Aromatic Chemistry (alevel) Flashcards
What is the empirical and molecular formula of benzene
CH , c6h6
What is Is Kekule structure of Benzne
Cyclic hexene - 3 double bonds
What is Benzene
A colourless , sweet smelling highly flammable liquid found naturally in crude oil is a component of petrol and in cigarette smoke . It is classified as a carcinogen and can cause cancer . It is classified as an aromatic hydrocarbon or arene. Many aromatic compounds can be synthesised from benzene
Why are arenes often called aromatic compounds
They were first extracted from sweet smelling oils like balsam
What is the evidence to disprove kekule’s model of benzene
- lack of reactivity of benzene
If benzene contained the c=c bonds as shown in kekules benzene it should decolourise bromine water in an electrophilic substitution reaction. However benzene does not decolourise bromine water under normal conditions which tells us benzene does not undergo electrophilic additition reactions leading us to suggest it doesn’t have c=c bonds in its structure
-bond lengths of benzene
Using a technique called x-ray diffraction , it is possible to measure bond lengths in a molecule . And it was found all carbon - carbon bonds in benzene are of equal length and is between the length of a double bond and single bond 0.139nm in length . However double bonds are usually shorter than single bonds (0.134 and 0.154) which shows kekules structure is incorrect - thermodynamic stability of benzene
Enthalpy of hydrogenation to cyclohexane was found to be -120 kjmol-1 so we would assume as benzene has 3 double bonds it would be -360 however it is less exothermic than expected and is -208 therefore benzene is more stable than we predict
What does the enthalpy evidence tell us about the structure
More energy is put in to break the bonds in benzene that would be needed for the Kekule structure . This 152kjmol is known as the delocalisation energy or resonance energy of benzene
how is a ring of electron density formed above and below the plane of the carbon atoms in benzene
each c atom uses 3/4 of its available electrons in bonding and so each c atom has 1 electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms . adjacent p orbital electrons overlap sideways in both directions and this overlapping creates a system of pi bonds which spread over all 6 of the atoms in the ring structure
what are the six electrons said to be
the 6 electrons occupying the system of pi bonds are said to be delocalised
what does the spreading of electrons do for benzene
stabilizes the molecule .
what are the rules for naming benzene containing molecules
- benzene considered parent chain eg nitrobenzene(NO2) or bromobenzene or ethylbenzene
- unless its attached to a alkyl chain with 7 or more carbon atoms or has a functional group
then you use the prefix phenyl eg 2-phenyloctane - if there is more than one substituent group , c=1 is one of the groups , if one of them is a methyl group then the name is methylbenzene , listed in alphabetical order with lowest possible numbering
what do you have to be careful about when naming benzene molecules
not adding too many carbon atoms to the alkyl chain on benzene
what are the 3 naming exceptions and draw these
benzoic acid C6H5COOH
phenylamine C6H5NH2
benzaaldehyde C6H5CHO
whats the difference between bonding in benzene vs alkenes
- in eg ethene pi bonds contain localised electrons above and below the plane of atoms and has a high electron density and this localised electron density will readily polarise
- however in benzene its delocalised so will not readily polarise as has delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure
therefore alkenes can induce a dipole in a non -polar molecule but benzene has insufficient pi electron density to polarise a molecule
what reactions does benzene undergo
- does not undergo addition reactions as would require electrons from delocalised system
-this would disrupt the delocalisation of the ring structure . this would result in the product being less stable than benzene which is energetically unfavourable .
-benzene takes part in substitution reactions , as the product retains the delocalised structure and the stability
what occurs in electrophilic substitution with benzene
- two of the delocalised electrons are donated to the electrophile forming a covalent bond
-the c-h bond breaks and two electrons are returned to the delocalised ring - the unstable intermediate breaks down to form the organic product and the H+ ion
what are the three reactions of benzene
- nitration of benzene
-halogenation of benzene
-acylation reactions
what happens in the nitration of benzene
- converts benzene into nitrobenzene(C6H5No2)
-is also a condensation reaction as produces water - react benzene with HNO3 to form nitrobenzene and h20
uses an acid catalyst (h2so4)
and reflux at 50c
50c is used to ensure there is only one substitution as if the temp increases there is a greater chance of getting more than one nitro group substituted into the ring
what is nitrobenzene used for
- preparation of dyes
- pesticides
-polishes - precursor for explosive
-important for pharmaceuticals
what are the steps for the nitration of benzene
first we generate our electrophile
HNO3+H2SO4—> NO2+ +HSO4- +h20
this electrophile accepts a pair of electrons from the ring to form a dative covalent bond
this disrupts the stable delocalised system and forms an unstable intermediate
finally the unstable intermediate breaks down to form nitrobenzene and the H+ ion to restire stabilitt rhe electrons in theC-h bond moves back into the ring
then the catalyst is regenerated from the H+ lost from the ring
H+ + (HSO4- )—> H2SO4
why does H2so4 act as a cataylst in this reaction
because it is regenerated at the end
what occurs in the halogenation of benzene
1) generation of electrophile , Br2+FeBr3—> Br+ + FeBr4-
then regenerated into
H+ + FeBr4- __> FeBr3 + HBr
why is a halogen carrier needed for the halogenation of benzene
benzene is too stable to react with non - polar halogen molecules
the electron density is insufficient to polarise a non - polar halogen molecule therefore a catalyst is required to polarise the halogen and then benzene can react with bromine at room temperature and pressure
