Benzene

Question 1

What family of organic compounds is benzene part of?

Answer 1

Aromatic

Question 2

What does the original kekule structure contain?

Answer 2

Alternate single and double C-C bonds

Question 3

What does benzene have that makes it more stable?

Answer 3

Delocalised Pi electrons make it more stable than it would be f it had 3Pi bonds between alternate single and double bonds (kekule)

Question 4

What is the value for delocalisation E of benzene?

Answer 4

+150

Question 5

The structure and bonding in benzene

Answer 5

• Planar with a ring of 6 carbon atoms
• each C atom forms 3 single covalent bonds, to 2 carbon atoms and 1 hydrogen
• the spare electrons in P orbital overlap sideways to form a Pi bond
• there is a Pi electron cloud above and below the plane of carbon atoms
• the Pi electrons are delocalised, benzene is more stable due to this
• C-C bond lengths are all equal
• C-C-C bond angle is 120

Question 6

Why should you be careful using benzene?

Answer 6

It is a liquid at room temperature and is a carcinogen (cancer causing)

Question 7

Why doesn’t benzene undergo Electrophic addition reaction?

Answer 7

As it is very stable so this would disrupt the delocalised Pi electrons and stability would be lost which would require lots of energy

Question 8

What does benzene undergo?

Answer 8

Electrophilic substitution - where an electrophiles substitutes an atom e.g hydrogen. This means Pi electrons aren’t disrupted, stability is not lost, and more energy isn’t required. But very reactive electrophiles are needed

Question 9

What are the conditions for nitration of benzene

Answer 9

Concentrated nitric and sulfuric acid

55 degrees

Question 10

What is the attacking electrophile in nitration of benzene?

Answer 10

NO2+

Question 11

What type of reactions are all benzene reactions?

Answer 11

Electrophilic substitution

Question 12

What are the conditions for halogen action of benzene?

Answer 12

No light, room temp, anhydrous

Question 13

What is the formula of the attacking species of halogenation of benzene?

Answer 13

Cl+ if using an AlCl3/FeCl3 catalyst

Br+ if using Br catalyst

Question 14

Why is a catalyst needed in halogenation of benzene?

Answer 14

As bromine and chlorine are not strong enough as electrophiles by themselves so a halogen carrier is used (catalyst)

Question 15

What is the purpose of a AL/Fe catalyst in halogenation of benzene?

Answer 15

It polarises the halogen as well as speeding up a reaction

Question 16

What does a freidal-crafts reaction involve?

Answer 16

Involves substitution of an alkyl group such as CH3 for a hydrogen atom on benzene ring

Question 17

What is Freidal crafts also known as

Answer 17

Alkylation

Question 18

What are the conditions for alkylation of benzene

Answer 18

Room temp , anhydrous

A catalyst is also used e.g AlCl3

Question 19

Why don’t aromatic compounds e.g chlorobutane undergo hydrolysis easily?

Answer 19

• when the halogen is attached to the rig it is less reactive than when it’s in a halogenoalkanes, meaning C-Cl bond is stronger in chlorobenzene
• this is due to the overlap of electrons in the P orbital of chlorine with the delocalised Pi electrons of the ring, to form an extended Pi system - so the C-Cl bond has some Pi/double bond character
• the overlap also reduces the delta + charge on the Carbon which is attached to chlorine, as it is spread over the delocalised System, so if the chlorine atom were subsituted it would result in the delocalised Pi system being destroyed and a loss of stability.

Question 20

Comparison of 1-chlorobutane and chlorobenzene

Answer 20

1-chlorobutane= larger delta +, weaker C-Cl bond

Chlorobenzene= smaller delta +, stronger C-Cl bond

Question 21

What are the attacking species in alkylation ?

Answer 21

The alkyl group with a +

E.g IN chloromethane = CH3+ is attacking species

Question 22

Naming benzene- what is NH2?

Answer 22

Aminobenzene / phenylamine

Question 23

Naming: what is COOH?

Answer 23

Benzenecarboxylic acid / benzoicacid

Question 24

Naming- what is OH?

Answer 24

Hydroxybenzene / phenol

Question 25

Naming- what is CH3?

Answer 25

Methylbenzene / toluene

Question 26

Give reaction equations for formation of the nitronium ion needed in nitration of benzene

Answer 26

1- H2SO4 + HNO3 –> H2NO3+ + HSO4-

2: H2NO3+ —-> H2O + NO2+

Question 27

Define delocalisation energy

Answer 27

Extra stability exhibited by benzene & other aromatic compounds due to delocalised Pi electrons

Question 28

How can you separate nitrobenzene? And what temp?

Answer 28

Fractional distillation, temp higher than 55 degrees as both are liquids at 55, at higher temps further nitration occurs

Question 29

Draw mechanism for formation of nitrobenzene

Answer 29

..

Question 30

Draw mechanism for formation of chlorobenzene using an AlCl3 catalyst

Answer 30

..

Question 31

Give all alkyl groups

Answer 31

CH3 methyl
C2H5 ethyl
C3H8 propyl
C4H9 butyl

Question 32

Describe a test to distinguish between aliphatic and aromatic halogen

Answer 32

Hydrolysis (NUCLEOPHILIC sub) add acidified silver nitrate- reflux and 1 atm

Aliphatic- PPT corresponding to halogen

Aromatic- no PPT, bond not broken too strong and no X- ions produced

Question 33

If you have a structure in electrophilic sub where a chlorine is attached: and they ask why that product wouldn’t be another molecule, why?

Answer 33

The p orbital electrons of the chlorine in the aromatic is incorporated into the delocalised Pi electron system of the ring

Question 34

Why is the Enthalpy change using a benzene ring (e.g hydrogenation) more positiv / larger than that of alkene to alkane ?

Answer 34

Removing a benzene ring which is a lot more stable (due to delocalisation. Energy) Than an alkene

Question 35

Why is benzene subseptible to electrophilic substitution?

Answer 35

Because the Pi electron cloud is electron rich and is attracted to the electron deficient species