Electrophilic Substitution Reactions Of Benzene

Question 1

In benzene, what reactions do they mainly undergo?

Answer 1

Electrophilic substitution.

Question 2

What is an equation for benzene undergoing electrophilic substitution?

Answer 2

R: A benzene ring with a curly arrow from the inner ring to the E+.
R: The inner ring loses 1/3 of its structure, the open end facing the electrophile, with a positive charge in the centre and the E and H branching out of benzene with a curly arrow from the H to the positive charge.
P: The benzene inner ring is reformed with the E in the same place branching off of the benzene ring with another product of H+.

Question 3

What is the overall equation for the general equation for electrophilic substitution?

Answer 3

Benzene + E+ = Benzene with electrophile attached + H+.

Question 4

What are the 3 written steps for the electrophilic substitution of benzene?

Answer 4

1. The positive nature of the electrophile means it is attracted to the electron density of the delocalised pi-ring in benzene.
2. This intermediate is formed. The electrophile is bonded to the benzene ring, but so still is the H atom. The delocalised pi system is temporarily broken, to leave a partially delocalised electron system and a positive charge.
3. The intermediate loses a proton, and the electrons from this C-H bond restore the delocalised pi system.

Question 5

What are 2 features of the delocalised pi system?

Answer 5

• Extemely stable.
• A region of high electron density.

Question 6

Why is the electrophilic substitution of benzene tricky?

Answer 6

Because electrophiles cannot simply be added into the mixture, there has to be certain reagents and condtions to allow them to be invovled.

Question 7

What are 2 examples of electrophilic reactions of benzene?

Answer 7

1. Nitration of benzene.
2. Halogenation of benzene.

Question 8

What happens in the nitration of benzene?

Answer 8

A hydrogen atom is replaced by a nitro (NO2-) group.

Question 9

What is the overall reaction of the nitration of benzene?

Answer 9

Benzene + HNO3 = Benzene with an NO2 group + H2O.

Question 10

What are the 3 steps in the electrophilic substitution of arenes?

Answer 10

1. Generation of an electrophile.
2. Electrophilic attack.
3. Regenerating aromaticy.

Question 11

What are the steps in the nitration of benzene?

Answer 11

• The electrophile is generated: NO2+ ion is generated by reacting conc. HNO3 and conc. H2SO4.
• Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring: the nitrating mixture of HNO3 and H2SO4 is refluxed with the arene at 25-60C.

Question 12

What is the equation for the formation of an electrophile in the nitration of benzene?

Answer 12

H2SO4 + HNO3 = NO2+ + HSO4- + H2O.
- Where NO2+ is the electrophile.
- Reaction at 50C.
- Acids are both concentrated.

Question 13

What is the mechanism for the nitration of benzene?

Answer 13

1. A benzene ring with a curly arrow coming off of the inner ring pointing to the NO2+.
2. This forms a benzene ring with 1/3 of the inner ring removed and a positive charge in the centre. There are two branches crossing over one another to form a whole with H on one and NO2 on the other with a curly arrow coming off of the H branch to the positive charge. THIS HAPPENS BECAUSE NO2+ ACCEPTS A PAIR OF E- FROM THE BENZENE RING TO FORM A DATIVE COVALENT BOND. THE ORGANIC INTERMEDIATE FORMED IS UNSTABLE AND BREAKS DOWN TO FORM NITROBENZENE AND A H+ ION.
3. This forms a benzene ring with an NO2 group and another product of H+.

Question 14

What is the equation for the reformation of the catalyst in the nitration of benzene?

Answer 14

H2SO4- + H+ = H2SO4 (catalyst).

Question 15

Why is a water bath used in the nitration of benzene? Or?

Answer 15

To maintain a stable temperature. Other wise, if the temperature rises above 50C, further substitution reactions may occur leading to the production of dinitrobenzene.

Question 16

What else can benzene react with in nitration? To form?

Answer 16

An excess nitric acid (2HNO3) above 50C to form 1,3-dinitrobenzene.

Question 17

What needs to be used in order for halogens to react with benzene? Example?

Answer 17

Halogen carrier catalysts, e.g. AlBr3.

Question 18

What is the equation for the bromination of benzene?

Answer 18

Benzene + Br2 = Bromobenzene + Hbr.
- (with AlBr3 or FeBr3 used to help).

Question 19

What are the steps of bromination of benzene?

Answer 19

1. Benzene is too stable to react with a non-polar bromine molecule. The electrophile is the bromonium ion, which is generated when the halogen carrier catalyst reacts with bromine in the first stage of the mechanism.
2. The bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The intermediate is unstable and breaks down to form bromobenzene, and an H+ ion.
3. The H+ ion formed in step 2 reacts with the AlBr4-/FeBr4- ion from step 1 to regenerate the AlBr3/FeBr3 catalyst.

Question 20

What are the 3 written steps of bromination of benzene?

Answer 20

1. Br2 + AlBr3 = AlBr4- + Br+.
2. Benzene ring with curly arrow from inner ring to Br+. FORMS an intermediate with 2/3 of inner circle with positive charge inside and bonds of H and Br going out and crossing over with a curly arrow from the H branch to the positive charge. FORMS a bromobenzene ring and a H+.
3. H+ + AlBr4- = AlBr3 + HBr.

Question 21

What are 2 other reactions of substitution?

Answer 21

Alkylation and acylation reactions.

Question 22

What is the alkylation of benzene? (Friedel-Crafts alkylation)?

Answer 22

The substitution of a hydrogen atom in the benzene ring by an alkyl group.

Question 23

How is an alkylation of benzene reaction carried out?

Answer 23

You react benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile.

Question 24

What does alkylation form?

Answer 24

It increases the number of carbon atoms in a compound by forming C-C bonds.

Question 25

What is the mechanism equation for alkylation?

Answer 25

Benzene + C2H5Cl = Ethylbenzene + HCl
- with AlCl3 as a halogen carrier catalyst.

Question 26

What happens in acylation reactions?

Answer 26

When a benzene and an acyl chloride are reacted together in the presence of an AlCl3 catalyst, an aromatic ketone is formed. C-C bonds are also formed.

Question 27

What is the mechanism reaction for acetylation?

Answer 27

Benzene + Ethanoyl chloride = Phenylethanone + HCl
- with AlCl3 as a halogen carrier catalyst.

Question 28

How can we compare the reactivity between alkenes with arenes?

Answer 28

Using the reaction between cyclohexene and bromine (alkenes decolourise bromine by an electrophilic substitution reactions). In ELECTROPHILIC ADDITION.

Question 29

What is the reaction between cyclohexene and bromine?

Answer 29

Cyclohexene + Br2 = 1,2-dibromocyclohexane.

Question 30

How does the reaction between cyclohexene and bromine work? Bromine adding across the double bond in cyclohexene? (3)?

Answer 30

1. The pi-bond in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density.
2. The localised electrons in the pi-bond induce a dipole in the non-polar bromine molecule making one bromine atom on the Br2 molecule slightly positive and the other bromine atom slightly negative.
3. The slightly positive bromine atom enables the bromine molecule to act like an electrophile.

Question 31

What is the structural mechanism between cyclohexene and bromine?

Answer 31

1. A curly arrow comes off of the cyclohexene double bond and pointing to the slightly positive Br on the dipole with the slightly negative Br with another curly arrow coming off of the middle bond in between the Br’s to the slightly negative Br.
2. THIS FORMS an intermediate carbocation of cyclohexane where Br is branched off of the molecule and there is a positive charge on one of the carbons with a curly arrow pointing to it from the Br- lone pairs (must have all electrons around the Br.
3. THIS FORMS 1,2-dibromocyclohexane.

Question 32

Why does benzene need halogen carrier catalysts to react with bromine but alkenes do not?

Answer 32

• Because benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure. The electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene.
• When a non-polar molecule such as bromine approaches the benzene ring there is insufficient pi-electron density around any two carbon atoms to polarise the bromine molecule. This prevents the reaction from taking place.