A Level Chemistry Ocr > Module 6 Chapter 25 > Flashcards

Module 6 Chapter 25 Flashcards

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1
Q

Who was benzene first isolated by

A

Michael Faraday in 1825

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2
Q

What is benzene used for

A

It is a major feedstock in many industries:
Polymers
Pharmaceuticals
Dyes
Explosives

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3
Q

What is a feature of benzene

A

It is highly carcinogenic, it is colourless, sweet smelling, highly flammable liquid found in crude oil and cigarette smoke.

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4
Q

What is benzene an example of

A

it is the simplest arena with the empirical formula of CH, and molecular formula of C6H6

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5
Q

What occurred in 1865 to do with benzene

A

August kekulé suggested a cyclical structure for benzene with three alternating carbon-carbon double bonds.

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6
Q

What were problems with the kekulé structure of benzene

A

It does not react in the same way as alkenes, and in general seems unreactive. This model does not explain all of its physical and chemical properties.

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7
Q

What did the kekulé model propose to get over this claim

A

That benzene’s low reactivity was due to a rapid equilibrium between two isomers

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8
Q

Draw the equilibrium between the two isomers of benzene

A

DRAW IT

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9
Q

What are problems with the kekulé model for benzene (1)

A

If benzene did contain these c=c bonds, it should decolourise bromine in an electrophilic addition reaction
—> Therefore there are no c=c double bonds within its structure

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10
Q

What are problems with the kekulé model for benzene (2)

A

isomers: compounds like 1,2 - dichlorobenzene should form isomers but only one form exists

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11
Q

What are problems with the kekulé model for benzene (3)

A

bond length: c-c and c=c bonds have different bonds lengths so benzene would be an irregular hexagon but imaging shows it’s regular with intermediate bond lengths

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12
Q

What are problems with the kekulé model for benzene (4)

A

Hydrogenation enthalpy: Benzene can be saturated with hydrogen to form cyclohexane
With the kekule model having an expected hydrogenation enthalpy of -360KJ/mol, whereas in reality it has a ^H of -208KJ/mol so therefore it’s bonds are more stable than alkenes

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13
Q

What are delocalised electrons

A

Bonding electrons that are not fixed between two atoms but shared between 3 or more atoms

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14
Q

What is the actual structure of benzene

A
  1. Each carbon atoms form there covalent sigma bonds
  2. The remaining electron is found in the p orbital at a right angle above and below the carbon atoms
  3. The p orbitals overlap sideways, crating a ring of delocalised pi electrons above and below the structure. Forming a ring of e- density
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15
Q

What does the ring in benzene allow

A

Charge to be evenly spread across the molecule making it stable and allowing equal bond lengths.

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16
Q

What is a feature of benzene

A

it is a planar molecule

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17
Q

What are the delocalised electrons shown as in benzene

A

A circle inside the benzene ring

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18
Q

What is the nomenclature of benzene

A

Some groups are shown as prefixes to benzene.

In aromatic compounds benzene ring is often considered the parent chain/main chain

But if it is attached to an alkyl chain with a functional group or an alkyl chain with 7 or more c it is considered to be a substitutent

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19
Q

What is it called in nomenclature when benzene is substituent

A

Prefix = phenyl

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20
Q

What are some naming exceptions for benzene

A

benzoic acid (Benzenecarboxylic acid)
Phenylamine (aminebenzene, small nitro group)
Benaldehyde (benzene carbaledhye)

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21
Q

What is a feature of benzene

A

It is the parent Arlene that others are derived from

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22
Q

What is a feature of naming arenes

A

The position of the substituted hydrogens are numbered to give the lowest possible numbers.
—> with one group always occupying position 1

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23
Q

When naming compounds with benzene what are the different suffixes and prefixes.

A

The ring can be either given of the suffix ‘benzene’ or as the prefix ‘phenyl’ if with alcohol or amine groups

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24
Q

What reactions can alkenes undergo that benzene can’t and why

A

Electrophilic addition reactions as it would disrupt the delocalised pi electron ring, resulting in the product being less energetically stable

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25
What type of reaction does benzene undergo
It undergoes electrophilic substitution reactions by replacing a hydrogen atom for another group —> retaining the energetically stable delocalised ring
26
What is the general equation for the electrophilic substitution of benzene
C6H6 + X+ —> C6G5X + H+ Whereas X+ is an electrophile
27
How does the electrophilic substitution reaction work (1)
The electrophilic is attracted to the ring due to the high electron density caused by delocalised pi electrons A pair of electrons from the ring are donated to the electrophile to form a covalent bond
28
How does the electrophilic substitution reaction work (2)
An unstable intermediate forms with an incomplete ring and a positive charge The bond with the hydrogen atom undergoes heterolytic fission to donate electrons to the ring structure The stable ring structure returns leading the substituted product and a hydrogen ion
29
Draw the mechanism for the general electrophilic substitution of a benzene ring
DRAW IT
30
What is required for the nitration of benzene
Reagent: concentrated nitric acid (HNO3) Catalyst: concentrated sulphuric acid (H2SO4) Conditions: 50-55*C If higher multiple substitutions occurs
31
Draw the overall reaction for the nitration of a benzene ring with one nitro group (1)
DRAW IT
32
Draw the overall reaction for the nitration of a benzene ring with two nitro groups
DRAW IT
33
What is the two half equation of nitronium ion formation and hence the overall equation
H2SO4 + HNO3 —> HSO4 - + H2NO3 + H2NO3 + —> H2O + NO2+ H2SO4 + HNO3 —> HSO4- + H2O + NO2+
34
What is step 1 in the substitution of a nitro group into benzene
The formation of the nitronium ions
35
What is step 2 in the substitution of the nitro group into benzene
The nitronium ion acts as an electrophile causing electrophilic substitution of the benzene rings. It accepts a pair of electrons from the benzene rings to form a dative covalent bond. Intermediate is formed which is unstable and breaks down to form stable nitrobenzene and H+ ion, ring is reformed too.
36
Draw the mechanism for step of the substitution of nitrogen into benzene
DRAW IT
37
What is step 3 in the substitution of nitro into benzene
Catalyst reforms The H+ ion formed by substitution reacts with the HSO4- ions to form H2SO4 regenerating the catalyst. H+ + HSO4- —> H2SO4
38
What do Friedol-craft reactions use and what are the different processes by which this can occur
Halogen carriers (AlX3, FeX3 or Fe) as catalysts to substitute R-CL groups to rings via: Halogenation - substitution of a halogen Alkylation - substitute an alkyl chain using haloalkanes Acylation - substitute a ketone chain using acytchlorides
39
What are acyl chlorides
They are similar to carboxylic acids but have a CL instead of an OH
40
What is a feature of the conditions of a Friedol-Craft reaction
They are performed under reflux in anhydrous conditions
41
What is the first step in the electrophilic substitution by halogenation and the equation
Electrophile formation The chlorinated group is polarised by the catalyst and generates the electrophile R-Cl + AlCl3 —> R+ + AlCl4 -
42
What is the second step in the electrophilic substitution in a halogenation reaction
The electropohiles attack the benzene ring structure to substitute the R group for a hydrogen
43
What is the third step in the electrophilic substitution in a halogenation reaction
The H+ ion formed by the substitution reacts with the chlorine donated to the halogen carrier to form HCL H+ + AlCl4- —> AlCl3 + HCL
44
What occurs in an alkylation reaction to benzene
H atom in ring is substituted with an alkyl group React benzene with haloalkene in presence of AlCl3 Alkylation increases the number of carbon atoms in the compound
45
What occurs in acylation to benzene
Benzene is reacted with an acyl chloride with AlCl3 catalyst and an aromatic ketone is formed
46
DRAW ethanoyl chloride
DRAW IT
47
Why are there differences in reactivity between benzene and alkenes
Bromine adds across the double bond in cyclohexene 1. Pi bond in Alkene contain localised e- above and below the plane of two C in C=C high electron density Dipole induced in non-polar bromine Slightly positive bromine acts AAs an electrophile
48
What are phenols
They are aromatic compounds that have an alcohol’s hydroxyl group attached directly to the ring
49
What are features of phenols
The hydroxyl group will always take position one unless there is a carboxylic acid group present The simplest phenol is (C6H5OH) with one OH attached to benzene
50
What type of molecules are not considered as phenols
Alcohols containing a benzene ring, but don’t have a hydroxyl group directly attached to it are called aromatic alcohols and not phenols
51
In what subshell are the electrons in the p orbital present in and what occurs to it in the phenol
The oxygen atom bonded to the benzene has two lone pairs of electrons in the p orbitals. As the ring is electron deficient one of the p orbitals overlaps into the ring structure to become delocalised.
52
What does the effect of the overlapping of the oxygens p orbital electrons do
It increases the ring’s electron density, allowing electrophilic substitution reactions to occur easily. Some similar reactions to alcohols will occur but some are different
53
Why is a phenol a weak acid and what is the equation for the dissacociation of phenol
The P orbital overlap into the ring weakens the O-H bond so phenol can donate H+ and form stable phenoxide ions when dissolved in water C6H5OH <-> C6H5O- + H+
54
What are features of phenol solutions
They are form solutions that are more acidic than alcohols and less acidic than carboxylic acids Ka (ethanoic acid) > Ka (phenol) > Ka alcohol
55
How can you different between alcohols, phenols and carboxylic acids
Alcohols don’t react with NaOH (SB) or Na2Co3 (WB) Phenols and CA react with solutions of strong bases Only CA are strong enough acid to react with Na2CO3 - this can be used to distinguish between CA and phenol.
56
What are the reactions of phenol with hydroxides and metals
C6H5OH + NaOH -> C6H5O-Na+ + H2O 2C6H5OH + 2NA —> 2C6H5O-Na+ + H2
57
What is the reaction between phenols and bromine water
They can react with bromine water by electrophilic substitution at RTP without the need for a catalyst. Decolourising bromine water and forming a white precipitate (2,4,6-tribromophenol)
58
Draw the reaction for the reaction between phenols and 3Br2
DRAW IT
59
What are the differences in reactivity with halogens between alkenes, phenols and arenes (alkenes)
The pi bond in alkenes provides and area of high electron density above and below the molecule —> this can induce a dipole in a bromine molecule
60
What are the differences in reactivity with halogens between alkenes, phenols and arenes (arenes)
The pi electrons in benzene are spread over the ring structure which lowers the overall electron density to increase stability. The lower electron density means that benzene is in unlikely to induce a dipole into a bromine molecule. Halogen carriers are therefore required to produce electrophile (Br+)
61
What are the differences in reactivity with halogens between alkenes, phenols and arenes (phenols)
The addition of p orbital electrons to the ring structure from the oxygen atom of a phenol increases the electron density. The increase in density is enough to induce a dipole in a bromine molecule allowing substitution without a catalyst.
62
What can the presence of a group on a benzene ring do
It will have an effect on the substitution of further groups known as the directing effect
63
What can the substituted groups be classified by
The effect that they have on the electron density of the ring
64
What are the two types of classification of the groups on the benzene ring
Releasing electrons - increase e- density Withdrawing electrons - decrease
65
What is disubstitution
When more than one functional group is substituted onto the ring
66
What do side chains that increase electron density do
They allow electrophiles to react faster than with benzene as they are ‘activating’ groups
67
What are the two examples of activating groups
OH as the o lone pair orbitals overlap NH2 as the N lone pair orbitals overlap
68
What do activating groups do
They direct substitution to positions 2 and 4 The weaker position at 3 is rarely substituted forming little to no isomers.
69
What is ortho
Position - 2 - on the benzene ring
70
What is meta
Position 3 - benzene
71
What is para
Position 4 on the benzene ring
72
What do different groups attached to the benzene ring do
They have directing effects on any second substituent on the benzene ring
73
What are 2-4 directing groups called and the exception and what are 3 directing groups called
They are activating groups except the halogens They are a deactivating groups
74
What are the 2 and 4 directing groups
-NH2 -OH
75
What are the 3 directing groups
NO2
76
Why are directing groups useful
As they can be used to our advantage when planning organic synthesis
77
What is required when multiple electrophilic substitutions take place
When one or more electrophilic substitutions take place you have to consider the order in which the reactions are carried out to ensure the correct sub-pattern is formed Ensuring everything is in the correct position
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