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
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
how does the Friedel-Crafts alkylation of benzene work?
- adding an alkyl group (carbon chain) to the benzene ring
- aluminium or iron (III) chloride required
- also the chloroalkane of the carbon chain you want to substitute onto the ring
- this reaction creates a carbocation and this is the electrophile that the benzene ring attacks
what are the conditions for the friedel-crafts alkylation of benzene?
- FeCl3/AlCl3
- the chloroalkane of the carbon chain you want to substitute onto the ring
- (the reactants should be heated under reflux in a non-aqueous solvent)
explain the delocalised electron system of benzene in terms of the types of bonds involved:
- the p orbital on each carbon atom in benzene overlaps with the p orbitals either side of them
- this series of overlaps produces a pi bond system
- these electrons are spread out over the whole carbon ring and therefore are described as being delocalised
what is the shape and bond angle of benzene?
- benzene is a planar, regular hexagon
- the shape around each carbon atom is trigonal planar
- bond angle of 120°
what is the general name for compounds containing a benzene ring?
- arenes or aromatic compounds
why does benzene not undergo electrophilic addition reactions?
- benzene’s delocalised ring of electrons is a region of high electron density which attracts electrophiles
- however, the ring of electrons is very stable so benzene does not undergo electrophilic addition as this would break up the delocalised ring of electrons
how is the acyl chloride electrophile made stronger for friedel-crafts acetylation?
- an acyl chloride is reacted with AlCl3
- the AlCl3 accepts a lone pair of electrons from the acyl chloride
- this increases the polarisation in the acyl chloride and it forms a carbocation
- this is a much stronger electrophile so its able to react with the benzene ring
give the equation for the reaction between aluminium chloride and ethanoyl chloride:
CH3COCl + AlCl3 —> CH3C+O + AlCl4-
explain how AlCl3 behaves as a catalyst in the Friedel-Crafts acyl of benzene:
- AlCl3 initially reacts with an acyl chloride to produce a reactive carbocation intermediate and AlCl4-
- at the end of the mechanism a hydrogen ion is released from the benzene ring
- this reacts with the AlCl4- to produce HCl and AlCl3
- therefore AlCl3 doesnt get used up during the reaction and so acts as a catalyst
why are electrophiles attracted to benzene?
- electrophiles are electron pair acceptors (electron deficient) so are attracted to areas of high electron density
- benzene has a ring of delocalised electrons
- this is an area of high electron density so it attracts electrophiles
what conditions ensures mononitration will occur in the nitration of benzene?
- mononitration is when only one NO2 group is added to benzene
- this occurs if the temperature is kept below 55°C
why is the C-Cl bond stronger in benzene than in a chloroalkane?
- the lone pair of electrons on the chlorine atom is delocalised over the benzene ring which strengthens the carbon-chlorine bond
why is chlorobenzene unreactive towards nucleophiles?
- the C-Cl bond in benzene is stronger than in halogenoalkanes
- this makes it harder to break and so is not easily attacked by nucleophiles
