Module 6.1 - Aromatic Compounds, Carbonyls and Acids

Question 1

What is benzene?

Answer 1

• naturally occurring aromatic hydrocarbon. Stable ring structure with delocalised electrons
• C₆H₆ (empirical formula: CH)
• liquid at room temperature

Question 2

Give one use of benzene.

Answer 2

Key ingredient in gasoline (which increases efficiency of car engine)

Question 3

Describe Kekulé’s model of benzene.

Answer 3

• 6 membered ring with alternating double and single bonds
• discovered when one group was added to benzene, only one isomer was ever made but when 2 groups were added there were 3 isomers

Question 4

What were the experimental evidence that didn’t support Kekulé’s model of benzene?

Answer 4

• benzene is resistant to addition reactions (unlike alkenes) (Kekulé tried to explain this saying double and single bonds alternate in very fast equilibrium)
• enthalpy of hydrogenation of benzene shows it’s more stable than predicted (can calculate enthalpy change of hydrogenation of cyclo-1,3,5-hexatriene, Kekulé model, but actual value is -208kJmol^-1, 152kJmol^-1 more energetically stable than predicted)
• all 6 carbon bonds in benzene are the same length (C-C bond length: 0.147nm, C=C bond length: 0.135nm, benzene: 0.140nm, Kekulé should have 3 longer and 3 shorter bond lengths, disproving Kekulé’s model)

Question 5

Describe the delocalised structure of benzene.

Answer 5

• bezene has a delocalised structure, explaining all 3 pieces of evidence disproving Kekulé’s model
• each of the 6 C atoms donates one e^- from p orbital
• e^- combine to form a ring of e^- above and below the plane of the molecule
• e^- said to be delocalised as able to move freely within ring and don’t belong to a single atom meaning all bonds in the ring are identical
• enthalpy of hydrogenation is 152kJmol^-1 more stable than expected w Kekulé as more energy needed to disrupt delocalisation so benzene is v stable and resistant to addition reactions

Question 6

What is a substitution reaction?

Answer 6

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

Question 7

What is a benzene derivative?

Answer 7

A benzene ring that has undergone a substitution reaction

Question 8

What is the prefix when a nitro functional group is substituted onto a molecule?

Answer 8

nitro-

functional group: -NO₂

Question 9

Name this molecule.

Answer 9

phenylethene

Aromatic ring isn’t main functional group so aromatic ring is a phenyl group as H atom removed. Alkene group used for addition polymerisation in this molecules

Question 10

Describe the general mechanism for the electrophilic substitution on a benzene molecule.

Answer 10

1. Electrons above and below plane of atoms in benzene ring attract electrophile
2. Electrophile accepts a pair of π electrons from delocalised ring and makes a covalent bond. This is the slowest, rate determining step
3. Reactive intermediate is formed where the delocalised electrons have been disrupted
4. Unstable intermediate releases a H⁺ ion and the stable product has formed (v fast step)

Question 11

Describe the nitration of benzene.

Answer 11

• electrophilic substitution reaction (H atom exchanged for a nitro group (-NO₂)
• reagent: conc nitric acid, catalyst: conc sulfuric acid
• C₆H₆ + HNO₃ –> C₆H₅NO₂ + H₂O
• initially, conc nitric acid and conc sulfuric acid mixed in a flask held in an ice bath. Benzene then added and reflux condenser set up, keeping mixture at 50°C to prevent further substitution occurring
• sulfuric acid needed to generate NO₂⁺ electrophile from nitric acid. Sulfuric acid is regenerated after nitration so is catalyst
• HNO₃ + H₂SO₄ –> NO₂⁺ + HSO₄^- + H₂O
• regeneration of catalyst: H⁺ + HSO₄^- –> H₂SO₄

Question 12

Describe the halogenation of benzene.

Answer 12

• halogen carrier needed as ring too stable to react directly w halogens
• halogen carriers:
• chlorination: AlCl₃/FeCl₃/Fe (Fe forms iron halide in situ)
• bromination: AlCl₃/FeCl₃/Fe (Fe forms iron halide in situ)
• halogen carrier used to generate positive halogen ion
• e.g. Br₂ + FeBr₃ –> Br⁺ + FeBr₄^- (creates positive bromine ion to act as an electrophile. It’s generated in situ. Can then attack benzene ring)
• halogen carrier is catalyst and gets regenerated at end of halogenation as H⁺ from benzene ring forms HBr:

FeBr₄^- + H⁺ –> HBr + FeBr₃

Question 13

Compare how bezene reacts with bromine water compared to cyclohexene. What does this show?

Answer 13

• cyclohexene: π bonds break in an addition reaction
• bezene: no addition reaction even though electrons in benzene are delocalised in π system so benzene must have a lower electron density between carbon atoms than an alkene
• when non polar molecules e.g. bromine approach ring there’s not enough electron density between C atoms to induce dipole and start a reaction (also the case when attempting to substitute alkyl halides like haloalkanes). Using a halogen carrier means a stronger electrophile can be generated and alkylation can occur

Question 14

What is a Friedel Crafts reaction?

Answer 14

A substitution reaction where hydrogen is exchanged for an alkyl or acyl chain. C-H bond broken and C-C bond formed

Question 15

Describe the alkylation of benzene.

Answer 15

• haloalkanes e.g. chloromethane mixed w halogen carrier e.g. AlCl₃
• anhydrous
• halogen carrier = catalyst and gets regenerated (Lewis acid also used as catalyst)
• reactive carbocation is made which undergoes electrophilic substitution within benzene ring
• multiple substitutions likely so mixture of products made. Products may be separated by fractional distillation or chromatography
• actual yield of substituted product can be improved by adding excess benzene
• mixture of products caused as each successive substitution makes delocalised π electrons more nucleophilic so more susceptible to electrophilic attack. Increase in reactivity due to alkyl chain donating electrons to aromatic ring

Question 16

Describe the acylation of benzene.

Answer 16

• acyl chloride: RCOCl (very reactive)
• can be used in Friedel-Crafts reaction as halogen carrier to substitute a H atom
• anhydrous
• as carbonyl group withdraws electrons from aromatic ring, a less reactive ketone is made, so only one substitution can occur
• reaction mixture at 60°C for 30 minutes under reflux for reaction to occur

Question 17

What is phenol?

Answer 17

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

Question 18

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

Answer 18

• phenol: hydroxyl group attached directly to aromatic ring
• aromatic alcohol: hydroxyl group attached to alkyl chain attached to aromatic ring

Question 19

Describe the acidity of phenol.

Answer 19

-weak acid that partially dissociates in water

C₆H₅OH + H₂O ⇌ H₃O⁺ + C₆H₅O^-

-acid as reacts w strong bases e.g. NaOH but only weak acid as doesn’t react w carbonates. Phenol won’t react w weak bases e.g Na₂CO₃

Question 20

Describe the reaction of phenol with strong bases.

Answer 20

-forms salt and water

C₆H₅OH + NaOH → C₆H₅O^-Na⁺ + H₂O

Question 21

How does the reactivity of phenol compare to that of benzene?

Answer 21

• phenol more reactive than benzene
• p orbital electrons from oxygen of hydroxyl group adding to delocalised electrons of aromatic ring so π system of aromatic ring becomes more nucleophilic
• increase in electron density allows aromatic ring in phenol to be mroe susceptible to electrophilic attack as can induce dipole in no polar molecules
• e.g. can induce dipole in bromine to react w it

Question 22

What is a directing effect?

Answer 22

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

Question 23

Describe the bromination of phenol.

Answer 23

-undergoes a triple substitution reaction w bromine water at room temp

C₆H₅OH + 3Br₂ –> C₆H₂Br₃OH + 3HBr

-resulting product is a white ppt of 2,4,6-tribromophenol (smells of antiseptic)

Question 24

Describe the nitration of phenol.

Answer 24

• dilute HNO₃ to undergo single substitution at room tempt
• forms a mixture of 2-nitrophenol and 4-nitrophenol

C₆H₅OH + HNO₃ –> C₆H₄(NO₂)OH + H₂O

• doesn’t require a H₂SO₄ catalyst
• when conc HNO₃ used a triple substitution occurs, forming 2,4,6-trinitrophenol

Question 25

How does a hydroxyl group (and other electron donating groups) on an aromatic ring affect the position of substitution?

Answer 25

• additional electrons in π system
• reactions mainly occur on 2 and 4 positions on ring (2- and 4-directing effect)
• hydroxyl group activates these carbon atoms so their rate of substitutions is faster than other positions
• effect more pronounced when NH₂ group attached to aromatic ring

Question 26

How does a nitro group (and other electron withdrawing) on an aromatic ring affect the position of substitution?

Answer 26

• 3-directing effect
• nitro groups withdraw electrons from π system making rate of substitution highest on 3rd C atom

Question 27

What is a nucleophile?

Answer 27

A species attracted to an electron deficient part of a molecule where it donates a pair of electrons to make a new covalent bond

Question 28

What is the ionic equation of the oxidation of ethanal to ethanoic acid?

Answer 28

3CH₃CHO_(l) + Cr₂O₇^2-_(aq) + 8H⁺_(aq) –> 2CH₃COOH_(aq) + 2Cr³⁺_(aq) + 4H₂O_(l)

Question 29

Describe the general nucelophilic addition reactions to carbonyls.

Answer 29

• carbonyls have a dipole over C=O functional group so susceptible to nucleophilic attack on δ⁺ C atom
• nucleophile donates lone pair of electrons to electron deficient carbon
• simultaneously, π bond in C=O breaks forming a reactive intermediate
• extra electron pair quickly donated to neighbouring hydrogen to form an alcohol group and the stable product

Question 30

Describe the reduction of carbonyls by sodium tetrahybridoborate(III).

Answer 30

• NaBH₄ is a reducing agent. Made of BH₄^- ion, a source of hydride ions, H^-
• hydride ion involved in electrophilic addition and reduction of carbonyls to alcohols
• CH₃COC₃H₇ + 2[H] –> CH₃CH(OH)C₃H₇
• hydride ion attacks δ⁺ C atom and forms a bond, intermediate forms that reacts with an ethanol/water molecule, to form an organic product (alcohol)

Question 31

Desccribe the reaction between hydrogen cyanide and carbonyls.

Answer 31

-HCN: weak acid that partially ionises in solution, forming a cyanide nucleophile with negative charge on carbon atom

HCN + H₂O ⇌ CN^- + H₃O⁺

• other sources of CN^- include NaCN
• cyanide ion can’t react directly w carbonyl compound but when reaction is acidified the carbonyl group becomes more reactive as polarity of C=O bond increases
• allows additional H atoms to be added, forming a hydroxynitrile (used in many industrial processes)

Question 32

Describe the reaction between 2,4-dinitrophenylhydrazine and aldehydes/ketones as a test for the carbonyl functional group of aldehydes/ketones only.

Answer 32

• Brady’s reagent: mixture of methanol, sulfuric acid and 2,4-DNP
• when added to aldehyde/ketone a yellow/orange ppt of 2,4-dinitrophenylhydrazone derivative is seen
• no ppt observed w carboxylic acid or ester even though they have C=O

Question 33

How would you identify the specific aldehyde/ketone after using Brady’s reagent to discover you have an aldehyde/ketone?

Answer 33

• 2,4-dinitrophenylhydrazone derivative ppt can be collected by filtration and purified using recrystallisation
• once dried, accurate melting point or pure product can be measured through experiment
• aldehyde/ketone identified by comparing melting point of 2,4-dinitrophenylhydrazone derivative ppt w a database

Question 34

When working out the specific aldehyde/ketone, why is the melting point of the 2,4-dinitrophenylhydrazone derivative precipitate used rather than the aldehyde/ketone?

Answer 34

• experimentally different for different ketones as have v similar mp
• ketones of similar chain length have similar bp so difficult to distinguish between
• 2,4-dinitrophenylhydrazone derivatives have v different mp and bps

Question 35

How do you make Tollens’ reagent and how do you use it to test whether you have an aldehyde/ketone?

Answer 35

Making:

1. NaOH solution added to AgNO₃ solution until brown ppt formed
2. Dilute NH₃ added drop wise until brown ppt redissolves

• used to distinguish between an aldehyde/ketone
• it’s a weak oxidising agent and can react w carbonyl functional group in aldehyde but not ketone
• when Tollens’ added to ketone, no reaction as ketones can’t be oxidised further
• when added to aldehyde a silver mirror is observed as redox reaction occurs between Tollens’ and aldehyde tro form carboxylic acid
• Ag⁺_(aq) + e^- → Ag(s)
• silver metal precipitated out appearing as silver mirror effect on inside of reaction vessel

Question 36

Describe the solubility (physical property) of carboxylic acids.

Answer 36

• small carboxylic acids are v soluble in polar solvent e.g. water as H bonds can form between carboxylic acid functional group and water
• as hydrocarbon chain length increases, solubility decreases as only polar COOH functional group can form H bonds w water so as molecule becomes non polar the solubility decreases

Question 37

Describe the partial ionisation of carboxylic acids.

Answer 37

• weak acid so only partially ionise to release H⁺ into solution from carboxylic acid group
• forms carboxylate ion

(-lone pair of electrons from O of H₂O to H causing fission of COOH O-H bond)

-reactions of carboxylic acids happen at slower rate w a strong acid as pH is higher so conc of H⁺_(aq) will be lower

Question 38

Describe the reaction between carboxylic acids and metals.

Answer 38

• reactive w metals above H in the reactivity series
• form H₂ and a metal salt
• name of salt generated from acid
  e. g.

sodium + ethanoic acid → sodium ethanoate + hydrogen

2Na + 2CH₂COOH → 2CH₃COONa + H₂

Question 39

Describe the reaction between carboxylic acids and metal oxides.

Answer 39

• metal oxides classified as bases as react w acids
• forms metal salt and water
  e. g.

magnesium oxide + methanoic acid → magnesium methanoate + water

MgO_(s) + HCOOH_(aq) → (HCOO)₂Mg_(aq) + H₂O_(l)

Question 40

Describe the reaction between carboxylic acids and metal hydroxides.

Answer 40

• group 1 metal hydroxides are soluble bases that release OH^-_(aq)
• form water and metal salt
  e. g.

potassium hydroxide + propanoic acid → potassium propanoate + water

KOH_(aq) + CH₃CH₂COOH_(aq) → CH₃CH₂COOK_(aq) + H₂O_(l)

Question 41

Describe the reaction between carboxylic acids and metal carbonates.

Answer 41

• metal carbonates are bases
• form water, carbon dioxide and a metal salt
  e. g.

sodium carbonate + methanoic acid → sodium methanoate + carbon dioxide + water

Na₂CO_3(s) + 2HCOOH_(aq) → 2HCOONa_(aq) + CO_2(g) + H₂O_(l)

-group 1 metals can also form metal hydrogencarbonates where carbonic acid only had 1 proton exchanged to form metal hydrogencarbonate (MHCO₃ where M is group 1 metal). Acids react with metal hydrogencarbonates forming salt, water and carbon dioxide

Question 42

Name this ester.

Answer 42

propyl butanoate

Question 43

Describe esterification from carboxylic acids and alcohols.

Answer 43

• alcohol and carboxylic acid heated gently in presence of H₂SO₄ catalyst
• reversible reaction and has a slow rate of reaction
• ester separated from reaction mixture by distillation as it’s volatile w lowest bp of the chemicals. Separation has to happen quickly to prevent reverse reaction occurring
• for larger esters: reaction mixture needs to be heated under reflux until equilibrium is established. Ester can be separated using fractional distillation
• this method of ester prep not suitable for phenols/its derivatives as reaction rate is so low

Question 44

Describe esterification from an acid anhydride and an alcohol.

Answer 44

• acid anhydride: acid derivative more reative than a similar carboxylic acid, made from removal of water from 2 carboxylic acid molecules
• acid anhydrides react w alcohols, including phenol/its derivatives making an ester
• not reversible so higher yield than carboxylic acid
• rate of reaction still slow but can be increased by gently warming the reaction mixture

Question 45

Describe the hydrolysis of esters in acidic conditions.

Answer 45

• refluxed w a catalyst of hot aqueous acids e.g. dilute H₂SO₄ or dilute HCl as source of H⁺_(aq)
• ester reversibly decomposes to an alcohol and carboxylic acid

Question 46

Describe the hydrolysis of esters in alkaline conditions.

Answer 46

• alkaline chemicals: bases that can dissolve in water
• ester refluxed w hot aqueous alkali e.g. KOH_(aq)/NaOH_(aq)
• decomposes to carboxylate salt and an alcohol
• not reversible
• alkaline hydrolysis of esters used to make soaps so it’s also called saponification

Question 47

Name this acyl chloride.

Answer 47

butanoyl chloride

Question 48

How can you prepare an acyl chloride?

Answer 48

• OH group on carboxylic acid must be substituted for chlorine atom
• SOCl₂ (thionyl chloride), liquid at room temp and reacts readily w carboxylic acids to form acyl chlorides
• SO₂ and hydrogen chloride gases made
• acyl chloride separated from mixture by distillation
  e. g.

CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

Question 49

How can acyl chlorides be used to make esters?

Answer 49

• react w alcohols
• not reversible so has a higher yield than w carboxylic acid
  e. g.

CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl

-can be used to make esters from phenols but reaction is violent and produces corrosive fumes of HCl

Question 50

How can acyl chlorides be used to make carboxylic acids?

Answer 50

• small acyl chlorides (e.g. ethanoyl chloride) is added to water, quickly hydrolyses to produce a carboxylic acid
• v exothermic reaction and misty fumes of HCl given off

CH₃COCl + H₂O → CH₃COOH + HCl

Question 51

How can acyl chlorides be used to make primary amides?

Answer 51

• primary amides made when acyl chlorides react w ammonia
• ethanamide: ethanoyl chloride added to conc ammonia solution. Quickly produces a mixture of solid ammonium chloride and ethanamide observed as white smoke
• some of the products remain in a colourless solution

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

Question 52

How can acyl chlorides be used to make secondary amides?

Answer 52

• formed when acyl chlorides react w primary amides
• N has 1 H directly bonded to it
• N also has 2 organic groups attached (often called an N-substituted amide)
• white solid compound of N-ethylethanamide can be made from ethanoyl chloride and cold conc solution of ethylamine

CH₃COCl + CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + HCl