Benzene and Aromatic

Question 1

What is the difference between aromatic and aliphatic structures?

Answer 1

aromatic

• ring of stable atoms
• alternating double and single bonds = same length
• cyclic, planar, p orbital on every atom of the ring, 4n+2 electrons

aliphatic

• straight chain, non-aromatic
• can be cyclic or not
• open chain = straight or branched
• saturated or unsaturated

Question 2

What is the test for aromatics?

Answer 2

have high carbon content = low C to H ratio
- burn with a smoky flame

aliphatic and cycloalkanes burn with a clear flame

Question 3

What is benzene more stable than expected? Why is benzene not cyclohexatriene?

Answer 3

cyclohexatriene enthalpy = -87
benzene experimental enthalpy = 50

benzene is more stable than expected and cannot be cyclohexatriene
- due to aromaticity

37 kcalmol = resonance stabilisation
- electrons are delocalised = p electrons

Question 4

What is resonance stabilisation? What is it also known as?

Answer 4

Wheland Intermediates

delocalised electrons within the molecule
- rearrangement of pi electrons

makes the molecule more stable and reduces reactivity

Question 5

What is kekule resonance? What is wrong with it?

Answer 5

kekule states that the bonds in benzene are alternating single and double bonds

• would cause distorted lengths and a distorted hexagon molecule
• benzene must have carbon bonds of the same length

states benzene must be cyclohexatriene

• benzene cannot be cyclohexatriene because it is more stable than it
• stability due to aromaticity and resonance stabilisation = delocalised 6e in pi orbitals

Question 6

What are the requirements for a molecule to be aromatic?

Answer 6

must be cyclic
must have p orbitals on every atom in the ring
must be planar
must have 4n+2 pi electrons

Question 7

How does sunscreen work?

Answer 7

SPF (sun screen protection factor) is the amount of protection from UV-B

• some sunscreens contain inorganic components such as zinc oxide that reflect light
• others contain compounds that absorb UV-B light

Question 8

What are the steps to electrophilic substitution?

Answer 8

1 - rate determining step = electrophilic addition

2 - fast step = proton elimination

Question 9

What is an electrophile?

Answer 9

positively charged or neutral atom that accepts electrons

- acts like a lewis acid

Question 10

What is a nucleophile?

Answer 10

negatively charged or neutral atom that donates electrons

- acts like a Lewis base

Question 11

Why does electrophilic aromatic substitution require a catalyst?

Answer 11

a catalyst is required to generate the electrophile
a catalyst is also required because the aromatic structures are less reactive due to their resonance stabilisation
- rearrangement of p electrons

Question 12

What are the types of aromatic electrophilic substitution?

Answer 12

aromatic nitration
aromatic halogenation
aromatic sulfonation
alkylation and acylation Friedel–Crafts reaction = reaction that attaches substituents to an aromatic ring

Question 13

What are the requirements for nitration of benzene?

Answer 13

catalyst - sulphuric acid
- generates the electrophile from nitric acid but reacting free proton with it

reaction must be done under reflux

Question 14

What are the requirements for the friedel craft’s reaction for acylation of benzene?

Answer 14

catalyst - aluminium chloride

- generates the electrophile from acetyl chloride

Question 15

Why is aluminium chloride a catalyst for Friedal crafts alkylation?

Answer 15

alkylation
- aluminium chloride is a catalyst because after it has eliminated the proton it cannot react again. this is because the alkyl group is not charged so it will not react agan

acylation

• aluminum chloride is not a catalyst because after the elimination of the proton it can react again
• the oxygen is charged so can react with the aluminium chloride and form a complex
• means more than one equivalent of AlCl3 is required for the reaction and water must be added at the end of the reaction to liberate the product from the complex