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Chemistry A-Level James > Chapter 6.1- Aromatic compounds, carbonyls and acids > Flashcards

Chapter 6.1- Aromatic compounds, carbonyls and acids Flashcards

1
Q

What is benzene?

A

naturally occurring aromatic compound, which is very stable planar ring structure with delocalised electrons

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2
Q

What is the Kekule model of benzene?

A

-six -membered carbon ring with alternating single and double bonds between the carbon atoms

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3
Q

What was Kekule’s evidence for his model?

A
  • the number of isotopes of benzene:
  • when you add one group to benzene there is 1 isomer
  • when you add two groups to benzene there are 3 isomers
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4
Q

What are the 3 problems with Kekule’s model?

A
  • unlike alkenes, benzene is resistant to addition reactions
  • enthalpy of hydrogenation of benzene shows that benzene is much more stable than was predicted
  • all six carbon atoms in benzene are the same length
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5
Q

Why does the enthalpy change of hydrogenation of benzene disprove the Kekule model?

A
  • the enthalpy change of hydrogenation of cyclohexane is -120KJmol^-1, and this contains one double bond. If you multiply this by 3 then the enthalpy change of hydrogenation of benzene should be -360KJmol^-1.
  • instead experimentally it was found that it was -208KJmol^-1 and therefore was 152KJmol^-1 more energetically stable than predicted
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6
Q

Why does the bond lengths in benzene disprove Kekule’s model?

A
  • X-ray diffraction techniques have shown that all six carbon bonds in benzene are 0.140nm, which is between a C-C bond at 0.147nm and a double bond at 0.135nm.
  • Kekule’s model suggests that there should be three shorter C=C double bonds and three longer C-C single bonds.
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7
Q

What is the delocalised structure of benzene?

A
  • each of the 6 carbon atoms donates one electron from its p-orbital. These electrons combine to form a ring of delocalised electrons above and below the plane of the molecule.
  • the electrons are said to be delocalised as they are able to move freely within the ring and do not belong to a single atom.
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8
Q

How does the delocalised structure of benzene explain the 3 problems with his model?

A
  • lots of energy is needed to disrupt the shared electrons so benzene will not readily undergo addition reactions
  • the delocalisation of electrons makes the molecule 152KJmol^-1 more stable than three separate double bonds
  • the electrons are shared evenly so all the bonds are the same length
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9
Q

What is a substitution reaction?

A

where a group or atom is exchanges for another group or atom in a chemical reaction

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10
Q

What is a benzene derivative?

A

benzene ring that has undergone a substitution reaction

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11
Q

How would you name a single-substituted benzene derivative?

A
  • prefixbenzene

- for example when a ethyl group replaces a hydrogen atom in benzene the name becomes ethylbenzene

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12
Q

How would you name a double-substituted benzene derivative?

A
  • if two different groups replace the hydrogen atoms on benzene, then the name is written as a prefix in alphabetical order. prefixbenzene
  • one group will be added first and be given carbon number 1, the second group will then be given a number to state which carbon atom it is on with respect to the first substituted group.
  • the smallest possible numbers should be used
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13
Q

How would you name a aromatic compound where benzene is not the main functional group?

A

-benzene is no longer part of the name
-the prefix changes and the name takes the form:
PhenylMainFunctionalGroup
-example: phenylethene, when ethene replaces a hydrogen atom on a aromatic ring

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14
Q

When you substitute one group into the benzene ring, how many isomers do you get?

A

1

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15
Q

When you substitute two groups into the benzene ring, how many isomers do you get?

A

3

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16
Q

What is electrophilic substitution?

A

substitution reaction where an electrophile is attracted to an electron-rich atom or part of a molecule and a new covalent bond is formed by the electrophile accepting an electron pair

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17
Q

What is a electrophile?

A

species that accepts an electron pair

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18
Q

What are the steps in the electrophilic substitution of benzene?

A
  1. Electrons above and below the plane of atoms in the benzene ring attract an electrophile
  2. The electrophile accepts a pair of pie electrons from the delocalised ring and makes a covalent bond. This is the slowest step and known as the rate determining step.
  3. A reactive intermediate is formed where the delocalised electrons have been disrupted
  4. The unstable intermediate releases an H+ ion and the stable product is formed. This is a very fast step
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19
Q

What is the molecular and empirical formula of benzene?

A

molecular: C6H6
empirical: CH

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20
Q

What is nitration of benzene?

A

electrophilic substitution reaction where a hydrogen atom is exchanged for a nitro group (-NO2)

21
Q

What are the reagent and what is the catalyst in the nitration of benzene?

A
  • reagent :concentrated nitric acid

- catalyst: concentrated sulfuric acid

22
Q

How is the N02+ electrophile generated and what is the equation?

A

HN03 + H2SO4——-NO2^+ +HS04^- + H20

23
Q

How is sulfuric acid the catalyst in the nitration of benzene?

A
  • the HSO4^- combines with the H+ ion that is released from the unstable intermediate and this forms sulfuric acid (H2S04)
  • H^+ + HSO4^- =====H2SO4
24
Q

What is halogenation?

A

this is an electrophilic substitution reaction where a halogen ion is the electrophile and so replaces a hydrogen atom in benzene

25
Q

What required for benzene to undergo halogenation?

A
  • halogen carrier

- benzene will not react with halogens because the electron ring is too stable

26
Q

What are the different halogen carriers which can be used in chlorination (Cl+ halogen ion is substituted for a H atom)?

A
  • AlCl3
  • FeCl3
  • Fe
27
Q

What are the different halogen carriers which can be used in bromination (Br^+ halogen ion is substituted for a H atom)?

A
  • AlBr3
  • FeBr3
  • Fe
28
Q

How is a Br+ halogen electrophile formed from the reaction with iron (III) bromide and bromine?

A

Br2 + FeBr3====Br^+ + FeBr4^-

29
Q

How are halogen carriers catalysts, using the example of the bromination of benzene with the FeBr3 halogen carrier?

A

FeBr4^- + H^+======HBr + FeBr3

30
Q

What is a Friedel-Crafts reaction?

A

a substitution where hydrogen is exchanged for an alkyl, or acyl, chain

31
Q

What happens when bromine water is shaken with alkenes?

A
  • this will cause decolorisation as the coloured bromine is used to form the colourless 1,2-dibromoethane.
  • This is because alkenes have a pie bond between the two carbon atoms. This is a region of high electron density and will therefor readily undergo addition reactions to become saturated
32
Q

What happens when benzene is mixed with bromine water?

A
  • no reaction occurs
  • this is because benzene is not able to induce a dipole in the non-polar bromine molecule and therefore electrophilic substitution does not occur
33
Q

How are haloalkanes used to attach alkyl groups to benzene rings?

A
  • mix the haloalkane (such as chloromethane) with a halogen carrier (such as iron(III) chloride) which generates the alkyl electrophile (CH3+)
  • the carbocation undergoes electrophilic substitution with the benzene ring
34
Q

In Friedel-Crafts reactions how is the halogen carrier acting as a catalyst and give a example using FeCl3

A
  • the FeCl4^- reacts with the H atom on the unstable intermediate
  • this forms HCL and FeCl3 (catalyst regenerated)
35
Q

What is the functional group of acyl chloride?

A

RCOCL

36
Q

What do acyl chlorides do in Friedel-Crafts reactions?

A

attach carbonyl groups (ketone) to the benzene ring

37
Q

How are carbonyl groups added to benzene rings through the use of acyl chlorides, using CH3COCL and FeCl3?

A
  1. mix the acyl chloride with a halogen carrier which generates the electrophile
  2. CH3COCL + FeCl3====CH3O^+ + FeCl4^-
  3. the electrophile undergoes electrophilic substitution with the benzene ring and a benzene ring attached to a carbonyl is made
38
Q

What are the conditions required in acylation?

A
  • 60 degrees for 30 minutes under reflux

- anhydrous conditions

39
Q

Why does only one substitution occur in acylation but multiple substitutions can occur in alkylation?

A
  • the carbonyl group removes electron density from the benzene ring and makes it less susceptible to attack from electrophiles so only one substitution occurs
  • in alkylation, multiple substitutions occur because the alkyl chain donates electrons to the aromatic ring making it more negative and more susceptible to attack by electrophiles
40
Q

What are phenols?

A

class of aromatic compounds where a hydroxyl group (-OH) is directly attached to the aromatic ring

41
Q

Is phenol a weak or strong acid?

A

-weak acid
-partially dissociates in water
C6H5OH + H20===H30^- + C6H50^-

42
Q

What happens when phenol reacts with NaOH?

A
  • neutralisation reaction which produces a salt and water

- C6H5OH +NaOH=====C6H5O^-Na^+. +H20

43
Q

What happens when phenol reacts with carbonates?

A

-doesn’t react with carbonates such as sodium carbonate as they are weak bases

44
Q

Why is phenol more reactive than benzene?

A
  • due to the p-orbital electrons from the oxygen of the hydroxyl group adding to the delocalised electrons of the aromatic ring
  • the pie-system becomes more nucleophilic as there is an increase in electron density which allows the aromatic ring in phenol to be more susceptible to electrophilic attack as it can induce a dipole in a non-polar molecule
45
Q

What is the bromination of phenol?

A
  • electrophilic substitution reaction
  • tripple substitution reaction
  • C6H5OH + 3Br2===C6H2Br3OH + 3HBr
  • white precipitate forms which smells of antiseptic
46
Q

What happens when phenol undergoes nitration with dilute nitric acid?

A
  • single substitution reaction with dilute nitric acid (HNO3)
  • reaction forms a mixture of either 2-nitrophenol and 4-nitrophenol
  • C6H5OH + HNO3==C6H4(NO2)OH + H20
47
Q

What happens when phenol undergoes nitration with concentrated nitric acid?

A
  • triple electrophilic substitution reaction occurs

- 2,4,6-trinitrophenol is formed

48
Q

What is the directing effect?

A

how a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution

Chemistry A-Level James (7 decks)

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