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aromatic compounds Flashcards

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1
Q

how does a halogen carrier allow benzene to react with chlorine (1)

A
  • introduce a dipole on the Cl2
  • AlCl3+Cl2->AlCl4- +ClI
  • AlCl3+Cl2->Cl+ -AlCl3-
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2
Q

state the name of the mechanism in which clhorine reacts with benzene (1)

A

electrophillic subsitution

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3
Q

In contrast to benzene the reaction of an alkene with bromine does need a halogen carrier. Compare the different reactivities of benzene and alkenes towards chlrine (3)

A
  • in benzene pi electrons are delocalised / spread out
  • in alkenes pi electrons are concentrated between two carbonds
  • electrophiles attracted more to greater electron density in alkenes
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4
Q

Describe the bonding and structure of a a benzene molecule

A
  • p orbitals overlap
  • above and below ring
  • form pi bonds/orbitals
  • pi bonds / electrons are delocalised
    other two points:
  • ring is planar
  • c-c bonds are equal length/ have intermediate length, between C=C/ or C-H
  • sigma bonds are between C-C and/or C-H
  • bond angles are 120 degrees
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5
Q

Describe the bonding in benzene
explain why electrophiles such as bromine react much more readily with phenol than wihth benzene (7)

A

Bond in benzene
* overlap of p orbitals / pi bonds / electrons
* above and below the ring
* electrons are delocalised
* C-C bonds are:same length/strength/in between single and double/ sigma bonded
greater reactivity of phenol
* the ring is activated ebcause the lone pair from O is delocalised into the ring
* so electron density is increased
* so electrophikes are more attracted 9to the ring)/ dipole in electrophile more easzily induced

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6
Q

Compare the reagants and conditions for the nitration of phenol with those used for the nitration of benzene
state and explain the effect of the OH group on the reactivity of the benzene ring in phenol (7)

A

conditions for nitration of benzene
* HNO3 is concentrated
* concentrate H2SO4 is present
* heating / 50 degress celcius
explanation for greater reactivity of phenol
* lone pair from the O atom is delocalised into the ring
* greater electron density around the ring
* the benzene ring in phenol is activated
* attractds electrophiles/NO2+ more/makes it more susceptible to electrophiles

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7
Q

Explain what is meant by the term delocalised pi bond electrons (2)

A
  • electrons are spread over more than two atoms
  • formed by overlap of p orbitals
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8
Q

Desribe and explain how a molecule of bromine acts as an electrophile
Use your answer to explain why bromine reacts much more readily with cyclohexene than it does with benzene (5)

A
  • bromine acts as an electrophile - electrophile accepts and electron pair
  • benzene is more stable/ more energy is required
  • benzene pi electrons are delocalised
  • benzene has a lower electron / charge density
  • so bromine is less polarised/ attracted to it / benzene is less susceptible to electrophiles
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9
Q

The compoind iodine monobromide IBr also reacts with benzene in an electrophillic reaction
which compound would bee the main broduct iodobenzene of bromobenzene explain your answer (2)

A
  • iodobenzene
  • Br is more electronegative than I
  • so the I atoms will be postivive/ the electrophile
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10
Q

Deduce an equation for the reaction of iodo monobromide with benzene (1)

A

C6H6 + IBr -> C6H5I +HBr

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11
Q

Describe the bonding in a benzene molecule (4)

A
  • 3 electrons from each carbon atoms used to form sigma bonds to 2C and H atom
  • 1 electron from each C atom/electron in p orbital overlap with electrons from adjacent C atoms/ electrons in p orbitals
  • creates a delocalised system of electrons
  • rings of charge / pi bonds above and below plane of C atoms
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12
Q

State how the Kekule model differs from the accepted model of bonding in benzene (1)

A

contains alternating double and sigma bonds

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13
Q

Describe the three pieces of evidence that led scientists to reject the kekue model of the structure of benzene (3)

A
  • benzene is less reactive than predicted
  • C-C bond lengths in benzene are all the same length
  • enthalpy change of hydrogenation is less negative than predicted
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14
Q

A group of different students are given some samples of ethanol, dilute ethanoic acid, and dilute solution of phenol in unlabelled bottles
* solution of sodium hydroxide to whcih a few drops of universal indicator has been added
* a solution of sodium carbonate
describe and explain how the students could use these solutions to identify the samples in the bottles. Give equations for any reactions that will occur during the prodcedure (6)

A
  • ethanol will not give a positive result
  • it is not acidic
  • ethanoic acid will fizz with sodium carbonate
  • and will react with sodium hydroxide causing indicator to turn red/orange/green
  • becasue ethanoic acid is a strong enough acid to react with both alkalis and carbonates
  • CH3COOH + NaOH ->CH3COO-Na+ +H2O
  • 2CH3COOH+Na2CO3->2CH3COO-Na+ + CO2 + H2O
  • phenol will not fizz with sodium carbonate
  • but will react with sodium hydroxide causing indicator to turn red/ orange/greem
  • C6H5OH +NaOH -> C6H5O-Na+ + H2O
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15
Q

Describe in terms of orbital overlap, the similarities and differences between bonding in the kekule model and the delocalised model of benzene (3)

A

similarities
* sideways overlap of p orbitals
* pi bond/system/ring above and below (bonding C atoms/ring/plane)
differences
* kukule has alternating pi bonds/ localised pi electrons and delocalised has pi ring system and p oritals overlap/electrons spready around ring/overlap in both directions

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16
Q

experimental evidence led to the gerneal acceptance of the delocalised model over the kekule model
describe two pieces of evidence to support the delocalised model of benzene (2)

A
  • C-C bond lenght is between single and double bond / C-C bond lengths are the same
  • enthalpy hydrogenation is less exothermic than expected
  • benzene is less reactive than alkenes/ benzene requires a catalyst/ does not decolourise bromine/reacts by substiution/ does not react by addition
17
Q

Explain why phenol is nitrated more readily than benzene (3)

A
  • in phenol a lone pair of electrons on O is delocalised / donated into the pi system/ring
  • electron denistry increases
  • is more susceptible to electrophiliic attack/ attracts electrophile/polarises electrophile more
18
Q

A chemist carries out a reaction uing 4.97g of benzoic acid
the chemist obtains 3-nitrobenzoic acid as an impure solid
the chemist purifies the solid to obtain4.85g of 3 - nitrobenzoic acid
describe a method to obtain a pure sample of 3-nitrobenzoic acid from the impure solid determine the percentage yield and check it’s purity (6)

A

purification
* recrystallisation
* dissolve impure solid in minimal volume of hot water/solvent
* cool solution and filter solid
* wash with cold water/solvent and dry
percentage yield
* 4.97/122=0.0407
* 4.85/167=0.0290
* 0.029/0.407x100=71.3%
chekcing purity
* obtain melting point
* compare to known values
* pure sample will have a (sharp) melting point very close to data book value
spectroscopy
* run an NMR/IR spectrum
* compare to database
* spectrum of pure sample will contains same peaks and not others
TLC
* run a TLc
* compare rf valye to known data
* pure sample will have a verry similar Rf

19
Q

State the trend in relative ease of nitration of phenol,benzene and benzoic acid (1)

A
  • phenol is the most easily nitrated / most reactive and benzoic acid is the least easily nitrated/ least reactive
20
Q

Apply your knowledge of the bonding in arenes to explain the trend between phenol, benzene and benzoic acid (3)

A

phenol
* lone pair of electrons on O is delocalised/donated into the pi system ring
reactivity of benzoic acid
* COOH group on benzoic acid is an electron withdrawing group
* in phenol electron denisty is higher and ring is more susceptible to attack

21
Q

Sulfuric acid is a catlayst in many reactions
State one other example of an oraganic reaction in which sulfuric acid is a catalyst (1)

A
  • elimination of H2O from alcohols
  • nitration of benzene
  • esterification
  • hydrolysis of esters/amides
22
Q

Polymer can not be disposed of in landfill sites as it is not biodegradable suggest one way of processing waste polymers other than landfill and recycling (1)

A
  • uses as an orgnic feedstock
  • combustion for energy production
23
Q

Describe with the aid of suitable diagrams showing the orbital overlap the difference in bonding between kekule model and benzene (4)

A
  • p orbitals overlap to form pi bonds
  • pi bond are delocalised in benzene
  • pi bonds are loaclised/ between two carbon atoms in kekule model
  • and both diagrams
24
Q

Explain why bromines reacts more readily with salicyclic acid than with benzene (3)

A
  • lone pair o electrons on O is delocalised into the ring
  • Br2 is more polarised
  • electron density increases
25
Q

give chemical explanations for carbon - carbon bonds in benzene are all the same length (1)

A

pi bonds in benzene are delocalised

26
Q

Explain experimental evidence that led to the development of the updated model from the kekule model of benzene
describe the bonding in the updated model of bezene (4)

A
  • carbon to carbon bond lengths are the same in benzene
  • enthalpy change of hydrogenation is less exothermic for benzene
  • benzene has a greater stability requires catalyst for chlorination alkenes don’t require a catalyst
  • p orbitals overlap to form pi bonds
  • pi electrons are delocalised
27
Q

Explain why chlorine reacts much more readily with C6H5N(Ch3)2 than with benzene (3)

A
  • lone pair of electrons on N is delocalised into the ring
  • electron densitry increases
  • electrophile is more polarised
28
Q

The student is told by a friend that the FeCl3 catalyst is not needed because quinol is more reactive than benzene
explain why the students friend is correct (3)

A
  • the lone pair of electrons on the oxygen atom
  • is donated/ partially delocalised into the pi system
  • making quinol more susceptible to an electrophillic attack

Chemistry mark scheme flashcards (7 decks)

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