Chapter 25 Benzene

Question 1

What is benzene?

Answer 1

An aromatic hydrocarbon with the molecular formula C6H6, naturally found in crude oil
(Colourless. Sweet smell. Carcinogenic)

Question 2

Describe the Kekulé model of benzene

Answer 2

6 carbon ring with altering C=C double bonds

Question 3

What evidence was there to disprove the Kékule model of Benzene?

Answer 3

Lack of reactivity of Benzene- does not decolourise bromine water at room temperature and pressure, cannot undergo electrophilic addition so does not contain C=C

The C-C bond lengths in Benzene are all the same length, in between C-C and C=C

The hydrogenation Enthalpy of benzene is less exothermic than expected- not 3 x cyclohexene, more stable than anticipated

Question 4

What is an arene?

Answer 4

An aromatic hydrocarbon

Question 5

What is the bonding in Benzene?

Answer 5

P- orbitals of the carbon are at 90 degrees to the plan
The p orbitals overlap sideways above and below the bonding atoms in both directions
Forms a delocalised ring of electrons, a system of pi bonds
Planar, cyclic, hexagonal molecule

Question 6

How do pi bonds form (when asked with a question relating to Benzene)?

Answer 6

Sideways overlap of p orbitals above and below the bonding atoms
Forms a localised pi bond

Question 7

What are the general rules for naming Benzene compounds?

Answer 7

Benzene is a function group when with alkyl chains smaller than 7 carbons e.g methylbenzene
When there is a chain with a functional group, this becomes the parent chain, and Benzene is now phenyl. E.g 2-phenyl-Ethan-1-ol think about priority!
When naming derivatives, use the most common stem as point one e.g 2chloro-methylbenzene, or what ever synthesised first

Question 8

What are common exception to the Benzene naming rule?

Answer 8

Phenylamine
Benzoic Acid
Benzaldehyde

Question 9

Compare the reactivity of an alkene compared to Benzene and explain why?

Answer 9

Alkenes contain a localised pi bond, has a higher electron density so can polarise molecules, easily undergoing electrophilic addition, so more reactive.
Benzene contains a delocalised ring of electrons /pi system which has a lower electron density. This cannot polarise the molecules, so cannot undergo Electrophilic addition, so less reactive.

Question 10

What are the conditions for nitration of benzene?

Answer 10

React with concentrated HNO3 with concentrated H2SO4 catalyst at 50 degrees in a water bath
Reacts very slowly with HNO3 under standard conditions

Question 11

Describe the reaction mechanism of benzene with nitric acid and acid catalyst

Answer 11

H2SO4 + HNO3 —> HSO4- + NO2+ +H2O
Curly arrow from benzene ring to NO2+, Arrow from C-H bond to intermediate positive charge
H+ + HSO4 - - > H2SO4

Question 12

What are the conditions for the halogenation of benzene?

Answer 12

React benzene with a halogen carrier such as ALCl3 or FeCl3 and the halogen (same species in carrier and element)
Halogen carrier formed in Situ

Question 13

Give the steps of the catalyst of the halogenation of benzene?

Answer 13

FeCl3 + Cl2 —> FeCl4 - +Cl+
Mechanism
H+ + AlCl4- —> HCl + ALCl3

Question 14

How do you convert benzene into ethyl benzene ?

Answer 14

React with chloroethane, and react with a halogen carrier
Cation= CH3CH2+

Question 15

What are acylation reactions in relation to benzene?

Answer 15

Reacting benzene with an acyl chloride, C=O with Cl on the same carbon
Cation= positive charge removing cl

Question 16

How do you know a compound is a phenol?

Answer 16

There is an OH directly bonded to the benzene ring, not to a side chain

Question 17

How do you name substituted benzene derivatives?

Answer 17

The most common benzene derivative e.g methylbenzene is used to number the rest of the functional groups as side
E.g 3-chloro-nitrobenzene

Question 18

Why are phenols weakly acidic?

Answer 18

OH group donates electrons to carbanion formed
Phenoxide ion
Resonance stabilised unlike alcohols

Question 19

How can you differentiate between alcohols and phenols?

Answer 19

Phenols will react with strong bases like NaOH whilst alcohols will not
See change in pH

Question 20

How can you identify phenol from carboxylic acids?

Answer 20

Only carboxylic acids can react with weak bases such as Na2CO3, not phenol
Carboxylic acids will fizz

Question 21

What are the conditions and observations of phenol reacting with bromine?

Answer 21

Under standard conditions, room temp and pressure
Decolorises bromine water and forms a white precipitate

Question 22

Why is phenol more reactive than benzene?

Answer 22

The lone pair of electrons from the oxygen p orbital of the OH group is donated and delocalised into to the pi system
This increases the electron density of the benzene ring, making the ring more susceptible for electrophilic attack, and able to polarise and attract electrophilic more easily than benzene

Question 23

What are the conditions for the nitration of phenol?

Answer 23

Room temperature and pressure, no catalyst needed
React with dilute HNO3

Question 24

What are the directing effects of mono-substituted benzene?

Answer 24

If electron donating functional group of bonded group, directing to position 2 (ortho) and position 4 (para)
If electron withdrawing, directing to position 3 (meta)

Question 25

Give meta directing functional groups

Answer 25

NO2
Ketones

Question 26

Give ortho and para directing groups of benzene

Answer 26

OH
NH2
Halogens
Alkyl groups

Question 27

Why are halogens ortho and para directing?

Answer 27

Although they are electron withdrawing as the inductive effects than resonance of the lone pair, the donation of the lone pair has a greater effect in stability, so ortho and para

Question 28

(Why do electron donating groups direct to ortho and para?)

Answer 28

These positions allow a resonance structure where the carbons of the benzene ring have full octet with the lone pair being brought down
The positive charge is on the functional group- this is the major resonance contributor, and is the most stable
This possible arrangement is not possible with meta, the positive charge cannot be on that position, also have one less resonance structure (3 instead) so less stable

Question 29

(Why are electron withdrawing groups meta directing?)

Answer 29

The resonance structures of meta avoid a positive charge on the carbon bonded to the electron withdrawing group- this position is a minor contributor and the least stable
Therefore, meta is the most stable position

Question 30

Where will directing groups be useful?

Answer 30

In synthesis
Determining position of functional groups so order of reactions

Question 31

What is important about ortho and para? What are exceptions to their abundance?

Answer 31

Normally 67 to 33 % as two positions are ortho
If there is a large groups such as tert butyl, the steric hindrance prevents much ortho, so predominately para

Question 32

How do you convert NO2 to NH2 on benzene?

Answer 32

React with:
1. Sn/ Conc HCl
2. NaOH

Question 33

(What are the rules for aromaticity?)

Answer 33

4n+2 Pi Electrons
Flat
Fully Conjugated
Closed Ring

Question 34

(Why, in terms of orbitals, are aromatic compounds particularly stable?)

Answer 34

Bonding molecular orbitals are of equivalent energy due to nodal planes rather than nodal positions when the phases the orbitals do not align
Phases are constantly changing (although with filling its lowest to higher energy)
So the molecular orbitals are more stable than other molecules, as the out of phase alignements are lower energy

Question 35

(Why, in terms of orbitals, are anti-aromatic compounds particularly unstable?)

Answer 35

Due to the filling of the molecular orbitals, there is a di radical nature to the pi electron distribution, as the bonding orbitals are equivalent in energy.
The nature of the molecular orbitals fills the electrons with 1 electron in each first, rather than pairing the electrons in one orbital
This effectively produces a molecule with 2 radicals

Question 36

How do you form a diazonium compound?

Answer 36

Phenylamine with HCl and NaNO2, (form HNO2 in situ, with H+ still used), to form Diazonium Salt when COLD temperatures (+N—N)Cl-
React with Phenol Salt to produce Azo Dye in ALKALINE conditions, where the positioning of the nitrogen is Ortho/Para directed

Question 37

Order a C-C, C=C, and benzene C-Cs in terms of bond length. Shortest first

Answer 37

C=C regular
Benzene C-C
C-C regular

Question 38

When the arenium ion is formed, carbocation intermediate, how many delocalised electrons are there and what is their position?

Answer 38

4 electrons remaining delocalised (like 2 double bonds, think electrophilic addition)
Across the 5 remaining carbon atoms

Question 39

How do you convert phenylamine to a halogenobenzene or benzene?

Answer 39

Reaction with NaNO2/HCl for diazonium
React with phosphoric acid for benzene
Or React with CuX for halogen replacement

Because N triple bond is a good leaving group, SN1