aromatic compounds Flashcards
What is benzene
- colourless, sweet smelling, highly flammable liquid
- found naturally in crude oil component of petrol and found in cigarette smoke
- classified as a carcinogen can cause cancer
Describe what a benzene molecule consists off
- a hexagonal ring of six carbon atoms with each carbon atom joined to two other carbon atoms and one hydrogen atom
What benzene classified as
aromatic hydrocarbon
What is the molecular formula fo benzene
C6H6
What is the kekule model
- the tructure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds
- he dreasmed that the ring shape of benzne is about a snake seizing it’s own tail
What were the three pieces of evidence to disprove kekule model
- lack of reactivity of benzene
- lengths of carbon carbon bonds in benzene
- hydrogentation enthalpies
how does the lack of reactivity of benzene disprove
- if benzene contained C=C bonds it would decolourise bromine in an electrophilic addition
- benzene does not undergo electrophillic addition reactions
- bezene does not decolourise bromine under normal conditions
- therefore benzene cannot have any C=C bonds in it’s structure
How does the lengths of carbon to carbon bonds in benzene disprove kekules model
- using a technique called x ray diffreaction it is possible to masure bond lengths in a molecule
- found that all the bonds in benzene were 0.139nm in length
- this bond length between the length of a single bond 0.153nm and a double bond 0.134nm
how does hydrogenation enthalpies disprove kekule model
- kekule structure contains alternate single and doouble bonds
- if benzene did have kekule structure it would be expected to have an expected enthalpy change of hydrogenaition three times thaat of cyclohexene
- when cyclohexee is hydrogention one double bond reacts with hydrogen
- the enthalpy change of hydrogenation is 120kJmol-1
- therefore kekule structure would predict to contain three double bonds and so the enthalpy change of hydrogenation would be -360KJmol-1
- the actual enthalpy change o hydrogentation of benzene is only 208KJmol-1
- means less energy is produced than expected
- so benzene is more stabe than the theoretical kekule model
Describe the delocalised model of benzene
- benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms
- each carbon atom uses three of its availabe four electrons in bonding to two other carbon atoms and to one hydrogen atom
- each cabron atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
- adjacten p orbital electrons overlap sideways in both directions above and below the plane of the carbon atoms to form a ring of electron density
- this overlapping of the p orbitals creates a system of pi bonds which spread over all six od carbon atoms in the ring structure
- the six electrons occupying the system of the pi bonds are said to be delocalised
Describe the mechanism for the reaction of benzene
- benzene and it’s derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replace d by another atom of a group of atoms
- benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophhilic substitution
Describe the nitration of benzene
- benzene reacts slowly with nitric acid to form nitrobenzene
- reaction is catalysed by sulfuric acid and heated to 50 degrees celcius
- water bath used to maintain the steady temperature
- NO2 group replaces the hydrogen
Why can’t the temperature be over 50 degrees during nitration of benzene
- further substitutition reactions may occur leading tot eh production of dinitrobenzene
What are the ues of nitrobenzene
- starting material in the preparation of dyes, pharmaceuticals and pesticides
- starting material in the preparation of paracetemol
What is the reaction mechanism for the nitration of benzene
electrophillic substitution
What i the electrophile in nitration of benzene
NO2+
how is the electrophile of benzene produced
- reaction of concentrated nitric acid and concentrated sulfuric acid
how does No2+ react with benzene ring
- No2+ accepts a pair of electrons from the benzene ring to form a dative covalent bonds
- the organic intermediate formed is unstabel and breaks down to form the organic product nitrobenzene and the H= ions
What is step 1 for nitration of benzene
HNo3 +H2SO4->No2+ +HSO4- +H2O
What is the final step for nitration of benzene
H+ +HSO4- ->H2SO4
Describe the halogenation of benzene
- halogens do not react with benzene unless a catlayst called a halogen carrier is present
- halogen carriers can be generated in situ (in the reaction vessel) from the metal and the halogen
What are common halogen carriers
- FeCl3
- AlBr3
- FeBr3
- AlCl3
describe the bromination of benzene
- at room temperatyre and pressure and in the prescence if a halogen carrier via electrophillic substitution
Why does bromination of benezene require a halogen carrier and describe the bromination of benzene
- too stable to react with a non polar bromine molecule
- the electropile a brominium on Br+ which is generated when the halogen carrier catalyst reacts with bromine in the first stage of a mechanism
- brominium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond
- organic intermediate is unstable and breaks down to form the organic product bromobenzene and H+ ion
Describe how the brominium ion is produced when in the bromination of benzene with the halogen carrier FeBr3
Br2 +FeBr3 - > FeBr4- +Br+
Describe how the halogen carrier FeBr3 is reformed after the bromination of benzene
H+ +FeBr4- ->FeBr3 + HBr
Describe the alkylation of benzene
- substiution of a hydrogen atom in the benzene ring by an alkyl group
- the reaction is carried out by reacting benzene with a haloalkane in the prescence of ALCl3
- acts as a halogen carrier catalyst generating the electrophile
*
What is the name of alkylation of benzene
friedal crafts alkylation
Describe acylation reactions with benzene
- when benzene reacts with acyl chloride in the prescence of an AlCl3 catalyst, an aromatic ketone is formed
- electrophillic substiution
*
What is the first member of the acyl chloride group and what is fromeed when it reacts with benzene
- ethanoyl chloride CH3COCl
- pheylethanone
decribe the reaction between bromine and the double bond in cyclohexene
- pi bond in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond
- produces an area of high electron density
- the localcised electrons in the pi bond induce a dipole in the non polar bromine molecule making one bromine atom of the Br2 molecule slightly positive and the other bromine atom alightly negative
- the slightly positive bromine atom enables the bromine molecule to act like an electrophiile
- via electrophillic addition
Why does benzene not react with bromine unless a halogen carrier is present
- benzene has delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure
- the electron denity around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene
- when non polar molecule such as bromine apporaches the benzene ring there is insufficient pi electron density around any two carbon atoms to polarise the bromine molecule
- preventing any reaction taking place
What is a phenol
- phenols are a type of organic chemical containing a hydroxyl group -OH and a functional group directly bonded to an aromatic ring
What is the simplest phenol
C6H5OH
Describe why phenol is a weak acid
- less soluble in water than alcohols due to the prescence of the non polar benzene ring
- when dissolved in water phenol partially dissociated froming the phenoxide ion and a proton
- partially dissociate to produce protons
Decribe how you can tell phenol i more acidic than alcohols but less acidic than carboxylic acids
- ethanol does not react with sodium hydroxide of sodium carbonate
- phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide
- only carboxylic acids are strong enough to reat with sodium carbonate
What is the test for a carboxylic acid from a phenol
- reaction with sodium carbonate can be used
- carboxylic acid reacts with sodium carbonate to produce carbon dioxide as a gas
What are the products of phenol and sodium hydroxide and what type of reaction is this
- form the salt sodium phnoxide and water in a neutralisation reaction
Describe the reactions that phenol undergoes
- electrophillic substitution
Describe the bromination of phenol and the conditions
- phenol reacts with aqeueous solution of bromine (bromine water) to form a white precirpitate 2,4,5 tribromophenol
- the reaction decolourises the bromeine water
- does not require a halogen carrier and reaction is carried out at room temperature
Describe the nitration of phenol
- phenol reacts readily with dilute nitricc acid at room temperature
- a mixture of 2-nitrophenol and 4-nitrophenol is formed
Compare the reactivity of phenol and benzene
- bromine and nitric acid react more readily with phenol than they do with benzene
- phenol is nitrated with dilute nitric acid rathre than needed concentrating nitric and sulfuric acids as with benzene
- increased reactivity is caused by a lone pari fo electrons from the oxygen p orbital of the OH group being donated into the pi system of pheno
- the electron density of the benzene ring in phenol is increased
- the increased electron density attrats electrophiles more strongly than with benzene
- the aromatic ring in phenol is therefore more susceptible to attack from elextrophiles that in benzene
- for bromine the electron denitry in the phenol ring structure is sufficient to polarise bromine molecule so no halogen carrier catalyst is required
Describe the difference between activation and deactivation with reactions with benzene given that it’s with phyelamine and bromine and nitrobenzene and bromine
- bromine requires a halogen carrier catalyst to react with benzene whereas bromine will react rapidly with phenylamine
nitrobenzene reacts slowly with bromine requiring both a halogen carrier catalysst and a high temepratyre - the benzene ring in nitrobenze is less susceptible to electrophillic substiution that benzene
- for example -NH2 groups activates the ring as the aromatic ring reacts more readily with electrophiles
- the -No2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles
- positions are also different NH2 is directed to positions 2 or 3 and No2 group directs the second substituent to position 3
What are 2 and 4 directing (ortho and para directing) group
- -NH2 or NHR
- OH
- OR
- R of C6H5
- F, Cl, Br I
What are 3 dircting (meta directing group)
- RCOR
- COOR
- SO3H
- CHO
- COOH
- CN
- NO2
- NR3+
Describe how the positioning groups are related to activation and deactivation
- 2 and 4 directing groups - ortho and para directors are activating groups with the exception of halogens
- all 3- directing groups - meta directors and deactivating groups
