Aromantic Chemistry Flashcards
What is the molecular formula of benzene?
C6H6
What is the structure of benzene proposed by August Kekulé?
It has a hexagonal structure with alternating single and double carbon-carbon bonds
What does August Kekulé’s structure of benzene suggest and how does this differ from reality?
It suggests that benzene should react like an unsaturated alkene, but it does not undergo reactions characteristic of alkenes. For example benzene does not react with an aqueous solution of bromine but requires vigorous conditions with a catalyst
What is the delocalisation energy of benzene?
The extra stability compared to if it had the structure of cyclohexa-1,3,5-triene
How can you estimate the stability of benzene?
Frrom the enthalpy changes when cyclohexene and benzene react with hydrogen to give cyclohexane
How was the cyclic structure or a benzene molecule confirmed and what was found?
It was confirmed by X-ray diffraction. It was found that all carbon-carbon bond lengths in benzene were about 0.140, halfway between that of a single or double bond. So this rebukes Kekulé’s theory
What is the currently accepted structure of benzene?
Each Carbon has three single bonds with its two adjacent carbon atoms and one hydrogen. These are sigma bonds. Its fourth bonding electron is shared in a pi bond. The pi bonds of the six carbon atoms overlap and the six electrons are shared in a doughnut shaped cloud above and below the plane of the flat hexagon of the carbon atoms. The electrons in these orbitals are said to be delocalised. This is what makes benzene more stable and explains it’s delocalisation energy.
When naming aromatic compounds what role can benzene play?
It can:
• form the root of the name wuch as in methylbenzene, nitrobenzene, chlorobenzene and benzenecarboxylic acid
• be regarded as a substituent in which case the C6H5 group is known as the phenyl group
What do we do if a benzene ring has two or more substituents?
We use numbers to describe the positions of the groups, making sure we give the lowest possible numbers to the substituents
What do the main reactions of benzene involve and what are they called?
The replacement of one or more of the six hydrogen atoms by halogen atoms or a functional group. The hydrogen atom is said to be ‘substitued by the halogen atom or functional group and so these are called substitution reactions
The substitution reaction of benzene is more accurately reffered to as electrophillic substitution. Why is this?
The delocalised electrons are a region of high electron density, so substitution reactions generally involve reaction with a positively charged ion or the positive end of a polar molecule that can accept electrons to form new covalent bonds. These are electrophiles
Why does electrophillic substitution, not addition occur with benzene?
Because of the stable nature of the benzene ring
What is the general reaction mechanism for the electrophillic substitution reactions of benzene and benzene derivatives?
Snap cameral roll
What are the three steps of the electrophillic substitution of benzene?
1) the electrophile ‘E +’ is attracted to the delocalised electron cloud of the benzene ring structure
2) the electrophile bonds to the benzene ring via two of the six delocalised electrons, leaving a positively charged partially delocalised system containing four delocalised electrons spread over 5 carbon atoms. This temporarily causes the loss of the natural stability of the siz electron delocalised benzene system
3) in order to regain the delocalisation energy of the benzene ring, an H+ ion is lost by breaking the C-H bond. The electrons from this bond re-form the stable delocalised siz-electron system
What are the conditions for the electrophillic substitution reactions of benzene?
- the electrophiles have to be produced in the reaction mixture by mixing appropriate reagents
- the reaction condiditons are generally severe: you have to use heat, concentrated reagents and catalysts to make the reaction take place
