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A-Level Organic Chemistry AQA (Paper 2) > Aromatic Chemistry > Flashcards

Aromatic Chemistry Flashcards

1
Q

What are arenes?

A

•Hydrocarbons based on benzene C6H6.

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2
Q

Draw the skeletal formula/symbol of benzene.

A

•Hexagon with a circle - there is one carbon atom and one hydrogen atom at each vertice.

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3
Q

What is the bonding of benzene? (3)

A
  • Despite it being unsaturated - does not readily undergo addition reactions.
  • All c-c bonds are the same.
  • Each bond is intermediate between a single and a double bond.
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4
Q

What is the structure of benzene? (2)

A
  • Flat, regular hexagon of carbon atoms, each bound to a single hydrogen atom, planar structure.
  • Molecular formular = C6H6.
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5
Q

What is delocalisation?

A

•When the electrons are spread over more than two atoms - in benzene over six that form the ring.

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6
Q

Describe the pi system of benzene. (4)

A
  • Each carbon has three covalent bonds - one to a hydrogen atom and the other two to carbon atoms.
  • The fourth electron of each carbon atom is in a p-orbital, there are six of these.
  • P-orbitals overlap and the electrons are delocalised.
  • A region of electron density above and below the ring is formed.
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7
Q

What does the pi system of benzene mean? (2)

A
  • It makes it have very strong bonding.
  • Extra stability (aromatic stability) - resistance for reacting ∴ only substitution reactions can occur where the H is replaced.
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8
Q

What is the enthalpy change for the hydrogenation (addition of hydrogen) of cyclohexene?

A

•ΔH = -120 kJmol^-1.

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9
Q

What would be the hypothetical enthalpy change for the hydrogenation of a ring with alternate double bonds (non-delocalised benzene, no p-orbital overlap)?

A

•ΔH = -360 kJmol^-1

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10
Q

What is the actual enthalpy change for the hydrogenation of benzene? (2)

A
  • ΔH = -208 kJmol^-1 = less negative than theoretical value.
  • Very little energy given out due to the stability of the molecule.
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11
Q

By how much is benzene more stable than the unsaturated structure?

A

•By 152 kJmol^-1 (the delocalisation energy), more than the theoretical difference.

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12
Q

What are the physical properties of arenes? (4)

A
  • Benzene is a colourless liquid at room temperature.
  • Boils at 353K (lower than hexane (354K)).
  • Freezes at 279K (higher than hexane (178K)) - due to benzene’s flat planar structure, the hexagonal molecules pack well together in the solid state, so they are harder to separate for the solid to melt.
  • Like other hydrocarbons that are non-polar, arenes do not mix with water but mix with other hydrocarbons and non-polar solvents.
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13
Q

What are the two factors important to the reactivity of aromatic compounds? (2)

A
  • The ring is in an area of high electron density, because of the delocalised bonding ∴ is attacked by electrophiles.
  • Aromatic ring is stable - needs energy to be put in to break the ring before the system can be destroyed (delocalisation energy) ∴ the ring almost remains intact in the reactions of arenes.
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14
Q

What are most of the reactions of aromatic systems?

A

•Electrophilic substitution.

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15
Q

How do arenes burn in the air (combustion)? (2)

A
  • WIth flames that are noticeable smokey - due to high carbon : hydrogen ratio compared with alkanes.
  • Unburnt carbon remaining, produces soot.
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16
Q

What does a smokey flame suggest?

A

•An aromatic compound.

17
Q

Describe the mechanism of electrophilic substitution reactions with aromatic rings. (6)

A
  • Delocalised system of the aromatic ring has a high electron density that attracts electrophiles.
  • Electrons are attracted towards the electrophile.
  • A bond forms between one of the carbon atoms and the electrophile.
  • To do this, the carbon must use electrons from the delocalised system, which destroys the aromatic system.
  • To get back the stability of the aromatic system, the carbon loses a H^+ ion with the electron in the C-H bond returning to the delocalised sstem.
  • The sum of these reactions is the substitution of H^+ by an electrophile^+.
18
Q

What is nitration? (6)

A

•The substitution of a NO2 group for one of the hydrogen atoms in an arene ring.
•NO2^+ generated in the reaction mixture of concentrated nitric and concentrated sulfuric acid:
H2SO4 + HNO3 —> H2NO3^+ + HSO4^-
•Sulfuric acid (catalyst) is a stronger acid than nitric acid and donates a proton, H^+, to HNO3.
•H2NO3^+ then loses a molecule of water to give NO2^+ (nitronium ion/nitryl cation).
H2NO3^+ —> NO2^+ + H2O

19
Q

What is the overall equation of the formation of the NO2^+ electrophile? (2)

A

HNO3 + 2H2SO4 —> +NO2 + 2HSO4^- + H3O^+
Or
HNO3 + H2SO4 —> +NO2 + H2O + HSO4^-

20
Q

What are nitrated arenes used for? (2)

A
  • They’re important in the production of explosives like TNT.
  • The first step in making aromatic amines (used to make industrial dye).
21
Q

What is TNT short for?

A

•Trinitrotoluene.

22
Q

How is TNT made?

A

•By nitrating methylbenzene (known as toluene).

23
Q

What is Friedel-Crafts acylation reaction? (8)

A

•Where aluminium chloride (AlCl3) is used as a catalyst.
•Substitution - with RCO substituting for a hydrogen on the aromatic ring.
•Acyl chlorides provide the RCO group, they react with AlCl3 to form AlCl4^- and RCO^+.
RCOCl + AlCl3 —> RCO^+ + AlCl4^-
•The aluminium atom in AlCl3 has only six electrons in its outer main level and readily accepts a lone pair from the chloride atom of RCOCl.
•RCO^+ is a good electrophile that is attacked by the benzene ring to form substitution products.
•AlCl3 is a catalyst - it is reformed by reaction of the AlCl4^- ion with H^+ from the benzene ring:
AlCl4^- + H^+ —> AlCl3 + HCl

24
Q

What is the functional group priority when naming compounds from highest to lowest? (9)

A
  • Carboxylic acid.
  • Ester.
  • Acyl chloride.
  • Nitrile.
  • Aldehyde.
  • Ketone.
  • Alcohol.
  • Amine.
  • Halogenoalkane.
25
Q

What are the conditions of nitration? (2)

A
  • Reagent: conc HNO3 and conc H2SO4.

* 50°C.

26
Q

What are the products of nitration? (3)

A
  • Aromatic nitro compounds used to make:
  • Aromatic amines (e.g. used further to make azo dyes).
  • Explosives (e.g. TNT/2,4,6-trinitromethylbenzene).
27
Q

What are the conditions of Friedel-Crafts acylation?

A
  • Reagent: Acyl chloride/acid anhydride and AlCl3 catalyst.

* Anhydrous, to prevent AlCl3 reacting.

28
Q

What are the products of Friedel-Crafts acylation?

A

•Aromatic ketones - useful for adding C atoms to aromatic rings, valuable in organic synthesis.

29
Q

Describe the bond lengths in benzene. (2)

A
  • Bond length intermediate is between single c-c, 0.154 nm and double c=c, 0.134 nm.
  • Provides evidence for delocalisation = stability.
30
Q

What is the bond angle between carbon atoms in benzene?

A

•120°.

31
Q

What are the uses of aromatic amines?

A 
Making...
•Dyes.
•Quaternary ammonium salts OR making (cationic) surfactants.
•Hair conditioner.
•Fabric softener.
•Detergents
32
Q

Give a reagent that could be used in a test-tube reaction to distinguish between benzene and cyclohexene and the observations made.

A

•Bromine water; benzene = NVC, cyclehexene = brown-orange solution to colourless solution.

33
Q

How do you name alcohols with a benzene ring?

A

•Use a number and phenol to describe which carbon the benzene ring is bonded to.

34
Q

When phenylamine is prepared by the reduction of nitrobenzene using tin and an excess of hydrochloric acid. Explain why an aqueous solution is obtained in this reduction even though phenylamide is insoluble in water.

A

•C6H5NH2 is present as an ionic salt = C6H5NH3^+Cl^-

A-Level Organic Chemistry AQA (Paper 2) (17 decks)

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