2.10 Reactions of alcohols
c. describe the following chemistry of alcohols:
i. combustion
ii. reaction with sodium
iii. substitution reactions to form halogenoalkanes, including
reaction with PCl5 and its use as a qualitative test for the presence
of the –OH group
iv. oxidation using potassium dichromate (VI) in dilute sulfuric acid
on primary alcohols to produce aldehydes and carboxylic acids
and on secondary alcohols to produce ketones
Connector
Draw the following functional groups as displayed
formulae:
primary alcohol
ketone
secondary alcohol
carboxylic acid
aldehyde
Crowe2009
Physical properties of alcohols
• Alcohols - boiling points relatively high
compared with corresponding alkanes
(e.g. ethanol, 78oC ethane -88.5oC).
• Smaller alcohols miscible with water, unlike the
alkanes.
• Suggest the reason for these.
• Hydrogen bonding between the molecules.
Combustion of alcohols
Alcohols are highly flammable and may be used as fuels.
Write a balanced equation for the complete combustion
of ethanol.
C2H5OH (l) + 3O2 (g)
2CO2 (g) + 3H2O (g)
HOMEWORK - Bioethanol
How is it produced?
How is it used?
Discuss the advantages and disadvantages of its use
as a fuel.
Reactions of alcohols
1. acidified potassium dichromate(VI)
• primary alcohols give aldehydes
K2Cr2O7/H+
H
HEAT
CH3CH2CH2OH
CH3CH2C=O
[-2H]
propanal
• What type of reaction is this?
• Excess acidified K2Cr2O7 oxidises the aldehyde
to the corresponding carboxylic acid:
OH
CH3CH2C=O propanoic acid
• Acidified KMnO4 is too strong an oxidising agent
to produce propanal, propanoic acid is obtained.
Reactions of alcohols
1. acidified potassium dichromate(VI)(continued)
• Secondary alcohols give ketones:
K2Cr2O7/H+
HEAT
(CH3)2CHOH
(CH3)2C=O
[-2H]
propanone
• Tertiary alcohols do not react with K2Cr2O7/H+
Reactions of alcohols
2. Breaking the O-H bond
i) reaction with sodium
sodium + ethanol  sodium ethoxide + hydrogen
write a balanced symbol equation
2Na + 2C2H5OH  2NaOC2H5 + H2
ii) esterification
H+
propan-1-ol + ethanoic acid  propyl
+ water
ethanoate
Write structural formulae of the reactants and
products.
Reactions of alcohols
3. Nucleophilic substitution (breaking the C-O bond)
What is a nucleophile? What is substitution?
R-OH + X-  R-X + OHR-OH + PCl5  RCl + POCl3 + HCl *
( * Test for –OH )
What would be observed?
What further tests would you carry out?
Rapid exothermic reaction producing misty
fumes of HCl (g),
which turn moist blue litmus paper red.
Reactions of alcohols
3. Nucleophilic substitution (breaking the C-O bond)
(continued)
R-OH + X-

R-X + OH-
conc.H2SO4
R-OH + Br
R-Br + OHreflux
conc.H2SO4
R-OH + NaBr

R-Br + H2O
reflux
(NaBr or KBr and conc. H2SO4 heat under reflux)*
*A metal halide and concentrated sulphuric acid should not be used
when making a bromoalkane or an iodoalkane, because sulphuric
acid is an oxidising reagent and unwanted side reactions occur.
Reactions of alcohols
3. Nucleophilic substitution (breaking the C-O bond)
(continued)
R-OH + X
R-X + OHred phosphorus
R-OH + I-

R-I
+ OH-
reflux
red phosphorus
3R-OH + PI3

reflux
3R-I
+ H3PO3
(Red phosphorus and bromine or iodine,
heat under reflux.)
Reactions of alcohols
4. Dehydration reactions
heat/conc. H2SO4
alcohol
alkene
-H2O
Write the symbol equations for the dehydration
reactions of, butan-1-ol and butan-2-ol. Name
the three products.