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
what halogen carriers can be used
- ALCl3 , FeCl3 , AlBr3, FeBr3
what are the two friedel craft reactions
friedel - craft alkylation and friedel - craft acylation
what is friedel craft alkylation
substitution of a hydrogen atom by an alkyl group
it is carried out by reacting benzene at room temp with a haloalkane in the prescence of AlCl3 whicb acts as a halogen carrier catalyst , generating the electrophile
RCl + AlCl3–> R+ + AlCl4-
what does alkylation do to the benzene ring
increases the number of carbon atoms in a compound by forming carbon - carbon bonds
draw the mechanism for alkylation
what is acylation
benzene reacts with an acyl chloride in the prescence of an AlCl3 to form an aromatic ketone
how is phenylethanone produced
between benzene and ethanoyl chloride and is used in the perfume industry
what is an acyl chloride
when we replace the ~oH group in a carboxylic acid we produce an acid derivative then we replace the OH group with a chlorine atom
draw the mechanism for acylation
what are phenols
a class of organic compound in which a hydroxyl group -OH is attached to one of the carbons in a benzene ring
what could you mistake for a phenol
an aromatic compound with an OH group attached to a side chain of benzene which is an aromatic alcohol not a phenol eg 2-phenylethanol
what are the physical properties of phenol
pure phenol is a white crystalline solid , smelling of disinfectant . it has to be handled with great care because is causes immediate white blistering to the skin. the crystals are often rather wet and discoloured
what are the uses of phenol
used in the production of disinfectants , detergents , plastics , paints and aspirin
what do we know about the solubility of phenol
phenol is less soluble in water than alcohols due to the non polar benzene ring. however phenols are less acidic than carboxylic acids
what happens when phenol is added to water
phenol partially disocciates in water forming the phenoxide ion and a proton .
this forms a weak acidic solution
with equilibrium lying to the left
what are the melting and boiling points of phenol
higher values than expected due to hydrogen bonding . hydrogen bonds can form between a lone pair on an oxygen on one molecule and and the hydrogen on the -oh group on one of its neighbours . so phenol is moderately soluble in water
how can we use ka values to distingusih between carboxylic acids , phenol and alcohols
ka values will be higher for stronger acids
what test can we use to distingusih between carboxylic acids and phenols
react both with sodium hydroxide or a weak base and only carboxylic acid will produce carbon dioxide as phenols do not react with sodium carbonate
what bases does phenol react with
- no reaction with weak bases only strong bases
therefiore
-phenol does not react with sodium carbonate , only sodium hydroxide
what base do carboxylic acids react with
-strong and weak bases
so sodium carbonate and sodium hydroxide
what bases do alcohols react with
not weak nor strong so no reaction with sodium carbonate or sodium hydroxide
draw the reaction between water and phenol
what is the phenoxide ion
phenol is able to lose a hydrogen ion because the phenoxide ion formed is stabilised through the negative charge on the oxygen atom being delocalised around the ring. increased reactivity of phenols results from one of the lone pairs on the oxygen atom ( from a p orbital) overlaps with the delocalised electrons on the benzene ring. as a result the negative charge is not entirely localised on the oxygen , but is delocalised around the whole ion. this means a higher electron density in the ring which we say is activated . this increased electron density attracts electrophiles more strongly than with benzene . spreading the charge aroun makes the ion more stabke than it would be if all the charge remained on the oxygen
what are the other reactions of phenol
- phenol forms a phenoxide on reaction with sodium hydroxide and phenol , producing water and sodium phenoxide
phenol reacts with sodium producing hydrogen gas and sodium phenoxide
bromine and phenol - will react will bromine water in the cold and in the absense of any catalyst this produces a white precipitate of 2,4,6 - tribromophenol and hbr and bromine water is decoloried
- nitration of phenol - phenol reacts readily with dilute nitric acid at room temperature . it forms a mixture of 2-nitrophenol and 4- nitrophenol
why is no halogen carrier required for phenol
the electron density in the phenol is now sufficient to polarise bromine molecules , so no halogen carrier is required the increased electron density of phenols ring of electrons increases the reactivity towards all electrophiles not just with bromine
what are directing groups
the reactivity of the aromatic ring is changed in the prescence of an attached group - a substituent and these groups make the ring more reactive ( activate the ring) or less reactive ( deactivate the ring)
what happens when phenylamine reacts with bromine
- NH2 activates the ring
- ring reacts more readily with electrophilles when NH2 is attached
-NH2 group increases the ring electron density
-does not require a halogen carrier catalyst
what happens in the reaction with bromine and nitrobenzene
bromine reacts slowly with nitrobenzene and it requires a halogen carrier and a high temperature (less susceotible to electrophillic substitution than benzene itself
the NO2 group deactivates the ring and the ring reacts less readily with electrophiles when the NO2 group is attached
-the NO2 group decreases the electron density
what do electron donating groups do to the ring
activate the ring
what do electron withdrawing groups do to the ring
deactivate the ring
what are directing effects
different groups on the benzene ring can have a directing effect on any substituent on the benzene ring
what are 2 and 4 directing groups
donate electrons onto the ring
reactivity increases
2nd subsitution at 2 or 4 positions
what are 3 directing groups
group withdraws electrons from the ring
reactivity decreases
2nd subsitition at 3rd position
describe the bonding in a benzene molecule
-3 electrons from each carbon atom is used to form sigma bonds to 2 carbon atoms and 1 hydrogen atom
-delocalised electron in a p orbtial overlaps with electrons from adjacent p orbtials
-this creates a delocalised system of electrons
-this creates a sytem of pi bonds above and below the plane of carbon atoms
