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Chemistry > Ch6.1 Aromatic compounds, carbonyls and acids > Flashcards

Ch6.1 Aromatic compounds, carbonyls and acids Flashcards

1
Q

What are the physical properties of benzene?

A
  • colourless
  • volatile
  • liquid
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2
Q

Why is benzene produced in huge quantities?

A
  • it has so many uses as a feedstock chemical
  • it’s used to produce many things such as: pharmaceuticals, plastics, weedkillers and pesticides
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3
Q

State the 3 problems with Kekule’s structure of benzene

A
  1. the bromination problem
  2. the bond length problem
  3. the hydrogenation problem
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4
Q

Explain the bromination problem of Kekule’s structure of benzene

A
  • if benzene contained C=C bonds it would decolourise bromine water, but it doesn’t
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5
Q

Explain the bond length problem of Kekule’s structure of benzene

A
  • if benzene contained alternating C-C bonds and C=C bonds, then the bond lengths would also alternate 0.13, 0.15, 0.13, 0.15 etc. around the ring
  • in fact, all the bonds have the same length (0.14nm)
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6
Q

Explain the hydrogenation problem of Kekule’s structure of benzene

A
  • when cyclohexene reacts with hydrogen, the enthalpy change is -120kJ/mol
  • if benzene had 3 C=C double bonds, hydrogenation should give an enthalpy change of -360kJ/mol
  • in reality it is -208kJ/mol which is less exothermic then expected
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7
Q

Describe the modern understanding of benzene

A
  • the pi electrons in each C=C bond delocalise to form 2 delocalised pi rings (this is what the circle in the hexagon represents when you draw benzene)
  • benzene is very stable and unreactive
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8
Q

Why is benzene stable and unreactive?

A
  • there are no regions of positive or negative charge for nucleophiles or electrophiles to attack
  • even radicals can’t attack the ring
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9
Q

How are benzene compounds named if benzene is the dominant group?

A
  • the stem is benzene and the prefix tells us which side groups are attached
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10
Q

How are compounds including benzene named if benzene isn’t the dominant group?

A
  • the benzene group is a side group which we refer to as a phenyl
  • the other part of the molecule that is dominant is the stem of the name
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11
Q

What are the 2 key reactions of benzene?

A
  • nitration
  • halogenation
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12
Q

Describe the very special conditions needed for nitration of benzene

A
  • nitrating mixture must be made (mixture of concentrated nitric and sulphuric acids)
  • we have to mix the acids in a flask which is sitting in an ice bath since the reaction between them is highly exothermic
  • when we reflux benzene with the nitrating mixture, we need to keep it below 50 degrees C, if not then multiple substitutions occur and the mixture becomes explosive
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13
Q

Explain the roles of the different components in the nitrating mixture for the nitration of benzene

A
  • nitric acid is the source of the nitro group (NO2)
  • the sulphuric acid is a catalyst
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14
Q

How does the nitrating mixture change during the nitration of benzene?

A
  • produces nitronium ions (NO2+)

HNO3 + H2SO4 –> NO2+ + HSO4- + H2O

  • then regenerates acid catalyst

HSO4- + H+ –> H2SO4

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15
Q

Describe the nitration reaction of benzene mechanism

A
  • the nitronium ion is an electrophile and it takes 2 [i electrons from the delocalised ring (full arrow from circle out to NO2+ ion)
  • the C-H bond has to break (heterolytic fission) and the two bonding electrons restore the delocalised pi network (circle splits to horseshoe with + in the middle and an arrow from C-H bond to plus)
  • (then benzene with an NO2 is drawn and a H+ is also produced) = these are the final products
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16
Q

Describe how halogenation of benzene is made possible

A
  • halogens produce negative halide ions which will not act as electrophiles
  • instead we have to make chloronium, fluoronium and bromonium ions (Cl+, F+, Br+)
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17
Q

How are chloronium, fluoronium and bromonium ions made?

A
  • we mix the halogen with a halogen carrier
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18
Q

Which halogen carriers are used for which halogens?

A
  • chlorine = AlCl3, FeCl3, Fe
  • bromine= AlBr3, FeBr3, Fe
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19
Q

What are the 2 chemical equations that occur during the halogenation of benzene (use chlorine as the halogen)

A

Cl2 + FeCl3 –> Cl+ + FeCl4

(Cl+ is a very powerful electrophile)

FeCl4 + H+ –> FeCl3 + HCl

(the FeCl4 reacts with the H+ ions displaced from benzene to regenerate the halogen carrier)

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20
Q

How can organic groups be added to benzene rings?

A
  • using Friedel-Crafts reactions
  • eg, if we want to add an alkyl group to benzene, we can generate an electrophile using a halogen carrier and a haloalkane
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21
Q

What are the equations for generating an electrophile from a halogen carrier and a haloalkana?

A

CH3Cl + FeCl3 –> CH3+ + FeCl4-

H+ + FeCl4- –> HCl + FeCl3

CH3+ is the electrophile

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22
Q

What is another example of a Friedel-Crafts reaction other than alkylation?

A

acylation

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23
Q

What are the 2 equations that occur when preparing an electrophile for acylation of benzene?

A

RCClO + FeCl3 –> RCO+ + FeCl4-

H+ + FeCl4- –> HCl + FeCl3

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24
Q

What are phenols?

A
  • aromatic compounds in which an OH group is attached directly to a benzene ring
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25
Q

Are phenols acidic or alkaline and why?

A
  • phenols are weakly acidic because the H in OH groups is labile
  • this means the OH bond breaks easily because of the proximity of the electronegative oxygen atom
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26
Q

What ions are formed when a phenol reacts with water?

A
  • phenoxide ion
  • oxonium ion
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27
Q

What can phenol react with due to it being a weak acid?

A
  • alkalis such as sodium hydroxide

phenol + sodium hydroxide ⇌ sodium phenoxide + water

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28
Q

What are the chemical tests for phenols?

A
  • turns blue litmus paper pink
  • if iron (III) chloride solution is added to phenol it turns purple
  • it won’t react with carbonates to make CO2 despite it being a weak acid
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29
Q

Are phenols more or less reactive than benzene?

A
  • phenols are more reactive than benzene
30
Q

Why is phenol more reactive than benzene?

A
  • benzene has an even distribution of electrons and no regions of concentrated positive or negative charge
  • phenol has the elctronegative oxygen which causes a dipole to form
  • the lone pairs of the oxygen also become part of the delocalised network which increases the electron density
31
Q

What does phenol’s higher electron density allow it to do differently to benzene?

A
  • react with halogens
  • it also often has multiple substitutions
32
Q

What are carbonyl compounds?

A
  • compounds with the C=O functional group
  • aldehydes and ketones are both carbonyl compounds
33
Q

Where is the carbonyl group located in aldehydes and ketones?

A
  • in aldehydes, the group is at the end of the carbon chain
  • in ketones, the group is somewhere in the middle of the carbon chain
34
Q

How can aldehydes and ketones be made from alcohols?

A

OXIDATION

  • primary alcohol to aldehyde to carboxylic acid
  • secondary alcohol to ketone
  • tertiary alcohol can’t be oxidised
35
Q

What colour does the oxidising agent turn whilst oxidising alcohols and name the ions for these colours

A

orange to green

(Cr2O7)2- to Cr3+

oxidation number change:
+6 to +3 (reduced)

36
Q

What are the chemical tests for carbonyl compounds?

A
  • carbonyl compounds can be detected by using 2,4 DNPH
    (gives an orange/red precipitate with an aldehyde or ketone)
  • aldehydes produce a silver mirror with Tollen’s reagent
  • aldehydes give a red precipitate with Fehlings solution
37
Q

What is Tollen’s reagent?

A
  • mixture of silver nitrate and nitric acid
38
Q

Are carboxylic acids weak or strong acids? Explain

A
  • weak
  • they undergo partial dissociation in the presence of water
39
Q

Describe the Ka values of carboxylic acids

A
  • they’re very low
40
Q

carboxylic acid + metal

A

carboxylate salt + hydrogen

41
Q

carboxylic acid + metal oxide

A

carboxylate salt + water

42
Q

carboxylic acid + metal hydroxide

A

carboxylate salt + water

43
Q

carboxylic acid + metal carbonate

A

carboxylate salt + water + carbon dioxide

44
Q

Are carboxylic acids soluble in water? Explain the trend with chain length

A
  • they are soluble in water
  • solubility decreases with chain length because the alkane group begins to dominate over the COOH group
45
Q

Is esterification fast or slow? How do we manage this?

A
  • esterification is a very slow process
  • for this reason, we use a concentrated sulphuric acid catalyst and reflux the mixture for half an hour
46
Q

How can esters be separated?

A
  • with distillation because they are very volatile
47
Q

How can esters be separated back into alcohols and carboxylic acids?

A
  • using hydrolysis
48
Q

What are the 2 types of hydrolysis and how do they differ?

A
  • acid and alkaline
  • they form different products
49
Q

Describe acid hydrolysis

A
  • the acid doesn’t take part in the reaction
  • the hydrolysis produces the original alcohol and carboxylic acid
  • reaction is reversible
50
Q

Describe alkaline hydrolysis

A
  • the alkali (usually NaOH) is a reactant
  • the process generates the original alcohol but the carboxylate salt of the carboxylic acid
  • the process is not reversible
51
Q

Why is alkaline hydrolysis sometimes referred to as saponification?

A
  • alkaline hydrolysis of fats is used to make soap
  • the Latin for soap is sapo
52
Q

What kind of compounds are fats and oils and what is the difference between them?

A
  • both esters (triglyceride esters)
  • the only difference between them is their melting and boiling points
53
Q

What do you get when you hydrolyse a fat or an oil?

A
  • glycerol and fatty acids
  • fatty acids are simply long-chain carboxylic acids
54
Q

What kind of fatty acids are there?

A
  • saturated, unsaturated or polyunsaturated
55
Q

Where are saturated fatty acids found and what is their effect on the human body?

A
  • lead to health problems such as heart disease and stroke since they lead to the buildup of fatty tissue
  • this and cholesterol narrow arteries and therefore increase blood pressure
  • they’re found in animal fats and dairy produce but also in palm oil
56
Q

What do unsaturated fatty acids contain?

A

C=C bond

57
Q

Why are reactions involving carboxylic acids slow and how can they be sped up?

A
  • slow since carboxylic acids are weak
  • we can speed the reactions up by using carboxylic acid derivatives
58
Q

Order the 2 derivatives of a carboxylic acid with carboxylic acids from least reactive to most reactive

A
  • carboxylic acid (least)
  • acyl chloride
  • acid anhydride (most)
59
Q

What is the functional group of a carboxylic acid?

A

HO-C=O

60
Q

What is the functional group of an acyl chloride?

A

Cl-C=O

61
Q

What is the functional group of an acid anhydride?

A

O=C-O-C=O

62
Q

What is produced when an acyl chloride reacts with alcohols or amines?

A

conc HCl

63
Q

What is the effect on substitutions onto a benzene ring if an OH or NH2 is already attached to the ring?

A
  • OH and NH2 are called activating groups
  • they both increase the reactivity of the benzene ring by donating pairs of electrons into the delocalised structure
  • this increases the electron density on the neighbouring carbon atoms as well as carbon 4
  • as a result, substitution of side groups occurs more readily in these positions
64
Q

What directing effect do OH and NH2 groups generate if they are directly attached to benzene before the substitutions take place?

A

2,4 directing effect

65
Q

What is the effect on substitutions onto a benzene ring if an NO2 is already attached to the ring?

A
  • nitrate groups withdraw electrons from the delocalised network in the benzene ring which deactivates it and reduces the electron density in the adjacent carbon atoms
  • this encourages substitution onto carbon 3 and 5
66
Q

What directing effect does an NO2 group generate if it is directly attached to benzene before the substitutions take place?

A

3,5 directing effect

67
Q

How is a carboxylic acid turned into an acyl chloride?

A
  • we need to react it with a molecule that’s rich in chlorine
  • the standard reagent for this is SOCl2 but PCl3 and PCl5 also work
68
Q

When turning a carboxylic acid to an acyl chloride, why is SOCl2 generally used over PCl3 or PCl5?

A
  • PCl3 and PCl5 produce conc HCl in the reaction so SOCl2 is safer
69
Q

What is the name of SOCl2?

A

thionyl chloride

70
Q

When turning a carboxylic acid to an acyl chloride, why is SOCl2 a useful reagent?

A

it is liquid at room temp

Chemistry (17 decks)

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