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Chemistry > (10) Aromatic > Flashcards

(10) Aromatic Flashcards

1
Q

what does aromatic mean?

A

particular sort of delocalised bonding

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2
Q

what are arenes?

A

aromatic hydrocarbons; based on benzene rings

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3
Q

benzene state and where it comes from

A

colourless liquid at RT and pressure
obtained from petroleum (crude oil)

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4
Q

three different bonding compounds of arenes

A
  • cyclohexene (one C=C)
  • cyclohexane (all C-C)
  • benzene (3 C=C, every other)
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5
Q

what is the kekulé structure

A

benzene has 3 C-C and 3 C=C

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6
Q

problems with kekule structure

A
  • alternating bonds, C-C and C=C have different lengths —> xray shows bonds are equal lengths
  • would be known as cyclohexa-1,2,3-triene; can hydrogenate = enthalpy -120 kJ mol-1 —> enthalpy = -208 kJ mol-1
  • undergoes electrophilic substitution (hydrogen replaced) —> undergoes electrophilic addition (pi-bond breaks = new bond)
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7
Q

what is the difference in enthalpy due to between kekule structure and benzene

A

the stability from the system of delocalised electrons

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8
Q

how many electrons are free to bond in a benzene ring and what are they used for

A
  • 30 (H 1 each, C 4 each)
  • 12 C electrons used for 6 C-H sigma bonds
  • 12 C electrons used for 6 C-C sigma bonds
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9
Q

explain the pi bonding in a benzene ring

A
  • each C has an electron not used in sigma bonds
  • p-orbitals overlap to form pi-bond delocalised* system of electrons above and below plane of hydrocarbon skeleton

*pi-bonds are spread over several atoms

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10
Q

what does delocalisation of electrons do in a benzene ring

A
  • lowers energy if structure (makes more stable)
  • difference in energy values = delocalisation energy
  • unique electronic structure has dominant influence on properties of molecule
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11
Q

structure of benzene ring

A

planar, hexagonal molecule with bond angle = 120°

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12
Q

what does the circle in a benzene ring represent ?

A

the delocalised pi-electron cloud

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13
Q

what type of reaction will alkenes readily undergo? and an example

A

electrophilic addition; eg. ethene and bromine (test for unsaturation)

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14
Q

why does benzene not undergo electrophilic addition?

A

unsaturated, doesn’t react with bromine due to stability of pi-delocalised system of electrons

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15
Q

what type of reaction does benzene undergo

A

electrophilic substitution where pi-electron cloud is preserved

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16
Q

substituted benzene compounds:
- phenol
- methylbenzene
- phenylamine
- iodobenzene
- bromobenzene
- benzoic acid
- methyl benzoate
- methyl-3-nitrobenzoate
- benzene diazonium chloride

A
  • C6H5OH
    -C6H5(CH3)
    -C6H5NH2
    -C6H5I
    -C6H5Br
    -C6H5COOH
    -C6H5COOCH3
    C6H4(NO2)COOCH3
    -C6H5N2+Cl-
17
Q

what are the two mechanisms for reactions of benzene

A
  • monohalogenation
  • mononitration
18
Q

main monohalogenation of benzene studied

A

monobromination of benzene using bromine in presence of iron/iron bromide (due to stability of delocalised system of electrons in benzene catalyst reuqired)

19
Q

explain monobromination mechanism reaction

A
  • Br2 is polarised as it approaches benzene
  • FeBr3 acts as halogen carrier
  • δ- Br atom transfers pair of electrons to iron in FeBr3// other Br atom accepts pair of electrons from delocalised cloud of elections in benzene
  • temporilary, delocalised pi-electron system stability lost
  • electrons in C-H go back to ring to reform electron cloud losing H+ ion
  • FeBr4- reacts with H+ —> FeBr3 (catalyst) and HBr
20
Q

when does mononitration of benzene occur

A

when one (or more) H atoms on benzene is replaced by nitrogen group (NO2)

21
Q

what mechanism is mononitration of benzene and how is the ion needed produced

A

electrophilic substitution, electrophile (nitronium ion) produced through reaction of conc nitric acid and conc sulfuric acid

HNO3 + 2H2SO4 —> NO2+ + H3O+ + 2HSO4-

22
Q

what is the nitro group and describe its appearance and use when attached to benzene

A

NO2
nitrobenzene = yellow oily liquid
dyes and explosives

23
Q

what happens when benzene reacts with halogenoalkanes and acyl chlorides and what is this called

A

alkylated and acylated benzene products (called friedel-crafts reactions)

24
Q

catalyst in friedel-crafts reactions

A

aluminium halides

25
Q

what is alkylation

A

substituting H atom for alkyl group

26
Q

describe alkylation reaction between chloroethane and ethyl benzene

A
  • occurs in presence of aluminium chloride (halogen carrier/catalyst)
  • aluminium chloride reacts with halogenoalkane to form electrophile
  • electrophile reacts with benzene
  • catalyst regenerated when AlCl4- reacts with H+
27
Q

what is acylation

A

substituting a H atom for an acyl group

28
Q

describe acylation reaction between ethanoyl chloride and phenylethanone

A
  • occurs in presence of aluminium chloride (catalyst)
  • aluminium chloride reacts with acyl chlorides to form electrophile
  • electrophile reacts with benzene
  • catalyst regenerated
29
Q

monobromination equation

A

FeBr3 + Br2 —> FeBr4- + Br+

30
Q

mononitration equation

A

HNO3 + 2H2SO4 —> NO2+ + H30+ + 2HSO4-

31
Q

monoalkylation equation

A

CH3CH2Cl + AlCl3 —> CH3CH2+ + AlCl4-

32
Q

monoacylation equation

A

CH3COCl + AlCl3 —> CH3CO+ + AlCl4-

33
Q

benzene ring we use in school (normal benzene toxic)

A

methyl-3-nitrobenzoate, prepared from methyl benzoate (ester of benzoic acid)

34
Q

describe prep of methyl 3 nitribenzoate

A
  • dissolve methyl benzoate in conc H2SO4
  • cool in ice
  • add conc H2SO4 to conc HNO3 (=NO2+)and cool
  • drop by drop nitrstion mixture to solution and keep below 10°
  • leave at room temp for 15mins
  • pour into crushed ice and stir until melted and crystalline methyl 3 nitrobenzoate fromed
  • filter using buchner filtration, wash w/ cold water
  • recrystallise from ethanol, filter off and dry
35
Q

purification of methyl 3 benzoate

A
  • recrystallise from min vol of hot ethanol
  • warm ethanol about 50° by beaker of hot water (no naked flames)
  • dissolve crystals in hot ethanol
  • filter using fluted filter paper
  • cool and crystallise
  • immerse in ice to complete crystallisation
    -suction filter and wash crystals with ice cold solvent
  • dry between filter paper and weigh
  • record mass
  • observe melting point (77.5°) - cream solid
36
Q

why is the reaction mixture kept below 10° (methyl-3-benzoate)

A

greater chance of multiple nitrations at higher temp (explosive)

37
Q

why is suction filtration used (methyl-3-benzoate)

A

faster and drier product

38
Q

why is fluted filter paper used in recrystallisation (methyl3benzoate)

A

minimises contact with cold funnel - could cause cause crystallisation

Chemistry (8 decks)

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