Topic 18.1 - Arenes Flashcards
What is the definition of an Arene?
Arenes are cyclic hydrocarbon compounds in which there are delocalised electrons that are held in a ring.
What is the definition of delocalised electrons?
Delocalised electrons are bonding electrons that are not fixed between two atoms in a bond but re shared between 3 or more atoms.
What are the 4 pieces of evidence that prove the actual structure of Benzene?
- Lack of isomers
- X-ray diffraction of bond lengths
- Stability
- Thermochemical data
How has the investigation into isomers of Benzene led to the discovery of its actual structure?
If you were to to a substitution reaction with a chlorine molecule you would expect there to be 2 isomers of 1,2-dichlorobenzene that are produced assuming that benzene has double bonds present in its structure. However, upon completing this reaction it was found that only 1 isomer could ever be made (no matter where you placed the chlorines on the ring only 1 isomer was ever made). This provided evidence that the structure of benzene consists of a delocalised ring of electrons that is stabilising with no double bonds present.
How has the investigation into the X-ray diffraction of bond lengths of Benzene led to the discovery of its actual structure?
If you were to perform an X-ray diffraction a molecule of Benzene you would expect the bond lengths to be different between consecutive carbons as you have single and double bonds (0.154 and 0.134 nm respectively). However, what was found when this X-ray diffraction was performed that the bond length between all the carbon atoms of the ring was the same, 0.144 nm roughly. Since all the bond lengths are identical, this provided evidence to support that the actual structure of Benzene consists of only single bonds and a delocalised ring of electrons that is stabilising.
How has the investigation into the stability of Benzene led to the discovery of its actual structure?
If you were to react Benzene assuming that it has double bonds in its structure you would expect addition reactions to occur and to generally be similar to alkenes. However, when this reaction was performed it was found that Benzene is much less reactive than alkenes and that they typically undergo substitution reactions instead of addition reactions. This provides evidence that the structure of Benzene consists of a delocalised ring of electrons that is stabilising with no double bonds present, only single.
How has the investigation into the Thermochemical Data of Benzene led to the discovery of its actual structure?
If you conducted a hydrogenation of the assumed structure of Benzene the estimated enthalpy change would be -360 kJmol(-1). However, when this reaction was actually conducted the found enthalpy change of the reaction was much less at -208kJmol(-1). Since Benzene had a lower enthalpy change it shows that the molecule is lower in energy and, therefore, that the actual structure is more stable that predicted. This provides evidence that the structure of Benzene consists of a delocalised ring of electrons that is stabilising with no double bonds present, only single.
Describe the bonding present in Benzene
Each carbon atom form 3 sp^2 orbitals with 2 neighbouring carbons and 1 hydrogen. The final electron in each carbon atom is in an atomic p orbital. These p orbitals are shared evenly between all six carbons and from rings of negative charge above and below the carbon plane. This is the delocalised (pi) electron ring system.
What is the name of the compound with C6H5Cl as its formula?
Chlorobenzene
What is the name of the compound with C6H5NO2 as its formula?
Nitrobenzene
What is the name of the compound with C6H5CH3 as its formula?
Methylbenzene
What is the name of the compound with C6H5OH as its formula?
Phenol
What is the name of the compound with C6H5NH2 as its formula?
Phenylamine
What is the name of the compound with C6H5CHO as its formula?
Benzaldehyde
What is the name of the compound with C6H5COOH as its formula?
Benzoic Acid
What is the name of the compound with C6H5OCH3 as its formula?
Methoxybenzene
What is the name of the compound with C6H5CH3 as its formula?
Methylbenzene
What is the Phenyl group?
C6H5
What is the formula for Benzene?
C6H6
How would you name a compound that has a benzene ring with a hydroxyl group on the first carbon and a chlorine group on the 2nd and 4th carbons?
2,4 - dichlorophenol
How would you name a compound that has a benzene ring with an amine group on the first carbon and a nitro group of the 3rd carbon?
3 - nitrophenylamine
How would you draw the skeletal formula 1-chloro-3-methylbenzene?
Draw a benzene ring, then a line with a chlorine on the end (the first Carbon) and then a line from the 3rd carbon to represent the methyl group
Are Arenas polar compounds?
No
What is the only intermolecular force holding Arenes together?
London Forces
What does the boiling temperature of Arenes depend on?
The size of the molecules - this is because as the size of the molecule increases the number of electrons with9in each molecule will also increase and, therefore, increase the strength of the London Forces.
How are London Forces formed?
- Electrons naturally move around a molecule which causes the electron density of areas of the molecule to fluctuate
- This forms instantaneous dipoles within the structure that are able to induce dipoles in other molecules
- These interactions are London Forces
What are the strongly electron donating groups?
- Amino (NH2)
- Hydroxy (OH)
- Alkoxy (OR)
What are the weakly electron donating groups?
- Alkyl (R)
What are the strongly electron withdrawing groups?
- Nitro (NO2)
- Cyano (CN)
What are the moderately electron withdrawing groups?
- Acyl (CHO)
- Carboxylic (COOH)
What are the weakly electron withdrawing groups?
- Halo groups
What is the definition of an electrophile?
A substance that will accept a pair of electrons
What is the definition of a substitution reaction?
Where an atom or group of atoms is swapped for something else/different group of atoms
Why does Benzene undergo electrophilic substitution reactions?
As substitution reactions allow us to keep the stability of the molecule from the delocalised ring of electrons as it is not broken
What is the order of reactivity of the molecules: Benzene, Methylbenzene and Methoxybenzene? Why?
MOST REACTIVE
- Methoxybenzene
- Methylbenzene
- Benzene
LEAST REACTIVE
Methoxybenzene is the most reactive as the CH3O group is strongly election donating as the two lone pairs of electrons on the oxygen are shared into he delocalised ring. This means that there is a higher negative charge on the benzene ring so it is more susceptible toto electrophilic attack. The Methyl group of Methykbenzene is only weakly electron donating so it will make the delocalised electron ring more susceptible to electrophilic attack more than Benzene but less than Methoxybenzene.
When is the only time Halogenation of Benzene will occur?
In the present of an iron catalyst
Why do Benzene and Bromine molecules not readily react with eachother?
The bromine molecule will have a partially positive charge and negative charge as it approaches the electron dense Benzene. However, the partially positive bromine is not a strong enough electrophile to react with Benzene du to the delocalised stabilising electron ring.
Why is an Iron catalyst required for the halogenation of Benzene?
To polarise the halogen and make it a stronger electrophile
Write the 2 equations for the 2 stages of producing the electrophile in the halogenation of Benzene (assuming bromine is the halogen)
STAGE 1 - producing the catalyst
2Fe + 3Br2 -> 2FeBr3 (the catalyst)
STAGE 2 - polarising the halogen to produce the stronger electrophile
FeBr3 + Br2 -> FeBr- + Br+ (the stronger electrophile)
Draw the mechanism for the reaction of Benzene with bromine in the presence of an iron catalyst (Halogenation of Benzene)
TYPE OF REACTION: ELECTROPHILIC SUBSTITUTION
- The Br+ electrophile attacks the electron dense delocalised electron ring of the Benzene. Draw an arrow from the circle of the Benzene ring to the Br+.
- The delocalised electron ring has been broken and now has a positive charge. Draw the bond between Benzene with Br and the Hydrogen on that carbon. Also draw a horseshoe in replacement of the circle with a + (positive charge). In this step the Hydrogen-Benzene bond breaks returning an electron tot he delocalised ring and making it whole again. Draw an arrow from the Hydrogen bond to the horseshoe.
- In the final step you have made bromobenzene which is drawn the same as Benzene but with a bromine on one of its carbons. You have also produced H+ which is then reformed to produce the catalyst.
Write the equation that shows the reforming of the catalyst from the halogenation of Benzene with bromine
H+ + FeBr4- -> FeBr3 + HBr
What are the: reagants, conditions and electrophile from the halogenation of Benzene with bromine?
REAGANTS: Bromine
CONDITIONS: Warm with Iron (III) Bromide catalyst (FeBr3) or with Fe that will make FeBr3
ELECTROPHILE:Br+
Can iodination of Benzene occur?
No, as iodine is unreactive, however, I-Cl can be used
Why is Iron usually used as the catalyst?
Its cheaper
In the chlorination of Benzene what are the two catalysts that can be used?
AlCl3 or FeCl3
What are the: reagents, conditions and electrophile required for the Nitration of Benzene?
REAGANTS: Concentrated Nitric Acid
CONDITIONS: Heat to 50C, Concentrated H2SO4
ELECTROPHILE: NO2+
How is the electrophile generated for the Nitration of Benzene?
By mixing nitric acid (HNO3) with sulphuric acid (H2SO4)
Write an equation to show how the electrophile for the Nitration of Benzene is generated
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
Draw the mechanism for the Nitration of Benzene
TYPE OF REACTION: ELECTROPHILIC SUBSTITUTION
- The NO2+ electrophile attacks the electron dense delocalised electron ring of the Benzene. Draw an arrow from the circle of the Benzene ring to the NO2+.
- The delocalised electron ring has been broken and now has a positive charge. Draw the bond between Benzene with NO2 and the Hydrogen on that carbon. Also draw a horseshoe in replacement of the circle with a + (positive charge). In this step the Hydrogen-Benzene bond breaks returning an electron tot he delocalised ring and making it whole again. Draw an arrow from the Hydrogen bond to the horseshoe.
- In the final step you have made nitrobenzene which is drawn the same as Benzene but with a -NO2 on one of its carbons. You have also produced H+ which is then reformed to produce the catalyst.
Write the equation that shows the reforming of the catalyst from the Nitration of Benzene
HSO4- + H+ -> H2SO4
What is the importance of the Nitration reaction?
The nitro group can readily be reduced to an amino group, a common functional group required in many pharmaceutical compounds.
What does the Friedel Crafts Alkylation reaction do?
It substitutes an alkyl group for a hydrogen atom on a benzene ring or arene.
Why can’t you use a regular halogenoalkane as the electrophile for the Friedel Crafts Alkylation reaction?
As it is not a strong enough electrophile - it needs to be reacted with something else in order to increase the positive nature of the electrophile
Write an equation for the production of the electrophile in a Friedel Crafts Alkylation when chloroethane is used
AlCl3 + CH3CH2Cl -> CH3C(+)H2 +AlCl4-
NB - the positive charge on the CH3CH2 is on the carbon that has lost the Cl
Draw the mechanism for the Friedel Crafts Alkylation reaction of chloroethane and Benzene
TYPE OF REACTION: ELECTROPHILIC SUBSTITUTION
- The CH3C(+)H2 electrophile attacks the electron dense delocalised electron ring of the Benzene. Draw an arrow from the circle of the Benzene ring to the positive carbon of CH3C(+)H2 .
- The delocalised electron ring has been broken and now has a positive charge. Draw the bond between Benzene with CH2CH3 (the alkyl group) and the Hydrogen on that carbon. Also draw a horseshoe in replacement of the circle with a + (positive charge). In this step the Hydrogen-Benzene bond breaks returning an electron to the delocalised ring and making it whole again. Draw an arrow from the Hydrogen bond to the horseshoe.
- In the final step you have made ehtylbenzene which is drawn the same as Benzene but with a -CH2CH3 on one of its carbons. You have also produced H+ which is then reformed to produce the catalyst.
Write the equation that shows the reforming of the catalyst from the Friedel Crafts Alkylation reaction
H+ + AlCl4- -> HCl + AlCl3
What are the: reagents, conditions and electrophile required for the Friedel Crafts Alkylation reaction of Benzene with a chloroalkane?
REAGANTS: Chloroalkane
CONDITIONS: Heat under reflux with AlCl3 catalyst
ELECTROPHILE: CH3C(+)H2
Write the word equation for the Friedel Crafts Alkylation reaction of Benzene with chloroethane
Benzene + CH3CH2Cl -> EthylBenzene + HCl
What is the importance of the Friedel Crafts Alkylation reaction?
It allows us to form a new C-C bond between Benzene and a carbon chain
Write the word equation for the Nitration of Benzene
Benzene + HNO3 -> Nitrobenzene + H2O
Write the word equation for the Halogenation of Benzene with Bromine
Benzene + Br2 -> Bromobenzene + HBr
What does the Friedel Crafts Acylation reaction do?
It substitutes an acyl (-COCl) group for a hydrogen atom on a benzene ring or arene.
Why can’t you use a regular acyl chloride as the electrophile for the Friedel Crafts Acylation reaction?
As it is not a strong enough electrophile - it needs to be reacted with something else in order to increase the positive nature of the electrophile
Write an equation for the production of the electrophile in a Friedel Crafts Acylation reaction when ethanoyl chloride is used
CH3COCl + AlCl3 -> AlCl4- + CH3C(+)O
NB - the positive charge on the CH3CO is on the carbon that has lost the Cl, the C=O
Draw the mechanism for the Friedel Crafts Acylation reaction of ethanoyl chloride and Benzene
TYPE OF REACTION: ELECTROPHILIC SUBSTITUTION
- The C(+)OCH3 electrophile attacks the electron dense delocalised electron ring of the Benzene. Draw an arrow from the circle of the Benzene ring to the C(+)OCH3.
- The delocalised electron ring has been broken and now has a positive charge. Draw the bond between Benzene with COCH3 and the Hydrogen on that carbon. Also draw a horseshoe in replacement of the circle with a + (positive charge). In this step the Hydrogen-Benzene bond breaks returning an electron to the delocalised ring and making it whole again. Draw an arrow from the Hydrogen bond to the horseshoe.
- In the final step you have made phenylethanone which is drawn the same as Benzene but with a -COCH3 on one of its carbons. You have also produced H+ which is then reformed to produce the catalyst
Write the equation that shows the reforming of the catalyst from the Friedel Crafts Acylation reaction
H+ + AlCl4- -> AlCl3 + HCl
What are the: reagents, conditions and electrophile required for the Friedel Crafts Acylation reaction of Benzene with an acyl chloride?
REAGANTS: Acyl Chloride
CONDITIONS: Heat with AlCl3 catalyst
ELECTROPHILE: CH3C(+)O
Write the word equation for the Friedel Crafts Acylation reaction of Benzene with ethanoyl chloride
Benzene + Ethanoyl Chloride -> Phenylethanone + HCl
What is the importance of the Friedel Crafts Acylation reaction?
The reaction forms several biological compounds including DNA
Describe the general mechanism of the Electrophilic Substitution reactions for Benzene
- A covalent bond is formed between the electrophile and the Benzene ring as a pair of electrons leaves the delocalised system.
- This disrupts the system forming an unstable intermediate.
- The electron pair in the C-H bond adjacent to the new C-electrophile bond moves back into the ring to restore the stability, the breaking the C-H bond.
- The hydrogen has been substituted by the electrophile.
Can Benzene undergo addition reactions?
Yes
Under what conditions do addition reactions of Benzene occur?
Severe
What is the reagent for the addition reaction of Hydrogen with Benzene?
Hydrogen (H2)
What are the conditions for the addition reaction of Hydrogen with Benzene?
- 200C
- 50atm
- Rayney nickel catalyst
What is the catalyst for the addition reaction of Hydrogen with Benzene?
Rayney nickel catalyst
What is a Rayney nickel catalyst?
A specially finely divided form of nickel
What is the word equation for the addition reaction of Benzene with Hydrogen?
Benzene + Hydrogen -> Cyclohexane
What is the equation for the addition reaction of Benzene with Hydrogen?
C6H6 + 3H2 -> C6H12
What are the reagents for the addition reaction of Benzene with chlorine (NOT HALOGENATION)?
Chlorine, Cl2
What are the conditions for the addition reaction of Benzene with chlorine (NOT HALOGENATION)?
UV light
NB - The Benzene must be boiling
Write the word equation for addition reaction of Chlorine and Benzene
Benzene + Cl2 -> 1,2,3,4,5,6-hexachlorocyclohexane
Write the equation for the reaction of Chlorine and Benzene
C6H6 + Cl2 -> C6H6Cl6
What is the change in shape from Benzene to 1,2,3,4,5,6-hexachlorocyclohexane during the addition reaction?
Planar -> Tetrahedral
Why was 1,2,3,4,5,6-hexachlorocyclohexane (C6H6Cl6) removed form use as an insecticides?
C6H6Cl6 has tetrahedral structures around each carbon which means that it can form geometric isomers. Some of these geometric isomers are harmful to humans and, since you can’t isolate the isomers form one another, the product had to be removed
What is the skeletal formula of phenol?
A Benzene ring with an -OH off of one of the carbons
What is the molecular formula of phenol?
C6H5OH
Is phenol soluble in water?
Yes, phenol is slightly soluble in cold water and this solubility increases as the water is warmed
Why is Phenol soluble in water?
As it has an Oxygen, with 2 lone pairs, bonded to a hydrogen to it is able to form hydrogen bonds with water
Why does Phenol have a higher melting point than other hydrocarbons of a similar Mr?
As it is able to form hydrogen bonds with other molecules of Phenol due to the O-H bond
What are the aspects of a hydrogen bonding diagram that you need to include when drawing hydrogen bonding between 2 Phenol molecules? Draw this hydrogen bonding.
- 180 degree angle for O-H-O bond
- Draw dashed lines between the bonding O and H
- The dashed lines are from the lone pair of electrons
- The delta + and delta - on the Oxygen atom and Hydrogen atom
What is the definition of a Brønsted-Lowry acid?
A proton donor
Write the equation to show how phenol forms an acidic solution
C6H5OH + H2O -> C6H5O- + H3O+
What is the name of the organic product that is formed when Phenol forms an acidic solution?
Phenoxide ion
Is Phenol a strong acid?
No, it is a weak acid
What is the definition of a weak acid?
A molecule that partially dissociate into its ions
Why is Phenol able to form acidic solutions but alcohols aren’t?
This is because the ion that Phenol forms from donating a proton is stable as the electrons on the oxygen, its negative charge, can be incorporated into the stabilising ring. Alcohols have nothing to stabilise the negative charge.
Are Phenols stronger or weaker acid in comparison to Carboxylic acids?
Phenols are weaker acids as they are they form the phenoxide ion which is less stable than the carboxylate ion
What are the 3 reactions that show Phenol acting as an acid?
- Sodium
- Metal (sodium) hydroxide
- Metal (sodium) carbonate
Write the equation for the reaction of Phenol with sodium
2C6H5OH + 2Na -> 2C6H5O-Na+ +H2
Write the equation for the reaction of Phenol with sodium hydroxide
C6H5OH + NaOH -> C6H5O-Na+ + H2O
What would happen if a strong acid was added to sodium phenoxide?
The strong acid would act as the acid and donate its proton to the sodium phenoxide and replace the Na returning it to phenol
Write the equation for the the reaction of HCl with sodium phenoxide including state symbols
C6H5O-Na+ (aq) + HCl (aq) -> C6H5OH (s) + NaCl
What does the reaction of sodium hydroxide tell you about the acidity of phenols versus alcohols?
Phenols engage in a neutralisation reaction with sodium hydroxide whereas alcohols do not react. Therefore, phenols are more acidic than alcohols.
Do Phenols react with sodium carbonate?
No
What does the reaction of sodium carbonate tell you about the acidity of carboxylic acids versus phenols?
Carboxylic acids react with sodium carbonate to produce CO2 in the observable form of effervescence. Phenols do not react with sodium carbonate. Therefore, carboxylic acids are stronger weak acids in comparison to phenols.
What is the order of acidity of alcohols, carboxylic acids and phenols from most acidic to least acidic?
MOST ACIDIC
Carboxylic Acids
Phenols
Alcohols
LEAST ACIDIC
Why is phenol not as strong a nucleophile as alcohols?
This is because the lone pair of electrons from the oxygen atom aren’t as readily available to act as a nucleophile a they are in alcohols. This is because these electron are incorporated into the delocalised ring system.
How can phenols be made into more reactive nucleophiles?
By adding an alkali
What is the equation for turning phenol into a stronger nucleophile?
C6H5OH + OH- -> C6H5O- + H2O
What type of flame do phenols burn with during combustion?
A smoky flame due to a high carbon:hydrogen ration
What do you observe when bromine water is added to phenol compounds?
A colour change - the solution turns from orange to colourless and a white precipitate forms
What type of reaction is the reaction of bromine and phenol?
electrophilic substitution
What is the equation for the reaction of bromine with phenol?
C6H5OH + 3Br2 -> C6H3Br3OH + 3HBr
What are the reagent’s, conditions and products for the reaction of bromine with phenol?
REAGANTS: Bromine water
CONDITIONS: Room temperature
PRODUCTS: 2,4,6-tribromophenol + HBr
What are the reagent’s, conditions and products for the reaction of nitric acid with phenol?
REAGANTS: dilute nitric acid
CONDITIONS: Room temperature
PRODUCTS: 2-nitrophenol, 4-nitrophenol and H2O
What happens when you add concentrated nitric acid to phenol instead of dilute nitric acid?
Multiple substitutions of -NO2 group occur onto the phenol molecule
Write the equation for the reaction of dilute nitric acid with phenol
2C6H5OH + 2HNO3 -> C6H4OHNO3 + C6H4OHNO3 +2H2O
Write the equation for the reaction of concentrated nitric acid with phenol
2C6H5OH + 2HNO3 -> C6H2OH(NO2)3 + 3H2O
Why does phenol undergo electrophilic substitution reaction more readily than benzene?
This is because phenol has a more electron dense delocalised ring due to the lone pair of electrons on the oxygen atom of the -OH group being incorporated into the ring. Therefore, phenol is more susceptible to electrophilic attack in comparison to benzene.
Why do phenols not undergo nucleophilic substitution reactions?
Nucleophiles are negative and so upon approaching phenols would be repelled by the negative charge of phenol due to the high electron density of the ring
Why can the hydroxyl group in phenols not. be oxidised to a carbonyl (C=O) group?
As you would have to break the stabilising delocalised ring to do so
What is the reason why 2-nitrophenol is less soluble in water in comparison to 4-nitrophenol?
The -OH and -NO2 groups on 2-nitrophenol are close together and so from an intramolecular hydrogen bond making the -OH group unavailable to form a hydrogen bond with water. Comparatively, the -OH and -NO2 groups in 4-nitrophenol are fully separated so the molecule can from intermolecular hydrogen bonding with water molecules
