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Chemistry module 6 > aromatic compounds > Flashcards

aromatic compounds Flashcards

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1
Q

What is benzene

A
  • colourless, sweet smelling, highly flammable liquid
  • found naturally in crude oil component of petrol and found in cigarette smoke
  • classified as a carcinogen can cause cancer
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2
Q

Describe what a benzene molecule consists off

A
  • a hexagonal ring of six carbon atoms with each carbon atom joined to two other carbon atoms and one hydrogen atom
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3
Q

What benzene classified as

A

aromatic hydrocarbon

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4
Q

What is the molecular formula fo benzene

A

C6H6

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5
Q

What is the kekule model

A
  • the tructure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds
  • he dreasmed that the ring shape of benzne is about a snake seizing it’s own tail
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6
Q

What were the three pieces of evidence to disprove kekule model

A
  • lack of reactivity of benzene
  • lengths of carbon carbon bonds in benzene
  • hydrogentation enthalpies
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7
Q

how does the lack of reactivity of benzene disprove

A
  • if benzene contained C=C bonds it would decolourise bromine in an electrophilic addition
  • benzene does not undergo electrophillic addition reactions
  • bezene does not decolourise bromine under normal conditions
  • therefore benzene cannot have any C=C bonds in it’s structure
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8
Q

How does the lengths of carbon to carbon bonds in benzene disprove kekules model

A
  • using a technique called x ray diffreaction it is possible to masure bond lengths in a molecule
  • found that all the bonds in benzene were 0.139nm in length
  • this bond length between the length of a single bond 0.153nm and a double bond 0.134nm
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9
Q

how does hydrogenation enthalpies disprove kekule model

A
  • kekule structure contains alternate single and doouble bonds
  • if benzene did have kekule structure it would be expected to have an expected enthalpy change of hydrogenaition three times thaat of cyclohexene
  • when cyclohexee is hydrogention one double bond reacts with hydrogen
  • the enthalpy change of hydrogenation is 120kJmol-1
  • therefore kekule structure would predict to contain three double bonds and so the enthalpy change of hydrogenation would be -360KJmol-1
  • the actual enthalpy change o hydrogentation of benzene is only 208KJmol-1
  • means less energy is produced than expected
  • so benzene is more stabe than the theoretical kekule model
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10
Q

Describe the delocalised model of benzene

A
  • benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms
  • each carbon atom uses three of its availabe four electrons in bonding to two other carbon atoms and to one hydrogen atom
  • each cabron atom has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
  • adjacten p orbital electrons overlap sideways in both directions above and below the plane of the carbon atoms to form a ring of electron density
  • this overlapping of the p orbitals creates a system of pi bonds which spread over all six od carbon atoms in the ring structure
  • the six electrons occupying the system of the pi bonds are said to be delocalised
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11
Q

Describe the mechanism for the reaction of benzene

A
  • benzene and it’s derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replace d by another atom of a group of atoms
  • benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophhilic substitution
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12
Q

Describe the nitration of benzene

A
  • benzene reacts slowly with nitric acid to form nitrobenzene
  • reaction is catalysed by sulfuric acid and heated to 50 degrees celcius
  • water bath used to maintain the steady temperature
  • NO2 group replaces the hydrogen
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13
Q

Why can’t the temperature be over 50 degrees during nitration of benzene

A
  • further substitutition reactions may occur leading tot eh production of dinitrobenzene
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14
Q

What are the ues of nitrobenzene

A
  • starting material in the preparation of dyes, pharmaceuticals and pesticides
  • starting material in the preparation of paracetemol
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15
Q

What is the reaction mechanism for the nitration of benzene

A

electrophillic substitution

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16
Q

What i the electrophile in nitration of benzene

A

NO2+

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17
Q

how is the electrophile of benzene produced

A
  • reaction of concentrated nitric acid and concentrated sulfuric acid
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18
Q

how does No2+ react with benzene ring

A
  • No2+ accepts a pair of electrons from the benzene ring to form a dative covalent bonds
  • the organic intermediate formed is unstabel and breaks down to form the organic product nitrobenzene and the H= ions
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19
Q

What is step 1 for nitration of benzene

A

HNo3 +H2SO4->No2+ +HSO4- +H2O

20
Q

What is the final step for nitration of benzene

A

H+ +HSO4- ->H2SO4

21
Q

Describe the halogenation of benzene

A
  • halogens do not react with benzene unless a catlayst called a halogen carrier is present
  • halogen carriers can be generated in situ (in the reaction vessel) from the metal and the halogen
22
Q

What are common halogen carriers

A
  • FeCl3
  • AlBr3
  • FeBr3
  • AlCl3
23
Q

describe the bromination of benzene

A
  • at room temperatyre and pressure and in the prescence if a halogen carrier via electrophillic substitution
24
Q

Why does bromination of benezene require a halogen carrier and describe the bromination of benzene

A
  • too stable to react with a non polar bromine molecule
  • the electropile a brominium on Br+ which is generated when the halogen carrier catalyst reacts with bromine in the first stage of a mechanism
  • brominium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond
  • organic intermediate is unstable and breaks down to form the organic product bromobenzene and H+ ion
25
Q

Describe how the brominium ion is produced when in the bromination of benzene with the halogen carrier FeBr3

A

Br2 +FeBr3 - > FeBr4- +Br+

26
Q

Describe how the halogen carrier FeBr3 is reformed after the bromination of benzene

A

H+ +FeBr4- ->FeBr3 + HBr

27
Q

Describe the alkylation of benzene

A
  • substiution of a hydrogen atom in the benzene ring by an alkyl group
  • the reaction is carried out by reacting benzene with a haloalkane in the prescence of ALCl3
  • acts as a halogen carrier catalyst generating the electrophile
    *
28
Q

What is the name of alkylation of benzene

A

friedal crafts alkylation

29
Q

Describe acylation reactions with benzene

A
  • when benzene reacts with acyl chloride in the prescence of an AlCl3 catalyst, an aromatic ketone is formed
  • electrophillic substiution
    *
30
Q

What is the first member of the acyl chloride group and what is fromeed when it reacts with benzene

A
  • ethanoyl chloride CH3COCl
  • pheylethanone
31
Q

decribe the reaction between bromine and the double bond in cyclohexene

A
  • pi bond in the alkene contains localised electrons above and below the plane of the two carbon atoms in the double bond
  • produces an area of high electron density
  • the localcised electrons in the pi bond induce a dipole in the non polar bromine molecule making one bromine atom of the Br2 molecule slightly positive and the other bromine atom alightly negative
  • the slightly positive bromine atom enables the bromine molecule to act like an electrophiile
  • via electrophillic addition
32
Q

Why does benzene not react with bromine unless a halogen carrier is present

A
  • benzene has delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure
  • the electron denity around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene
  • when non polar molecule such as bromine apporaches the benzene ring there is insufficient pi electron density around any two carbon atoms to polarise the bromine molecule
  • preventing any reaction taking place
33
Q

What is a phenol

A
  • phenols are a type of organic chemical containing a hydroxyl group -OH and a functional group directly bonded to an aromatic ring
34
Q

What is the simplest phenol

A

C6H5OH

35
Q

Describe why phenol is a weak acid

A
  • less soluble in water than alcohols due to the prescence of the non polar benzene ring
  • when dissolved in water phenol partially dissociated froming the phenoxide ion and a proton
  • partially dissociate to produce protons
36
Q

Decribe how you can tell phenol i more acidic than alcohols but less acidic than carboxylic acids

A
  • ethanol does not react with sodium hydroxide of sodium carbonate
  • phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide
  • only carboxylic acids are strong enough to reat with sodium carbonate
37
Q

What is the test for a carboxylic acid from a phenol

A
  • reaction with sodium carbonate can be used
  • carboxylic acid reacts with sodium carbonate to produce carbon dioxide as a gas
38
Q

What are the products of phenol and sodium hydroxide and what type of reaction is this

A
  • form the salt sodium phnoxide and water in a neutralisation reaction
39
Q

Describe the reactions that phenol undergoes

A
  • electrophillic substitution
40
Q

Describe the bromination of phenol and the conditions

A
  • phenol reacts with aqeueous solution of bromine (bromine water) to form a white precirpitate 2,4,5 tribromophenol
  • the reaction decolourises the bromeine water
  • does not require a halogen carrier and reaction is carried out at room temperature
41
Q

Describe the nitration of phenol

A
  • phenol reacts readily with dilute nitricc acid at room temperature
  • a mixture of 2-nitrophenol and 4-nitrophenol is formed
42
Q

Compare the reactivity of phenol and benzene

A
  • bromine and nitric acid react more readily with phenol than they do with benzene
  • phenol is nitrated with dilute nitric acid rathre than needed concentrating nitric and sulfuric acids as with benzene
  • increased reactivity is caused by a lone pari fo electrons from the oxygen p orbital of the OH group being donated into the pi system of pheno
  • the electron density of the benzene ring in phenol is increased
  • the increased electron density attrats electrophiles more strongly than with benzene
  • the aromatic ring in phenol is therefore more susceptible to attack from elextrophiles that in benzene
  • for bromine the electron denitry in the phenol ring structure is sufficient to polarise bromine molecule so no halogen carrier catalyst is required
43
Q

Describe the difference between activation and deactivation with reactions with benzene given that it’s with phyelamine and bromine and nitrobenzene and bromine

A
  • bromine requires a halogen carrier catalyst to react with benzene whereas bromine will react rapidly with phenylamine
    nitrobenzene reacts slowly with bromine requiring both a halogen carrier catalysst and a high temepratyre
  • the benzene ring in nitrobenze is less susceptible to electrophillic substiution that benzene
  • for example -NH2 groups activates the ring as the aromatic ring reacts more readily with electrophiles
  • the -No2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles
  • positions are also different NH2 is directed to positions 2 or 3 and No2 group directs the second substituent to position 3
44
Q

What are 2 and 4 directing (ortho and para directing) group

A
  • -NH2 or NHR
  • OH
  • OR
  • R of C6H5
  • F, Cl, Br I
45
Q

What are 3 dircting (meta directing group)

A
  • RCOR
  • COOR
  • SO3H
  • CHO
  • COOH
  • CN
  • NO2
  • NR3+
46
Q

Describe how the positioning groups are related to activation and deactivation

A
  • 2 and 4 directing groups - ortho and para directors are activating groups with the exception of halogens
  • all 3- directing groups - meta directors and deactivating groups

Chemistry module 6 (2 decks)

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