Aromatic compounds Flashcards
Defintion of aromatic.
Some or all of the carbons atoms are in a benzene ring.
Defintion of aliphatic.
Carbons atoms that are joined together in a chain.
What is the moleular formula of benzene?
C6H6
What is the empirical formula of benzene?
CH
What is the IUPAC name for benzene?
Cyclohex-1,3,5-triene.
Explain why Kekule’s structure was proven to be incorrect (3 points).
- All C-C bonds were of the same length.
- Enthalpy change of hydrogenation was less exothermic than expected.
- Lack of reactivity- benzene will not decolourise bromine water.
Describe the bonding in benzene (4 points).
- Sideways overlap of p-orbitals to form a delocalised pie electron ring (pie bond over all 6 carbons).
- 6 delocalised electrons in the pie electron ring.
- The C-C bond and C-H single bonds are sigma bonds.
- The molecule is polar, bond angles are 120 degrees.
Compare the Kekule and the delocalised structure of benzene (2 points).
- Kekule has 3 pie bonds; delocalised structure contains a pie ring.
- The electrons are localised in the bonds in the Kekule structure; the electrons are delocalised in the delocalised structure of benzene.
*Describe how the pi ring forms around benzene.
*Refer to aromatic compounds notes.
What kind of reactions does benzene undergo?
Electrophilic substitution.
Definition of an electrophile.
Electron pair acceptor.
Definition of substitution.
Replacing a hydrogen with an electrophile.
What are the reagents for the nitration of benzene?
Concentrated HNO3(aq) and benzene.
What catalyst is used for the nitration of benzene? What is the condition?
H2SO4(aq).
Reflux
Write the equation for the formation of the electrophile used in the nitration of benzene with HNO3 as the reagent and H2SO4 as the catalyst.
HNO3 + H2SO4 …. NO2+ + H2O + HSO4-
Write the overall equation for the nitration of benzene using HNO3 as the reagent.
C6H6 + HNO3 ….. C6H5NO2 + H2O
*Describe the mechanism of the nitration of benzene using HNO3 as the reagent and H2SO4 as the catalyst.
*Refer to aromatic compounds notes.
Write out the equation for the regeneration of the catalyst for nitration of benzene.
HSO4- + H+ ….. H2SO4
Describe, in words, the mechanism for the nitration of benzene (4 points).
- Electron pair donated to NO2+ from pie electron ring.
- Intermediate formed with 4 electrons delocalised over 5 carbons.
- The pair of electrons from the C-H bond are donated into the pie electron ring.
- The product has 6 electrons shared over 6 carbons.
What reagents are used for halogenation of benzene?
Br2 / Cl2
What are the conditions for the halogenation of benzene?
AlBr3 catalyst (a halogen carrier) pr Fe and Br2 mixed together; FeBr3.
Write the overall equation for halogention of benzene with Br2 as the reagent.
C6H6 + Br2 …. C6H5Br + HBr
What is the equation for the regeneration of AlBr3 in halogenation of benzene?
AlBr4- + H+ …. HBr + AlBr3
*If benzene had Kekule’s structure and the halogenation of benzene was at room temperature with Br2(aq), what would the reaction look like? Name the type of reaction. Does this reaction occur?
*Refer to aromatic compounds notes.
Addition reaction.
The fact that this reaction does not occur is more evidence for the benzne delocalised structure.
Suggest reagents, conditions, an equation for the formation of the electrophile and an overall equation for the chlorination of benzene.
Reagents: Cl2.
Condition: AlCl3 catalyst.
Formation of electrophile: Cl2 + AlCl3 …. Cl+ + AlCl4-
Overall equation: C6H6 + Cl+ …. C6H5Cl + HCl
What are the reagents for the alkylation of benzene?
Benzene and a haloalkane, e.g. CH3Cl
What are the conditions for the alkylation of benzene?
AlCl3 catalyst (a halogen carrier).
Write the equation for the formation of an electrophile: CH3+ for the alkylation of benzene.
AlCl3 + CH3Cl ….. AlCl4- + CH3+
Write the overall equation for the alkylation of benzene with CH3Cl as the reagent and AlCl3 as a catalyst.
C6H6 + CH3Cl …. C6H6CH3 + HCl
*Describe the mechanism for the alkylation of benzene with CH3Cl as the reagent and AlCl3 as the catalyst.
*Refer to aromatic compounds notes.
What is the reagent for the acylation of benzene to form a ketone?
An acyl chloride, e.g. CH3COCl
What are the conditions for the acylation of benzene?
A halogen carrier as a catalyst, e.g.AlCl3.
*Write the overall equation for the reaction between benzene and ethanoyl chloride. Name the product formed.
*Refer to aromatic compounds notes.
Phenylethanoate.
What’s the name for the reactions that introduce a side-chain onto benzene?
Friedal-Crafts reactions.
*Write an equation for the reaction of phenylethene and Br2(aq). Name this reaction.
*Refer to aromatic compounds notes.
Electrophilic addition.
*Write the equation for the reaction of benzene with Br2(aq).
*Refer to aromatic compounds notes.
Describe and explain why alkenes are more reactive than benzene (4 points).
- C=C in alkene has localised pi electrons over 2 carbons.
- Electron density of C=C is greater than electron density of benzene ring.
- Alkene polarises the bromine molecule more than benzene does.
- Benzene’s delocalised electron pi ring stabilises the benzene molecule so more energy is needed to break bonds.
*Write an equation for the reaction between phenol and sodium. Name the product formed and state any observations.
*Refer to aromatic compounds notes.
Sodium phenoxide.
Effervescence and solid dissolves.
*Write an equation for the reaction between phenol and NaOH. Name the product formed.
*Refer to aromatic compounds notes.
Sodium phenoxide.
*Write an equation for the reaction between phenol and MgO. Name the product and state any observations.
*Refer to aromatic compounds notes.
Magnesium phenoxide.
Solid dissolving.
Compare the reactivity of phenol and benzene and explain why (3 points).
- The lone pair in the p-orbital on the oxygen becomes delocalised into the pi electron ring.
- This increases the electron density around the ring.
- Phenol polarises electrophiles more than benzene.
*Write the equation for the reaction between bromine and phenol. Name the product and state any observations.
*Refer to aromatic compounds notes.
2,4,6-tribromophenol.
Orange to colourless ( decolourises in bromine water) and white precipitate: 2,4,6-tribromophenol.
*Write the equation for the reaction between nitric acid and phenol. Name the product.
*Refer to aromatic compounds notes.
2 structural isomers are produced; 2-nitrophenol, 4-nitrophenol.
Which position does OH direct the next group to substitute onto the benzene ring?
Carbons 2, 4 and 6.
Which position does NO2 direct the next group to substitute onto the benzene ring?
3
Which position does NH2 direct the next group to substitute onto the benzene ring?
2 and 4.
