Benzene And Its Compounds Flashcards
What is an arene?
Hydrocarbon containing one or more benzene rings.
Describe the structure of benzene
Each of the six carbon atoms in the hexagonal planer ring is SP2 hybridised.
There are three sigma bonds on each carbon atom and one pie bond.
The pie bonds are spread over all six carbon atoms and are said to be delocalised.
The pie bonding is formed by the overlap of carbon P atomic orbitals.
The bond angle is 120
Why do benzene not undergo addition reactions?
Addition reactions into the benzene ring would disrupt the aromatic stabilisation achieved by the complete delocalisation in the ring.
by substituting an atom the highly stable delocalised ring of pie bonding remains intact.
Describe electrophilic substitution with chlorine or bromine with benzene.
Benzene will react with bromine or chlorine in the presence of anhydrous aluminium bromide (AlBr3) and anhydrous aluminium chloride (AlCl3).
The electrophile is created when an aluminium bromide molecule polarises a bromine molecule. The Br2 molecule forms a dative coordinate bond with aluminium bromide by donating a lone pair of electrons from one of its bromine atoms into an empty 3P orbital in aluminium. This draws electrons from the other bromine atoms in the Br2 molecule making it partially positive and creating an electrophile. The electrophile is the Br+ cation.
The Br+ cation and the electron rich benzene ring are attracted to each other in a electrophilic substitution reaction .
You should know the mechanism for this.
Products of this reaction are chlorobenzene and HCL.
The catalyst in this reaction is known as halogen carriers.
What is electrophilic substitution?
The replacement of an atom by another atom or group of atoms after initial attack by an electron deficient species.
What are the electron donating groups?
NH2
OH
R
Cl
When these groups are bonded directly to benzene ring they direct the incoming electro file to attack the two, four and/or six positions. These groups activate attack by electrophiles because they tend to donate electrons into the benzene ring.
What happens when methylbenzene reacts with chlorine gas using anhydrous aluminium chloride?
Two products can be made. 2-chloromethylbenzene and 4-chloromethylbenzene.
Excess chlorine gas is bubbled through then we conform three compounds with the six position as well .
Note that the methyl side chain of the benzene ring is not affected under the conditions above however chlorine will react with alkanes in the presence of UV light. This reaction is free radical substitution reaction. Note that there is no substitution into the benzene ring under these conditions.
In excess chlorine, eventually all three of the hydrogen atoms on the methyl side chain will be replaced by chlorine atoms.
Describe the nitration of benzene
Electrophillic substitution reaction
Concentrated nitric acid and concentrated sulfuric acid react to give nitronium ion which is the electrophile
HNO3 + 2H2SO4 ➡️ NO2+ + 2HSO4- + H3O+
The nitrating mixture is refluxed with benzene at 25-60 degrees Celsius to make nitrobenzene
The NO2 group is electron withdrawing so any further substitutions are directed to 3,5 positions.
H+ ion is also a product
What are the electron withdrawing groups?
-NO2
-COR
-CHO
-COOH
These groups when directly bonded to the benzene ring direct the incoming electro file to attack the three or five position. These groups deactivate attack by electrophile because they tend to withdraw electrons from the benzene ring.
Describe alkylation or acylation of benzene (Friedel-crafts reactions)
Friedel-crafts reactions is the electrophilic substitution of an acyl or an alkyl group into a benzene ring. They result in the introduction of a side chain into a benzene ring.
The electrophile is formed by adding an aluminium chloride catalyst with heat under reflux to:
*a halogenoalkane to make alkylbenzene
*An acyl chloride to make an acylbenzene
This creates a carboncation electrophile to attack the benzene in the first step.
Example of generating electrophile:
C2H5Cl- + AlCl3 ➡️ CH3CH2+ [AlCl4-]
The Carbocaine electro file made then attacks a benzene ring. The aluminium chloride catalyst is regenerated in the final step.
Further alkylation of the benzene ring can take place as the reaction proceeds.
Describe the oxidation of the side chain in Arenes
Oxidation reaction can happen in alkylarenes. The alkane side-chain is oxidised to form a carboxylic acid.
The reagent is (hot alkaline KMNO4) from ms aqueous or K2CR207 aqueous
The condition is heat under reflux.
The reagent is then acidified with dilute sulfuric acid.
Alkyl group is always oxidised regardless of the chain length. If more than one alkyl group each one is oxidised.
Describe the hydrogenation reaction benzene
The reaction is carried out with benzene being heated with hydrogen gas and a nickel or platinum catalyst. Benzene is converted into cyclohexane.
This is a type of electrophilic addition reaction.
Describe phenol
Is a crystalline solid that melts at 43°. The melting point of phenol is relatively high for an aryl compound of its molecular mass because of the hydrogen bonding between its molecules however the large polar benzene ring makes phenol on only slightly soluble in water as a disrupt hydrogen bonding with water molecules.
How can phenol be prepared?
We can prepare phenol by reacting phenyl amine with nitric acid using ice to keep the temperature below 10°C. The reaction gives an unstable diazonium salt. We then warm the diazonium salt in the aqueous solution to produce phenol.
- NaNO2 + HCl ➡️ HNO2 (nitric acid) + NaCl
- Phenyl amine + HNO2 + HCl ➡️ benzene diazonium chloride (diazonium salt)
The diazonium salt formed is unstable and will decompose easily on warming with water giving of nitrogen gas. - C6H5N2+Cl- + H2O ➡️ C6H5OH + HCL + N2
Describe the acidity of phenol
Phenol is weakly acidic losing an H+ ion from its hydroxide group. Phenol is stronger than water which is stronger than ethanol in acidity.
The order of acidity is explained by the conjugate basis formed. The phenoxide ion C6H50 minus has its negative charge spread over the whole ion as one of the lone pairs on the oxygen atom overlaps with the delocalised pi bonding system in the benzene ring. This reduces the charge density of the negative charge. Therefore H plus ions are not strongly attracted to the phenoxide ion as they are to hydroxide or oxide ions.
Phenol ionises to form a more stable negative ion than water and ethanol with its charge spread out. This stability means that the ionisation of phenol is more likely to happen. So a higher proportion of phenol molecules donate an H plus ion then in dissociation for water and ethanol.
Why is ethanol a weaker acid than water?
Ethanol is a weaker acid than water because of electron donating alkyl group attached to the oxygen atom in the oxide ion. This has the effect of concentrating more negative charge on this oxygen atom which more readily accepts an H plus ion.
Describe the reaction of phenol with sodium hydroxide.
A weak acid, so it will react with an alkali to give a salt plus water
Phenol + NaOH ➡️ Sodium phenoxide (salt) + H2O
Sodium phenoxide is soluble in water .
Describe the reaction of phenol with sodium metal
Molten phenol reacts vigourously with sodium metal giving off hydrogen gas and again forming sodium phenoxide.
Compare the reactions of phenyl and benzene with electrophiles
Compared with benzene phenol reacts more readily with electrophiles. The overlap of one of the lone pair of electrons on the oxygen atom in the OH group with the pie bonding system increases the electron density of the benzene ring and phenol. This makes phenols benzene ring more open to attract from electron deficient electrophiles than benzene itself. The electron donating OH group activates the benzene ring directing substitution to positions 2 4 and 6.
Describe the bromination of phenol
Phenol undergoes the reaction under milder conditions than benzene.
For example, bromine water will not react with benzene at room temperature. To produce Bromo benzene we need pure bromine and an aluminium bromide catalyst. However, bromine water reacts readily with phenol. Phenol decolourises the orange bromine solution and forms a white PPT of 2,4,6- tribromoethanol .
Describe the nitration of phenol.
With benzene we need a mixture of concentrated nitric acid and sulphuric acid to reflux with benzene between 25°C and 60°C for nitration to take place. However, the activated ring and undergo nitration with nitric acid at room temperature.. if we use concentrated nitric acid we get 2,4,6- nitrophenol.
What is the test for phenol
When excess bromine reacts with phenol a white ppt is formed (tribromophenol) forms due to the substitution of bromine atoms onto the aromatic ring.
Suggest how sigma and pi bonds are formed in a benzene molecule
Sigma bonds are formed from end-on-end overlap between C-H/C-C/C-N.
Pi bonds are formed by sideways overlap of p orbitals between C-N/C-C
How can the formation of a by-product be minimised
Use an excess of reactant (thing you are reacting with) or limit the amount of reactant (the thing which is initially meant to get react)
