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Chemistry A2 Rafa > 6.1 Aromatic compounds, carbonyls and acids > Flashcards

6.1 Aromatic compounds, carbonyls and acids Flashcards

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1
Q

Define benzene

A

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

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2
Q

Define model

A

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

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3
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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4
Q

Define benzene derivative

A

Benzene ring that has undergone a substitution reaction

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5
Q

Define electrophilic substitution

A

a substitution reaction 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.

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6
Q

Define reaction mechanism

A

A model with steps to explain and predict a chemical reaction.

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7
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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8
Q

What are phenols?

A

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

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9
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.

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10
Q

Define nucelophile.

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.

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11
Q

What did Kekule discover?

A

When one group was attached to the benzene, only one isomer was ever made
When two groups were added there were always three structural isomers.

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12
Q

Problems with Kekule model

A
  • Benzene is resistant to addition reactions
  • Enthalpy of hydrogenation shows that benzene is much more stable then predicted
  • All six carbon bonds were the same length
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13
Q

How did Kekule explain the resistance to reaction?

A

By saying the double and single bonds changed position in a very fast equilibrium.

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14
Q

Explain the delocalised structure of benzene

A

Each of the six carbon atoms donates one electron from its p-orbital.
Form ring above and below.

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15
Q

Why is benzene very stable?

A

A lot of energy is needed to disrupt the delocalisation.

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16
Q

What do hydrogen carriers include?

A

Iron
Iron halides
Aluminium halides

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17
Q

Nitration of benzene describe process

A

Nitric and sulphuric acid mixed in a flask and held in an ice bath
Benzene added under reflux
Condenser
Keep at 50 degrees

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18
Q

Why is the sulphuric acid needed in nitration of benzene?

A

To generate the NO2+ electrophile from the nitric acid

Catalyst as it is regenerated

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19
Q

What is a base?

A

A chemical that will react with an acid

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20
Q

What do you need when you add alkyl groups to benzene?

A

Strong Lewis acid

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21
Q

What is a Lewis acid?

A

Accepts a pair of electrons

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22
Q

Functional group of acyl chloride

A

RCOCl

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23
Q

Use of acyl chloride

A

Used in friedel crafts reaction as the halogen carrier

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24
Q

Conditions for acylation of benzene

A 
60 degrees
30 mins
Reflux 
AlCl3 catalyst
Anhydrous conditions
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25
Testing for carbonyl functional group
Brady's reagent Orange Mix of methanol, sulfur if acid and a solution of 2,4-dinitrophenylhydrazone
26
What does Brady's reagent identify?
Ketone or aldehyde but not carboxylic acid or ester
27
What do you need when you add alkyl groups to benzene?
Strong Lewis acid
28
What is a Lewis acid?
Accepts a pair of electrons
29
Use of acyl chloride
Used in friedel crafts reaction as the halogen carrier
30
What do you use to attach carbonyl groups to aromatic compounds?
Acyl chlorides
31
Why can we only get single substitution when we add carbonyl groups on benzene?
The carbonyl group removes electron density from the ring
32
Why is phenol a weak acid?
Because it partially dissociates in water and releases H+
33
why doesn't phenol react with weak bases?
Because it is a weak acid
34
Why is phenol more reactive then benzene
Higher electron density, non bonding electrons join delocalised system
35
Things that smell like antiseptic
tri bromo and chloro phenol
36
when is the directing effect more noticable?
when we have a NH2 substituted instead of phenol
37
When do we get a 3 directing effect?
with NO2 group attached
38
Why is there a colour change from orange to green during the oxidation of aldehydes?
the oxidation state of the chromium changes
39
source of H- ions
sodium tetrahydrideborate (III), NaBH4
40
formula of cyanide ions
CN-
41
Source of cyanide ions
HCN and NaCN
42
Why do we acidify the solution before adding CN- to increase chain length?
increases the polarity go C=O group
43
What is Brady's reagent made from?
methanol, sulphuric acid and a solution of 2,4 dinitrophenylhydrazine (2,4 - DNP)
44
What does bradys tell us?
if there is an aldehyde or ketone present
45
what happens when bradys is added to aldehyde or ketone
orange yellow precipitate
46
Why do we measure the boiling points of the precipitates?
bigger difference than the substances themselves
47
how do we make Tollen's reagents?
Add sodium hydroxide to silver nitrate solution until a brown precipitate is formed add dilute ammonia drop by drop until it redissolves
48
What is Tollen's reagent
A weak oxidising agent
49
Why is a silver mirror formed?
Silver is oxidised, the Ag+ ions turn into solid silver
50
How do we make esters using carboxylic acids?
``` Alcohol and carboxylic acid sulphuric acid and gentle heating if they are large esters we need to heat under reflux reversible not suitable for phenol + derivatives ```
51
What are the two main ways of making esters?
Using carboxylic acids | Using acid anhyrides
52
How do we make esters from acid anhydrides?
``` acid anhydrides react with alcohols non reversible reactions react with phenol + derivatives higher yield produces ester and carboxylic acid ```
53
How can we reverse esterification?
Using hydrolysis
54
What happens if we refiux an ester with a hot aqueous acid (acid hydrolysis)
reversible decomposition acid catalyst alcohol and carboxylic acid
55
Describe alkaline hydrolysis
Irreversible decomposition | forms a carboxylate salt and alcohol
56
Whats alkaline hydrolysis used for ?
saponification
57
What are acyl chlorides?
Derivative of carboxylic acids where the -OH group is replaced by a -Cl group
58
Properties of short chained acyl chlorides?
fuming colourless liquids
59
How do we make acyl chlorides?
carboxylic acid and SOCl where Cl replaces the OH group
60
Why is it better to use acyl chlorides to make esters?
Higher yield
61
Bad thing about using acyl chlorides to form esters
HCl fumes
62
How do we get ester involved with aromatic rings?
Phenol and carboxylic acid won't react using with acyl chlorides violent reactions that produce corrosive fumes
63
Describe the reaction to form carboxylic acid from acyl chloride
Add water very exothermic misty HCl fumes
64
Describe forming primary amides using acyl chlorides
React with conc ammonia | an ammonium salt is also made
65
Forming secondary amides using acyl chlorides
React with primary amines | called N substituted amides (secondary amide another name for)
66
What happens when acyl chlorides react with secondary amines?
N,N amides which have two alkyl groups on the nitrogen
67
How do you make Tollen's reagent?
1) Add sodium hydroxide to silver nitrate solution until a brown precipitate forms 2) Add dilute ammonia drop by drop until the precipitate re-dissolves
68
Why does Tollen's reagent work?
its a weak oxidising agent and will oxidise aldehydes but not ketones as they cannot be further oxidised When oxidising, the Ag+ ions are reduced to solid silver
69
Why are carboxylic acids more soluble than their similar alcohols?
Because they can form 2 hydrogen bonds per molecule
70
Properties of short chained acyl chlorides
fuming colourless liquids which are very reactive and Cl is replaced by other groups
71
CH3COOH + SOCl2 = ?
CH3COCl + SO2 + HCl | and then distil
72
Why are triglycerides soluble in non-polar solvents and not in water?
-There is van der Waals between triglycerides. - There is van der Waals between triglycerides and (non-polar) solvent. - Triglycerides cannot hydrogen bond (to water)(enough). -Because there are not enough suitable sites/oxygen atoms -Or long hydrocarbon chains do not hydrogen bond/would interfere with hydrogen bonding in water
73
Conditions for alkylation of benzene
CH3Cl | AlCl3
74
Conditions for Friedel Crafts acylation of benzene
- reflux - 60 degrees - 30 mins - anhydrous conditions - single substitution only - CH3COCl - AlCl3
75
Conditions for nitration of benzene
- conc nitric and sulphuric mixed in ice bath - benzene added - reflux condenser set up at 50 degrees
76
Why is the mixture kept at 50 degrees for nitration of benzene?
to prevent further substitutions
77
Equation for adding sulphuric acid and nitric acid together in an ice bath
H2SO4 + HNO3 = NO2+ + HSO4- + H2O
78
What conditions are needed for the halogenation of benzene?
- X2 | - AlX3 or Fe or FeX3
79
How does iron act as a halogen carrier?
Forms a iron halide in situ
80
Conditions for the reduction of nitrobenzene
- Reflux at 100 degrees with Sn/conc. HCl | - After refluxing for 20 mins, add NaOH to remove excess acid
81
Why is the 3 directing effect seen on -NO2 groups are attached to the aromatic ring?
the nitro groups withdraw electrons from the pi-system
82
Which groups do 2- and 4- directing effect?
phenol | NH2
83
Which groups do 3 directing effect?
NO2
84
Conditions for adding a nitro group to phenol?
dilute HNO3 for single substitution | conc. HNO3 for triple substitution
85
What happens when you add NaOH to phenol?
Na+O- salt forms
86
Why are all of the bonds in benzene the same length?
the pi bonds are delocalised
87
Acidic hydrolysis of esters
alcohol + COOH | reversible
88
Alkaline hydrolysis of esters
alcohol and carboxylate salt | not reversible
89
What do you observe when you make a primary amide from an acyl chloride?
white smoke | some products remain in colourless solution
90
What do you observe when you make a secondary amide from an acyl chloride?
white solid
91
What did they expect the enthalpy of hydrogenation of benzene to be and what was it actually?
``` expected = -360kJmol-1 actual = -208kJmol-1 ```
92
When is nucleophilic addition?
C=O
93
When is electrophilic substitution?
Benzene and phenol
94
Why does CH3COCl produce acidic solutions?
in water it produces a carboxylic acid and HCl | the carboxylic acid dissociates
95
How to test for phenol?
- add NaOH to form colourless solution | - doesnt effervesce when sodium carbonate or hydrogencarbonate ions are added

Chemistry A2 Rafa (8 decks)

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