Aromaticity Flashcards
Describe benzene
C6H6
Bond angle: 120°
Bond length: 0.139nm
C-C and C-H covalent bonds
Delocalised ring of electrons in the centre caused by sideways overlapping of p-orbitals
Delocalisation gives a more stable shape and spreads over the whole structure above and below the plane of carbon atoms
Describe the differences in bond lengths between the Kekule model and the Delocalised model of benzene
Kekule model: C-C= 0.154nm C=C= 0.134nm, alternating bond lengths
Delocalised model: C-C= 0.139nm, equal bond lengths
Describe the reaction with hydrogen between the Kekule model and the Delocalised model of benzene
Kekule model: Electrophilic addition forms cyclohexane
Delocalised model: Electrophilic substitution forms cyclohexane
Describe the enthalpy of hydrogenation between the Kekule model and the Delocalised model of benzene
Kekule model: -360KJmol-1
Delocalised model: -208KJmol-1
Why is the Delocalised model of benzene more stable than the Kekule model in terms of enthalpy of hydrogenation?
Due to the delocalised ring of electrons making the delocalised model 152KJmol-1 more stable.
Describe the reactions with bromine between the Kekule model and the Delocalised model of benzene
Kekule model: decolourises bromine at room temperature due to the C=C
Delocalised model: would not decolourise bromine at room temperature
Describe the isomers of 1,2-chlorobenzene between the Kekule model and the Delocalised model of benzene
Kekule model: has two isomers
Delocalised model: has one isomer
How is the delocalised ring of electrons in benzene formed?
The fourth outer shell electron of a carbon atom is in a 2p orbital, above and below the plane of the carbon atoms. These p-orbitals overlap to give a delocalised electron structure, above and below the plane of carbon atoms.
Why can benzene only undergo substitution reactions?
Because if benzene underwent an addition reaction, the process would disrupt the stable delocalised electron system and the resulting product would be less stable. It undergoes substitution as the delocalised system of electrons is retained.
What does benzenes delocalised ring make it susceptible to attack from?
Electrophiles (electron deficient species that can accept a pair of electrons).
What are the three different substitution reactions that benzene can undergo?
Nitration
Halogenation
Friedal-Crafts alkylation
Describe the general mechanisms for benzene’s substitution reactions
A hydrogen atom is replaced by the incoming electrophile. To replace the hydrogen atom by an electrophile the stability of the ring needs to be disturbed, giving an unstable intermediate, which then loses a H+ in a rapid step.
What is the electrophile, reagents and conditions for nitration?
Electrophile: NO2+
Reagents: Concentrated nitric acid and concentrated sulphuric acid
Conditions: Heat to 50°C
What happens if the temperature for nitration of benzene exceeds 50°C?
1,3-dinitrobenzene is also produced as a product.
How can nitrobenzene be reduced?
It can be reduced to phenylamine using tin metal and hydrochloric acid.
What catalysts can be used in a halogenation reaction?
Iron fillings
Iron (III) bromide/chloride
Anhydrous aluminium bromide/chloride
What is the purpose of using a catalyst in halogenation?
To make the polarisation of the halogen more pronounced since the molecule is not sufficiently nucleophilic enough.
What are the conditions and reagents for the bromination of benzene?
Reagents: Br2 and FeBr3
Conditions: room temperature
What can occur if an excess of chlorine is used in halogenation?
A mixture of 1,2-dichlorobenzene and 1,4-dichlorobenzene is produced as a product.
What is the purpose and problem with DDT?
Insecticide made from chlorobenzene. Use is heavily restricted because of problems connected with its toxicity, its persistence in the environment and its presence in the food chain.
What is Friedal-Crafts alkylation used for?
Producing a new carbon to carbon bond, creating alkyl derivatives such as methylbenzene.
What catalyst is used in Friedal-Crafts alkylation?
Anhydrous aluminium chloride.
What reacts with benzene in Friedal-Crafts alkylation?
A halogenoalkane.
What are some problems associated with Friedal-Crafts alkylation?
It can activate the ring towards further alkylation, to reduce this, the halogenoalkane is added slowly during the reaction.
A primary carbocation formed may rearrange to form a secondary carbocation.
How can chloroalkanes form primary alcohols?
Heating under reflux and adding aqueous sodium hydroxide (NaOH).
Why is an aromatic chlorine bond shorter and stronger than an aliphatic chlorine bond?
it results from a non-bonding p electron pair on chlorine overlapping with the ring of electrons on benzene, resulting in a stronger, shorter bond.
How can you produce phenol from chlorobenzene?
Adding aqueous NaOH at 300C and increased pressure.
What is an electrophile?
A positively charged species that can attack an electron-rich centre.
