Upload
jeffydaniel1972
View
94
Download
2
Tags:
Embed Size (px)
DESCRIPTION
A very concise study material for Edexcel A2 Level. Few related past paper questions were incorporated for practice.
Citation preview
VIHS/DEPARTMENT OF CHEMISTRY Page 1
5.3 ARENES: BENZENE 2014
Syllabus specification
Arenes: benzene
a. use thermochemical, x-ray diffraction and infrared data as evidence for the structure and
stability of the benzene ring.
Students may represent the structure of benzene as
or
as appropriate in equations and mechanisms
b. describe the following reactions of benzene, limited to:
i) combustion to form a smoky flame.
treatment with:
ii) bromine.
iii) concentrated nitric and sulfuric acids.
iv) fuming sulfuric acid.
v) halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts
reaction).
vi) addition reactions with hydrogen.
c. describe the mechanism of the electrophilic substitution reactions of benzene in
halogenation, nitration and Friedel-Crafts reactions including the formation of the
electrophile.
d. carry out the reactions in 5.4.1b where appropriate (using methylbenzene or
methoxybenzene).
e. carry out the reaction of phenol with bromine water and dilute nitric acid and use these
results to illustrate the activation of the benzene ring.
Introduction:
Arenes are hydrocarbons with a ring or rings of carbon atoms in which there are delocalised
electrons. Benzene, the simplest arene with a molecular formula C6H6, is an important and
useful chemical which is obtained by the catalytic reforming of fractions from crude oil.
Arenes are sometimes called aromatic compounds.
Study of the structure of benzene is an another example that shows how scientific models
develop in response to new evidence. This links to further investigations of the models that
chemists use to describe the mechanisms of organic reactions.
VIHS/DEPARTMENT OF CHEMISTRY Page 2
5.3 ARENES: BENZENE 2014
General properties of benzene
It is a Colourless liquid with a characteristic odour.
Boils at 80oC and freezes at 6oC.
Immiscible with water but soluble in organic solvent.
Gives smoky luminous sooty flame on burning.
Structure of benzene:
Benzene, C6H6, is a cyclic compound that has six carbon atoms in a hexagonal ring. Several
structures for benzene have been proposed. Early theories suggested that there were
alternative single and double bonds between the carbon atoms(fig 5.3.1), but this did not fit
with later experimental evidence. It was shown that all the carbon-carbon bonds are the
same length and that the molecule is planar.
Two modern theories are used to explain the structure.
The Kekule’ version assumes that benzene is a resonance hybrid between
the two structures as given below. This model can be used to explain many
chemical properties and reaction of benzene.
Fig 5.3.2 The displayed formula of kekule’s benzene ring structure
The other theory assumes that each sp2 hybridized carbon atom is joined by
a σ- (sigma) bond to each of its two neighbours, and by a third σ- sigma bond
to s-orbital of hydrogen atom forming a hexagonal planar ring. The fourth
bonding electron is in p-orbital(called as non-hybrid p—orbital) in the right
angle to the planar of σ- (sigma) bonds. This p-orbital overlap side way, and
the six p-orbitals overlap above and below the plane of the ring of carbon
atoms. This produces a delocalised π-(pi)bonding system of electrons, as in:
Fig. 5.3.1 Simplified structure of benzene
VIHS/DEPARTMENT OF CHEMISTRY Page 3
5.3 ARENES: BENZENE 2014
Fig. 5.3.3 The delocalisation of the electrons in the π-bonds of the
symmetrical six-membered ring structure of benzene
Evidences for structure and extra stability of benzene
(i) Thermochemical evidence: via enthalpy of hydrogenation.
Benzene is more stable than ‘cyclohexatriene’, which is the theoretical compound
with three single and three localised double carbon-carbon bonds. The amount by
which it is stabilised can be calculated from the enthalpies of hydrogenation.
For example, the enthalpy of hydrogenation of one mole cyclohexene is -120 kJ.
+ H2(g) ∆H = -120 kJ mol—1
Cyclohexene Cyclohexane
Therefore , ∆H for the addition to three localised double bonds in ‘cyclohexatriene’
would be 3 x (-120) = -360 kJ mol—1. However for benzene:
+ 3H2(g) ∆H = -208kJ mol—1
Benzene cyclohexane
Thus,152 kJ less energy is given out because of benzene’s unique structure. This is
called the delocalisation stabilisation energy or resonance energy and can be
shown in an enthalpy-level diagram.
VIHS/DEPARTMENT OF CHEMISTRY Page 4
5.3 ARENES: BENZENE 2014
‘Cyclohexatriene’
∆H = -360 kJ mol-1 ∆H = -152 kJ (resonance energy)
Benzene
∆H = -208 kJ mol-1
Cyclohexane, C6H12
Fig.5.3.4 Enthalpy-level diagram for the hydrogenation of benzene and cyclohexatriene.
Thermo-chemical evidence: via bond enthalpies
The amount by which benzene is stabilised can also be calculated from average
bond enthalpies. The enthalpy of formation of gaseous benzene is +83 kJ mol-1.
The value for the theoretical molecule ‘cyclohexatriene’ can be found using the
Hess’s law cycle below:
6C(s) + 3H2(g) C6H6(g)
6C(g) + 3H2(g) 6C(g) + 6H(g)
Step 1 equals 6 x enthalpy of atomisation of carbon(∆Hatm[C(s)]) = 6 x (+715)
= +4290 kJ
Step 2 equals 3 x H—H bond enthalpy = 3 x (+436) = + 1308kJ
Step 3 equals enthalpy change of bonds made, which is calculated as below
Three C—C = 3 x (-348) = -1044 kJ
Three C=C = 3 X (-612) = -1836 kJ
Six C—H = 6 x (-412) = -2472 kJ
Total = - 5362 kJ
Enthalpy
kJmol-1
∆Hf
Step 1
Step 2
Step 3
VIHS/DEPARTMENT OF CHEMISTRY Page 5
5.3 ARENES: BENZENE 2014
Hence the ∆Hf of ‘cyclohexatriene’ = ∆Hstep 1 + ∆Hstep 2 + ∆Hstep 3
= +4290 + 1308 +(-5352)
= +246 kJ mol-1.
The actual enthalpy of formation of gaseous benzene is +83 kJ mol-1. The value
calculated above is 163 kJ more and approximately equals the resonance energy of
benzene. Hence, the structure with the delocalised electron system is energetically
more stable.
X-ray diffraction evidence
X-ray diffraction shows the position of the centre of atoms. If the diffraction pattern of
benzene is analysed, it clearly shows that all the bond lengths between the carbon
atoms are the same. Which is not the same in the case of cyclohexene.
Table 1. comparison of bond length in benzene and cyclohexene.
Bond Bond length/nm
All the six carbon-carbon bonds in
benzene
0.140
Carbon-carbon single bond in
cyclohexene
0.154
Carbon-carbon double bond in
cyclohexene
0.134
Fig.5.3.5 Electron density map of
benzene.
Electrons are equally distributed
over six carbon atoms due to
delocalisation of the pi- bonding
electron system.
If benzene has cyclohexatriene
structure, equal distribution of
electrons cannot be seen on the
carbon ring.
Thus, benzene is thermodynamically
more stable due to its delocalized pi-
bonding system.
0.140 nm
VIHS/DEPARTMENT OF CHEMISTRY Page 6
5.3 ARENES: BENZENE 2014
Infra red evidence:
Comparison of the infrared spectrum of aromatic compounds with those of aliphatic
compounds containing a C=C group showed slight differences. The C—H stretching
vibration in benzene is at 3036cm-1 and the C=C stretching is at 1479cm-1, whereas
the equivalent vibrations in an aliphatic compound such as cyclohexene are at 3023
and 1438cm-1.
Naming benzene derivatives.
The derivatives of benzene are named either as substituted products of benzene or
as compounds containing the phenyl group, C6H5—. The names and structures of
some derivatives of benzene are given below.
Systematic name Substituent group Structure
Chlorobenzene Chloro, -Cl C6H5-Cl
Nitrobenzene Nitro, -NO2 C6H5-NO2
Methylbenzene Methyl,-CH3 C6H5-CH3
Phenol Hydroxyl, -OH C6H5-OH
Phenylamine Amine, -NH2 C6H5-NH2
Phenylethanone Ethanone,-COCH3 C6H5-COCH3
Phenylmethanol Methanol,-CH2OH C6H5-CH2OH
When more than one hydrogen atom is substituted, numbers are used to indicate the
positions of substituent on the benzene ring. The ring is usually numbered clockwise
and the numbers used are the lowest ones possible. In some cases, the ring is
numbered anticlockwise to get the lowest possible numbers.
Fig. 5.3.6 IR spectra for (a)
cyclohexzene and (b) benzene.
VIHS/DEPARTMENT OF CHEMISTRY Page 7
5.3 ARENES: BENZENE 2014
In phenyl compounds, such as phenol and phenylamine, the –OH and –NH2, groups
are assumed to occupy the 1 position.
Fig. 5.3.7 Naming substituted benzene compounds.
Reactions of benzene
(i) combustion:
Benzene burns in a limited amount of air with a smoky flame. Combustion is
incomplete and particles of carbon are formed.
The complete combustion of benzene requires large volume of oxygen.
2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O(l)
(ii) Addition:
The double bond in benzene is not as susceptible to addition as is the double bond in
alkenes. However, it does react with hydrogen in the presence of a hot nickel catalyst
to form cyclohexane.
+ 3H2
Benzene cyclohexane
Reagents: Hydrogen gas.
Conditions: In the presence of Raney nickel(finely divided with a very large surface
area and very high catalytic activity)catalyst at high temperature(about 150oC).
Reaction type: Electrophilic addition.
VIHS/DEPARTMENT OF CHEMISTRY Page 8
5.3 ARENES: BENZENE 2014
Electrophilic substitution:
(iv) Halogenation
Dry benzene reacts with chlorine gas in the presence of iron (or a catalyst of
anhydrous iron(III) chloride). Steamy fumes of hydrogen chloride are given off and
chlorobenzene(C6H5Cl) is formed.
Cl
+ Cl2(g) + HCl(g)
Benzene chlorobenzene
Reagents: Chlorine gas.
Conditions: Room temperature and pressure, in the presence of anhydrous FeCl3.
Reaction type: Electrophilic substitution.
Mechanism: Heterolytic electrophilic substitution.
The mechanism for this reaction is as follows.
Step 1: The catalyst, anhydrous iron(III) chloride , is made by the reaction of iron with
chlorine
Fe + 1½ Cl2 FeCl3
This reacts with more chlorine, forming the electrophile Cl+
Cl2 + FeCl3 Cl+ + [FeCl4]—
electrophile
Step 2: The Cl+ attacks the π-electrons in the benzene ring, forming an intermediate
with a positive charge. Finally, the [FeCl4]— ion removes an H+ ion from benzene,
producing chlorobenzene(C6H5Cl) and reforming the catalyst(FeCl3)
H+ + [FeCl4]— HCl + FeCl3
VIHS/DEPARTMENT OF CHEMISTRY Page 9
5.3 ARENES: BENZENE 2014
Reaction with nitric acid: Nitration.
When benzene is warmed with a mixture of concentrated nitric and sulfuric acid, a
nitro-group(NO2) replaces a hydrogen atom in the benzene ring. Nitrobenzene and
water are produced.
NO2
+ HNO3(conc.) + H2O
Benzene nitrobenzene
Reagents: A mixture of Conc.H2SO4 and Conc.HNO3(nitrating mixture)
Conditions: Warm under reflux at 50oC.
Reaction Type: Electrophilic substitution.
Mechanism: Heterolytic electrophilic substitution.
The mechanism for this reaction is as follows.
Step 1:The sulfuric acid reacts with the nitric acid to form the electrophile NO2+. The
temperature must not go above 50oC or some dinitrobenzene(C6H4(NO2)2) is formed.
2H2SO4 + HNO3 2HSO4¯ + H3O+ + NO2
+
Acid base electrophile
Step 2: The NO2+ attacks the π-electrons in the benzene ring, forming an
intermediate with a positive charge. Finally, the HSO4— ion removes an H+ ion from
benzene, producing nitrobenzene(C6H5NO2) and reforming the catalyst(H2SO4).
Note: The addition of Cl+ to benzene is similar to the first step of the addition of
chlorine to ethene. The difference arises at the next step. The benzene
intermediate loses an H+, thus regaining the stability of the delocalised system,
whereas the intermediate with ethene adds Cl—ion.
A catalyst must be present for the addition of Cl+ to benzene, because the
activation energy of the first step is higher than that for the addition to ethene.
Conc.H2SO4
50oC
VIHS/DEPARTMENT OF CHEMISTRY Page 10
5.3 ARENES: BENZENE 2014
HSO4— + H+ H2SO4
Role of H2SO4
Acts as a catalyst, as it increases the rate of reaction and remains chemically
unchanged as it is being regenerated at the end of the reaction.
Acts as an acid(proton donor), as it donates protons in the reaction.
Role of HNO3
It generates nitronium ion,NO2+, which acts as an electrophile in the
mechanism.
It acts as a base by accepting protons.
Exercise
(01) Benzene prefers to undergo substitution reaction rather than addition reactions.
Explain.
(02) In the nitration of benzene sulphuric acid acts as an acid whereas nitric acid acts
as a base. Show by an equation how this is so.
VIHS/DEPARTMENT OF CHEMISTRY Page 11
5.3 ARENES: BENZENE 2014
(03) Why Raney nickel is used in the manufacture of cyclohexane from benzene?
(04) Explain why smoky flame are seen during the combustion of benzene.
(05) Write an equation for the bromination of benzene. By using appropriate arrow
draw the mechanism of this reaction.
VIHS/DEPARTMENT OF CHEMISTRY Page 12
5.3 ARENES: BENZENE 2014
Reaction with fuming sulphuric acid: Sulfonation.
When benzene is warmed with fuming sulfuric acid, benzenesulfonic acid is
produced. Fuming sulphuric acid is a solution of sulphur trioxide in sulphuric acid.
The electrophile is the SO3 molecule.
SO3H
+ SO3
Benzene benzenesulfonic acid
Reagents: fuming sulphuric acid
Conditions: Heat under reflux
Reaction Type: Electrophilic substitution.
Mechanism: Heterolytic electrophilic substitution.
The mechanism for this reaction is as follows.
Step 1
Step 2
This reaction is important in the manufacture of detergents, where a substituted
benzene ring is sulfonated and the final product is neutralised.
Friedel-Crafts reaction: (i) Reaction with halogenoalaknes
In the presence of an anhydrous aluminium chloride catalyst, alkyl group(eg C2H5)
can be substituted into the ring.
VIHS/DEPARTMENT OF CHEMISTRY Page 13
5.3 ARENES: BENZENE 2014
For example, In the reaction between benzene and chloroethane, the products are
ethylbenzene and hydrogen chloride.
C2H5
+ C2H5 Cl + HCl
benzene ethylbenzene
Reagents: Halogenoalkanes
Conditions: Heat under reflux at 50oC, in the presence of anhydrous AlCl3 as a
catalyst.
Reaction Type: Electrophilic substitution.
Mechanism: Heterolytic electrophilic substitution.
Note:The reaction mixture must be dry.
The mechanism for this reaction is as follows.
Step 1: The electrophile, +CH2CH3, is produced by the reaction of the catalyst with
the halogenoalkane:
CH3CH2Cl + AlCl3 +CH2CH3 + [AlCl4]
—
Chloroethane electrophile
Step 2: The positive carbon atom attacks the π–system in the benzene ring:
Step 3: The intermediate loses a H+ ion so as to regain the stability of the benzene
ring.
Finally, the catalyst is regenerated by the reaction:
H+ + [AlCl4]— HCl + AlCl3.
VIHS/DEPARTMENT OF CHEMISTRY Page 14
5.3 ARENES: BENZENE 2014
In this reaction, a catalyst is used to increase the positive nature of the electrophile
and make it better at attacking benzene rings. AlCl3 acts as a Lewis Acid and helps
break the C—Cl bond.
Friedel-Crafts reaction: (ii) Reaction with acyl(acid) chlorides.
In the presence of an anhydrous aluminium chloride catalyst, benzene reacts with
acylchlorides to form ketones.
For example, In the reaction between benzene and ethanoyl chloride, the products
are phenylethanone and hydrogen chloride.
COCH3
+ CH3COCl + HCl
Benzene phenylethanone
Reagents: Acyl(acid) chlorides
Conditions: Heat under reflux at 50oC, in the presence of anhydrous AlCl3 as a
catalyst.
Reaction Type: Electrophilic substitution.
Mechanism: Heterolytic electrophilic substitution.
The mechanism for this reaction is as follows.
Step 1: The electrophile, CH3C+O is produced by the reaction of the acylchloride with
the catalyst:
CH3COCl + AlCl3 CH3C+O + [AlCl4]
—
ethanoyl chloride electrophile
Step 2: The positive carbon atom attacks the π-system in the benzene ring.
Step 3: The intermediate loses a H+ ion so as to regain the stability of the benzene
ring.
VIHS/DEPARTMENT OF CHEMISTRY Page 15
5.3 ARENES: BENZENE 2014
Finally, the catalyst is regenerated by the reaction:
H+ + [AlCl4]— HCl + AlCl3.
Phenol
Phenol(C6H5OH) contains an –OH group on a benzene ring. A lone pair of electron
on the oxygen atom becomes part of the delocalised π-system and makes phenol
much more susceptible to attack by electrophiles.
simple structure of phenol.
Fig. 5.3.8 Orbital structure of phenol.
Properties of phenol.
Phenol is less acidic than carboxylic acid but more acidic than alcohol(-COOH >
phenol > -OH). Therefore it can easily loses a proton and form stable phenoxide ion.
C6H5OH(aq) C6H5O¯(aq) + H+(aq)
Phenoxide ion
It is a solid at room temperature.
VIHS/DEPARTMENT OF CHEMISTRY Page 16
5.3 ARENES: BENZENE 2014
It is partially soluble in water due to the formation of hydrogen bond with water.
It is more reactive than benzene.
It can be used as antiseptic compounds.
Reactions of phenol.
(i) Reaction with aqueous sodium hydroxide.
Phenol reacts with sodium hydroxide to form a salt - sodium phenoxide. it is ionic and
water soluble
C6H5OH(aq) + NaOH(aq) C6H5O¯ Na+(aq) + H2O(l)
This reaction is an evidence for the acidic character of phenol.
(ii) Reaction with sodium metal.
Phenol reacts with sodium to form an ionic salt - sodium phenoxide and hydrogen.
This reaction is similar to that with aliphatic alcohols such as ethanol
2C6H5OH(s) + 2Na(s) 2C6H5O¯ Na+(s) + H2(g)
(iii) Reaction with carbonates and hydrogen carbonates.
Phenol does not react with carbonates and hydrogen carbonates as is is weakly
acidic.
(iv) Electrophilic substitution:
The OH group in phenol is electron releasing therefore it increases the electron
density of the delocalised system which makes substitution much easier compared to
benzene as a p orbital on the oxygen overlaps with the p orbitals in benzene
Fig. 5.3.9 p-orbitals in the system. The p orbital on the Oxygen
overlaps with the p orbitals in
the ring.
VIHS/DEPARTMENT OF CHEMISTRY Page 17
5.3 ARENES: BENZENE 2014
The electron density is greatest at the 2,4 and 6 positions which results in the substitution
takes place at the 2,4 and 6 positions.
Reaction with aqueous bromine.
The electron rich ring in phenol is attacked by bromine water, in an electrophilic substitution
reaction. The brown bromine water is decolorised and a white precipitate of 2,4,6-
tribromophenol and a solution of hydrogen bromide are formed. No catalyst is needed.
Phenol 2,4,6-tribromophenol
(white precipitate)
Reagents : Aqueous bromine
Conditions: Room temperature and pressure.
Reaction type: Electrophilic substitution
Observation: Orange colour decolourises/ formation of white ppt./ misty fumes.
Reaction with nitric acid:
The ring is sufficiently activated for nitration to take place with dilute nitric acid. At room
temperature, the organic product is a mixture of 2-nitrophenol and 4-nitrophenol.
OH OH OH
NO2
+ HNO3(aq) + + H2O
NO2
Phenol 2-nitrophenol 4-nitrophenol
6
6
6
6
6
2
4
VIHS/DEPARTMENT OF CHEMISTRY Page 18
5.3 ARENES: BENZENE 2014
Reagents : Dilute nitric acid
Conditions: Room temperature and pressure.
Reaction type: Electrophilic substitution
If the mixture is heated 2,4 and 2,6 dinitrophenol are formed as well. If concentrated nitic
acid is used, 2,4,6-trinitrophenol is the product.
Checklist
After studying this topic, you should be able to:
Define electrophile.
Estimate resonance energy of benzene from hydrogenation and bond enthalpy data.
Write equations and state conditions for the reactions of benzene and phenol with
bromine and nitric acid and benzene with sulphuric acid and the friedel- crafts
reactions.
Draw mechanisms for the halogenations, nitration and friedel- crafts reactions of
benzene.
Explain why the ring in methylbenzene is slightly activated and that in phenol very
activated.
Practice questions
(01) Explain why phenol can be nitrated under much milder conditions than those
required to nitrate benzene.
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
..................................................................................................................................................
(02) In the reaction shown below, the aromatic compound 1,4-dimethylbenzene reacts
with 2-bromobutane. The reaction is catalysed by aluminium chloride, AlCl3, which
dissolves in the reaction mixture.
VIHS/DEPARTMENT OF CHEMISTRY Page 19
5.3 ARENES: BENZENE 2014
(a) (i) Name the type of reaction and the mechanism.
...................................................................................................................................................
(ii) Write the equation to show how the attacking species forms and give the mechanism for
the reaction.
Equation:
Mechanism:
VIHS/DEPARTMENT OF CHEMISTRY Page 20
5.3 ARENES: BENZENE 2014
(03) Some reactions of benzene are shown below.
(a) (i) Write the equation to show how the catalyst, AlCl3, reacts with reagent A to form the
species which attacks the benzene ring.
(ii) Draw the structure of the intermediate ion formed when the species in (ii) attacks the
benzene ring.
(b) The methylbenzene formed in reaction 1 generally reacts in a similar way to benzene
but faster, as the ring is said to be activated.
(i) Explain how the presence of a methyl group activates the benzene ring.
...................................................................................................................................................
...................................................................................................................................................
VIHS/DEPARTMENT OF CHEMISTRY Page 21
5.3 ARENES: BENZENE 2014
...................................................................................................................................................
...................................................................................................................................................
(ii) Use your answer to (i) to explain why methylbenzene reacts faster.
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
(c) (i) Draw the structural formula of compound X, formed in reaction 2.
(ii) The organic product of reaction 2 is also formed when the same reactants, but with an
aluminium catalyst, are heated using microwave radiation. Suggest two reasons why this
technique may be considered ‘greener’.
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
(d) Name reagent B needed for reaction 3.
VIHS/DEPARTMENT OF CHEMISTRY Page 22
5.3 ARENES: BENZENE 2014
(04) Explain, in terms of the bonding in the benzene ring, why the enthalpy of hydrogenation
is less exothermic than would be expected from a molecule with three double bonds.
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
(05)(i) Explain why phenol, C6H5OH, and methoxybenzene, C6H5OCH3, are much more
reactive than benzene with bromine.
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
..................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
...................................................................................................................................................
(ii) Write the equation for the reaction between phenol and bromine water. State symbols are
not required.