Aromatic compounds

Question 1

Defintion of aromatic.

Answer 1

Some or all of the carbons atoms are in a benzene ring.

Question 2

Defintion of aliphatic.

Answer 2

Carbons atoms that are joined together in a chain.

Question 3

What is the moleular formula of benzene?

Answer 3

C6H6

Question 4

What is the empirical formula of benzene?

Answer 4

CH

Question 5

What is the IUPAC name for benzene?

Answer 5

Cyclohex-1,3,5-triene.

Question 6

Explain why Kekule’s structure was proven to be incorrect (3 points).

Answer 6

1. All C-C bonds were of the same length.
2. Enthalpy change of hydrogenation was less exothermic than expected.
3. Lack of reactivity- benzene will not decolourise bromine water.

Question 7

Describe the bonding in benzene (4 points).

Answer 7

1. Sideways overlap of p-orbitals to form a delocalised pie electron ring (pie bond over all 6 carbons).
2. 6 delocalised electrons in the pie electron ring.
3. The C-C bond and C-H single bonds are sigma bonds.
4. The molecule is polar, bond angles are 120 degrees.

Question 8

Compare the Kekule and the delocalised structure of benzene (2 points).

Answer 8

1. Kekule has 3 pie bonds; delocalised structure contains a pie ring.
2. The electrons are localised in the bonds in the Kekule structure; the electrons are delocalised in the delocalised structure of benzene.

Question 9

*Describe how the pi ring forms around benzene.

Answer 9

*Refer to aromatic compounds notes.

Question 10

What kind of reactions does benzene undergo?

Answer 10

Electrophilic substitution.

Question 11

Definition of an electrophile.

Answer 11

Electron pair acceptor.

Question 12

Definition of substitution.

Answer 12

Replacing a hydrogen with an electrophile.

Question 13

What are the reagents for the nitration of benzene?

Answer 13

Concentrated HNO3(aq) and benzene.

Question 14

What catalyst is used for the nitration of benzene? What is the condition?

Answer 14

H2SO4(aq).
Reflux

Question 15

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.

Answer 15

HNO3 + H2SO4 …. NO2+ + H2O + HSO4-

Question 16

Write the overall equation for the nitration of benzene using HNO3 as the reagent.

Answer 16

C6H6 + HNO3 ….. C6H5NO2 + H2O

Question 17

*Describe the mechanism of the nitration of benzene using HNO3 as the reagent and H2SO4 as the catalyst.

Answer 17

*Refer to aromatic compounds notes.

Question 18

Write out the equation for the regeneration of the catalyst for nitration of benzene.

Answer 18

HSO4- + H+ ….. H2SO4

Question 19

Describe, in words, the mechanism for the nitration of benzene (4 points).

Answer 19

1. Electron pair donated to NO2+ from pie electron ring.
2. Intermediate formed with 4 electrons delocalised over 5 carbons.
3. The pair of electrons from the C-H bond are donated into the pie electron ring.
4. The product has 6 electrons shared over 6 carbons.

Question 20

What reagents are used for halogenation of benzene?

Answer 20

Br2 / Cl2

Question 21

What are the conditions for the halogenation of benzene?

Answer 21

AlBr3 catalyst (a halogen carrier) pr Fe and Br2 mixed together; FeBr3.

Question 22

Write the overall equation for halogention of benzene with Br2 as the reagent.

Answer 22

C6H6 + Br2 …. C6H5Br + HBr

Question 23

What is the equation for the regeneration of AlBr3 in halogenation of benzene?

Answer 23

AlBr4- + H+ …. HBr + AlBr3

Question 24

*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?

Answer 24

*Refer to aromatic compounds notes.
Addition reaction.
The fact that this reaction does not occur is more evidence for the benzne delocalised structure.

Question 25

Suggest reagents, conditions, an equation for the formation of the electrophile and an overall equation for the chlorination of benzene.

Answer 25

Reagents: Cl2. Condition: AlCl3 catalyst. Formation of electrophile: Cl2 + AlCl3 .... Cl+ + AlCl4- Overall equation: C6H6 + Cl+ .... C6H5Cl + HCl

Question 26

What are the reagents for the alkylation of benzene?

Answer 26

Benzene and a haloalkane, e.g. CH3Cl

Question 27

What are the conditions for the alkylation of benzene?

Answer 27

AlCl3 catalyst (a halogen carrier).

Question 28

Write the equation for the formation of an electrophile: CH3+ for the alkylation of benzene.

Answer 28

AlCl3 + CH3Cl ..... AlCl4- + CH3+

Question 29

Write the overall equation for the alkylation of benzene with CH3Cl as the reagent and AlCl3 as a catalyst.

Answer 29

C6H6 + CH3Cl .... C6H6CH3 + HCl

Question 30

*Describe the mechanism for the alkylation of benzene with CH3Cl as the reagent and AlCl3 as the catalyst.

Answer 30

*Refer to aromatic compounds notes.

Question 31

What is the reagent for the acylation of benzene to form a ketone?

Answer 31

An acyl chloride, e.g. CH3COCl

Question 32

What are the conditions for the acylation of benzene?

Answer 32

A halogen carrier as a catalyst, e.g.AlCl3.

Question 33

*Write the overall equation for the reaction between benzene and ethanoyl chloride. Name the product formed.

Answer 33

*Refer to aromatic compounds notes. Phenylethanoate.

Question 34

What's the name for the reactions that introduce a side-chain onto benzene?

Answer 34

Friedal-Crafts reactions.

Question 35

*Write an equation for the reaction of phenylethene and Br2(aq). Name this reaction.

Answer 35

*Refer to aromatic compounds notes. Electrophilic addition.

Question 36

*Write the equation for the reaction of benzene with Br2(aq).

Answer 36

*Refer to aromatic compounds notes.

Question 37

Describe and explain why alkenes are more reactive than benzene (4 points).

Answer 37

1. C=C in alkene has localised pi electrons over 2 carbons. 2. Electron density of C=C is greater than electron density of benzene ring. 3. Alkene polarises the bromine molecule more than benzene does. 4. Benzene's delocalised electron pi ring stabilises the benzene molecule so more energy is needed to break bonds.

Question 38

*Write an equation for the reaction between phenol and sodium. Name the product formed and state any observations.

Answer 38

*Refer to aromatic compounds notes. Sodium phenoxide. Effervescence and solid dissolves.

Question 39

*Write an equation for the reaction between phenol and NaOH. Name the product formed.

Answer 39

*Refer to aromatic compounds notes. Sodium phenoxide.

Question 40

*Write an equation for the reaction between phenol and MgO. Name the product and state any observations.

Answer 40

*Refer to aromatic compounds notes. Magnesium phenoxide. Solid dissolving.

Question 41

Compare the reactivity of phenol and benzene and explain why (3 points).

Answer 41

1. The lone pair in the p-orbital on the oxygen becomes delocalised into the pi electron ring. 2. This increases the electron density around the ring. 3. Phenol polarises electrophiles more than benzene.

Question 42

*Write the equation for the reaction between bromine and phenol. Name the product and state any observations.

Answer 42

*Refer to aromatic compounds notes. 2,4,6-tribromophenol. Orange to colourless ( decolourises in bromine water) and white precipitate: 2,4,6-tribromophenol.

Question 43

*Write the equation for the reaction between nitric acid and phenol. Name the product.

Answer 43

*Refer to aromatic compounds notes. 2 structural isomers are produced; 2-nitrophenol, 4-nitrophenol.

Question 44

Which position does OH direct the next group to substitute onto the benzene ring?

Answer 44

Carbons 2, 4 and 6.

Question 45

Which position does NO2 direct the next group to substitute onto the benzene ring?

Answer 45

3

Question 46

Which position does NH2 direct the next group to substitute onto the benzene ring?

Answer 46

2 and 4.