6.1 Flashcards
Define benzene.
Benzene is a naturally occurring aromatic structure, which is very stable planar ring structure with delocalised electrons.
Define a model.
a model is a simplified version that allows us to make predictions and understand observations more easily.
What homologous series is benzene in?
Benzene is the simplest member of the arene homologous series.
What are benzene/’s empirical and molecular formula?
It has the molecular formula C6H6 and therefore an empiricle formula of CH.
What are the uses of benzene?
It is a liquid at room temperature and is a key ingredient added to gasoline as it increases the efficiency of the car engine.
Describe Kekules model of benzene.
Although the empirical and molecular formula of benzene has been known since the 1850’s, the structure was difficult to determine.
Friedrich August Kekule was German chemist who, in 1865, suggested that benzene was a six-membered ring with alternating single and double bonds between the carbon atoms.
What evidence led kekule to creating his model?
He discovered that when one group was added to benzene, only one isomer was ever made; but when two groups were added; there was always three structural isomers produced.

What were the problems with Kekulé’s model?
There are three pieces of experimental evidence that do not support the Kekulé model:
- Unlike alkanes, benzene is resistant to addition reactions.
- Enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted.
- All six carbon bonds in benzene are the same length.
How did Kekule explain how benzene did not undergo similar reactions to alkenes?
Ethene will readily undergo addition reactions but benzene tends to undergo substitution of a hydrogen atom rather than addition reactions. Kekulé tried to explain this by saying the double and single bonds changed position in very fast equilibrium.

Describe how the enthalpy of hydrogenation of benzene shows that benzene is much more stable than predicted, and therefore is a problem with Kekules model?
Hydrogenation is the addition of hydrogen to an unsaturated chemical. Using bond enthalpy data we can calculate the enthalpy change for the complete hydrogenation of cyclohexane and cycle-1,3,5-hecatriene. Cycle-1,3,5-hexatrene is he Kekulé model of benzene. However, experimentally it is found that the enthalpy change for hydrogenation of benzene is -208kJ mol-1, which shows it is 152 kJ mol-1 more energetically stable than predicted.

Describe how the bond lengths of the carbon - carbon bonds in benzene is a problem with Kekules model?
X-ray diffraction techniques have shown that all six carbon bonds in benzene are 0.140 nm, which is between a C-C single bond at 0.147 nm and a C=C double bond at 0.135 nm. Kekule’s structure suggests that there should be three shorter C=C double bonds and three longer C-C single bonds. This evidence disproved the Kekule structure as all six bonds are the same length.
Describe the delocalised structure of benzene.
It is now thought that benzene has a delocalised electron structure. The delocalised model can explain all three pieces of experimental evidence that do not support Kekules rule.
In the delocalised structure, each of the six carbon atoms donates one electron from its p-orbital. These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule. The electrons in the rings are said to be delocalised as they are able to move freely within the ring and do not belong to a single atom. Therefore unlike Kekules structure, all bonds in this ring are identical, so they are the same length.
What does the delocalisation of electrons in benzene lead to benzene being?
The delocalisation of electrons leads to benzene about 152 kJ mol-1 more stable than expected when using the kekulé model. Becuase so much energy is needed to disrupt this delocalisation, benzene is very stable and resistant to addition reactions.
Define a substitution reaction.
a substitution reaction is where a group or atom is exchanged for another group or atom in a chemical reaction.
Define benzene derivative?
A benzene derivative is a benzene ring that has undergone a substitution reaction.
Define a benzene derivative.
A benzene derivative is a benzene ring that has undergone a substitution reaction.
What type of reaction will benzene undergo?
As benzene has delocalised electrons it ios energetically more stable than initially calculated. So it rarely undergoes addition reactions but it will undergo substitution reactions. In these reactions, a hydrogen atom is substituted with a different group and a benzene derivative is formed.
What are the common groups that will substitute a hydrogen atom on a benzene ring?

Single-substituted benzene derivative. What is the name of this compound?

Ethylbenzene
- Stem - the longest chain of carbon atoms is the aromatic ring and has the stem benzene.
- Prefix - there is one ethyl group on the ring so there is no need to number.
How would you name a double-substituted benzene derivative?
(two hydrogen atoms on different cari=bons of benzene can be replaced by two groups)
- If the groups are different, the name is written in alphabetical order. One group will be added first and be given carbon number 1.
- When naming double-substituted benzene derivatives, the smallest possible numbers should be used.
Define electrophilic substitution.
Electrophilic substitution is a substitution reaction where an electrophile is attracted an electron-rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair.
Define a reaction mechanism.
A reaction mechanism is a model with steps to explain and predict a chemical reaction.
What does the electron dense structure of benzene mean it is susceptible to?
Electrophilic attack.
Name the reaction steps that can be associated with electrophilic substitution.
- Electrons above and below the plane of atoms in the benzene ring attract an electrophile.
- The electrophile accepts a pair of π electrons from the delocalised ring and makes a covalent bond. This is the slowest step and known as the rate-determining step.
- A reactive intermediate is formed where the delocalised electrons have been disrupted.
- The unstable intermediate releases an H+ ion and the stable product is formed. This is a very fast step.































