Module 6.1 - Aromatic Compounds, Carbonyls and Acids

Question 1

What is Benzene?

Answer 1

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

Question 2

What is a model?

Answer 2

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

Question 3

What homologous series does benzene belong to?

Answer 3

arenes

Question 4

What is the molecular formula of benzene?

Answer 4

C6H6

Question 5

What is the empirical formula of benzene?

Answer 5

CH

Question 6

What state is benzene at room temperature?

Answer 6

liquid

Question 7

What can benzene be used for?

Answer 7

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

Question 8

What is Kekulé’s model of benzene?

Answer 8

• 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

Question 9

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

Answer 9

• 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

Question 10

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

Answer 10

• 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

Question 11

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

Answer 11

• 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

Question 12

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

Answer 12

• 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

Question 13

Describe the delocalised structure of benzene.

Answer 13

• 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

Question 14

What is a substitution reaction?

Answer 14

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

Question 15

What is a benzene derivative?

Answer 15

A benzene ring that has undergone a substitution reaction

Question 16

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

Answer 16

NO2, nitro-

Question 17

What is the difference between a phenyl group and benzene?

Answer 17

Phenyl: H atom removed from benzene ring

Question 18

What is electrophilic substitution?

Answer 18

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

Question 19

What is a reaction mechanism?

Answer 19

A model with steps to explain and predict a chemical reaction

Question 20

Describe the general steps of the electrophilic substitution of benzene.

Answer 20

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)

Question 21

Describe the nitration of benzene.

Answer 21

• 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

Question 22

Describe the bromination of benzene.

Answer 22

-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

Question 23

Describe the chlorination of benzene.

Answer 23

-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

Question 24

What is a Friedel Crafts reaction?

Answer 24

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

Question 25

Describe the electrophilic substitution of haloalkanes to an aromatic ring.

Answer 25

-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

Question 26

What is the functional group of an acyl chloride?

Answer 26

RCOCl (very reactive)

Question 27

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

Answer 27

- 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)

Question 28

What is a phenol?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

Describe and explain the acidity of phenols.

Answer 30

-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

Question 31

Describe and explain the reactivity of phenols.

Answer 31

- 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

Question 32

What is meant by the directing effect?

Answer 32

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

Question 33

Describe the bromination of phenol.

Answer 33

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

Question 34

Describe the nitration of phenol.

Answer 34

-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

Question 35

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

Answer 35

- 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