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Arenes Flashcards

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1
Q

What are arenes?

A

hydrocarbons that possess at least one benzene ring

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2
Q

Why is benzene stable?

A

The continuous overlap of six unhybridised p orbitals results in the delocalisation of π electrons above and below the ring, this resonance structure gives benzene extra stability

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3
Q

Why do benzenes not undergo addition reaction and what reactions do they undergo instead?

A

Benzene is less susceptible to addition reactions because it will destroy the aromatic character, it is more likely to undergo substitution reaction to retain the stable delocalised π electron system

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4
Q

What reactions do benzene undergo?

A
  1. Combustion
  2. Electrophilic substitution
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5
Q

What kind of flame do benzene combust in and why?

A

Sooty flame due to its high carbon content

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6
Q

What types of electrophilic substitution reactions of benzene are there?

A
  1. Nitration
  2. Halogenation
  3. Friedel-craft alkylation
  4. Friedel-craft acylation
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7
Q

What are the reagents and conditions for nitration of benzene?

A

conc. HNO3, conc. H2SO4, maintained at 55 degree celsius

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8
Q

What is the first step of nitration of benzene?

A

HNO3 + 2H2SO4 ⇌ NO2⁺ + 2HSO4⁻ + H3O⁺

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9
Q

What are the reagents and conditions of halogenation of benzene?

A

X2, Fe or anhydrous FeX3 or anhydrous AlX3

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10
Q

Why is F2 and I2 not used for the halogenation of benzene?

A
  • F2 is too reactive and will give low yields of mono-substituted products
  • I2 is too unreactive towards benzene ring
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11
Q

What is the first step of halogenation of benzene?

A

If FeCl3 used: FeCl3 + Cl2 → FeCl4⁻ + Cl⁺
If Fe used: Fe + 3/2Cl2 → FeCl3, FeCl3 + Cl2 → FeCl4⁻ + Cl⁺
If AlCl3 used: AlCl3 + Cl2 → AlCl4⁻ + Cl⁺

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12
Q

What does Fe, FeX3 and anhydrous AlX3 act as in halogenation of benzene and why are they needed?

A

They are catalysts which acts a Lewis acid which accepts electron pair from Cl⁻ because halogens are not very strong electrophiles and this converts them to more reactive electrophiles to attack stable π electron system of benzene ring

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13
Q

What are the reagents and conditions of Friedel-crafts alkylation?

A

RCl, anhydrous AlCl3 or FeCl3

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14
Q

What is the first step of Friedel-crafts alkylation?

A

Using AlCl3: AlCl3 + CH3CH2Cl → AlCl4⁻ + CH3CH2⁺
*note that it is the second C of CH2⁺ that attacks the benzene ring

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15
Q

What are the reagents and conditions of Friedel-crafts acylation?

A

RCOCl, anhydrous AlCl3

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16
Q

What is the first step of Friedel-crafts acylation?

A

AlCl3 + RCOCl → AlCl4⁻ + RCO⁺

17
Q

Why is methylbenzene a useful organic solvent?

A

It is preferred over benzene since its fumes are considerably less toxic than benzene

18
Q

Why is the boiling point of methylbenzene higher than benzene?

A

Methyl benzene has a greater number of electrons thus more energy is required to overcome the stronger id-id interactions

19
Q

Why is melting point of methylbenzene lower than benzene?

A

methylbenzene is less symmetrical, thus it has less efficient packing and lower energy is required to overcome the less extensive id-id

20
Q

What reactions do methylbenzene undergo?

A
  1. side chain reactions: free radical substitution, oxidation
  2. electrophilic substitution of benzene ring
21
Q

What are the reagents and conditions of oxidation of methylbenzene under acidic conditions?

A

acidified KMnO4, heat under reflux

22
Q

What are the observations when methylbenzene undergo oxidation under acidic condition?

A

Purple KMnO4 turns colourless, when reaction mixture is cooled, white precipitate of benzoic acid is formed
(benzoic acid is insoluble in cold water but soluble in hot water)

23
Q

What are the reagents and conditions of oxidation of methylbenzene under alkaline conditions?

A

alkaline KMnO4, heat under reflux

24
Q

What is the side chain after methylbenzene undergo oxidation under alkaline conditions?

A

COO⁻

25
Q

What is the product and observation after alkyl side chain of benzene undergoes oxidation?

A

benzoic acid + CO2 + 2H2O (can think of it like a combustion for alkane)
Observation: Purple KMnO4 turns colourless, when reaction mixture is cooled, white ppt. forms, when gaseous product is passed into limewater, white ppt. formed

26
Q

How does the group substituted to the benzene ring affect the benzene?

A
  1. reactivity of aromatic ring (rate of reaction)
  2. site of attack
  3. number of substitutions
27
Q

How does the substituent affect reactivity of aromatic ring?

A
  • Electron-donating group increase the electron density on the benzene ring, making the ring more susceptible to electrophilic attack as well as stabilise arenium ion, thus reaction conditions are milder
  • Electron-withdrawing groups decrease the electron density on the benzene ring, making the ring less susceptible to electrophilic attack as well as destabilise the arenium ion, reaction conditions required are harsher
28
Q

What are activating groups?

A

Substituent that makes the benzene ring more reactive, activates the ring, they are electron-donating groups

29
Q

What are deactivating groups?

A

Substituent that make the benzene ring less reactive, deactivates the ring, they are electron-withdrawing groups

30
Q

How does the substituent on benzene affect the site of attack?

A

The nature of the substituent determines the position of the second substituent (2,4-directing or 3-directing)

31
Q

What are the reagents and conditions of nitration of methylbenzene?

A

conc. HNO3, conc. H2SO4, maintained at 30 degree celsius

32
Q

What is the use of methylbenzene?

A

-common solvent to dissolve paints, rubber
-octane booster in gasoline fuels used in internal combustion engines

33
Q

What is the use of benzoic acid?

A

-food preservatives
- treatment of fungal skin disease

34
Q

What is the use of napthalene?

A

-raw material in manufactures of dyes, insecticides
-fumigant for repelling moths

Chemistry (21 decks)

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