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Georges chem > Aromatic compounds chp 25 > Flashcards

Aromatic compounds chp 25 Flashcards

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1
Q

what are some characteristics of benzene

A
  • aromatic ring
  • contains delocalised pi electron ring
  • undergoes electrophilic substituion
  • very stable (unreactive)
  • soluble in organic solvents due to being non-polar
  • high boiling and melting point
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2
Q

what is benzene classified as

A

Benzene is classified as an aromatic hydrocarbon or arene.

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3
Q

What is the molecular formula of benzene

A

C6H6

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4
Q

What was the structure proposed by kekule

A

In 1865 kekule suggested that the structure of benzene was based on a six membered ring of carbon atoms joined by alternate single and double bonds.

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5
Q

What evidence is there that disproves kekules model of benzene

A
  • The lack of reactivity of benzene (dont declourise bromine water)
  • The lengths of the carbon-carbon bonds in benzene
  • Hydrogenation enthalpies (less exothermic than kekules structure)
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6
Q

How does lack of reactivity of benzenes provide evidence against kekules structure

A
  • if benzene contained the C=C bonds it should decolourise bromine in an electrophilic addition reaction
  • Benzene does not undergo electrophilic addition reactions
  • suggests that benzene cannot have any C=C bonds in its structure
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7
Q

What can be used to measure bond length
How can bonds length disprove kekules structure of benzene

A
  • X-ray diffraction
  • all the bonds in benzene are between the length of double and single bonds
    ^0.139 nm
  • so no bonds in benzene are single or double
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8
Q

How does hydrogenation enthalpies provide evidence against kekules structure

A
  • as enthalpy of hydrogenation is less exothermic is means the structure of benzene is more stable that the proposed structure
    ^led scientists to develop delocalised ring model
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9
Q

How is the delocalised ring formed in the benzene

A
  • Adjacent p-orbitals overlap sideways create ring of pi electron density
    ^above and below plane of carbon ring
  • 6 electrons (from the carbons) occupy this system of π-bonds
  • said to be delocalised.
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10
Q

What is an aromatic compound with 1 substituent group called

A

Monosubstituent

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11
Q

When would the prefix phenyl be used in benzene nomenclature

A
  • When a benzene ring is attached to an alkyl chain with a functional group/seven or more carbon atoms

dont get confused with phenol

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12
Q

what are some exceptions to the nomenclature conventions of benzene

A
  • Benzoic acid (benzenecarboxylic acid)
  • phenylamine
  • benzaldehyde (benzenecarbaldehyde)
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13
Q

What are compounds with 2 substituent groups called
what does this mean for the nomenclature

A
  • disubstituent compounds have 2 substituent groups
  • ring is numbered, like carbon chain,
  • substituent groups listen in alphabetical order using the smallest numbers possible
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14
Q

what is a substituent group

A

an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon

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15
Q

What reaction mechanism does benzene and its derivates undergo

A

Benzene and its derivatives undergo electrophilic substitution

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16
Q

How does benzene react with nitric acid

A
  • reacts slowly with nitric acid to form nitrobenzene
  • conditions are H2SO4 and 50 degree celsius
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17
Q

What are the conditions for the reaction of nitric acid and benzene

A
  • The reaction is catalysed by sulphuric acid and heated to 50 () to obtain a good rate of reaction. A water bath is used to maintain the steady temperature
  • If the temperature of the reaction rises above 50 (), further substitution reactions may occur leading to the production of dinitrobenzene.
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18
Q

what is nitrobenzene used for

A

Nitrobenzene is an important starting material in the preparation of dyes, pharmaceuticals and pesticides. It can be used as a starting material in the preparation of paracetamol.

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19
Q

Draw the full reaction mechanism for the nitration of benzene

A
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20
Q

Name examples of halogen carriers

A

AlCl3, FeCl3, AlBr3, FeBr3

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21
Q

what are halogen carriers and what is their function

A
  • allow for the halogenation of benzene
  • they weaken bond in halogen molecule, ionising it to an electrophile
  • this electrophile can then react with benzene which it couldn’t do before
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22
Q

What are the conditions necessary for the bromination of benzene

A

At room temperature and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction

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23
Q

Draw the full reaction mechanism for the bromination of benzene

A

chlorine will react in the same way

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24
Q

why is a halogen carrier needed in the bromination of benzene and how is it regenarated

A
  • Benzene stable, so don’t react with non-polar halogen
  • halogen carrier ionises halogen to form halide cation and halogen carrier anion
  • halide ion accepts electron pair from benzene ring (dative covalent bond formed).
  • intermediate is unstable, breaks down to form halidebenzene and H+ ion
  • H+ formed in reacts with the halogen carrier anion to form halogen carrier and hydrogen halide

chlorine will react in the same way

25
Q

What is meant by the alkylation of benzene

A

The alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group

26
Q

What are the conditions needed for the alkylation of benzene

A

The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile

27
Q

What is an acylation reaction and what is Friedel-Crafts acylation

A
  • the transfer of an acyl group (RCO-) from one molecule to another
  • when benzene react with an acyl chloride in the presence of AlCl3 catalyst forming an aromatic ketone
28
Q

what is the first chemical in the acyl chloride homologous series

A

Ethanoyl chloride, CH3COCl, is the first member

29
Q

what type of reaction is the bromination of cyclohexane

A

electrophilic addition

30
Q

Draw the reaction mechanism for the addition of bromine across the double bond in cyclohexene
explain at crucial points

A
  • pi bond (double bond) contains localised electrons above and below plane of molecule and is an area of high electron density
  • ^induces dipole in bromine molecule
  • dipole enables one bromine atom to act like electrophile
31
Q

why does benzene not react with bromine in the same way that an alkene would

A
  • pi-electron density around any 2 carbons in benzene ring is less than in a C=C bond in an alkene.
  • When non-polar molecule such as bromine approaches benzene ring there is insufficient pi-electron density around any 2 carbons to polarise bromine molecule.
    ^This prevents any reaction taking place.
32
Q

how do alkenes react with bromine and how does benzene react with bromine

A
  • Alkenes react with bromine by electrophilic addition.
  • Benzene reacts with bromine by electrophilic substitution.
33
Q

what are phenols

A
  • Phenols are a type of organic chemical containing a hydroxyl, -OH, functional group directly bonded to an aromatic ring.
  • For it to be a phenol group then the OH group must be bonded directly to the aromatic ring
34
Q

What is the molecular structure of the first phenol and what is its name

A
  • C6H5OH, has the same name as the group, phenol
35
Q
  • How are phenols manufactured in the current day
  • write equation
A
  • from benzene, propene and oxygen in multi-step reaction.
  • The other product is propanone
36
Q

Is phenol more or less soluble than alcohols in water and why is this

A

phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring.

37
Q

How can phenol act ac a weak acid
draw it

A
  • can partially dissociation to give a phenoxide ion and a proton
38
Q

What does the acid dissociation constant of alcohols, phenols and carboxylic acids tell us

A

Phenol is more acidic than alcohols but less acidic than carboxylic acids.

39
Q

How can we tell that phenols are more acidic than alcohols but less acidic than carboxylic acids

A
  • Ethanol does not react with sodium hydroxide (a strong base) or sodium carbonate (a weak base)
  • Phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide
  • Only carboxylic acids are strong enough acids to react with the weak base, sodium carbonate out of the 3
40
Q

How can you distinguish between phenols and carboxylic acids

A
  • A reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid
  • the carboxylic acid reacts with sodium carbonate to produce carbon dioxide
  • phenols do not react with sodium carbonate
41
Q
  • what type of reaction is phenol reacting with sodium hydroxide
  • write reaction
A
  • Phenol reacts with sodium hydroxide in condensation reaction
  • sodium phenoixde and water are formed
42
Q

What reactions do phenols most typically undergo

A
  • electrophilic substitution reactions.
  • reactions take place more readily than reactions of benzene
43
Q
  • draw how phenol reacts with bromine
  • what is produced
  • name conditions needed
A
  • Phenol reacts with an aqueous solution of bromine (bromine water) to from a white precipitate of 2,4,6 -tribromophenol.
  • The reaction decolourises the bromine water (orange to colourless).
  • With phenol, a halogen carrier catalyst is not required and the reaction is carried out at room temperature.
44
Q

How does phenol react with nitric acid, draw reaction

A
  • electrophilic substituion with dilute nitric acid at room temperature.
  • mixture of 2-nitrophenol and 4-nitrophenol is formed
45
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 rather than needing concentrated nitric and sulfuric acids as with benzene.

46
Q

Explain the chemistry behind the difference in reactivity between phenol and benzene

A
  • The increased reactivity is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group being donated in the pi-system of phenol.
  • The electron density of the benzene ring in phenol is increased. The increased electron density attracts electrophiles more strongly than with benzene.
  • The aromatic ring in phenol is therefore more susceptible to attack from electrohphiles than in benzene.
  • For bromine, the electron density in the phenol ring structure is sufficient to polarise bromine molecules and so no halogen carrier catalyst is required
47
Q

what is formed from the electrophilic substituion of phenol by nitric acid

A

It can form 2 isomers, 2-nitrophenol and 4-nitrophenol

48
Q

What conditions are needed for nitrobenzene to react with bromine

A

a halogen carrier catalyst and high temperatures

49
Q

what is meant by ortho, meta and para

A

Ortho (o-), meta (m-) and para (p-) refer to positions on a benzene ring relative to the group at position 1

50
Q

what are the 2 catergories of directing groups

A
  • 2-, 4- directing group, activating group
  • 3- directing group, deactivating group
51
Q

what are some examples of 2-, 4- directing groups

A
52
Q

Are 2-, 4- directing groups and 3- directing groups activating or deactivating groups

what is the expetion

A
  • All 2-, 4- directing groups are activating groups, with the exception of halogens
  • All 3- directing groups are deactivating groups
53
Q

what other factor can also affect the proportions of different substituted isomers

A
  • A large substituted group may get in the way, this is called a steric effect.
54
Q

How can the directing effect be used in chemistry

A
  • The directing effect of substituent groups can be used when planning an organic synthesis.
  • When carrying out more that 1 electrophilic substitution reaction on an aromatic compound, you may have to consider the order is which the reactions are carried out to ensure the correct substitution pattern and that the required product is prepared
55
Q

what are examples of 3- directing groups

A
56
Q

what is the molecular formula of benzene

A
  • C6H6
57
Q

How is each carbons 4 electrons used in a benzene ring

A
  • Each carbon atom uses 3/4 electrons in bonding to 2 other carbons and a hydrogen
  • Each carbon has 1 electron in a p-orbital at a right-angle to the plane of the molecule
58
Q
A

Georges chem (29 decks)

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