Unit 14B
P2
What is benzene?
Benzene is an important aromatic compound with a unique structure that has been
studied extensively. Benzene is a colorless, flammable liquid with a sweet odor. It's a
well-known human carcinogen that's used in many industrial and consumer products.
Benzene is a natural part of crude oil, gasoline, and cigarette smoke. It can also be
produced by volcanoes and forest fires.
Industrial processes are the main source of benzene in the environment. Benzene is
used as a solvent in the chemical and pharmaceutical industries.
It's used to make plastics, resins, synthetic fibers, lubricants, dyes, detergents, drugs,
and pesticides. Benzene can cause bone marrow depression, aplastic anemia, and
leukemia.It's also linked to non-Hodgkin's lymphoma.
Structure of Benzene
Experimental data from X-ray diffraction show that all carbon-carbon bonds in benzene
have an equal bond length of 0.139 nm. This is between the length of a single and
double bond which could suggest that benzene has a delocalised electron system.
Measurements of hydrogenation enthalpy offer more proof of benzene's durability. A
hypothetical cyclohexatriene should emit -360 kJ/mol upon hydrogenation, whereas
benzene only releases -208 kJ/mol, making it 152 kJ/mol more stable than anticipated.
A planar, cyclic, aromatic chemical with a hexagonal ring structure is benzene (C₆H₆). Its
distinctive stability and characteristics are attributed to the sigma (σ) and pi (π) bonding.
The image below shows the structure of Benzene.
,Hybridisation of carbon in benzene sigma and pi bonding, and delocalisation of
electrons
The overlapping of orbitals is called hybridisation.
Three sigma (σ) bonds are formed via the sp2 hybridisation of each carbon atom in
benzene, from adjacent carbon atoms, two C-C sigma bonds are formed. One
hydrogen atom and one C-H sigma bond. Benzene has a completely planar form thanks
to the hexagonal ring structure created by these sigma bonds, which have bond angles
of 120°.
Each carbon has one unhybridised 2p orbital perpendicular to the ring plane for its Pi
(π) Bonding and Delocalisation. In benzene, every carbon atom undergoes sp2
hybridisation. In order to create three comparable sp2 hybrid orbitals, the carbon atom
hybridises by combining one 2s orbital with two 2p orbitals. Each carbon atom has one
electron in its unhybridized p orbital, which is the leftover p orbital. A delocalised pi (π)
bond system is created when these p orbitals laterally overlap above and below the
plane of the carbon atoms.
The aromatic stability of benzene is aided by the delocalised π electrons that are
dispersed throughout the whole hexagonal ring. Six sigma (σ) bonds are formed inside
the hexagonal ring when these sp2 hybrid orbitals overlap with the sp2 hybrid orbitals of
,nearby carbon atoms. Above and below the ring, a continuous π-electron cloud is
formed by the sideways overlap of these p orbitals.The six π-electrons are delocalised
throughout all six carbon atoms, forming a resonance structure in place of localised
double bonds. Electrophilic addition reactions do not occur in benzene. Rather than
localised double bonds, it favours electrophilic substitution, which further suggests
delocalised π-electrons. The aromatic stability of benzene is aided by the delocalised π
electrons that are dispersed throughout the whole hexagonal ring.
The Kekulé structure of benzene
A hexagonal ring of carbon atoms with alternating single and double bonds makes up
the Kekulé structure of benzene. It takes the name of August Kekulé, a German
scientist who initially proposed the structure in 1865.
, According to the Kekulé structure, the chemical compound C6H6 is formed by one
hydrogen atom being linked to each carbon atom in the ring. The size of benzene bonds
should be 1.54 angstroms for single C-C bonds and 1.34 angstroms for double C=C
bonds due to their alternating double bonds. The experimental findings refuted the
earlier, inaccurate assumptions regarding the structure of benzene.
Evidence Supporting Benzene's Structure
Infared
The IR spectrum of benzene shows the presence of delocalised pi electrons across the
ring, which results in all carbon-carbon bonds being of equal length and not exhibiting a
distinct double bond character. This is in contrast to the traditional absorption bands for
isolated C=C double bonds, which would be expected if benzene had alternating single
and double bonds as suggested by the Kekulé structure.
X-Ray
By demonstrating that all of the carbon-carbon bonds in the benzene ring are the same
length, which is in between a single and double bond, X-ray diffraction offers solid proof
for the delocalised structure of benzene. The concept of delocalised pi electrons in
benzene is supported by the fact that the electrons are evenly distributed throughout the
ring rather than alternating between single and double bonds as suggested by the
Kekulé model.The six hydrogen and six carbon atoms in benzene are all in the same
plane, according to the diffraction pattern. This is in line with benzene's planar molecular
structure.The bond angles in benzene are all 120 degrees, as seen by the diffraction
pattern, which is indicative of a hexagonal ring structure.
Thermochemical
The presence of delocalised electrons in a ring system, which is the main characteristic
of the accepted benzene structure, is indicated by thermochemistry. This shows that
benzene is much more stable than predicted based on a simple cyclic structure with
alternating single and double bonds (like cyclohexatriene). This added stability is seen
when measuring the heat released during hydrogenation reactions, where benzene
releases less heat than expected, indicating a lower energy state due to delocalisation.
P2
What is benzene?
Benzene is an important aromatic compound with a unique structure that has been
studied extensively. Benzene is a colorless, flammable liquid with a sweet odor. It's a
well-known human carcinogen that's used in many industrial and consumer products.
Benzene is a natural part of crude oil, gasoline, and cigarette smoke. It can also be
produced by volcanoes and forest fires.
Industrial processes are the main source of benzene in the environment. Benzene is
used as a solvent in the chemical and pharmaceutical industries.
It's used to make plastics, resins, synthetic fibers, lubricants, dyes, detergents, drugs,
and pesticides. Benzene can cause bone marrow depression, aplastic anemia, and
leukemia.It's also linked to non-Hodgkin's lymphoma.
Structure of Benzene
Experimental data from X-ray diffraction show that all carbon-carbon bonds in benzene
have an equal bond length of 0.139 nm. This is between the length of a single and
double bond which could suggest that benzene has a delocalised electron system.
Measurements of hydrogenation enthalpy offer more proof of benzene's durability. A
hypothetical cyclohexatriene should emit -360 kJ/mol upon hydrogenation, whereas
benzene only releases -208 kJ/mol, making it 152 kJ/mol more stable than anticipated.
A planar, cyclic, aromatic chemical with a hexagonal ring structure is benzene (C₆H₆). Its
distinctive stability and characteristics are attributed to the sigma (σ) and pi (π) bonding.
The image below shows the structure of Benzene.
,Hybridisation of carbon in benzene sigma and pi bonding, and delocalisation of
electrons
The overlapping of orbitals is called hybridisation.
Three sigma (σ) bonds are formed via the sp2 hybridisation of each carbon atom in
benzene, from adjacent carbon atoms, two C-C sigma bonds are formed. One
hydrogen atom and one C-H sigma bond. Benzene has a completely planar form thanks
to the hexagonal ring structure created by these sigma bonds, which have bond angles
of 120°.
Each carbon has one unhybridised 2p orbital perpendicular to the ring plane for its Pi
(π) Bonding and Delocalisation. In benzene, every carbon atom undergoes sp2
hybridisation. In order to create three comparable sp2 hybrid orbitals, the carbon atom
hybridises by combining one 2s orbital with two 2p orbitals. Each carbon atom has one
electron in its unhybridized p orbital, which is the leftover p orbital. A delocalised pi (π)
bond system is created when these p orbitals laterally overlap above and below the
plane of the carbon atoms.
The aromatic stability of benzene is aided by the delocalised π electrons that are
dispersed throughout the whole hexagonal ring. Six sigma (σ) bonds are formed inside
the hexagonal ring when these sp2 hybrid orbitals overlap with the sp2 hybrid orbitals of
,nearby carbon atoms. Above and below the ring, a continuous π-electron cloud is
formed by the sideways overlap of these p orbitals.The six π-electrons are delocalised
throughout all six carbon atoms, forming a resonance structure in place of localised
double bonds. Electrophilic addition reactions do not occur in benzene. Rather than
localised double bonds, it favours electrophilic substitution, which further suggests
delocalised π-electrons. The aromatic stability of benzene is aided by the delocalised π
electrons that are dispersed throughout the whole hexagonal ring.
The Kekulé structure of benzene
A hexagonal ring of carbon atoms with alternating single and double bonds makes up
the Kekulé structure of benzene. It takes the name of August Kekulé, a German
scientist who initially proposed the structure in 1865.
, According to the Kekulé structure, the chemical compound C6H6 is formed by one
hydrogen atom being linked to each carbon atom in the ring. The size of benzene bonds
should be 1.54 angstroms for single C-C bonds and 1.34 angstroms for double C=C
bonds due to their alternating double bonds. The experimental findings refuted the
earlier, inaccurate assumptions regarding the structure of benzene.
Evidence Supporting Benzene's Structure
Infared
The IR spectrum of benzene shows the presence of delocalised pi electrons across the
ring, which results in all carbon-carbon bonds being of equal length and not exhibiting a
distinct double bond character. This is in contrast to the traditional absorption bands for
isolated C=C double bonds, which would be expected if benzene had alternating single
and double bonds as suggested by the Kekulé structure.
X-Ray
By demonstrating that all of the carbon-carbon bonds in the benzene ring are the same
length, which is in between a single and double bond, X-ray diffraction offers solid proof
for the delocalised structure of benzene. The concept of delocalised pi electrons in
benzene is supported by the fact that the electrons are evenly distributed throughout the
ring rather than alternating between single and double bonds as suggested by the
Kekulé model.The six hydrogen and six carbon atoms in benzene are all in the same
plane, according to the diffraction pattern. This is in line with benzene's planar molecular
structure.The bond angles in benzene are all 120 degrees, as seen by the diffraction
pattern, which is indicative of a hexagonal ring structure.
Thermochemical
The presence of delocalised electrons in a ring system, which is the main characteristic
of the accepted benzene structure, is indicated by thermochemistry. This shows that
benzene is much more stable than predicted based on a simple cyclic structure with
alternating single and double bonds (like cyclohexatriene). This added stability is seen
when measuring the heat released during hydrogenation reactions, where benzene
releases less heat than expected, indicating a lower energy state due to delocalisation.
