Syllabus for Aromatic compounds
Syllabus for Aromatic compounds
Syllabus for Aromatic compounds
Syllabus for Aromatic compounds
Naming of Benzene ring
Rules for Nomenclature of Benzene ring:
1- In order to indicate the relative position of two groups, we do the
numbering on the benzene ring.
2- The carbon number “one” is that which has more potential group
present. The numbering goes to that side from where the second substitute
comes earlier.
3- However, The order of priority of groups from left to right.
COOH＞-C☰N > -COH > -COCH3 > -OH > -NH2 > OR > -X>-R
———————————————————-〉
preference decrease
Naming of aromatic compounds
4- In the case of disubstituted derivative the word ortho (o),
meta (m) and para (p) are also use.
5- 1,2- disubstituted derivative is called ortho-derivative, 1,4disubstituted derivative is called para derivative.
6- Therefore, If two substituent are different and they are not
present at priority order, then they are name in alphabetical
order. The last name substituent will be at position-1.
7- When a substituent on the ring have a special name to the
molecule, then special name is use as a parent name in the
molecule.
Aromatic hydrocarbons
Aromatic hydrocarbons
Aromatic compounds
Aromatic compounds
Aromatic compounds
Aromatic compounds
Aromatic compounds
Aromatic Compounds
BONDING IN BENZENE
Hybridisation sp2
Because each carbon is only joining to three other atoms, when the carbon atoms
hybridise their outer orbitals before forming bonds, they only need to hybridise
three of the orbitals rather than all four. They use the 2s electron and two of the 2p
electrons, but leave the other 2p electron unchanged.
The new orbitals formed are called sp2 hybrids, because they are made
by an s orbital and two p orbitals re-organising themselves. The three
sp2 hybrid orbitals arrange themselves as far apart as possible - which
is at 120° to each other in a plane. The remaining p orbital is at right
angles to them.
• Each carbon atom now looks like the diagram on the right. This is all
exactly the same as happens in ethene.
Structure of benzene
• Notice that the p electron on each carbon atom is overlapping with those on
both sides of it. This extensive sideways overlap produces a system of pi
bonds which are spread out over the whole carbon ring. Because the
electrons are no longer held between just two carbon atoms, but are spread
over the whole ring, the electrons are said to be delocalised. The six
delocalised electrons go into three molecular orbitals - two in each.
• In common with the great majority of descriptions of the bonding in
benzene, we are only going to show one of these delocalised molecular
orbitals for simplicity.
• In the diagram, the sigma bonds have been shown as simple lines to make
the diagram less confusing. The two rings above and below the plane of the
molecule represent one molecular orbital. The two delocalised electrons can
be found anywhere within those rings. The other four delocalised electrons
live in two similar (but not identical) molecular orbitals.
Structure of benzene
Structure of benzene
Structure of benzene
Orbital picture of benzene
• Benzene is a planar regular hexagon, with bond angles of 120°. This is
easily explained. It is a regular hexagon because all the bonds are
identical. The delocalisation of the electrons means that there aren't
alternating double and single bonds. It is planar because that is the only
way that the p orbitals can overlap sideways to give the delocalised pi
system.
The shape of benzene
The energetic stability of benzene
• This is accounted for by the delocalisation. As a general
principle, the more you can spread electrons around - in other
words, the more they are delocalised - the more stable the
molecule becomes. The extra stability of benzene is often
referred to as "delocalisation energy".
The reluctance of benzene to undergo addition reactions
• With the delocalised electrons in place, benzene is about 150
kJ mol-1 more stable than it would otherwise be. If you added
other atoms to a benzene ring you would have to use some of
the delocalised electrons to join the new atoms to the ring.
That would disrupt the delocalisation and the system would
become less stable.
The shape of benzene
Since about 150 kJ per mole of benzene would have to be supplied to break up
the delocalisation, this isn't going to be an easy thing to do.
The symbol for benzene
Although you will still come across the Kekulé structure for benzene, for most
purposes we use the structure on the right.
The hexagon shows the ring of six carbon atoms, each of which has one hydrogen
attached. (You have to know that - counting bonds to find out how many
hydrogens
to add doesn't work in this particular case.)
The circle represents the delocalised electrons. It is essential that you include the
circle. If you miss it out, you are drawing cyclohexane and not benzene.
The shape of benzene
Physical properties of benzene
• Benzene is a cyclic hydrocarbon with a chemical formula C6H6,
that is, each carbon atom in benzene is arranged in a sixmembered ring and is bonded to only one hydrogen atom.
According to molecular orbital theory for benzene structure,
benzene ring involves the formation of three delocalized π –
orbitals spanning all six carbon atoms, while the valence bond
theory describes two stable resonance structures for the ring.
Benzene is one of the most fundamental compounds used in the
manufacturing of various plastics, resins, synthetic fibers, rubber
lubricants, dyes, detergents, drugs, and pesticides. Benzene is
found to exhibit a unique set of physical and chemical properties.
Some physical and chemical properties of benzene are mentioned
below:
Properties of benzene
Physical and chemical properties of benzene:
• Benzene belongs to the family of aromatic hydrocarbons which are
nonpolar molecules and are usually colourless liquids or solids with a
characteristic aroma.
• Benzene being non-polar is immiscible with water but is readily
miscible with organic solvents.
• Upon combustion of benzene sooty flame is produced.
Benzene
Physical Properties of Benzene
(i) Colorless, volatile liquid with special odour and it is more toxic &
carcinogenic
(ii) Specific gravity : 0.88; lighter than water.
(iii) It is a good solvent for fat, wax, sulpher, resin etc.
(iv) M.P. ,40°C, B.P, 80.°C
(v) Insoluble in water but soluble in organic solvent.
(vi) During combustion, it produces black flame.Sooty flame.
Electrophilic Substitution
Electrophilic substitution: the replacement of an atom by
another atom or group of atoms after initial attack by an
electron-deficient species.
Most reactions of benzene and other arenes maintain the highly
stable delocalised ring of pi bonding electrons intact. This
occurs by substituting an atom, or group of atoms, for one or
more hydrogen atoms attached to the benzene ring.
Reactions of arenes
Addition reactions into the benzene ring would disrupt the “aromatic
stabilization” achieved by the complete delocalisation in the ring. The
initial attack is usually by an electrophile, which is attracted to the high
electron density around the benzene ring.
Electrophilic substitution with chlorine or bromine
Benzene will react with bromine in the presence of an
anhydrous aluminium bromide catalyst. The substitution
reaction is:
Electrophilic substitution in Arenes
At first sight, the electrophile that starts the attack on benzene is not obvious.
The electrophile is created when an aluminium bromide molecule polarizes a
bromine molecule. The Br2 molecule forms a dative (co-ordinate) bond with
aluminium bromide by donating a lone pair of electrons from one of its
bromine atoms into an empty 3p orbital in the aluminium. This draws electrons
from the other bromine atoms in the Br2 molecule, making it partially positive,
and creating the electrophile.
Electrophilic substitution in Arenes
• We can think of the electrophile as a Br + cation:
The Br+ cation and the 'electron-rich' benzene ring are attracted to each
other, as the mechanism of electrophilic substitution below shows.
Remember that the curly arrows show the movement of a pair of
electrons.
Electrophilic substitution in Arenes
A similar reaction happens when chlorine gas is bubbled through
benzene at room temperature in the presence of a catalyst, such as
aluminium chloride. The products of this electrophilic substitution are
chlorobenzene and hydrogen chloride. The catalysts in these reactions,
AIBr3 and AlC13, are known as halogen carriers
When we halogenate methylbenzene or other alkylarenes, the halogen
atom substitutes into the benzene ring at positions 2 or 4. These
positions are 'activated' by any electron-donating groups bonded
directly to the benzene ring (see Table 25.4 at the end of the chapter).
Other examples of benzene compounds that are activated in these
positions are phenol (C6H5OH) and phenylamine (C6H5NH2).
• So when we react methylbenzene with chlorine gas, using an
anhydrous aluminium chloride catalyst, two products can be
made:
• If excess chlorine gas is bubbled through, we can form 2,4dichloromethylbenzene, 2,6-dichloromethylbenzene and
2,4,6-trichloromethylbenzene. (Remember that the 2 and 6
positions in substituted arenes are equivalent.)
• The carbon-halogen bond in halogenoarenes is stronger than
the equivalent bond in a halogenoalkane, making the
halogenoarenes much less reactive. This is because one of the
lone pairs on the halogen atom overlaps slightly with the it
bonding system in the benzene ring. This gives the carbon—
halogen bond in a halogenoarene a partial double bond
character.
Reactions of Benzene
Notice that the methyl side-chain off the benzene ring is not affected
under the conditions used in the reaction above. However, we
learned that chlorine will react with alkanes in the presence of
ultraviolet (UV) light or strong sunlight. This reaction of the alkanes
is a free-radical substitution reaction. So, if the chlorine gas is passed
into boiling methylbenzene in the presence of UV light, the following
reaction takes place:
Note that there is no
substitution into the
benzene ring under
these conditions.
Nitration of benzene
• In excess chlorine, eventually all three of the hydrogen atoms on the
methyl side-chain will be replaced by chlorine atoms.
Nitration of benzene
The nitration of benzene is another example of an electrophilic
substitution. Nitration refers to the introduction of the -NO2 group into a
molecule. In this reaction the electrophile is the NO2+ ion, known as the
nitronium ion (or nitryl cation). This is made from a mixture of
concentrated nitric acid and concentrated sulfuric acid:
Nitration of benzene
Heat or
reflux at
60oC
The mechanism of the electrophilic substitution in this nitration is:
Nitration of benzene
• Benzene does not react with nitric acid, even when the acid is concentrated. But a
mixture of concentrated Nitric acid and concentrated sulfuric acid produces
nitrobenzene.
• This 'nitrating mixture' is refluxed with benzene at between 25 °C and 60 °C to make
nitrobenzene:
• Nitrating mixture means benzene or any other arenes, Concentrated
H2SO4 and HNO3
• Conditions: Heat or reflux at 60oC
• In stage I in the mechanism, the electrophile, NO2+, is attracted to the high
electron density of the it bonding system in benzene. A pair of electrons from the
benzene ring is donated to the nitrogen atom in NO 2+, and forms a new covalent
bond. At this point, benzene's delocalised ring of electrons is disrupted. There are
now four it bonding electrons and a positive charge spread over five carbon
atoms.
• Further nitration to dinitrobenzene (and also some oxidation) can occur unless the
temperature is controlled to be below about 55 °C.
• The role of the concentrated sulfuric acid in this reaction is to produce the powerful
electrophile needed to nitrate the benzene ring. By analogy with bromination.
The mechanism of the nitration of benzene
• Sulfuric acid is a stronger acid than nitric acid. It is so strong
that it donates a proton to nitric acid. In this reaction, nitric acid
is acting as a base.
H2SO4 + HNO3
HSO4 + H2NO3+
• The protonated nitric acid then loses a water molecule, and this
is then protonated by another molecule of sulfuric acid:
H2NO3+
H2O + NO2+
H2SO4 + H2O
HSO4 + H3O +
The nitronium ion then attacks the benzene ring in the usual
way, forming the carbocation intermediate, which subsequently
loses a proton:
The nitration of benzene and other arenes is an important
reaction in the production of explosives.
Also, via subsequent reduction, nitration is an important route to aromatic
amines, which are used to make a variety of dyes.
Alkylation (the Friedel-Crafts reaction)
In 1877 the French chemist Charles Friedel and his co-worker, American chemist
James Crafts, discovered that when benzene is heated with a chloroalkane in the
presence of aluminium chloride, the alkyl group attaches to the benzene ring.
The reaction goes via the formation of an intermediate carbocation.
The carbocation is the electrophile.
Acylation (Friedel-Crafts acylation)
• The reaction between an acyl chloride and aluminium chloride
produces the acylium ion. The intermediate is formed from the
attack of the acylium ion on the benzene ring.
The reaction between an acyl chloride and aluminium chloride
produces the acylium ion. The intermediate is formed from the attack
of the acylium ion on the benzene ring
Acylation (Friedel-Crafts acylation)
Summary
Question
Draw the structural formulae of the products you would expect from the
reaction of benzene and aluminium chloride with
What organochlorine compounds are needed to synthesise the following
compounds from benzene?
Electrophilic substitution in substituted arenes —the
orientation of the incoming group
• When methylbenzene is treated with nitric and sulfuric acids,
the three possible mono-nitro compounds are formed in the
following ratios:
Electrophilic substitution in substituted arenes —the
orientation of the incoming group
• If the NO2+ electrophile had attacked the ring in a purely random way,
the distribution should have been 40:40:20 (there being two 2positions and two 3-positions, but only one 4-position). This nonrandom attack is seen in other reactions too (see Table 25.5).
Orientation of electrophilic substitution
The data in Table above can be interpreted as follows.
•The orientation of the incoming group (NO2 or Br) depends on the substituent
already in the ring, and not on the electrophile.
• Some substituents favour both 2- and 4-substitution, whereas other substituents
favour 3-substitution, at the expense of both 2- and 4-substitution.
If we look closely at the types of substituents that are 2,4-directing, we find that
either they are capable of donating electrons to the ring by the inductive effect, or
they have a lone pair of electrons on the atom joined to the ring. This lone pair
can be incorporated into the pi system by sideways overlap of p orbitals.
On the other hand, all those substituents that favour 3-substitution have a
atom
joined directly to the ring (see Table below).
Ar - OCH3
Orientation in Electrophilic substitution
(a) Methylbenzene undergoes electrophilic substitution with nitronium
ions, NO2+.
Nitronium ions are generated by the reaction between concentrated
sulfuric acid and concentrated nitric acid.
(i) Construct an equation for the formation of nitronium ions, NO2+, by
this method.
(ii) Complete the scheme to show the mechanism for this reaction. Use
curly arrows to show the movement of electron pairs.
Question
Complete the mechanism by using curly arrow.
ASSORTED REACTIONS OF BENZENE AND METHYLBENZENE
• Combustion
Like any other hydrocarbons, benzene and methylbenzene burn in a
plentiful supply of oxygen to give carbon dioxide and water. For
example:
For benzene:
and methylbenzene:
OTHER REACTIONS OF BENZENE AND METHYLBENZENE
• However, for these hydrocarbons, combustion is hardly ever
complete, especially if they are burnt in air. The high
proportion of carbon in the molecules means that you need a
very high proportion of oxygen to hydrocarbon to get
complete combustion. The arenes tend to burn in air with
extremely smoky flames - full of carbon particles.
• The arenes tend to burn in air with extremely smoky flames full of carbon particles.
• You almost invariably get incomplete combustion, and the
arenes can be recognised by the smokiness of their flames.
Addition reactions
• Hydrogenation
Hydrogenation is an addition reaction in which hydrogen atoms are
added all the way around the benzene ring. A cycloalkane is formed.
For example:
With benzene:
and methylbenzene:
Addition reactions
• These reactions destroy the electron delocalisation in the original
benzene ring, because those electrons are being used to form
bonds with the new hydrogen atoms.
• Although the reactions are exothermic overall because of the
strengths of all the new carbon-hydrogen bonds being made,
there is a high activation barrier to the reaction.
• The reactions are done using the same finely divided nickel
catalyst that is used in hydrogenating alkenes and at similar
temperatures (around 150°C), but the pressures used tend to
be higher.
Sulphonation
Sulphonation involves replacing one of the hydrogens on a benzene ring
by the sulphonic acid group, -SO3H.
The sulphonation of benzene
There are two equivalent ways of sulphonating benzene:
Heat benzene under reflux with concentrated sulphuric acid for several
hours.
Warm benzene under reflux at 40°C with fuming sulphuric acid for 20
to 30 minutes. Fuming sulphuric acid, H2S2O7, can usefully be thought
of as a solution of sulphur trioxide in concentrated sulphuric acid.
Side chain oxidation in alkylbenzenes
Methylbenzene is heated under reflux with a solution of potassium
manganate(VII) made alkaline with sodium carbonate. The purple
colour of the potassium manganate(VII) is eventually replaced by a dark
brown precipitate of manganese(IV) oxide.
The mixture is finally acidified with dilute sulphuric acid.
Overall, the methylbenzene is oxidised to benzoic acid.
Side chain oxidation in alkylbenzenes
The presence of the benzene ring in an alkylarene, such as methylbenzene, can
affect the characteristic reactions of its alkyl side-chain. For example, alkanes are
not usually oxidised by a chemical oxidizing agent such as potassium
manganate(VII). However, in alkylarenes, the alkane side-chain is oxidised to form
a carboxylic acid. For example, methylbenzene produces benzoic acid when
refluxed with alkaline potassium manganate(VII), and then acidified with dilute
sulfuric acid:
Reactions
31 Halogen compounds
1.
Recall the reactions by which halogenoarenes can be
produced:
(a) substitution of an arene with Cl2 or Br2in the presence
of a catalyst, Al Cl3 or Al Br3 to form a halogenoarene,
exemplified by benzene to form chlorobenzene and
methylbenzene to form 2-chloromethylbenzene and
4-chloromethylbenzene
2. Explain the difference in reactivity between a
halogenoalkane and a halogenoarene as exemplified by
chloroethane and chlorobenzene
Halogenoarenes
Bromobenzene and chlorobenzene can be made from
benzene. The reactions of the ring in halogenobenzenes are similar to
those of benzene. Halogenoarenes undergo electrophilic substitution,
and can be nitrated:
Haloarenes
However, unlike halogenoalkanes, halogenoarenes cannot be
hydrolysed, even by boiling in aqueous sodium hydroxide. The
carbon-halogen bond is stronger in halogenoarenes than it is in
halogenoalkanes, possibly due to an overlap of p electrons similar
to that in phenol. In addition to this, the carbon attached to the
halogen is not accessible to the usual nucleophilic reagents that
attack halogenoalkanes, since its δ+ charge is shielded by the
negative π cloud of the ring. This means that halogenoarenes are inert
to all nucleophiles.
Certain halogenoarencs find important uses as insecticides and
herbicides.
Structure of haloarenes
Delocalisation of electron
pair in chlorobenzene
Orbital structure of chlorobenzene
The structure of aryl halides is basically the same as that of benzene, except
that the • halogen bonded to a carbon of the ring has a p orbital containing a
lone pair of electrons. This
Aryl halides
p-orbital of halogen interacts with the pi orbital
enveloping the ring, to form an extended pi orbital. The
extended pi orbital envelopes the six carbons of the
ring as also the halogen. In other words, the six
electrons of the ring carbons (one belonging to each
carbon) and two electrons of the halogen' are now
associated with all the seven atoms. ;This
delocalisation of electrons lends some double bond
character to the carbon-halogen bond.
The electronic structure of aryl halides could be represented by the following
canonical forms. Thus aryl halides are regarded as resonance hybrid of the four
contributing forms. For this the C-X (or C-Cl bond)bonds of aryl halides
(Chlorobenzene) are shorter and stronger than those of the alkyl halides. There is
partial double bond character between carbon and halogen in haloarenes.
The electrophilic substitution reactions in haloarenes takes place in 2 and 4
positions as a major product. Here the halogen is not a good donor of
electrons by resonance so electron density rs relatively higher in 2 and 4
positions. This is due to the higher electronegativity of halogen (chlorine)
atom, and because of the poor 2p (C) – 3p (Cl) overlap. Thus the inductive
effect of chlorine wins over the resonance effect, and the chlorine atom
deactivates the benzene ring but not very much.
*Explain the difference in reactivity between a halogenoalkane
and a halogenoarene as exemplified by chloroethane and
chlorobenzene