuric acid or phosphoric acid • product is an alkene