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a level chemistry > 6.1 Aromatic compounds and carbonyls > Flashcards

6.1 Aromatic compounds and carbonyls Flashcards

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1
Q

what is the formula of benzene

A

C6H6

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2
Q

what did Kekule suggest the structure of benzene was like

A

he suggested benzene was a six-membered ring with alternating double and single bonds

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3
Q

what were the 3 problems with Kekules structure

A

unlike alkenes benzene was resistant to electrophilic addition reactions

the enthalpy of hydrogenation was lower than expected telling us benzene was more stable than predicted

all six of the carbon bonds were the same lengths

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4
Q

How did Kekule try to explain the resistance of electrophilic addition reactions

A

He said that 2 forms of benzene existed and that the double and single bonds changed position in a very fast equilibrium - resonance

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5
Q

What was odd about the enthalpy of hydrogenation of benzene

A

The expected enthalpy of hydrogenation was -360kj/mol however the real value was -208kJ/mol telling us it was 152kJ/mol more energetically stable than predicted

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6
Q

Describe the bond lengths in benzene compared to Kekules model

A

benzene had equal bond lengths of 0.140nm, whereas in C-C bond it is 0.147nm and C=C bond it is 0.135nm

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7
Q

Describe the structure of benzene

A

Benzene is regular, planar and hexagonal with covalent bonds and the carbon bonds are all of equal length

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8
Q

what is the bond angle in benzene

A

120

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9
Q

Describe the delocalised structure of benzene

A

The carbon atoms each donate one electron from their p-orbitals which combine and form a ring of delocalised electrons above and below the plane of the molecule

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10
Q

how does the delocaised structure of benzene explain the 3 problems with kekules model

A

the electrons in the ring are delocalised so they can move freely within the structure, so all the bonds are the same lengths

The delocalisation also leads to benzene being more stable because a lot of energy is needed to disrupt this delocalisation

A lot of energy is needed to disrupt the delocalisation so benzene is resistant to undergo electrophilic addition

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11
Q

define substitution reaction

A

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

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12
Q

define electrophilic substitution

A

a substitution reaction where an electrophile is attracted to an electron rich atom, or a part of a molecule and a new covalent bond is formed

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13
Q

Explain why 2-phenylbutane isn’t called 2-butylbenzene

A

This is because the phenyl group is a substituent on carbon number 2, not a straight-chained butyl group

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14
Q

what are the 4 instances when the root of the compounds name is benzene

A

an alkyl group
a halogen
a nitro group (NO2)
a carboxylic acid

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15
Q

When is the root of the compounds name phenyl

A

With most other functional groups, the benzene can be regarded as a substituent

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16
Q

what is an amine group

A

NH2

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17
Q

define electrophile

A

electron pair acceptor

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18
Q

what are the 4 main differences between ethene and benzene

A

benzenes electrons are delocalised

benzene can’t induce a dipole

benzene doesn’t readily undergo electrophilic substitution

Benzene has a lower electron density

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19
Q

What is the formula for the nitration of a benzene ring

A

C6H6 + HNO3 –> C6H5N02 + H20

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20
Q

what are the reagents for the nitration of benzene

A

concentrated nitric and sulfuric acid and benzene

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21
Q

What are the conditions for the nitration of benzene

A

The concentrated nitric acid and sulfuric acid are initially mixed together in a flask held in an ice bath.

Then benzene is added, and a reflux condenser is set up, keeping the mixture at 50C to prevent multiple substitutions

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22
Q

state the formula of the formation of the electrophile for the nitration of benzene

A

HNO3 + H2SO4 –> NO2+ + HS04- + H20

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23
Q

State the formula for the regeneration of the catalyst for the nitration of benzene

A

HS04- + H+ –> H2S04

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24
Q

Describe the difference between the reactions of halogens and benzene compared to cyclohexene

A

Benzene doesn’t react directly with halogens as the aromatic ring is too stable. So, a halogen carrier such as iron (which forms an iron halide in situ). The halogen carrier will generate a positive halide ion

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25
Q

give the equation for the bromination of benzene

A

C6H6 + Br2 –> C6H5Br + HBr

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26
Q

state the reagents in the bromination of benzene

A

benzene, halogen carrier, halogen

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27
Q

Describe how the electrophile is generated in the bromination of benzene

A

FeBr3 + Br2 –> FeBr4- + Br+

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28
Q

Describe the regeneration of the catalyst in the bromination of benzene

A

FeBr4- + H+ –> FeBr3 –> HBr

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29
Q

What is a Friedel-Crafts reaction

A

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

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30
Q

describe the reaction that occurs between haloalkanes and benzene

A

haloalkanes are mixed with a halogen carrier and a reactive carbocation is made which undergoes electrophilic substitution with the benzene ring

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31
Q

Describe the generation of the electrophile between the reaction of a haloalkane and benzene

A

Rcl+FeBr3 –> FeBr4- + R+

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32
Q

Describe the structure of an acyl chloride

A

RC=OCl

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33
Q

State the equation for the reaction between benzene and ethanoyl chloride

A

C6H6 + CH3COCl –> C6H5CH3CO + HCl

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34
Q

What are the reagents for the Friedel craft acylation reaction

A

Acyl chloride, halogen carrier catalyst and benzene

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35
Q

What are the conditions for the reaction between an acyl chloride and benzene

A

Reflux at 60C for 30 mins and anhydrous so that acyl chloride doesn’t hydrolyse before it had the chance to react

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36
Q

Describe how the electrophile is generated in a reaction between an acyl chloride and benzene

A

CH3COCl + FeCl3 –> CH3CO+ + FeCl4 -

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37
Q

Describe how the catalyst is regenerated between the reaction of an acyl chloride and benzene

A

FeCl4- + H+ –> FeCl3 + HCl

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38
Q

What is a phenol

A

The first member of a type of aromatic compounds where the -OH group is attached directly to an aromatic ring

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39
Q

What is the difference between a phenol and aromatic alcohol

A

If the hydroxyl group is attached to an alkyl chain on the aromatic ring rather than just the hydroxyl on the aromatic ring then it is an aromatic alcohol

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40
Q

What kind of bases will phenol react with

A

strong bases but it will not react with carbonates

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41
Q

Why is phenol partially acidic

A

phenol can lose a hydrogen ion because the phenoxide ion is stabilised to some extent
The negative charge on the oxygen atom is delocalised around the ring
The more stable the ion is the more likely it is to form

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42
Q

Why is phenol more reactive than benzene

A

This is because the p-orbital electrons from the oxygen add to the delocalised electrons, so the pi-system of the aromatic ring becomes more nucleophilic, making it more susceptible to electrophilic attack

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43
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

44
Q

Describe the bromination of phenol

A

Phenol will undergo a triple substitution reaction with bromine water at room temperature resulting in a white precipitate and antiseptic smell

45
Q

What is the name of the product of the bromination of phenol

A

2,4,6-tribromophenol

46
Q

Describe the nitration of phenol

A

Phenol will undergo a single substitution reaction with dilute nitric acid at room temperature
and a triple substitution if concentrated nitric acid is used

47
Q

What are the names of the products of the nitration of phenol

A

2-nitrophenol 4-nitrophenol
2,4,6-trinitrophenol

48
Q

How does the hydroxyl group in the phenol affect position of substitution?

A

It pushes additional electrons into the pi-system making substitution reactions mainly occur on the 2nd and 4th positions on the aromatic ring

It activates those carbon atoms so that their rate of substitution is faster called a 2- and 4-directing effect

49
Q

What is the effect of nitrate groups on positions of substitution?

A

3-directing effect

50
Q

What are the 2 ways of purifying an organic solid

A

Filtration under reduced pressure
recrystallisation

51
Q

Describe filtration under reduced pressure

A

Place a Buchner funnel into the top of a filter flask and for small quantities use a Hirsch funnel

Connect the tubing from the vacuum pump to the side arm of the clamped filter flask and start the suction

place a piece of filter paper and dampen it using distilled water

pour your mixture into the funnel and wash out the vessel with distilled water and the solid

collect the solid onto a watch plate

52
Q

Describe the process of recrystallisation

A

Dissolve the crude product into the minimum volume of hot water and then it should quickly undergo gravity filtration using fluted filter paper

let the mixture cool down and the product will form

if no crystals appear to be forming scratch the sides of the conical flask with a glass rod

The purified product can be collected using vacuum filtration and the crystals should be washed

53
Q

How do you check purity

A

measuring melting points

54
Q

Describe how you would check purity

A

In a sealed capillary tube put a few grains of the organic solid and using a small rubber band attach it to a thermometer and submerge into the oil of the Thiele tube ensuring the rubber band is above the oil line.

Using a micro burner with a small flame heat the side arm of the Thiele tube and note the temperature range whne the organic product starts and finishes melting

55
Q

define nucleophile

A

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

56
Q

what is a carbonyl functional group

A

C=O

57
Q

Describe nucleophilic addition reactions in a carbonyl compound

A

The C=O is susceptible to nucleophilic attack due to the dipole on the carbon atom. A nucleophile donates a pair of electrons to the electron-deficient carbon and simultaneously the pi bond in the C=O breaks and the electrons go to the oxygen molecule. The extra electron pair is then quickly donated to the neighbouring hydrogen to form an alcohol group and stable the product

58
Q

What is the name of NaBH4

A

Sodium tetrahydridoborate(III)

59
Q

What is sodium tetrahydridoborate commonly used for

A

It is a reducing agent commonly used in organic synthesis

60
Q

How does NaBH4 act as a reducing agent

A

The compound is made of BH4- ions, which act as a source of hydride ions (H-) which is a species involved in the reduction of carbonyl compounds to alcohols

61
Q

Give the equation for the reduction of pentan-2-one

A

CH3COC3H7 2[H] –> CH3CH(OH)C3H7

62
Q

What is hydrogen cyanide

A

It is a weak acid that will partially ionise in solution and a cyanide nucleophile with a negative charge on the carbon atom is formed

63
Q

Give the equation for the dissociation of hydrogen cyanide

A

HCN +H2O<–> CN- + H30+

64
Q

What are some other sources of cyanide

A

NaCN

65
Q

Why can’t the cyanide ion react directly with carbonyl compounds

A

When the reaction is acidified the carbonyl functional group becomes more reactive as the polarity of the C=O bond is increased

66
Q

Describe how you would use 2,4-DNP to test for a carbonyl functional group

A

When this is added to an aldehyde or a ketone a yellow/orange precipitate is formed.

67
Q

What is Brady’s reagent

A

A mixture of methanol, sulfuric acid and 2,4-DNP

68
Q

Describe how you would distinguish between a ketone and an aldehyde after using Brady’s reagent

A

Collect the precipitate by filtration and purify using crystallisation and after the accurate melting point of the pure product can be measured using a thiele tube etc.

69
Q

What is tollens reagent

A

Ammonical silver nitrate

70
Q

What can be used to test for a carbonyl

A

Brady’s and Tollen’s reagents

71
Q

Describe how you would prepare Tollen’s reagent

A

Sodium hydroxide solution is added to silver nitrate solution until a brown precipitate is formed
Then dilute ammonia is added dropwise till the brown precipitate redissolves

72
Q

What is the positive test for tollens reagent

A

A silver mirror precipitate will occur if there is an aldehyde

73
Q

Describe how tollens reagent produces the positive result

A

When aldehydes react with Tollen’s reagent a redox reaction occurs the silver ions are reduced, and the aldehyde functional group is oxidised
Ag+ +e- –> Ag
Aldehyde +[o] –> carboxylic acid

74
Q

Describe the solubility of carboxylic acids

A

Small carboxylic acids are very soluble in polar solvents like water this is because hydrogen bonds can be formed between the carboxylic acid functional group and water

As the hydrocarbon chain of a carboxylic acid increases in size the solubility decreases because only the functional group can form hydrogen bonds with the water, so, as the molecule becomes more non-polar the solubility decreases.

75
Q

Describe the chemical properties of carboxylic acids

A

They are weak acids so they partially ionise in solution to form a carboxylate ion

76
Q

Describe the reaction of carboxylic acids with metals

A

Carboxylic acids will react with metals above hydrogen in the reactivity series to make hydrogen and a metal salt
(carboxylate)

2Na + 2HCOOH → 2HCOO- Na+ + H2

77
Q

Describe the reaction of metal oxides with carboxylic acids

A

Metal oxides can be classed as bases therefore they will react to form a metal salt and water
MgO(s) + CH3COOH(aq)→ (CH3COO)2Mg (aq) + H2O

78
Q

Describe the reaction of metal carbonates and carboxylic acids

A

they will form a metal carbonate water and carbon dioxide

Na2CO3 + 2HCOOH → 2HCOONa + H20+ CO2

79
Q

What is the ester functional group

A

R-COO-R

80
Q

How would you name an ester?

A

it has the suffix oate
The end of the name comes from the carboxylic acid and the start from the alcohol

81
Q

What are the 2 main ways of esterification?

A

Reacting a carboxylic acid with an ester
Reacting an acid anhydride with water

82
Q

Describe esterification using carboxylic acid and alcohol

A

They are heated gently in the presence of a sulfuric acid catalyst
produces an ester and water

83
Q

Describe the drawbacks of esterification using carboxylic acid and alcohol

A

it has a slow rate of reaction and is also reversible
This method of preparation is not suitable for phenol or its derivatives as the rate of reaction is too slow

84
Q

Describe how you would obtain the ester after reacting alcohol with a carboxylic acid

A

As the ester is volatile with the lowest boiling point of the chemicals it can be separated from the reaction mixture using distillation

To prepare the larger esters the reaction mixture will need to be heated under reflux until equilibrium is reached and the ester can be seperated using fractional distillation

85
Q

Describe the reaction between an acid anhydride and alcohol

A

This method is not reversible and has a higher yield and the rate of the reaction can be increased by gently heating the mixture
the products are the ester and a carboxylic acid

86
Q

Give the products for the reaction between ethanoic anhydride and methanol

A

Methyl ethanoate + ethanoic acid

87
Q

Define hydrolysis

A

A chemical reaction where water causes the breaking of bonds in a decomposition reaction

88
Q

What are the 2 ways of hydrolysing an ester

A

Acidic conditions
Alkaline conditions

89
Q

Describe how you would hydrolyse an ester under acidic conditions

A

When esters are refluxed with a catalyst of hot aqueous acid such as dilute sulfuric acid the ester will decompose reversibly into an alcohol and a carboxylic acid

90
Q

Give an example of ester decomposition under acidic conditions

A

(H2SO4(aq))
propyl ethanoate + H2O ⇌ Ethanoic acid + propanol

90
Q

Describe hydrolysis of an ester under alkaline conditions

A

An ester is refluxed with hot aqueous alkali such as sodium hydroxide it will decompose into an alcohol and carboxylate salt and the reaction is not reversible

91
Q

Give an example of alkaline hydrolysis of esters

A

ethyl propanoate + NaOH → sodium propanoate + ethanol

92
Q

What is the functional group of an acyl chloride

A

R-COCL

93
Q

What is the suffix for an acyl chloride

A

-oyl chloride

94
Q

How do you prepare an acyl chloride

A

The -OH group of a carboxylic acid must be substituted with a chlorine atom.
One method is to use SOCl2 which is a liquid at room temperature and readily reacts with a carboxylic acid to make the desired product
The acyl chloride is seperated using distillation

95
Q

Give the products for the reaction between thionyl chloride and ethanoic acid

A

ethanoyl chloride sulfur dioxide and hydrogen chloride gas

96
Q

What are the uses of acyl chlorides as reagents in organic synthesis

A

making esters
carboxylic acids
primary amides
secondary amides

97
Q

Describe how acyl chlorides can be used to make esters

A

They will react with alcohols to make an ester
This reaction is not reversible and has a high yield

98
Q

Give the equation for making an ester from ethanoyl chloride

A

ethanoyl chloride + ethanol → ethyl ethanoate + corrosive HCL gas

99
Q

Describe how acyl chlorides can be used to make carboxylic acids

A

when a small acyl chloride is added to water it quickly hydrolyses to produce a carboxylic acid
this is a very exothermic reaction and mist HCl fumes are given off

100
Q

Give the equation for making a carboxylic acid from acyl chlorides

A

Ethanoyl chloride + water →ethanoic acid + HCl

101
Q

Describe how primary amides are made from acyl chlorides

A

When an acyl chloride reacts with ammonia a primary amide is produced
The acyl chloride is added to concentrated ammonia solution producing solid ammonium chloride and an amide observed as white smoke

102
Q

Give the equation for the reaction of ethanoyl chloride to make a primary amide

A

ethanoyl chloride + concentrated ammonia solution → ethanamide (observed as white smoke) + ammonium chloride (s)

103
Q

Describe how secondary amides are made from acyl chloride

A

When an acyl chloride reacts with a primary amine it produces a secondary amide (N-substituted amide)
Awhite solid compound is made with a cold concentrated solution of the primary amine

104
Q

Give the equation for the reaction of an acyl chloride to form a secondary amide

A

Ethanoyl chloride + ethylamine → N-ethylethanamide + HCl

105
Q

give the symbol equation for the reaction of ethanoyl chloride to form a primary amine

A

CH3COCl + 2NH3 →CH3CONH2 +NH4Cl

106
Q

Give the symbol equation for the reaction of ethanoyl chloride to form a secondary amide

A

CH3COCl + CH3CH2NH2 → CH3CONHCH2CH3 +HCl

a level chemistry (14 decks)

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