4.2 aromaticity Flashcards
what does aromatic mean?
study of compounds containing carbon rings
what is an aromatic compound defined as?
- a planar and cyclic structure with more stability than other geometric arrangements of atoms with the same molecular formula
what is an arene defined as?
- a hydrocarbon with sigma bonds and delocalised pi electrons between carbon atoms forming a circle
what is the molecular formula of benzene?
C6H6
what is the structure and bonding in benzene?
- hexagon made of 6 carbon atoms in a planar ring
- each carbon covalently bonded to two other carbons and a hydrogen
- (this means each carbon atom has one unpaired electron in the outer p-orbital. these unpaired electrons combine to form a delocalised ring of electrons)
- each bond angle in the structure is 120°/trigonal planar arrangement
- 4th electron (2p) forms a 𝛑 bond by overlap with the 2p electron from adjacent carbons. this results in a ‘ring’ of delocalised 𝛑 electrons above and below the plane of the benzene molecule
- this results in double bond character between C atoms
- each C-C in the benzene ring had a length between that of a double and single bond which suggests that the carbon-carbon bonds are an intermediate between single and double bonds
- ⏣
what is the cyclohexatriene structure of benzene?
- Kekule structure (proposed by August Kekule in 1865)
- the structure suggests that benzene is cyclic with 3 x C=C bonds
- consists of alternate single and double carbon bonds
- the structure is still used today to represent benzene but is not the official accepted structure of benzene as this model had some flaws
- ⌬
what are the ‘resonance’ versions of Kekule’s structures?
- kekule proposed the structures were not static and alternated between each structure (double bonds differed but still separated as usual)
- we call these the ‘resonance’ versions
- ⌬
what were some of the problems with the Kekule structure?
- if benzene is unsaturated, it should undergo electrophilic addition reactions readily, but it doesnt
- when benzene is chlorinated in the presence of an aluminium chloride catalyst to form 1,2-dichlorobenzene, only 1 structure can be formed; which indicates that all the hydrogens in benzene are equivalent
- measurements from xray diffraction studies show that the electron density around carbon atoms is equal and that all the C-C bond lengths are equal and measure 0.140nm - between that of a double and single bond
how can the stability of benzene be demonstrated?
by comparing the enthalpies of hydrogenation of similar compounds to benzene
what is the predicted enthalpy of hydrogenation of cyclohextriene (kekule’s structure)?
what is the actual value?
- if the enthalpy of hydrogenation of cyclohexane is -120KJmol^-1 for hydrogenating 1 double bonds
- cyclohexadiene should be -249KJmol^-1 for 2 double bonds
- so cyclohextriene (kekule’s structure) should be -360KJm^-1 for 3 double bonds
- however when experimentally performed on benzene, it showed a hydrogenation value of -208KJmol^-
benzene is (more/less) stable than predicted by the hydrogenation of enthalpies?
more
(151KJmol^-1 more)
- which is why benzene is described as being aromatic and has stability
what is the value that benzene is stabilised by sometimes referred to as?
- stabilisation energy/resonance energy/delocalisation energy of benzene
why is benzene more stable?
- any compound gains stability if its able to move electrons around (delocalised electrons)
- all of the double bonds in benzene overlap and therefore lots of delocalisation occurs - which enhances the stability
- this is sometimes described as conjugation which confers stability to the benzene ring
- more conjugation (delocalisation) = more stability
more overlapping p orbitals, the (more/less) stable it is?
more
what does alicyclic mean?
- a term used to describe an organic compound which contains a ring but is not aromatic (benzene ring)
how do you name aromatics?
- mostly the same but with benzene as the suffix
- number the carbons in the ring from the carbon with a substituent on
- always number using the lowest number combination
- e.g 1-2, dichlorobenzene
- benzaldehyde
- benzoic acid/benzene carboxylic acid
what type of reactions does benzene undergo?
- electrophilic substitution reactions
why does benzene undergo electrophilic substitution reactions and not behave like a normal alkene and undergo electrophilic addition reactions?
- benzene is resistant to reacting due to its enhanced stability
- it will therefore not behave like a normal alkene and undergo electrophilic addition reactions
- to keep its stability, benzene undergoes electrophilic substitution reactions
- addition reactions to benzene would destroy the stability - which is energetically unfavourable
- the region of high electron density above and below the plane of the molecule results in benzene being attacked by electrophiles
what are the 3 reactions of benzene?
- halogenation - adding a halogen
- nitration - adding a nitro, NO2 group
- friedel-crafts alkylation
(all electrophilic substitution)
for the halogenation of benzene, why wouldn’t a reagent like HCl not work?
- as the chloride is negatively charged and would be repelled by the high electron density in the benzene ring
how does the halogenation of benzene work?
e.g - cant use a reagent like HCl as its Cl-
- so instead need to make a ‘positive chloride ion’
- this is done by reacting chlorine gas with anhydrous AlCl3 (catalyst)
AlCl3 + Cl2 —> AlCl4- + Cl+ (electrophile)
- eventually produces H+ and chlorobenzene
AlCl4- + H+ —> AlCl3 (regenerated catalyst) + HCl
- an electrophilic substitution reaction
how does the nitration of benzene work?
- same mechanism of electrophilic substitution
- higher temp causes multi substitution on the benzene ring (so not higher than 55°C)
- adding conc nitric and conc sulfuric acids generate the NO2+ ion which is known as the nitronium ion
cH2SO4 + cHNO3 —> HSO4- + NO2+ (nitronium ion) + H2O
what are the conditions for the halogenation of benzene?
- room temperature
- chlorine gas, Cl2
- anhydrous AlCl3 (catalyst)
what are the conditions for the nitration of benzene?
- cH2SO4 AND cHNO3
- 55°C
- reflux