H.
L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 14: Applications of Organic Chemistry
Assignment 14B: Understand the reactions and properties of
aromatic compounds
Benzene
Structure Figure 1- Kekulé
structure of benzene
Benzene has the formula, C6H6, with a planar cyclic structure- (2)
6 carbon atoms joined together in a flat ring (1).
The initial structure of benzene was proposed by the chemist,
Kekulé, which consisted of carbons arranged in a hexagon
with alternating double and single bonds between them (2). Kekulé also
proposed that each carbon atom has a hydrogen atom attached- see Figure 1.
Figure 2- Accepted
Over the years, the Kekulé structure was proved wrong as there structure of benzene
were multiple issues with the chemistry. The accepted structure of (2)
benzene has no alternating single/double bonds (see Figure 2).
The Kekulé structure displayed that benzene had three double
bonds; because of this, it should be able to undergo addition
reactions like the molecule ethene. However, this is not the case for benzene as
it generally undergoes substitution reactions where a hydrogen atom is replaced
by a group (2). In addition to this, benzene is a planar cyclic molecule and if it
had the Kekulé structure then the cyclic structure would be irregular with
alternating shorter and longer sides (2). This irregular cyclic structure is caused
by the different lengths of the single and double bonds in the Kekulé structure of
benzene- C-C bond is 0.154nm whereas the C=C bond is 0.134nm. The accepted
structure of benzene only consists of single bonds, so the cyclic structure is
regular.
Figure 3- Delocalised ring of electrons in benzene (1)
As seen in Figure 2, benzene
has a ring of carbon atoms,
and these atoms form single
covalent bonds to the carbons
on either side of it and to a
single hydrogen atom (1). As a
result, there is an unpaired
electron on each carbon atom
that is in a p-orbital below the plane of the ring (1). The p-orbitals on each
carbon atom then join to form a ring of delocalised electrons- see Figure 3. The
ring of delocalised electrons causes benzene to be a very stable molecule-
discussed in detail below (1).
To form the single carbon bonds in benzene, it must undergo sp 2 hybridisation- a
process where three orbitals combine to form 3 new identical orbitals that each
have an unpaired electron. Firstly, an electron from the 2s orbital is promoted to
the 2p orbital which is empty, resulting in 4 unpaired electrons-see Figure 4 (2).
The singly occupied 2s and two of the 2p orbitals hybridise to form three
identical sp2 orbitals. The remaining 2p orbital is unhybridized as each carbon is
only joining to three other atoms- see Figure 5 (2).
1
, H.
L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 14: Applications of Organic Chemistry
Figure 5-
Hybridisation of
benzene (2)
In terms of the arrangement, the sp2 hybrids formed have a trigonal planar
shape as they all repel each other. They have a bond angle of 120˚ with the
unchanged 2p orbital at right angles to them (90˚). Each carbon atom forms
sigma bonds with 2 other carbons and a single hydrogen atom (2). The sigma
bonds form as the sp2 hybrids of each carbon overlap, causing a single carbon
bond (sigma bond) to form. The two 2p hybrids overlap sideways which results in
the formation of a system of pi bonds which occur over the whole ring. As the
electrons are spread over the whole ring, they are known to be delocalised- the
six delocalised electrons go into three orbitals with two in each (2). The
delocalisation of electrons shows that the carbon-carbon bonds in benzene are
identical and have single and double bond character (4).
Moreover, there is additional evidence that supports the structure of benzene as
being stable and regularly shaped in the form of infrared, x-ray, and
thermochemical analysis.
Figure 6- Infrared evidence for benzene (3)
Infrared evidence, as seen in Figure 6, shows that
benzene has peaks at around 1450 cm-1 , 1500 cm-1
and 1580 cm-1 which are not characteristic of double
carbon-carbon bonds (1650 cm-1) (4). This evidence,
therefore, disproves the Kekulé structure as it
identifies benzene does not have any carbon-carbon
double bonds since it absorbs energy of different
frequencies. The Kekulé structure shows that
benzene does contain carbon-carbon double bonds,
so this evidence goes against the Kekulé structure of
benzene, disproving it as a result.
Figure 7- X-ray evidence for benzene (5)
X-ray evidence, shown in Figure 7, confirmed that benzene had a
regular hexagonal shape with bonds of equal length (6).
This disproves the Kekulé structure, consisting of bonds with
different lengths- 0.154nm C-C bonds and 0.134nm C=C bonds.
Also as mentioned before, the Kekulé structure has an irregular
2
L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 14: Applications of Organic Chemistry
Assignment 14B: Understand the reactions and properties of
aromatic compounds
Benzene
Structure Figure 1- Kekulé
structure of benzene
Benzene has the formula, C6H6, with a planar cyclic structure- (2)
6 carbon atoms joined together in a flat ring (1).
The initial structure of benzene was proposed by the chemist,
Kekulé, which consisted of carbons arranged in a hexagon
with alternating double and single bonds between them (2). Kekulé also
proposed that each carbon atom has a hydrogen atom attached- see Figure 1.
Figure 2- Accepted
Over the years, the Kekulé structure was proved wrong as there structure of benzene
were multiple issues with the chemistry. The accepted structure of (2)
benzene has no alternating single/double bonds (see Figure 2).
The Kekulé structure displayed that benzene had three double
bonds; because of this, it should be able to undergo addition
reactions like the molecule ethene. However, this is not the case for benzene as
it generally undergoes substitution reactions where a hydrogen atom is replaced
by a group (2). In addition to this, benzene is a planar cyclic molecule and if it
had the Kekulé structure then the cyclic structure would be irregular with
alternating shorter and longer sides (2). This irregular cyclic structure is caused
by the different lengths of the single and double bonds in the Kekulé structure of
benzene- C-C bond is 0.154nm whereas the C=C bond is 0.134nm. The accepted
structure of benzene only consists of single bonds, so the cyclic structure is
regular.
Figure 3- Delocalised ring of electrons in benzene (1)
As seen in Figure 2, benzene
has a ring of carbon atoms,
and these atoms form single
covalent bonds to the carbons
on either side of it and to a
single hydrogen atom (1). As a
result, there is an unpaired
electron on each carbon atom
that is in a p-orbital below the plane of the ring (1). The p-orbitals on each
carbon atom then join to form a ring of delocalised electrons- see Figure 3. The
ring of delocalised electrons causes benzene to be a very stable molecule-
discussed in detail below (1).
To form the single carbon bonds in benzene, it must undergo sp 2 hybridisation- a
process where three orbitals combine to form 3 new identical orbitals that each
have an unpaired electron. Firstly, an electron from the 2s orbital is promoted to
the 2p orbital which is empty, resulting in 4 unpaired electrons-see Figure 4 (2).
The singly occupied 2s and two of the 2p orbitals hybridise to form three
identical sp2 orbitals. The remaining 2p orbital is unhybridized as each carbon is
only joining to three other atoms- see Figure 5 (2).
1
, H.
L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 14: Applications of Organic Chemistry
Figure 5-
Hybridisation of
benzene (2)
In terms of the arrangement, the sp2 hybrids formed have a trigonal planar
shape as they all repel each other. They have a bond angle of 120˚ with the
unchanged 2p orbital at right angles to them (90˚). Each carbon atom forms
sigma bonds with 2 other carbons and a single hydrogen atom (2). The sigma
bonds form as the sp2 hybrids of each carbon overlap, causing a single carbon
bond (sigma bond) to form. The two 2p hybrids overlap sideways which results in
the formation of a system of pi bonds which occur over the whole ring. As the
electrons are spread over the whole ring, they are known to be delocalised- the
six delocalised electrons go into three orbitals with two in each (2). The
delocalisation of electrons shows that the carbon-carbon bonds in benzene are
identical and have single and double bond character (4).
Moreover, there is additional evidence that supports the structure of benzene as
being stable and regularly shaped in the form of infrared, x-ray, and
thermochemical analysis.
Figure 6- Infrared evidence for benzene (3)
Infrared evidence, as seen in Figure 6, shows that
benzene has peaks at around 1450 cm-1 , 1500 cm-1
and 1580 cm-1 which are not characteristic of double
carbon-carbon bonds (1650 cm-1) (4). This evidence,
therefore, disproves the Kekulé structure as it
identifies benzene does not have any carbon-carbon
double bonds since it absorbs energy of different
frequencies. The Kekulé structure shows that
benzene does contain carbon-carbon double bonds,
so this evidence goes against the Kekulé structure of
benzene, disproving it as a result.
Figure 7- X-ray evidence for benzene (5)
X-ray evidence, shown in Figure 7, confirmed that benzene had a
regular hexagonal shape with bonds of equal length (6).
This disproves the Kekulé structure, consisting of bonds with
different lengths- 0.154nm C-C bonds and 0.134nm C=C bonds.
Also as mentioned before, the Kekulé structure has an irregular
2
