Chapter 14 - Alcohols

PHYSICAL properties

-they have a polar O-H bond because of the difference in electronegativity

-have weak London forces but very strong hydrogen bonding (O-H)

trend in SOLUBILITY

as the alkyl chain length increases, the solubility of alcohols decreases

-larger alcohols have long, non-polar alkyl chains as the influence of the -OH group decreases so they can no longer form hydrogen bonds with water
-so larger alcohols tend to be insoluble in water

trend in VOLATILITY

measure of how easy it is for a substance to evaporate

-alkanes are more volatile than alcohols
-alcohols have hydrogen bonding which is the strongest type whereas alkanes only have London forces which is the weakest type so alcohols have a higher boiling point

trend in BOILING POINT

as the carbon chain length increases, the boiling point increases
-as the surface area increases so more points of contact between adjacent molecules so the number and strength of London forces increases so more heat energy needed to overcome

classification of alcohols

-primary alcohol = 1 alkyl group

-secondary alcohol = 2 alkyl groups

-tertiary alcohol = 3 alkyl groups

COMBUSTION of alcohols

-complete: C2H5OH + 3O2 → 2CO2 + 3H2O

-incomplete: C3H7OH + 4.5O2 → 3CO + 4H2O

OXIDATION of alcohols -oxidising agent

-agent used = acidified potassium dichromate solution (VI) or K2Cr2O7/H2SO4

-changes colour when alcohol is oxidised
-dichromate ion (Cr2O7^2-) within potassium dichromate is orange
-is reduced to Cr³⁺ ion which is green
orange → green

[O] is used to represent

OXIDATION of alcohols -primary alcohols - partial

-can be partially oxidised to form aldehydes by oxidising agent

RCH2OH + [O] → RCHO + H2O
alcohol + oxidising agent → aldehyde + water

to prevent complete oxidation
-primary alcohol is in excess
-aldehyde is distilled off immediately
-needs gentle heating

OXIDATION of alcohols -primary alcohols - complete

-can be completely oxidised to form carboxylic acids by excess oxidising agent under reflux

RCH2OH + 2[O] → RCOOH + H2O
primary alcohol + oxidising agent → carboxylic acid + water

to ensure complete oxidation
-oxidising agent is in excess
-reaction mixture is heated under reflux
-needs strong heating

reflux

the constant boiling and condensing of a reaction mixture

-ensures that any aldehyde formed initially in reaction also undergoes oxidation to form carboxylic acid

OXIDATION of alcohols -secondary alcohols

-can be oxidised to form ketones by oxidising agent

R₁CH(OH)R₂ + [O] → R₁COR₂ + H2O
secondary alcohol + oxidising agent → ketone + water

reaction conditions
-needs strong heating
-mixture heated under reflux - no risk of further oxidation of ketone

OXIDATION of alcohols -tertiary alcohols

are resistant to oxidation

-TIB of the lack of hydrogen atoms on the carbon atom

DEHYDRATION of alcohols

-water is lost to form organic compound

-type of elimination reaction

-converts alcohols into alkenes

alcohol → alkene + water

reaction conditions
-acid catalyst = concentrated sulphuric/phosphoric acid
-needs to be heated under reflux

HALIDE SUBSTITUTION of alcohols

-converts alcohols into haloalkanes

1) hydrogen halide is made in situ: salt + acid → hydrogen halide
2) then used to react with alcohol: ROH + HX → RX + H2O, alcohol + hydrogen halide → haloalkane + water

overall equation:
ROH + NaX + H2SO4 → RX + NaHSO4 + H2O

HALIDE SUBSTITUTION of alcohols -simpler

-converts alcohols into haloalkanes

-reaction conditions:
-needs sodium halide NaX + H2SO4 acid
-then warm