AS Chemistry
Unit 5: Organic Chemistry II
Section 1: Alcohols
Part 1: Introduction
Task 1
Can you solve the anagrams and complete the gaps in the notes below using
your knowledge of IGCSE chemistry?
The alcohols consist of a veloltcany bonded hydroxyl group __________
attached to a carbon loseenkt. The general formula for this ooousmlhgo series
is:
The most widely used alcohol is __________, CH3CH2OH.
Ethanol production
Two main processes are currently used to make ethanol. The process chosen
depends on coocimen factors and on the end use of the ctdporu.
Fermentation
The process uses a sugar such as slugeco. In fermentation, the sugar is slowly
mopsodcede by __________ to produce ethanol and __________:
The enzymes are made by singroams called __________.
At low pratmusteera the reaction is __________. At high temperatures, the
yeast cannot revivus and the enzymes are __________. The process is normally
carried out at a temperature of __________oC.
When the concentration of ethanol reaches about __________%, the yeasts
and their enzymes no longer inofcnut and fermentation stops. __________ is
used to achieve higher concentrations of ethanol in the end product.
Direct hydration
The second process used to product ethanol is the direct hydration of
__________. __________ is currently available in huge quantities from the
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Unit 5: Organic Chemistry II
__________ of crude oil. __________ and __________ react together in the
presence of a __________ acid catalyst at __________K and __________
MPa pressure.
The overall equation is represented in the following chemical equation:
This latter method is currently favoured for the intoproudc of ethanol for
ilstrandui use. However, as this method uses ethene as a raw airmetal, it may
become less ropalup compared to fermentation when oil supplies begin to run
out.
Task 2
Complete the table below to compare the two methods
Method
Rate
reaction
of
Quality
product
of
Raw
material
hydration
ethene
fermentation
sugars
Type
process
of
Renewability
of
raw
material
Classifications and reactions
There are three main classes of alcohols: primary, secondary and tertiary. As
for the haloalkanes, this classification is based on the number of alkyl groups
(R) attached to the carbon atom bearing the –OH group.
Task 3
Draw structural formulae for each of the following substances and state
whether each is a 1o, 2o or 3o alcohol:
a.
b.
c.
butan-2-ol
2-methylbutan-2-ol
cyclohexanol
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AS Chemistry
Unit 5: Organic Chemistry II
Summarise your findings. What structural group do primary, secondary and
tertiary alcohols contain?

Primary

Secondary

Tertiary
Many reactions of the –OH functional group are the same in all alcohols (we will
look at this later), independent of where it is attached to the carbon skeleton.
However, the three types of alcohol differ in their reactions with mild oxidising
agents such as acidified potassium dichromate(VI), K2Cr2O7.
Oxidation of alcohols
Primary alcohols are oxidised first to aldehydes, such as ethanal. Aldehydes
contain the functional group RCOH.
Task 4
Use structural formulae to show the oxidation of ethanol to ethanal in the space
below. Show all of the bonds in the functional groups. You may use [O] to
represent the oxidising agent.
An aldehyde still has one hydrogen atom attached to the carbonyl carbon, so it
can be oxidised further to a carboxylic acid. You studied carboxylic acids at
IGCSE. As before, write a balanced equation to show this oxidation step.
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Unit 5: Organic Chemistry II
N.B. When using [O] to represent an oxidant you must still ensure the equation
balances.
The 1o alcohol, e.g. ethanol is dripped into a warm solution of the oxidising agent.
The aldehyde, e.g. ethanal, which forms, immediately distils off (due to its lower
boiling point), preventing further oxidation to the carboxylic acid.
Can you think of a reason why aldehydes have a lower boiling point than the
corresponding alcohol?
If oxidation of ethanol to ethanoic acid is required, the reagents must be
heated under reflux (to prevent the aldehyde evaporating).
2o alcohols are oxidised to ketones. This family contains the RCOR grouping.
Task 5
Can you write a balanced equation for the oxidation of propan-2-ol to propanone.
These have no hydrogen atoms attached to the carbonyl carbon and so cannot
be easily oxidised further.
3o alcohols are not easily oxidised.
When orange potassium dichromate (VI) acts as an oxidising agent, it is reduced
to green chromium (III) ions. This colour change can be used to distinguish 1o
and 2o alcohols from tertiary alcohols.
To distinguish between 1o and 2o alcohols, you must do tests on their respective
oxidation products (i.e. on the aldehydes and ketones).
References
A-level Chemistry pages 335-340
Chemistry in Context pages 466-468, 465-476
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AS Chemistry
Unit 5: Organic Chemistry II
Learning Objectives
Candidates should be able to:
 recall the chemistry of alcohols, as exemplified by ethanol, including their
oxidation to carbonyl compounds and carboxylic acids.
 classify hydroxy compounds into primary, secondary and tertiary alcohols.
 suggest characteristic distinguishing reactions, e.g. mild oxidation.
 describe the formation of carboxylic acids from alcohols and aldehydes.
Cambridge A-level Centre
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Unit 5: Organic Chemistry II
Section 1: Alcohols
Part 2: Physical properties and general chemistry
Task 1
Consider the table below.
Alcohol
ethanol
propan-1-ol
butan-1-ol
propane
butane
pentane
i)
Mr
46
60
74
44
58
72
Boiling point (K)
352
371
390
231
273
309
How do the boiling points of the alcohols compare with that of the
alkanes of similar Mr?
..........................................................................................................................................
ii)
Boiling point is determined by the strength of the intermolecular forces.
Can you use this information to explain any difference in the boiling
points of the alkanes and the alcohols.
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
iii)
Why do we consider compare molecules of similar Mr rather than those
containing the same number of carbon atoms?
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
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Unit 5: Organic Chemistry II
Solubility of alcohols in water
The small alcohols are completely soluble in water. Whatever proportions you
mix them in, you will get a single solution.
However, solubility falls as the length of the hydrocarbon chain in the alcohol
increases. Once you get to four carbons and beyond, the fall in solubility is
noticeable, and you may well end up with two layers in your test tube.
The solubility of the small alcohols in water
Consider ethanol as a typical small alcohol. In both pure water and pure ethanol
the main intermolecular attractions are hydrogen bonds.
In order to mix the two, you would have to break the hydrogen bonds between
the water molecules and the hydrogen bonds between the ethanol molecules. It
needs energy to do both of these things.
However, when the molecules are mixed, new hydrogen bonds are made between
water molecules and ethanol molecules.
The energy released when these new hydrogen bonds are made more or less
compensates for that needed to break the original ones.
Imagine what happens when you have got, say, 5 carbon atoms in each alcohol
molecule.
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AS Chemistry
Unit 5: Organic Chemistry II
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 "tail" doesn't form hydrogen bonds
That means that quite a lot of the original hydrogen bonds being broken aren't
replaced by new ones.
Some other chemical reactions of the alcohols
Combustion
The simplest way to oxidise alcohols is to burn them. Ethanol burns with a pale
blue flame.
Task 1
Can you write a balanced equation for the complete combustion of ethanol?
What could be produced in a limited supply of oxygen? Why is this a cause for
concern?
..................................................................................................................................................
.....................................................................................................................................................
The reaction between sodium and ethanol
Details of the reaction
If a small piece of sodium is dropped into some ethanol, it reacts steadily to
give off bubbles of hydrogen gas and leaves a colourless solution of sodium
ethoxide.
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AS Chemistry
Unit 5: Organic Chemistry II
Although at first sight you might think this was something new and complicated,
in fact it is analogous to the reaction between sodium and water - something you
have probably known about for years.
Task 2
Can you write balanced equations for the reactions between sodium and water,
and sodium and ethanol?
Can you think where this latter reaction may be useful?
.....................................................................................................................................................
.....................................................................................................................................................
Substitution to form halogenoalkanes
Halogenoalkanes are important intermediates in the formation of many other
compounds. They can be formed from alcohols using a variety of reactions.
Reactions involving hydrogen halides
The general reaction looks like this:
Reaction with hydrogen chloride
Tertiary alcohols react reasonably rapidly with concentrated hydrochloric acid,
but for primary or secondary alcohols the reaction rates are too slow for the
reaction to be of much importance. A better method for preparing the
chloroalkanes is given later.
Replacing -OH by bromine
Rather than using hydrobromic acid, you usually treat the alcohol with a mixture
of sodium or potassium bromide and concentrated sulphuric acid. This produces
Cambridge A-level Centre
AS Chemistry
Unit 5: Organic Chemistry II
hydrogen bromide in situ which reacts with the alcohol. The mixture is warmed
to distil off the bromoalkane.
Replacing -OH by iodine
In this case the alcohol is reacted with a mixture of sodium or potassium iodide
and concentrated phosphoric(V) acid, H3PO4, and the iodoalkane is distilled off.
Task 3
Can you think why phosphoric(V) acid is used instead of concentrated sulphuric
acid?
Iodoalkanes can also be made by warming an alcohol with mixture of red
phosphorus and iodine:
This then reacts with the alcohol to give the corresponding halogenoalkane
which can be distilled off.
Chloroalkanes can be made by reacting alcohols with phosphorus(III) chloride,
PCl3, phosphorus(V) chloride, PCl5, or sulphur dichloride oxide (thionyl chloride,
SOCl2).
Reaction with phosphorus(III) chloride, PCl3
Alcohols react with liquid phosphorus(III) chloride to make chloroalkanes.
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Unit 5: Organic Chemistry II
Reaction with phosphorus(V) chloride, PCl5
Solid phosphorus(V) chloride reacts violently with alcohols at room temperature,
producing clouds of hydrogen chloride gas.
Reaction with sulphur dichloride oxide (thionyl chloride)
Sulphur dichloride oxide (thionyl chloride) has the formula SOCl2.
The sulphur dichloride oxide reacts with alcohols at room temperature to
produce a chloroalkane. Sulphur dioxide and hydrogen chloride are given off.
Care would have to be taken because both of these are poisonous. However,
because they are gases they are easy to separate from the halogenoalkane
product.
Elimination (when a small molecule is removed from a larger molecule – converts
a single bond to a double bond)
Some alcohols can be dehydrated to alkenes. This is an elimination reaction. The
conditions are 170oC in the presence of excess c.H2SO4 (or c. phosphoric acid),
or by passing the alcohol vapour over a hot, porous ceramic surface. N.B. the
acid must be in excess to avoid formation of an ether.
Task 3
Can you write a balanced equation for the dehydration of propan-2-ol to
propene?
References
A-level Chemistry pages 335-340
Chemistry in Context pages 468, 472-474
Cambridge A-level Centre
AS Chemistry
Unit 5: Organic Chemistry II
Learning Objectives
Candidates should be able to:
recall the chemistry of alcohols, exemplified by ethanol:
• combustion
• substitution to give halogenoalkanes
• reaction with sodium
• dehydration to alkenes
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AS Chemistry
Unit 5: Organic Chemistry II
Section 2: Aldehydes and Ketones
You can differentiate between an aldehyde and a ketone by testing how easily
they are oxidised. Any such test must involve the use of mild oxidising agents
(to prevent side reactions such as the breaking of C-C bonds). You should know
two such reagents:

Tollen’s reagent – this contains the complex [Ag(NH3)2]+. It is made in
the reaction between silver nitrate solution and an excess of aqueous
ammonia.
With gentle warming a silver mirror is formed in the presence of an
aldehyde.

Fehling’s solution – this contains a deep blue copper complex. On warming
this is reduced by aldehydes to form a red precipitate of Cu2O.
N.B. these reagents are not affected by ketones! The aldehydes are oxidised to
carboxylic acids.
Reduction of carbonyl compounds
Aldehydes and ketones can be reduced to 1o and 2o alcohols respectively. These
reactions are the reverse of the oxidation reactions outlined on the previous
page. This does not take place readily. There are two common methods employed
(for CIE the focus is on method 2):

Catalytic hydrogenation
This involves reacting the carbonyl compound with hydrogen in the
presence of a nickel or platinum catalyst. This is an equivalent reaction to
that involved in the hydrogenation of alkenes. If a molecule contains both
types of double bond then it will become completely saturated when
catalytically hydrogenated.
Task 1
Can you draw and name the reactants and products of the following reduction
reactions.
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
Unit 5: Organic Chemistry II
butanone

prop-2-enal

Reaction with sodium tetrahydridoborate (III) (NaBH4) in methanol
NaBH4 is a source of the hydride ion, H-, and a powerful reducing agent. This
reaction proceeds via nucleophilic attack on the carbonyl group. The
intermediate ion which is formed produces the appropriate alcohol on
acidification. No reaction occurs with alkenes which are not susceptible to
nucleophilic attack.
Task 2
Can you draw and name the reactants and products of the following reactions.
You can represent the reducing agent by [H].
ethanal

propanone

prop-2-enal

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Unit 5: Organic Chemistry II
Formation of nitriles
The elements of HCN add to the carbonyl group of aldehydes and ketones to
yield compounds known as hydroxynitriles (or cyanohydrins). These contain the
following functional group:
Addition appears to involve nucleophilic attack on carbonyl carbon by the
strongly basic cyanide ion; subsequently (or possibly simultaneously) oxygen
accepts a hydrogen ion from a HCN molecule (or a molecule of water) to form
the cyanohydrin product.
Task 3
Can you draw the mechanism for the nucleophilic addition of hydrogen cyanide
to propanal.
The reaction is catalysed by the presence of CN- from the addition of a small
amount of NaCN. The cyanide ion is a stronger nucleophile than hydrogen
cyanide.
Reactions of this type are important because they introduce another carbon
atom onto the carbon chain.
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Unit 5: Organic Chemistry II
Task 4
Can you think of another type of reaction which introduces the formation of a
new carbon-carbon bond? Write an equation for such a reaction.
All nitriles, R-C≡N, undergo hydrolysis to form carboxylic acids. This is achieved
by refluxing with aqueous acid or aqueous alkali. Alternatively, reduction of the
nitrile group produces an amine.
Task 5
Can you write equations for the hydrolysis and reduction of ethanenitrile in the
space below?
Reduction is carried out using sodium and ethanol.
Testing for aldehydes and ketones
Test:
Result:
Add a solution of 2,4-dinitrophenylhydrazine (2,4-DNPH).
a deep yellow or orange precipitate
Equation: Please copy from page 354 of ‘AS level Chemistry’.
Cambridge A-level Centre
AS Chemistry
Unit 5: Organic Chemistry II
References
A-level Chemistry pages 350-354
Chemistry in Context pages 482-489
Learning Objectives
Candidates should be able to:





describe the reduction of aldehydes and ketones e.g. using NaBH4.
describe the mechanism of the nucleophilic addition reactions of
hydrogen cyanide with aldehydes and ketones.
describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect the
presence of carbonyl compounds.
deduce the nature (aldehyde or ketone) of an unknown carbonyl compound
from the result of simple tests (i.e. Fehling’s or Tollens’ reagents; ease of
oxidation)
describe the formation of carboxylic acids from nitriles.
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Unit 5: Organic Chemistry II
Section 3: Carboxylic acids
Carboxylic acids are compounds which contain a -COOH group.
Task 1
Can you draw structural formulae for the following carboxylic acids: methanoic
acid, 2-methylbutanoic acid and hexanedioic acid?
The name counts the total number of carbon atoms in the longest chain including the one in the -COOH group. Notice that, if you have side groups
attached to the chain, you always count from the carbon atom in the -COOH
group as being number 1.
N.B. Carboxylic acids are made in the lab by the oxidation of primary alcohols or
aldehydes, and by the hydrolysis of nitriles.
The acidity of the carboxylic acids
Task 2
Use your knowledge of acid/base equilibria to complete the notes below.
Why are carboxylic acids acidic?
Using the definition of an acid as a "_________________", the carboxylic
acids are acidic because of the hydrogen in the -COOH group.
In solution in water, a hydrogen ion is transferred from the -COOH group to a
_________________. For example, with ethanoic acid, you get an ethanoate
ion formed together with a ________________ion, H3O+.
This reaction is _______________ and, in the case of ethanoic acid, no more
than about 1% of the acid has reacted to form ions at any one time. (This is a
rough-and-ready figure and varies with the concentration of the solution.)
Cambridge A-level Centre
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Unit 5: Organic Chemistry II
They are therefore _______________ acids.
The dissociation of ethanoic acid in water can be represented by the equation
given below?
The pH of carboxylic acid solutions
The pH depends on both the __________ of the acid and how easily it
_________ hydrogen ions from the -COOH group.
Ethanoic acid is typical of the acids where the -COOH group is attached to a
simple alkyl group. Typical lab solutions have pHs in the __________ range,
depending on their concentrations.
Reactions of the carboxylic acids
With metals
Carboxylic acids react with the __________ reactive metals to produce a
__________ and __________. The reactions are just the same as with acids
like hydrochloric acid, except they tend to be rather __________.
For example, dilute ethanoic acid reacts with __________. The magnesium
reacts to produce a colourless solution of magnesium __________, and
hydrogen is given off. If you use magnesium ribbon, the reaction is less
__________ than the same reaction with hydrochloric acid, but with
magnesium __________, both are so fast that you probably wouldn't notice
much difference.
The reaction of magnesium with ethanoic acid can be represented by the
equation below.
With metal hydroxides
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Unit 5: Organic Chemistry II
These are simple __________ reactions and are just the same as any other
reaction in which __________ ions from an acid react with __________ ions.
They are most quickly and easily represented by the equation:
The reaction of ethanoic acid with sodium hydroxide can be represented by the
equation below.
With carbonates and hydrogencarbonates
In both of these cases, a salt is formed together with carbon dioxide and
water.
The following ionic equations represent the two general reactions outlined
above.
References
A-level Chemistry pages 357-359
Chemistry in Context pages 498-501
Learning Objectives
Candidates should be able to:

describe the reactions of carboxylic acids in the formation of salts.
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Unit 5: Organic Chemistry II
Section 4: Esters
Esters have a hydrocarbon group of some sort replacing the hydrogen in the COOH group of a carboxylic acid. We shall just be looking at cases where it is
replaced by an alkyl group, but it could equally well be an aryl group (one based
on a benzene ring).
A common ester - ethyl ethanoate
The most commonly discussed ester is ethyl ethanoate. In this case, the
hydrogen in the -COOH group has been replaced by an ethyl group. The formula
for ethyl ethanoate is:
Notice that the ester is named the opposite way around from the way the
formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl"
bit comes from the ethyl group on the end.
Esters
Use the information in your textbooks and your scientific knowledge to answer
the questions below.
1.
Which functional group do all esters contain? Draw the displayed formula
in the space below.
2.
Esters are made by the reaction between which two families of
compounds?
……………………………………………………………………………………….................................................
……………………………………………………………………………………….................................................
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3.
Unit 5: Organic Chemistry II
Which term is used to describe this type of reaction
.......................................................................................................................................
4.
The name of an ester comes partly from the parent alcohol and partly
from the parent acid. The alcohol part of the name is placed first and is
separated by a space before the acid part of the name. Can you draw full
structural formulae for each of the following esters?
(a) propyl ethanoate
(b) ethyl propanoate
(c) methyl butanoate
(d) butyl methanoate
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5.
Unit 5: Organic Chemistry II
The four compounds above have something in common. What word could
be used to describe them?
……………………………………………………………………………………….................................................
6.
Using full structural formulae, write a balanced equation for the reaction
between ethanol and ethanoic acid.
7.
What substance is used as a catalyst in this reaction?
..........................................................................................................................................
8.
What happens if some ethyl ethanoate is heated with water containing
sulphuric acid?
……………………………………………………………………………………….................................................
……………………………………………………………………………………….................................................
9.
What term is used for this type of reaction?......................................................
10.
Why is alkaline hydrolysis usually preferred over acid hydrolysis?................
……………………………………………………………………………………….................................................
11.
Using full structural formulae, write a balanced equation for the
hydrolysis of ethyl ethanoate in alkaline conditions.
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Unit 5: Organic Chemistry II
12.
Fats and oils are esters of which alcohol?.............................................................
13.
What is saponification?..............................................................................................
……………………………………………………………………………………….................................................
……………………………………………………………………………………….................................................
……………………………………………………………………………………….................................................
……………………………………………………………………………………….................................................
13.
Esters are widely used in industry. Can you list 4 uses of esters below?
(a)………………………………………………………………………………..............................................
(b)………………………………………………………………………………..............................................
(c)………………………………………………………………………………..............................................
(d)………………………………………………………………………………..............................................
References
A-level Chemistry pages 360-363
Chemistry in Context pages 505-510
Learning Objectives
Candidates should be able to:



describe the formation of esters from carboxylic acids using ethyl
ethanoate as an example.
describe the acid and base hydrolysis of esters.
state the commercial use of esters, e.g. solvents; perfumes; flavourings.
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