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F324 and F325 Complete revision notes .pdf


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Topic 1 – Arenes
Revision Notes
1.

Structure of Benzene
a) History




Molecular formula is C6H6
Structure proposed by Kekulé had ring of carbons with alternating single and double
bonds (double bonds are shorter than single bonds)
There are problems with this structure. Firstly, all of the C-C bond lengths in benzene
are the same and are in between the length of a C-C and a C=C. Secondly, if
benzene contained double bonds it would undergo addition reactions (like alkenes).
However, benzene actually undergoes substitution rather than addition

b) Enthalpies of hydrogenation


The enthalpy of hydrogenation of cyclohexene is -120 kJ mol-1



If benzene had alternating double and single bonds, we would expect its enthalpy of
hydrogenation to be 3 x -120 = -360 kJ mol-1
However, its actual enthalpy of hydrogenation is only -208 kJ mol-1





Benzene is 152 kJ mol-1 lower in energy (more stable) than the hypothetical structure
containing alternating double and single bonds

Energy

c) Delocalisation of electrons






2.

The accepted structure for benzene is a planar (flat) ring of 6 carbon atoms, each of
which is also bonded to an H
Each carbon has a spare p-orbital. These overlap sideways to form -bonds (which
are rings of delocalised electrons, one above the plane and one below the plane).

Delocalisation of electrons gives benzene thermodynamic stability. (Stability means
lower in energy.)
Benzene undergoes substitution reactions rather than addition to maintain
delocalisation of electrons

Reactions of Arenes



a)

The high electron density in the -bonds make benzene attractive to electrophiles
(electron pair acceptors).
The mechanism for the following reactions of benzene is electrophilic substitution
Nitration





Equation
Reagents
Conditions




Generation of electrophile
Product is nitrobenzene



Mechanism:

b)

C6H6 + HNO3  C6H5NO2 + H2O
concentrated HNO3 and concentrated H2SO4 (the nitrating mixture)
60C
HNO3 + H2SO4  NO2+ + HSO4- + H2O

Halogenation



Equation
Reagents

C6H6 + Cl2  C6H5Cl + HCl
Cl2 and halogen carrier (Fe, FeCl3 or AlCl3)





Generation of electrophile
Cl2 + AlCl3  Cl+ + AlCl4Product is chlorobenzene, halogen carrier acts as a catalyst
NOTE – It works in exactly the same way for Br 2 with FeBr3 or AlBr3



c)

Mechanism:

Comparison with alkenes





3.

An alkene has a double bond, which means it reacts readily with electrophiles.
Benzene is less reactive with electrophiles because its delocalised electrons make it
more stable. The delocalised electrons are not easily disrupted so the activation
energy for benzene is higher than for an alkene.
Benzene is, therefore, more resistant to bromination than an alkene such as
cyclohexene and benzene needs a catalyst to polarise the halogen.

Properties of Phenol



Phenol is more reactive than benzene
Phenol is weak acid (proton donor). However, it is a stronger acid than ethanol
C6H5OH  C6H5O- + H+



The phenol functional group has antiseptic properties

4.

Reactions of Phenol

a)

Bromination
Phenol reacts with bromine to form 2,4,6-tribromophenol and HBr.
C6H5OH + 3Br2  C6H2Br3OH + 3HBr

Goes from orange to colourless and white precipitate formed
Bromination of phenol is easier than bromination of benzene:
 The OH activates the benzene ring
 The electron-pair from an oxygen p-orbital is donated to the benzene ring
 There is more electron density on the ring
 This attracts electrophiles more

Compared with benzene, phenol does not need a catalyst to react with bromine
Phenol also tri-substitutes whereas benzene mono-substitutes
b)

With sodium
Phenol reacts with sodium to form sodium phenoxide and hydrogen (effervescence
seen)
C6H5OH + Na  C6H5O-Na+ + ½H2

c)

With aqueous alkalis
Phenol reacts with sodium hydroxide to form sodium phenoxide and water .this is a
neutralisation reaction
C6H5OH + NaOH  C6H5O-Na+ + H2O

d)

Uses of phenols
Phenols are used in the production of antiseptics (like TCP, trichlorophenol),
disinfectants, plastics and resins for paints

5.

Naming Arenes
On a ring, the first substituent determines which carbon is numbered 1.

Methylbenzene

1,2-dimethylbenzene

3-chloromethylbenzene

Topic 2 – Carbonyl compounds
Revision Notes
1.

Introduction





2.

AS Recap




3.

Aldehydes and ketones are carbonyl compounds
They contain the carbonyl group C=O
The functional group in aldehydes is –CHO on the end of a chain e.g. ethanal
CH3CHO
The functional group in ketones is C=O not at the end of a chain e.g. propanone
CH3COCH3

Primary alcohols are oxidised by acidified potassium dichromate. An aldehyde is
produced first and this can be further oxidised to a carboxylic acid. To get the
aldehyde, it must be distilled off as it is formed. To get the acid, heat under reflux.
Secondary alcohols are oxidised to ketones by acidified K2Cr2O7. Colour change is
orange to green.
The C=O bond in aldehydes, ketones, carboxylic acids and esters can be identified by
infrared spectroscopy. It produces a large peak around 1700 cm-1.

Reduction of Aldehydes and Ketones
Reduction, here, means addition of hydrogen.
a)

Reduction using NaBH4

A specific reducing agent for aldehydes and ketones is sodium borohydride, NaBH 4.
In equations the reducing agent is represented by [H].
Aldehydes are reduced to primary alcohols by NaBH4 e.g.
CH3CHO + 2[H]  CH3CH2OH
Ketones are reduced to secondary alcohols by NaBH 4 e.g.
CH3COCH3 + 2[H]  CH3CH(OH)CH3
Other points to note about this reaction are:
 It is an addition reaction (there is only one product)
 The mechanism is called nucleophilic addition
 The nucleophile is H- which is provided by NaBH4

b)

Comparison with hydrogen gas

NaBH4 will reduce C=O double bonds but it will not reduce C=C double bonds
e.g.

CH2=CH-CHO + 2[H]  CH2=CH-CH2OH

To reduce both C=O and C=C use H2 with Ni catalyst
e.g.

4.

CH2=CH-CHO +2H2  CH3CH2CH2OH

Reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)




5.

Aldehydes and ketones react with 2,4-DNPH to give an orange precipitate (solid)
This is used a test for the presence of an aldehyde or ketone
The particular aldehyde or ketone can be identified by purifying (recrystallisation) the
precipitate, measuring its melting point and comparing this with the melting points of
known compounds (from a data book).

Reaction with Tollen’s Reagent




Tollen’s reagent is ammoniacal silver nitrate.
It is a mild oxidising agent that is used to distinguish between aldehydes and
ketones. The compound to be tested is warmed with Tollen’s reagent.
Aldehydes are oxidised by Tollen’s reagent which is reduced to silver metal
CH3CHO + [O] 
Ag+(aq) + e- 



CH3COOH
Oxidation
Ag(s)
Reduction
silver mirror on inside of test tube

Ketones do not react with Tollen’s reagent because they are not easily oxidised.

Topic 3 – Carboxylic acids and esters
Revision Notes
1.

Carboxylic acids



Carboxylic acids contain the functional group –COOH on the end of a chain.
They are weak acids (H+ donors). The acidic H is in the –COOH group e.g.
CH3COOH  CH3COO- + H+ (note – reversible reaction so  not )



They are soluble in water because they can hydrogen bond to water molecules
a)

Reactions of carboxylic acids

As they are acids they will react with metals, carbonates and bases e.g.
CH3COOH + Na 
Ethanoic acid

CH3COONa + ½H2
sodium ethanoate

Fizzing seen
Sodium dissolves

2CH3COOH + Na2CO3  2CH3COONa + H2O + CO2 Fizzing seen
Carbonate dissolves
CH3COOH + NaOH  CH3COONa + H2O

2.

Esters





Esters contain the functional group –COOR on the end of a chain
Making esters is called esterification
Esters can be made in two ways: carboxylic acid + alcohol or acid anhydride +
alcohol
Esters are sweet smelling and are used as flavourings and perfumes in food.
a)

Esterification of carboxylic acid with alcohol

Carboxylic acids react with alcohols to make and ester and water e.g.
CH3COOH + C2H5OH  CH3COOC2H5 + H2O
Ethanoic acid
ethyl ethanoate
Conditions:
b)


Reflux with concentrated H2SO4 (acts as a catalyst)

Esterification of acid anhydride with alcohol

Acid anhydrides can be thought of as 2 molecules of acid that have lost a molecule of
water e.g. propanoic anhydride, (CH3CH2CO)2O

(CH3CH2CO)2O + CH3OH 
Propanoic anhydride
c)


CH3CH2COOCH3 + CH3CH2COOH
methyl propanoate propanoic acid

Acid hydrolysis of esters

This is the reverse of esterification
CH3COOC2H5 + H2O  CH3COOH + C2H5OH
Ethyl ethanoate



For acid hydrolysis, heat the ester with a dilute acid such as HCl
d)



Alkaline hydrolysis

This is similar to acid hydrolysis but produces the carboxylate salt of acid rather than
acid itself. This is not reversible.
CH3COOC2H5 + NaOH 
Ethyl ethanoate



CH3COONa + C2H5OH
sodium ethanoate

For alkaline hydrolysis, heat the ester with dilute NaOH.

Topic 4 – Triglycerides, unsaturated and saturated fats
Revision Notes
1.

Triglycerides



Triglycerides are more commonly known as fats and oils
A triglyceride is a tri-ester of glycerol and 3 fatty acids e.g.
O
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

C

O

CH2

O

CH

O

CH2

O
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

C
O

CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2



C

A fatty acid is an unbranched, long chain carboxylic acid e.g. octadecanoic acid. The
shorthand formula for a fatty acid shows the number of carbons and the number and
position of any double bonds. Octadecanoic acid has 18 carbons and no double bonds
so its shorthand formula is 18, 0
O
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2 CH2

C

O

H

Source of these 2 diagrams - http://www.chemsheets.co.uk/



Glycerol is propane-1,2,3-triol



In a triglyceride the 3 fatty acids do not have to be the same

2.

Saturated and unsaturated fats





In a saturated fat there are no double bonds in the fatty acids from which the
triglyceride was formed
Unsaturated fats are formed from one or more fatty acids that contain a double bond
The systematic name for oleic acid, shown below, is octadec-9-enoic acid which
indicates that the double bond starts on carbon 9. The shorthand formula for this
acid is 18, 1(9)
Linoleic acid has 2 double bonds starting on carbons 9 and 12 so its systematic name
is octadec-9,12-dienoic acid and the shorthand formula is 18, 2(9,12)


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