Benzene Flashcards
Molecular formula of benzene
C6H6
Describe the nature of the carbon-carbon bonds in benzene
They are all the same lengths
The bond length in benzene is an intermediate between C=C and C-C
Shape of the benzene molecule
Planar
What structure of benzene did Kukule suggest
A cyclic arrangement of carbon atoms joined by alternate single and double bonds
What was the evidence that led scientists to doubt and reject Kukule’s model
The carbon-carbon bond length is intermediate between C=C and C-C
Benzene only reacts with bromine in presence of a halogen carrier so is less reactive than alkenes
Enthalpy change of hydrogenation for benzene is less exothermic/negative than that of cycloheaxa-1,3,5-triene
Benzene does not readily react by addition but reacts by substitution
Why does benzene react by substitution and not by addition
Benzene does not have three C=C double bonds but has a delocalised pi electron system.
The 6 p orbitals overlap sideways, above and below the ring to form a system of pi bonds.
The pi electrons are delocalised over all the 6 carbon atoms in the ring which gives the molecule greater stability.
When benzene reacts by substitution the delocalised pi electron system is retained, whereas in addition reactions the delocalised pi electron system is disrupted. Therefore substitution is energetically more favourable than addition.
Bond angles in benzene
120 degrees
How are the carbon atoms bonded in benzene
Each carbon atom is bonded to two other carbon atoms and to one hydrogen atom using three of its available 4 electrons.
Why is benzene stable
Each carbon atom has one electron in a p orbital. The six p orbitals overlap sideways, above and below the ring to form a system of pi bonds. The pi electrons are delocalised over all the six carbon atoms in the ring which gives the molecule a greater stability.
Explain in terms of structure and bonding why benzene and alkenes react differently with electrophiles like bromine
Benzene does no have 3 C=C bonds but has a delocalised pi electron system.
The six p orbitals overlap above and below the ring to form a system of pi bonds.
The pi electrons are delocalised over all six carbon atoms in the ring which gives the molecule greater stability.
Benzene reacts by substitution rather than addition.
When benzene reacts by substitution the delocalised pi electron system is retained, whereas in addition reactions the delocalised pi electron system is disrupted.
Differences between the pi bonding in Kekule’s model and the accepted model of benzene
Kekule’s model has three pi bonds. The two electrons in each pi bond are localised between the two carbon atoms in the double bond.
Benzene has a system of pi bonds where the pi electrons are delocalised over all six carbon atoms in the ring.
The p orbitals in Kekule’s model overlap in one direction, whereas the p orbitals in benzene overlap in both directions.
Although the enthalpy of hydrogenation of benzene might be expected to be -360 kJ/mol it was found to be -208 kJ/mol. Give a reason why -360 was not the actual value.
The six p orbitals overlap sideways to form a system of pi bonds.
The pi electrons are delocalised over all six carbon atoms in the ring which gives the molecule greater stability. More energy is needed to break the bonds in benzene, hence the enthalpy change of hydrogenation of benzene is less exothermic.
Observation for the combustion of benzene
Burns with a yellow smoky flame and leaves a black residue
What type of reaction is the nitration of benzene
Electrophilic substitution
Reagents needed for the nitration of benzene
Conc nitric acid and conc sulphuric acid
