Chapter 14 - Alcohols Flashcards
Functional group of alcohols & use
-OH functional group, known as hydroxyl group
methanol used as high-performance fuel due to its efficient combustion
–> important chemical feedstock (starting material in many industrial syntheses)
Physical properties of alcohols
Compared to alkanes, alcohols are less volatile, higher melting point, greater water solubility than corresponding alkanes
polar as alcohols have polar O-H bond because of the difference in electronegativity (alcohols are polar)
Intermolecular forces: very weak London forces but much stronger hydrogen bonds
Volatility and boiling points of alcohols
In liquid state, intermolecular hydrogen bonds hold alcohol molecules together
–> bonds must be broken to change liquid to gas
–> requires more energy than overcoming weaker London forces in alkanes
–>: alcohols are less volatile than alkanes with same no. of C atoms
Solubility of alcohols in water
Since it can form H bonds, it is much more water-soluble
–> alcohols are polar and they can form hydrogen bonds between alcohol molecules and water molecules
–> the greater the hydrocarbon chain, the smaller the influence OH hydroxyl group has and solubility decreases
Primary alcohols
when the OH group is attached to a carbon with 2 hydrogen atoms and one alkyl group
Secondary alcohols
when the OH group is attached to a carbon with 1 hydrogen atoms and 2 alkyl group
Tertiary alcohols
when the OH group is attached to a carbon with 0 hydrogen atoms and 3 alkyl group
Combustion of alcohols
Alcohols burn completely in plentiful oxygen to produce CO2 and water
C2H5OH(l) + 3O2(g) –> 2CO2(g) + 3H2O (l)
exothermic and releases large quantities of energy as heat
Oxidation of alcohols
-can be oxidised by oxidising agent (solution of potassium dichromate VI K2CR2O7 acidified with dilute sulfuric acid H2S04 )
–> if alcohol is oxidised, orange solution of dichromate ions is reduced to green solution with chromium ions
Cr2O7 2- –> Cr3+
dichromate (VI) ions –> chromium (III) ions
Oxidation of primary alcohols
Primary alcohols can make either aldehydes or carboxylic acids
Preparation of aldehydes
gentle heating of primary alcohols with acidified potassium dichromate and is distilled out of reaction mixture (prevents any further reaction with oxidising agent
Preparation of carboxylic acids
strongly heating a primary alcohol under reflux + excess acidified potassium dichromate (VI)
–> excess ensures all alcohol is oxidised
Oxidation of secondary alcohols & tertiary
Alcohol –> ketone
heated under reflux with oxidising agent
Tertiary: no reaction because all carbons are bonded to other carbons
Dehydration of alcohols
alcohol –> alkene
any reaction in which a water molecule is removed
–> heat under reflux in presence of acid catalyst ( CONC. H3PO4 or H2SO4)
alcohol –> alkene + water
Substitution reactions of alcohols
NaBr (s) + H2SO4(aq) –> NaHSO4(aq) + HBr (aq)
–> heated under reflux with sulfuric acid and sodium halide (hydrogen bromide formed in situ)
Alcohol + hydrogen halide –> haloalkanes + water
