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Chemistry > Module 6.1 - Aromatic Compounds, Carbonyls and Acids > Flashcards

Module 6.1 - Aromatic Compounds, Carbonyls and Acids Flashcards

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1
Q

What is Benzene?

A

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

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2
Q

What is a model?

A

A simplified version that allows us to make predictions and understand observations more easily

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3
Q

What homologous series does benzene belong to?

A

arenes

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4
Q

What is the molecular formula of benzene?

A

C6H6

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5
Q

What is the empirical formula of benzene?

A

CH

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6
Q

What state is benzene at room temperature?

A

liquid

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7
Q

What can benzene be used for?

A

Key ingredient added to gasoline as it increases the efficiency of a car engine

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8
Q

What is Kekulé’s model of benzene?

A
  • benzene had alternating single and double bonds in the 6 carbon ring
  • discovered when 1 group was added to benzene only 1 isomer was ever made, but when 2 groups added, there was always 3 structural isomers
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9
Q

What are the problems with Kekulé’s model of benzene?

A
  • unlike alkenes, benzene is resistant to addition reactions
  • enthalpy of hydrogenation of benzene shows its much more stable that was predicted
  • all 6 C bonds are the same length
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10
Q

How does benzene’s resistance to reaction disproves Kekulé’s model?

A
  • using his model, would expect Benzene to undergo similar reactions to alkenes
  • ethene readily undergoes addition reactions but benzene tends to undergo substitution of a H atom rather than addition
  • Kekulé tried to explain this by saying double + single bonds changed positions in a very fast equilibrium
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11
Q

How does benzene’s enthalpy of hydrogenation disproves Kekulé’s model?

A
  • hydrogenation: addition of H to an unsaturated chemical
  • using bond enthalpy data we can calculate enthalpy change for complete hydrogenation of cyclohexene + cyclo-1,3,5-hexatriene (Kekulé’s model)
  • experimentally found to be -208kJmol^-1, 152kJmol^-1 more energetically stable than predicted
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12
Q

How do the bond lengths in benzene disprove Kekulé’s model?

A
  • xray diffraction shows all 6 C bonds are 0.140nm, between length of a single bond (0.147nm) and a double bond (0.135nm)
  • Kekulé’s model suggest 3 shorter C=C bonds and 3 longer C-C bonds
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13
Q

Describe the delocalised structure of benzene.

A
  • has a delocalised electron structure that can support all 3 pieces of evidence that don’t support Kekulé’s
  • each C atom donates 1 electron from its p orbital
  • electrons combine to form a ring of delocalised electrons above + below plane of molecule
  • electrons in rings said to be delocalised as are able to move freely within ring + don’t belong to a single atom
  • unlike Kekulé’s model, all bonds in ring are identical (same length)
  • delocalisation of benzene being about 152kJmol^-1 more than expected using Kekulé’s model. As so much energy need to disrupt this delocalisation, benzene is v stable + resistant to addition reactions
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14
Q

What is a substitution reaction?

A

Where a group/atom is exchanged for another group/atom in a chemical reaction

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15
Q

What is a benzene derivative?

A

A benzene ring that has undergone a substitution reaction

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16
Q

What is the naming prefix when a nitro functional group is added?

A

NO2, nitro-

17
Q

What is the difference between a phenyl group and benzene?

A

Phenyl: H atom removed from benzene ring

18
Q

What is electrophilic substitution?

A

A substitution 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

19
Q

What is a reaction mechanism?

A

A model with steps to explain and predict a chemical reaction

20
Q

Describe the general steps of the electrophilic substitution of benzene.

A
  1. Electrons above + below the plane of atoms in benzene ring attract an electrophile
  2. Electrophile accepts pair of pi electrons from delocalised ring + makes a covalent bond. Slowest step + known as rate determining step
  3. A reactive intermediate is formed where delocalised electrons have been disrupted
  4. Unstable intermediate releases an H+ ion + the stable formed (v fast step)
21
Q

Describe the nitration of benzene.

A
  • electrophilic substitution (H atom exchanged for a nitro, NO2, group)
  • reagent is conc HNO3, with conc H2SO4 catalyst
  • C6H6 + HNO3 –> C6H5NO2 + H2O
  • initially conc HNO3 + conc H2SO4 mixed together in flask held in ice bath
  • benzene is added + reflux condenser set up, keeping mixture at 50°C to prevent further substitution occurring
  • H2SO4 needed to generate NO2+ electrophile from nitric acid. Sulfuric acid regenerated after nitration so is catalyst
  • HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
  • H+ + HSO4- –> H2SO4
22
Q

Describe the bromination of benzene.

A

-doesn’t react directly w halogens as ring too stable
-halogen carrier (AlBr3, FeBr3 or Fe which forms FeBr3 in situ) generates a positive halogen ion
-Br reacts w FeBr3 to form Br+ ion
Br2 + FeBr3 –> Br+ + FeBr4-
-Br+ generated in situ, can attack ring by electrophilic substitution
-halogen carrier is a catalyst + regenerated at end of halogenation as released H+ from benzene ring forms HBr
FeBr4- + H+ –> HBr + FeBr3

23
Q

Describe the chlorination of benzene.

A

-doesn’t react directly w halogens as ring too stable
-halogen carrier (AlCl3, FeCl3 or Fe which forms FeCl3 in situ) generates a positive halogen ion
-Cl reacts w FeCl3 to form Cl+ ion
Cl2 + FeCl3 –> Cl+ + FeCl4-
-Cl+ generated in situ, can attack ring by electrophilic substitution
-halogen carrier is a catalyst + regenerated at end of halogenation as released H+ from benzene ring forms HCl
FeCl4- + H+ –> HBCl + FeCl3

24
Q

What is a Friedel Crafts reaction?

A

A substitution reaction where hydrogen is exchanged for an alkyl or acyl chain

25
Q

Describe the electrophilic substitution of haloalkanes to an aromatic ring.

A

-mixed w halogen carriers eg FeCl3, which acts as a catalyst + is regenerated at end of reaction
-reactive carbocation is made which undergoes electrophilic substitution w benzene ring
R-Cl + FeCl3 –> R+ + FeCl4-
-multiple substitutions likely so mixture of products made. Products can be separated using fractional distillation or chromatography. Yield of a singly substituted product can be improved by adding more benzene
-mixture of products caused as each successive substitution makes the delocalised pi electrons more nucleophilic so more susceptible to nucleophilic attack
-this increase in reactivity due to alkyl chain donating electrons to aromatic ring

26
Q

What is the functional group of an acyl chloride?

A

RCOCl (very reactive)

27
Q

Describe the electrophilic substitution of an acyl chlrodie to an aromatic ring.

A
  • used as the halogen carrier to substitute a H atom
  • carbonyl group withdraws electrons from aromatic ring, so less reactive ketone is made so only one substitution can occur
  • reaction mixture held at 60°C + held for 30 mins under reflux for reaction to occur in anhydrous conditions (AlCl3 catalyst)
28
Q

What is a phenol?

A

A class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring

29
Q

What is the difference between a phenol and an aromatic alcohol?

A
  • phenol: OH group attached directly to aromatic ring

- aromatic alcohol: OH group attached to an alkyl chain of the aromatic ring

30
Q

Describe and explain the acidity of phenols.

A

-phenol is a weak acid, as partially dissociates in water
C6H5OH + H2O ⇌ H3O+ + C6H5O-
-as a weak acid, phenol reacts w strong bases to form salt + water
C6H5OH + NaOH –> C6H5O-Na+ + H2O
-phenol is an acid as reacts w strong bases eg NaOH. Weak acid as doesn’t react w carbonates. Phenol won’t react w weak bases eg Na2CO3

31
Q

Describe and explain the reactivity of phenols.

A
  • more reactive than benzene due to p orbital electrons from O of hydroxyl group adding to the delocalised electrons of aromatic ring
  • so pi system of aromatic ring become more nucleophilic
  • increase in electron density allows aromatic ring in phenol to be more susceptible to electrophilic attack as can induce dipole in non polar molecules eg bromine molecules so phenol can undergo direct halogenation unlike benzene
32
Q

What is meant by the directing effect?

A

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

33
Q

Describe the bromination of phenol.

A
  • triple substitution w bromine + water at room tempC6H5OH + 3Br2 –> C6H2Br3OH + 3HBr
  • resulting product is a white ppt of 2,4,6-tribromophenol
34
Q

Describe the nitration of phenol.

A

-triple substitution reaction w dilute nitric acid at room temp
-reaction forms a micture of 2-nitrophenol + 4-nitrophenol
C6H5Oh + HNO3 –> C6H4(NO2)OH + H2O
-unlike nitration w benzene, this reaction doesn’t require conc HNO3 or H2SO4 catalyst
-if conc HNO3 is used, a triple substitution occurs, forming 2,4,6-trinitrophenol

35
Q

Describe how the directing effect can influence the position of substitution.

A
  • substitution reactions can be more favourable for certain C atoms when a functional group directly affected by aromatic ring
  • phenol: hydroxyl group pushes additional electrons to pi system making substitution reactions mainly occure on 2 + 4 positions on aromatic ring
  • hydroxyl group activates these C atoms so their rate of substitution is faster than other positions, known as the 2,4 directing effect
  • effect more pronounced in aromatic compounds w an NH2 group directly attached to aromatic ring
  • when NO2 groups directly attached to aromatic ring, 3 directing effect seen. Nitro group withdraws electrons from pi system + makes substitution highest on 3rd C atom

Chemistry (13 decks)

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