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58 Cards in this Set
- Front
- Back
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Where do arenes occur naturally?
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materials such as crude oil, coal and can be produced in refineries from crude oil
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What is a benzene derivative?
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A benzene ring where one of the hydrogen atoms has been replaced by an atom or group of atoms
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What can benzene be used for?
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Synthesis of other aromatic materials, such as ethylbenzene, phenol and styrene. Small quantities can be used to make materials such as detergents, explosives, pharmaceuticals or dyestuffs
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What are the properties of benzene?
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Colourless liquid
Sweet odour Highly flammable |
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List two natural sources of aromatic compounds
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volcanoes
forest fires |
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Comment on the suggested linear structure of benzene, with several double bonds
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Benzene was rather unreactive for a molecule that was considered to be highly unsaturated; it did not take part in many reactions of alkenes.
Therefore this suggested structure was unlikely to be correct. |
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What was Kekulé's proposed structure of benzene?
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A six-membered carbon ring with alternating single and double bonds.
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What are the problems with Kekulé's proposed structure of benzene?
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It failed to explain the chemical and physical properties of benzene fully, such as:
low reactivity carbon-carbon bond lengths hydrogenation |
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Explain the low reactivity of Kekulé's proposed structure of benzene
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If C=C double bonds were present, then benzene would react in a similar way to alkenes.
Each C=C double bond would be expected to decolourise bromine water. This does not happen, nor does benzene take part in other electrophilic addition reactions expected from C=C double bonds in alkenes. |
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How did Kekulé account for the low reactivity in his proposed structure of benzene?
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He suggested that benzene existed in two different forms, differing only by the positions of the C=C double bonds. These forms are in such a rapid equilibrium that approaching bromine molecules could not be attracted to a double bond before the structure changed.
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Explain the problem with the carbon-carbon bond lengths in Kekulé's proposed structure of benzene
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The Kekulé structure of alternating single and double bonds was represented a symmetrical molecule. However, C-C bonds and C=C double bonds have different lengths.
X-ray studies revealed that the carbon-carbon bond lengths in benzene all had the same length, 0.139 nm. This is between the C-C bond length of 0.153 nm, and the C=C double bond length of 0.134 nm. This was important evidence to show that Kekulé's proposed structure of benzene was incorrect. |
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Explain the problem with the hydrogenation of Kekulé's proposed structure of benzene
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When cyclohexene reacts with H, the enthalpy change of hydrogenation is -120 kJ/mol.
Therefore, for cyclohexa-1,3,5-triene, Kekulé's proposed structure of benzene, the enthalpy change of hydrogenation is expected to be -320 kJ/mol as there are 3 double bonds. However, the actual enthalpy change of hydrogenation of benzene is -208 kJ/mol. Therefore the actual structure of benzene has less energy than Kekulé's proposed structure and is 152 kJ/mol more stable. This is known as the resonance energy. |
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Benzene does not, under normal conditions (i.e without the presence of a halogen carrier):
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decolourise bromine water
react with strong acids, such as HCl react with halogens: chlorine, bromine or iodine |
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Why does benzene not undergo addition reactions?
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in an addition reaction, electrons from the delocalised system would need to bond to the atom or group of atoms being added.
This would result in a product less stable than benzene, and is therefore not energetically favourable. Addition reaction would disrupt the delocalisation of the benzene structure |
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Why does benzene take part in substitution reactions?
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The organic product retains delocalisation, and hence the stability of the benzene ring
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Exposure to benzene can cause:
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cancer in humans. Continual exposure can cause anaemia or leukaemia (carcinogen).
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Who was credited for the determination for the structure of the benzene?
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Kathleen Lonsdale
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Who discovered the ring structure of benzene?
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Friedrich August Kekulé von Stradonitz
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How is the nitryl cation formed, in the preparation of nitration of benzene?
Include intermediate reactions, and a net reaction |
Sulfuric acid dissociates, forming HSO4- + H+
Nitric acid is protonated, forming H2NO3+ . This dehydrates, forming H2O (water) and the nitryl cation (NO2+). Net reaction: H2SO4 + HNO3 --> HSO4- + NO2+ + H2O |
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After the nitryl cation has been formed, what reaction takes place in the nitration of benzene?
What type of reaction is this? |
benzene + NO2+ --> nitrobenzene + H+
This is an electrophilic substitution reaction. |
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After benzene has been nitrated, what happens with the extra proton (H+)?
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Reacts with the hydrosulphate ion formed in the production of the nitryl cation.
HSO4- + H+ --> H2SO4 |
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What is the overall net reaction of the nitration of benzene?
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Benzene + Nitric Acid --> Nitrobenzene + Water
C6H6 + HNO3 --> C6H5NO2 + H2O |
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Name the reagents and conditions in the nitration of benzene
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concentrated nitric and concentrated sulfuric acid (catalyst)
Heated between 50-55 degrees celsius. |
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What is a consequence of excessive heating in the nitration of benzene?
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polynitration may occur
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Outline the process of nitration of benzene
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1. Nitrating mixture is prepared in a pear shaped, or round bottomed flask. Concentrated nitric and concentrated sulfuric acid are mixed together carefully while cooling the beaker of cold water.
2. Benzene is then added carefully to the nitrating mixture, keeping the temperature below 50 degrees celsius. 3. After all the benzene has been added, a reflux condenser is fitted to the flask, and the mixture is heated between 50-55 degrees celsius in a water bath. |
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What are the properties of nitrobenzene?
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Pale yellow liquid
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What are the uses of nitrobenzene?
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Starting material in the preparation of dyes, pesticides and pharmaceuticals (such as paracetamol)
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Comment on the nitration of methylbenzene, comparing it to the nitration of benzene
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Nitration of methylbenzene is 25 times faster than the nitration of benzene.
Produces 2,4,6-trinitromethylbenzene, also known as TNT (trinitrotoluene) |
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Comment on the halogenation of benzene. What is required for halogenation, and what type of reaction is it?
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For halogenation of benzene, it needs to be boiled with the halogen and exposed to UV light. Alternatively, a halogen carrier catalyst is required, such as AlCl3, FeCl3, AlBr3, FeBr3 or Fe.
This is an electrophilic substitution reaction |
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Show the formation of a suitable electrophile in the preparation for the halogenation of benzene
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Cl2 + FeCl3 --> Cl+ + FeCl4-
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After a suitable electrophile has been formed, show the step for the chlorination of benzene
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C6H6 + Cl+ --> C6H5Cl + H+
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After chlorobenzene has been formed, write an equation to show what happens with the proton (H+).
Comment on this result. |
FeCl4- + Cl+ --> FeCl3 + HCl
FeCl3 has been regenerated, therefore it is a catalyst. |
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What is the overall net reaction of the halogenation of benzene (using chlorine)?
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Benzene + Chlorine --> Chlorobenzene + Hydrogen Chloride
C6H6 + Cl2 --> C6H5Cl + HCl |
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What can chlorobenzene be used for?
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Used in solvents and in the production of pesticides
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What can bromobenzene be used for?
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Used in preparation of pharmaceuticals
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Outline how benzene undergoes electrophilic substitution
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1. Electron dense ring attracts an electrophile.
2. The electrophile accepts a pair of pi-electrons from the delocalised ring to form a covalent bond. 3. An intermediate forms, containing both the electrophile and the hydrogen atom being substituted. The delocalised pi-electron cloud has been disrupted and the intermediate is less stable than benzene. 4. The unstable intermediate rapidly loses a proton (H+). The delocalised ring of electrons reforms and stability is restored. |
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Why is a halogen carrier needed in the halogenation of bromine?
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Benzene is too stable to react with halogens. There is not enough electron density in the benzene ring to polarise the halogen molecule.
Therefore, a much more powerful electrophile is needed, such as a halide ion. This can be formed in the presence of a halogen carrier. |
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What do you observe if bromine is added to cyclohexene?
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Solution changes colour from orange to colourless
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Outline the process of the bromination of cyclohexene
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1. pi-bonds contain localised electrons sited above and below the two carbon atoms in the double bond. This produces a region of high electron density.
2. When bromine approaches cyclohexene, the electrons in the pi-bond repel the electrons in the Br-Br bond, inducing a dipole in the Br2 molecule. The bromine molecule has now been polarised. 3. The pi-electron pair from the double bond is attracted to the slightly positive bromine atom, causing the double bond to break. 4. A new bond forms between one of the carbon atoms and a bromine atom, forming a positively charged carbocation. The bond between the two Br atoms breaks by heterolytic fission, forming a bromide ion (Br-). 5. The bromide ion is attracted towards the intermediate carbocation, forming a covalent bond. The final product, 1,2-dibromocyclohexane has now been formed. |
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What do you observe if bromine is added to benzene?
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No reaction occurs, bromine mixture remains orange.
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What do you observe if bromine is added to benzene, in the presence of a suitable halogen carrier catalyst, such as Fe, FeBr3 or AlCl3?
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Solution changes colour from orange to colourless.
White fumes of hydrogen bromide gas are observed. |
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Why does bromine only react with benzene in the presence of a halogen carrier catalyst?
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1. Benzene has delocalised pi-electrons spread over all 6 carbon atoms in the ring structure.
2. Compared to alkenes, benzene has a low electron density. 3. When non-polar molecules, such as bromine approach benzene, there is insufficient pi-electron density above and below any two carbon atoms to cause the necessary polarisation of the bromine molecule. 4. A halogen carrier is needed to produce a more powerful electrophile, Br+. The greater charge on the Br+ ion is able to attract the pi-electrons from benzene so that the reaction can take place. |
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What are the properties of polychlorinated biphenyls?
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fire resistant
relatively unreactive poor conductors of electricity |
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What are the uses of polychlorinated biphenyls?
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coolants
insulating fluids for transformers and capacitors plasticisers in cement and paint carbonless copy paper |
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Why are PCBs harmful?
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High fat solubility, allowing them to be stored in the body.
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What is an aromatic alcohol?
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A benzene ring with an -OH group attached to a side chain.
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What is a phenol?
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A benzene ring with an -OH group attached DIRECTLY to the benzene ring.
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What are the properties of phenol?
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Solid at RTP
Slightly soluble in water |
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Comment on the solubility of phenol in water
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-OH group forms hydrogen bonds with water molecules.
However, presence of a benzene ring makes phenol less soluble in water than alcohols |
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How is the phenoxide ion produced?
Include an equation in your answer |
Phenol is dissolved in water, forming a weak acidic solution by losing an H+ from the -OH group.
Phenol + aq <-> Phenoxide ion + proton C6H5OH + aq <-> C6H5O- + H+ |
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After the phenoxide ion has been produced, how is sodium phenoxide produced from sodium hydroxide?
Include an equation to show the net reaction. |
C6H5O- + H+ + NaOH --> C6H5O-Na+ + H2O
Net reaction: C6H5OH + NaOH --> C6H5O-Na+ + H2O |
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After the phenoxide ion has been produced, how is sodium phenoxide produced from sodium?
Include an equation to show the net reaction. |
2C6H5O- + 2H+ + 2Na --> 2C6H5O-Na+ + H2
Net reaction: 2C6H5OH + 2Na --> 2C6H5O-Na+ + H2 |
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What can be observed when reacting phenol with a metal?
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Metal effervesces, producing hydrogen gas
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What is Joseph Lister famous for?
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The first person to treat wounds with dressing coated in carbolic acid (phenol). He insisted that surgeons washed their hands and sterilised their instruments with a solution of 5% phenol before carrying out operations.
It was also used on the the whole operating theatre, killing bacteria before wounds could be infected. This increased the success rate of amputations, however phenol was found to be caustic and made some surgeons very ill. Lister then found that boric acid was a better and safer antiseptic. |
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Comment on the bromination of phenol. What do you observe?
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Phenol undergoes electrophilic substitution with bromine at room temperature without the need of a halogen carrier.
When the bromine is added to an aqueous solution of phenol, the orange bromine colour disappears and a white precipitate of 2,4,6-tribromophenol is formed. |
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What is a test for phenol?
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The bromination of phenol
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Comment on the relative ease of bromination of phenol compared to benzene
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1. Increased reactivity occurs because a lone pair of electrons occupying a p-orbital on the oxygen atom in the phenol group is drawn into the benzene ring.
2. This creates a high electron density in the ring structure and the ring is activated. 3. Increased electron density is sufficient to polarise incoming bromine molecules, which are then attracted more strongly to the ring structure than in benzene. |
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What are some uses of phenols?
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alkyl phenols:
- found in surfactants and detergents chlorophenols: - found in antiseptics and disinfectants such as TCP and dettol salicyclic acid: - preparation of aspirin and other pharmaceuticals bisphenol: - production of epoxy resins for paints |
