6.1.1 Aromatic Compounds Flashcards
what is the formula of benzene
C6H6
what are characteristics of benzene
- colourless, sweet smelling and highly flammable
- found in crude oil, cigarette smoke and petrol
- classified as a carcinogen
what is the structure of benzene
- hexagon with circle in middle
- hexagon with double bond on ever other bond
what is the Kekule model of benzene
- hexagon, with alternating double and single bonds
what are the 3 evidence points disproving the Kekule model
1) lack of reactivity
2) length of the C=C bond
3) hydrogenation enthalpies
how does lack of reactivity disprove Kekule’s model
- if benzene did contain C=C
- it would discolour bromine
HOWEVER - benzene does not undergo electrophilic addition
- so it does not discolour bromine under normal conditions
- so must not contain C=C
how does the C=C bond length disprove Kekule
you can measure bond lengths using x-ray diffraction:
- length of C-C is 1.53 nm
- length of C=C is 1.34 nm
HOWEVER
- bromine contains only one bond length
- equal to 1.39 nm in between both
- so must not contain those two bonds
- (evidence examined by crystallographer Kathleen Lonsdale)
how does hydrogenation enthalpies disapprove Kekules theory
- Kekule’s benzene would be called cyclohexatriene
- so would expect its enthalpy to be three times that of cyclohexene
HOWEVER:
1) for cyclohexene, enthalpy change is -120kJmol
2) for cyclohexadiene, its -240kJmol, as expected
3) HOWEVER, for benzene it is -208kJmol, which is 152 kJ mol less energy than expected - so actual structure of benzene must be more stable than expected
what is the actual model of benzene called
the delocalised model
what is the structure of the delocalised model of benzene
- two rings connecting the top and bottom p-orbitals of all the C atoms in benzene
- called the pi-bond system
explain the delocalised model of benzene
- each C-atom uses 3/4 of its available electrons
- each carbon has 1 electron left in a p-orbital perpendicular to the plane of carbon and hydrogen atoms
- adjacent p-orbital electrons overlap sideways in both directions above and below plane of C-atoms
- forms a ring of e-density, creating a pi-bond system spread across all C-atoms
- electrons in pi-bond are delocalised
- DELOCALISED RING OF E- DENSITY ABOVE AND BELOW PLANE OF CARBON ATOMS
how do you name aromatic compounds
- need to use benzene
- and then add prefixes
1) for short alkyl chains (1,4-dimethylbenzene)
2) halogens (bromobenzene)
3) nitro groups NO2 (1,3-dinitrobenzene) - all mono-substituted
when is benzene not the main element of a compound
- when attached to an alkyl chain with a functional group
- when attached to an alkyl group with more than 7 carbon atoms
what is the prefix used for benzene
phenyl-
what is benzene attached to COOH called
benzoic acid
what is benzene attached to NH2 called
phenylamine
what is benzene attached to CHO called
benzaldehyde
what is benzene attached to HC=CH2
phenylethene
what is benzene attached to OH called
phenol
what is benzene attached to CH2COCH3 called
phenylpropanone
what do you need to remember when drawing out anything with benzene
which molecule is attached to the benzene
- eg for phenol IT MUST BE THE O ATTACHED TO THE LINE, AND THEN H AFTER, NOT JUST IN THE MIDDLE
what mechanism does benzene undergo
electrophilic substitution
what is the typical overall equation of electrophilic substitution of benzene
benzene + E+ ===> Ebenzene + H+
- where E+ represents an electrophile
what is the typical mechanism for electrophilic substitution of benzene
1) the E+ attracts an electron pair from the dense pi-bond (shown via curly arrow going from circle in benzene to the E+)
2) the H bonded to the carbon that has gained the E gives its e- pair to repair the pi-bond (shown by curly arrow going from C-H bond to the broken horseshoe bond in benzene, with a + towards the bottom of it)
3) the H+ leaves the molecule and the pi-bond is repaired, leaving the product
what are the uses of nitrobenzene
used in:
- dyes
- pesticides
- starting material of paracetamol
what is the overall equation of the nitration of benzene
benzene + nitric acid ===> nitrobenzene + water
( H2SO4 and 50C on the arrow)
why is benzene heated to 50C in nitration
- obtains a good reaction rate
- not too high that further substitution could occur, resulting in dinitrobenzene from forming
- can put in water bath to maintain temperature
what are the 3 step mechanisms of nitration
1) HNO3 + H2SO4 ===> NO2+ + HSO4- + H2O
- where the nitronium NO2+ ion is used as the electrophile
- both HNO3 and H2SO4 are concentrated
2) the mechanism (check overall for example)
3) HSO4- + H+ ===> H2SO4
why can H2SO4 be described as a catalyst when talking about it in nitration
it is regenerated in the final step, so isn’t used up
what does the halogenation of benzene require
a halogen carrier catalyst, which is generated in situ (in the reaction vessel)
- e.g. AgCl3 and FeBr3
what are the reaction conditions of the halogenation of benzene
room temperature and pressure
what is the overall equation of the bromination of benzene
benzene + Br2 ===> bromobenzene + HBr
- FeBr3 or AlBr3 goes on the arrow
what is the 3 step mechanism of the bromination of benzene
1) Br2 + AlBr3 ===> AlBr4- + Br+
- where the bromonium ion is used as the electrophile ( generated via a catalyst as benzene is too stable to react with non-polar benzene)
2) same mechanism for electrophilic substitution as before
3) FeBr4- + H+ ===> FeBr3 + HBr
- showing that FeBr3 acts as a catalyst
what is the alkylation of benzene
where the H off one of one of the carbon is substituted with an alkyl group
- could also be called Friedel-Crafts
what is the reaction conditions required for the alkylation of benzene
presence of AlCl3
- acts as a halogen-carrier catalyst
- generated the electrophile needed
what is the overall equation for the alkylation of benzene
benzene + R-Cl ===> Rbenzene + HCl
- where R refers to the alkyl chain
- and AlCl3 is on the arrow
what is the acylation of benzene
the reaction of benzene with an acyl chloride and AlCl3 catalyst
- forms an aromatic ketone
what is the structure of acylchloride
alkyl-C=O
-Cl
what is the overall reaction of the acylation of benzene
benzene + R-C=O(-C) ===> benzene-C=O(R) + HCl
- where the R is the alkyl chain
- and AgCl3 goes on the arrow
which mechanisms do alkenes use to react with bromine
electrophilic addition
what is the overall equation of cyclohexene and bromine
cyclohexene + Br2 ===> 1,2-dibromocyclohexane
why can alkenes react with bromine
- have a localised pi-bond above and below the plane of the 2 carbon atoms
- creating an area of high e-density
- which is able to polarise a dipole onto Br2
explain the mechanisms of electrophilic addition
1) arrow goes from double bond to the slightly positive bromine, and arrow goes from bromine bond to the slightly negative bromine
2) one bromine bonds to the alkane, and a carbocation is formed at the other carbon. curly arrow goes from the Br- to the +
3) the final product is formed, with both bromines
why can benzene not react via electrophilic addition
- it contains delocalised pi-electrons
- so the electron density between any 2 carbon atoms is less than in a C=C
- so when non-polar Br2 approaches, there is insufficient pi-electron density around any 2 carbon atoms to polarise it
- so no reaction
what is important to remember when naming phenols
- need to consider which groups take priority
-e.g. NO2 attached to phenol is 2-nitrophenol - BUT a COOH group attached takes priority, so is called 2-hydroxybenzoic acid
why is phenol an acid
what is the solubility of phenol as compared to benzene
benzene: cannot dissolve, as only contains L-forces, and is liquid
phenol: can dissolve as has L-forces, but also H-bonding on the OH group, and is a solid as has stronger IMF
(phenol is less soluble than OH tho as it has the presence of the non-polar benzene ring)
explain why phenol is a weak acid
it only partially dissociates in water, forming a phenoxide ion
what bases can phenol, alcohol and carboxylic react with
alcohol= none
phenol= only strong bases
carboxylic acids= weak and strong bases
how does phenol react with a base
phenol + base ===> salt + water
what would you get with ohenol and sodium hydroxide
phenol + NaOH ===> sodium phenoxide salt + H2O
how would you draw a phenoxide salt
- benzene
- ATTACHED to the O-
- with the metal+ next to it
- NEVER do a bond between metal and O tho, as is not a covalent bond
- for more than monobasic bases, would just do the metal sandwiched between multiple O- benzenes
how would phenol react with a metal
phenol + metal ===> metal phenoxide + 1/2H2
is benzene or phenol more reactive
phenol
why is phenol more reactive than benzene
- lone pair on the oxygen of OH
- partially delocalises into the pi-bond system
- increasing the electron density (8e- with 7 atoms vs 6e- with 6 atoms)
- more reactive as is able to induce dipoles on non polar molecules
- more susceptible to electrophilic attack
- shown by phenol with lone pair on the OH with curly arrow going to the circle of electrons
how would u show the overall pi bond in phenol
- draw out phenol skeletally
- add pi orbitals above and below the Cs and the O
- attach them together in a circle with a bit bumping out
what can be said about the mechanisms in which phenol reacts
- via electrophilic substitution
- with more milder and readily conditions than with benzene
equation for the bromination of phenol
phenol + 3Br2 ===> 2,4,6-tribromophenol + 3HBr
what are the observations of the bromination of phenol
1) would turn from orange to colourless
2) form white precipitate
- at room temp, and NO catalyst needed
what is the equation of the nitration of phenol
phenol + nitric acid ===> either 2-nitrophenol or 4-nitrophenol + H2O
- no 6-nitrophenol needed to be stated as the same as 2- (why you form more of 2 than 4, as 6 is included)
- at room temp
what is disubstitution
- when substituted aromatic compounds go through a second substitution
- e.g. 2-nitrophenol can react again with a nitro group
what are directing groups
groups which have an impact on the second substitution of benzene
what are the activating groups
- increase the electron density, electron donating and are more reactive than benzene with electrophiles
- direct at the 2,4(,6) positions (except halogens which can react here)
- NH2 and OH
what are deactivating groups
- decrease electron density
- electron withdrawing
- less reactive than benzene with electrophiles
- direct at 3 an 5
- NO2
why are directing factors useful
useful for organic synthesis, where can be used to decide which steps are taken for the products to end up on that position
