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A level chemistry > 6.1 Aromatic compounds and carbonyls > Flashcards

6.1 Aromatic compounds and carbonyls Flashcards

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
give the equation for the bromination of benzene
C6H6 + Br2 --> C6H5Br + HBr
26
state the reagents in the bromination of benzene
benzene, halogen carrier, halogen
27
Describe how the electrophile is generated in the bromination of benzene
FeBr3 + Br2 --> FeBr4- + Br+
28
Describe the regeneration of the catalyst in the bromination of benzene
FeBr4- + H+ --> FeBr3 --> HBr
29
What is a Friedel-Crafts reaction
A substitution reaction where hydrogen is exchanged for an alkyl or acyl chain
30
describe the reaction that occurs between haloalkanes and benzene
haloalkanes are mixed with a halogen carrier and a reactive carbocation is made which undergoes electrophilic substitution with the benzene ring
31
Describe the generation of the electrophile between the reaction of a haloalkane and benzene
Rcl+FeBr3 --> FeBr4- + R+
32
Describe the structure of an acyl chloride
RC=OCl
33
State the equation for the reaction between benzene and ethanoyl chloride
C6H6 + CH3COCl --> C6H5CH3CO + HCl
34
What are the reagents for the Friedel craft acylation reaction
Acyl chloride, halogen carrier catalyst and benzene
35
What are the conditions for the reaction between an acyl chloride and benzene
Reflux at 60C for 30 mins and anhydrous so that acyl chloride doesn't hydrolyse before it had the chance to react
36
Describe how the electrophile is generated in a reaction between an acyl chloride and benzene
CH3COCl + FeCl3 --> CH3CO+ + FeCl4 -
37
Describe how the catalyst is regenerated between the reaction of an acyl chloride and benzene
FeCl4- + H+ --> FeCl3 + HCl
38
What is a phenol
The first member of a type of aromatic compounds where the -OH group is attached directly to an aromatic ring
39
What is the difference between a phenol and aromatic alcohol
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
40
What kind of bases will phenol react with
strong bases but it will not react with carbonates
41
Why is phenol partially acidic
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
42
Why is phenol more reactive than benzene
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
43
What is the directing effect
How a functional group attached directly to an aromatic ring affects which carbon atoms are more likely to undergo substitution
44
Describe the bromination of phenol
Phenol will undergo a triple substitution reaction with bromine water at room temperature resulting in a white precipitate and antiseptic smell
45
What is the name of the product of the bromination of phenol
2,4,6-tribromophenol
46
Describe the nitration of phenol
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
What are the names of the products of the nitration of phenol
2-nitrophenol 4-nitrophenol 2,4,6-trinitrophenol
48
How does the hydroxyl group in the phenol affect position of substitution?
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
What is the effect of nitrate groups on positions of substitution?
3-directing effect
50
What are the 2 ways of purifying an organic solid
Filtration under reduced pressure recrystallisation
51
Describe filtration under reduced pressure
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
Describe the process of recrystallisation
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
How do you check purity
measuring melting points
54
Describe how you would check purity
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
define nucleophile
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
what is a carbonyl functional group
C=O
57
Describe nucleophilic addition reactions in a carbonyl compound
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
What is the name of NaBH4
Sodium tetrahydridoborate(III)
59
What is sodium tetrahydridoborate commonly used for
It is a reducing agent commonly used in organic synthesis
60
How does NaBH4 act as a reducing agent
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
Give the equation for the reduction of pentan-2-one
CH3COC3H7 2[H] --> CH3CH(OH)C3H7
62
What is hydrogen cyanide
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
Give the equation for the dissociation of hydrogen cyanide
HCN +H2O<--> CN- + H30+
64
What are some other sources of cyanide
NaCN
65
Why can't the cyanide ion react directly with carbonyl compounds
When the reaction is acidified the carbonyl functional group becomes more reactive as the polarity of the C=O bond is increased
66
Describe how you would use 2,4-DNP to test for a carbonyl functional group
When this is added to an aldehyde or a ketone a yellow/orange precipitate is formed.
67
What is Brady's reagent
A mixture of methanol, sulfuric acid and 2,4-DNP
68
Describe how you would distinguish between a ketone and an aldehyde after using Brady's reagent
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
What is tollens reagent
Ammonical silver nitrate
70
What can be used to test for a carbonyl
Brady's and Tollen's reagents
71
Describe how you would prepare Tollen's reagent
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
What is the positive test for tollens reagent
A silver mirror precipitate will occur if there is an aldehyde
73
Describe how tollens reagent produces the positive result
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
Describe the solubility of carboxylic acids
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
Describe the chemical properties of carboxylic acids
They are weak acids so they partially ionise in solution to form a carboxylate ion
76
Describe the reaction of carboxylic acids with metals
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
Describe the reaction of metal oxides with carboxylic acids
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
Describe the reaction of metal carbonates and carboxylic acids
they will form a metal carbonate water and carbon dioxide Na2CO3 + 2HCOOH → 2HCOONa + H20+ CO2
79
What is the ester functional group
R-COO-R
80
How would you name an ester?
it has the suffix oate The end of the name comes from the carboxylic acid and the start from the alcohol
81
What are the 2 main ways of esterification?
Reacting a carboxylic acid with an ester Reacting an acid anhydride with water
82
Describe esterification using carboxylic acid and alcohol
They are heated gently in the presence of a sulfuric acid catalyst produces an ester and water
83
Describe the drawbacks of esterification using carboxylic acid and alcohol
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
Describe how you would obtain the ester after reacting alcohol with a carboxylic acid
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
Describe the reaction between an acid anhydride and alcohol
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
Give the products for the reaction between ethanoic anhydride and methanol
Methyl ethanoate + ethanoic acid
87
Define hydrolysis
A chemical reaction where water causes the breaking of bonds in a decomposition reaction
88
What are the 2 ways of hydrolysing an ester
Acidic conditions Alkaline conditions
89
Describe how you would hydrolyse an ester under acidic conditions
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
Give an example of ester decomposition under acidic conditions
(H2SO4(aq)) propyl ethanoate + H2O ⇌ Ethanoic acid + propanol
90
Describe hydrolysis of an ester under alkaline conditions
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
Give an example of alkaline hydrolysis of esters
ethyl propanoate + NaOH → sodium propanoate + ethanol
92
What is the functional group of an acyl chloride
R-COCL
93
What is the suffix for an acyl chloride
-oyl chloride
94
How do you prepare an acyl chloride
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
Give the products for the reaction between thionyl chloride and ethanoic acid
ethanoyl chloride sulfur dioxide and hydrogen chloride gas
96
What are the uses of acyl chlorides as reagents in organic synthesis
making esters carboxylic acids primary amides secondary amides
97
Describe how acyl chlorides can be used to make esters
They will react with alcohols to make an ester This reaction is not reversible and has a high yield
98
Give the equation for making an ester from ethanoyl chloride
ethanoyl chloride + ethanol → ethyl ethanoate + corrosive HCL gas
99
Describe how acyl chlorides can be used to make carboxylic acids
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
Give the equation for making a carboxylic acid from acyl chlorides
Ethanoyl chloride + water →ethanoic acid + HCl
101
Describe how primary amides are made from acyl chlorides
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
Give the equation for the reaction of ethanoyl chloride to make a primary amide
ethanoyl chloride + concentrated ammonia solution → ethanamide (observed as white smoke) + ammonium chloride (s)
103
Describe how secondary amides are made from acyl chloride
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
Give the equation for the reaction of an acyl chloride to form a secondary amide
Ethanoyl chloride + ethylamine → N-ethylethanamide + HCl
105
give the symbol equation for the reaction of ethanoyl chloride to form a primary amine
CH3COCl + 2NH3 →CH3CONH2 +NH4Cl
106
Give the symbol equation for the reaction of ethanoyl chloride to form a secondary amide
CH3COCl + CH3CH2NH2 → CH3CONHCH2CH3 +HCl

A level chemistry (14 decks)

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