6.1 Flashcards
6.1.1 a) Compare Kekulé’s model of benzene with the subsequent, accepted model of benzene.
Benzene is an aromatic hydrocarbon with a molecular formula of C6H6 - it is arranged as a planar, hexagonal ring. The Kekulé model of benzene proposes a 6 carbon ring, with alternating single and double bonds. However, this has since changed: it is now proposed that the p-orbitals of each carbon overlap to create a delocalised π-system. Each of the six carbon atoms donates one electron from its p-orbital, forming a ring of delocalised electrons above and below the plane of the molecule.
6.1.1 b) Outline the evidence for a delocalised, rather than Kekulé, model of benzene.
- In Kekulé’s model of benzene, the alternating single and double bonds suggest three shorter C=C bonds (0.135nm), and three longer C-C bonds (0.147nm). However, x-ray diffraction techniques reveal that the carbon-carbon bonds in benzene are of the same length (0.140nm - inbetween a C-C bond and C=C bond).
- Hydrogenation is the addition of hydrogen to an unsaturated chemical. The enthalpy change of hydrogenation of cyclohexene (containing one C=C double bond) is -120 kJ/mol. The enthalpy of hydrogenation of Kekulé’s model of benzene (containing three C=C double bonds) should therefore be -360kJ/mol). However the experimental enthalpy of benzene is -208 kJ/mol. Benzene is therefore more energetically stable than Kekulé’s model suggests (this is a result of the stable delocalised ring of electrons, which requires more energy to disrupt).
- Alkenes readily undergo addition reactions, a result of their C=C double bond. Using Kekulé’s model of benzene, the three C=C double bonds should similarly react. However, benzene is more prone to substitution reactions. It doesn’t, for example, react with bromine water (no colour change is observed), whereas an alkene would become saturated (decolourising the orange bromine water).
6.1.1 c) Explain how the rules of nomenclature can be employed for systematically naming substitued aromatic compounds.
A benzene derivative (otherwise known as an arene or an aromatic compound) is a benzene ring that has undergone a substitution reaction. The prefix depends on the substitute (e.g. chloro, bromo, nitro or methyl), and in the case of multiple substitutions, the carbons they are attached to should be numbered, with the prefixes written in alphabetical order (adding di, tri or tetra where necessary). When the longest chain of carbon atoms is the aromatic ring, the stem is benzene (e.g 1-bromo-4-methylbenzene). Phenols are a class of aromatic compounds where a hydroxyl group is directly attached to the aromatic ring - in this case the stem is phenol, not benzene (e.g. 2-ethylphenol). But if the hydroxyl group is attached to an alkyl chain on the aromatic ring, then the compound is no longer a phenol derivative - it would be described as an aromatic alcohol (e.g. phenylenthanol). In fact, when a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, then benzene is considered to be a substituent and the prefix is phenyl (e.g. phenylethanone, 2-phenyloctane). The exceptions are benzoic acid, phenylamine, and benzaldeyde.
6.1.1 d) i) Outline the mechanism for nitration of benzene.
Nitration is an electrophilic substitution reaction, where one hydrogen is exchanged for a nitro group (-NO2).
C6H6 + HNO3 → C6H5NO2 + H2O
The reagent is concentrated nitric acid, HNO3, with concentrated sulfuric acid, H2SO4, acting as a catalyst:
HNO3 + H2SO4 → NO2+ + HSO4- + H2O
The nitronium ion, NO2+, is the electrophile.
1. The nitronium ion accepts a pair of π-electrons from the delocalised ring and forms a dative covalent bond with benzene (the arrow goes to the electrophile, from the benzene ring).
2. A reactive intermediate is formed (with both the hydrogen and the nitro group bonded to a carbon). The delocalised electrons are disrupted (represented by the gap in the benzene ring, which is now positive: + ). The hydrogen therefore donates a pair of electrons to the disrupted delocalised electrons (the arrow goes from the H-C bond to the gap in the benzene ring).
3. A H+ ion is released, and a stable product (C6H5NO2) is formed. The sulfuric acid is also regenerated:
H+ + HSO4- → H2SO4
To prevent further substitution (so that mononitration occurs), temperature must be kept below 55
6.1.1 d) ii) Outline the mechanism for halogenation of benzene.
Benzene does not directly react with halogens (the delocalised ring is too stable). Halogen carriers (such as iron, iron halides or aluminium halides) are used to produce halide ions. For example, bromine can react with iron(III) bromide to form a positive bromine ion:
Br2 + FeBr3 → Br+ + FeBr4-
The bromonium ion, Br+, acts as the electrophile.
1. The bromonium ion accepts a pair of π-electrons from the delocalised ring and forms a dative covalent bond with benzene (the arrow goes to the electrophile, from the benzene ring).
2. A reactive intermediate is formed (with both the hydrogen and the bromine bonded to a carbon). The delocalised electrons are disrupted (represented by the gap in the benzene ring, which is now positive: + ). The hydrogen theredore donates a pair of electrons to the disrupted delocalised electrons (the arrow goes from the H-C bond to the gap in the benzene ring).
3. A H+ ion is released, and a stable product (C6H5Br) is formed. The halogen carrier, acting as a catalyst, is regenerated:
FeBr4- + H+ → FeBr3 + HBr
6.1.1 iii) Outline the mechanism for Friedal-Crafts alkylation.
A Friedal-Crafts alkylation enables any alkyl group to be attached to a benzene ring, using a haloalkane and a halogen carrier:
R-Cl + AlCl3 → R+ + AlCl4-
e.g. CH3Cl + AlCl3 → +CH3 + AlCl4-
The reactive carbocation on the alkyl group, +CH3, allows it to act as an electrophile.
1. The carbocation accepts a pair of π-electrons from the delocalised ring and forms a dative covalent bond with benzene (the arrow goes to the electrophile, from the benzene ring).
2. A reactive intermediate is formed (with both the hydrogen and the alkyl group bonded to a carbon). The delocalised electrons are disrupted (represented by the gap in the benzene ring, which is now positive: + ). The hydrogen therefore donates a pair of electrons to the disrupted delocalised electrons (the arrow goes from the H-C bond to the gap in the benzene ring).
3. A H+ ion is released, and a stable product (C6H5CH3/C7H8 - methylbenzene) is formed. The halogen carrier, acting as a catalyst, is regenerated:
AlCl4- + H+ → AlCl3 + HCl
[Note: the alkyl chain donates electrons to the aromatic ring, increasing its reactivity and making it more susceptible to electrophilic attack. A mixture of products is therefore formed, with multiple substitution reactions occurring - this is because each successive substitution makes the delocalised π-electrons more nucleophilic, and therefore more likely to react with an electrophile]
6.1.1 iii) Outline the mechanism for Friedal-Crafts acylation.
A Friedal-Crafts acylation allows any acyl group to attach to a benzene ring, using an acyl chloride and a halogen carrier:
RCOCl + FeCl3 → RC+O + FeCl4-
e.g. CH3COCl + FeCl3 → CH3C+O + FeCl4-
The reactive carbocation allows the acyl group,
CH3C+O, to act as an electrophile.
1. The carbocation accepts a pair of π-electrons from the delocalised ring and forms a dative covalent bond with benzene (the arrow goes to the electrophile, from the benzene ring).
2. A reactive intermediate is formed (with both the hydrogen and the acyl group bonded to a carbon). The delocalised electrons are disrupted (represented by the gap in the benzene ring, which is now positive: + ). The hydrogen therefore donates a pair of electrons to the disrupted delocalised electrons (the arrow goes from the H-C bond to the gap in the benzene ring).
3. A H+ ion is released, and a stable product (C6H5COCH3 - phenylethanone) is formed. The halogen carrier, a catalyst, is regenerated:
FeCl4- + H+ → FeCl3 + HCl
The product usually formed is a phenylketone, however if HCOCl is used, an aldehyde is produced.
[Only one substitution can occur: the carbonyl group withdraws electrons from the aromatic ring, so a less reactive ketone is made]
6.1.1 f) Explain the relative resistance to bromination of benzene when compared with alkenes.
Alkenes readily undergo addition reactions, a result of their C=C double bond. For example, they become saturated when added to bromine water (decolourising the orange solution). In this electrophilic addition reaction, the localised electron density of the π-bond is high enough to induce a dipole in the non-polar bromine molecule. This allows bromine to be added across the double bond.
In contrast, benzene is more prone to substitution reactions. It doesn’t, for example react with bromine water (no colour change is observed). This is because the delocalised electron density of the π-system in benzene is so low, that it can’t polarise a bromine molecule. This prevents any reaction from taking place.
6.1.1 f) Explain why phenol is considered a weak acid.
Phenol is considered to be a weak acid for two reasons:
1) It is less soluble in water than alcohols (this is due to the presence of the non-polar benzene ring). When dissolved in water, it partially dissociates (forming the phenoxide ion and releasing a proton):
C6H5OH + H2O ⇄ H3O+ + C6H5O-
or C6H5OH ⇄ H+ + C6H5O-
2) While more acidic than alcohols, phenol is less acidic than carboxylic acids. i.e. it reacts with strong bases to form a salt and water, but it doesn’t react with carbonates.
e.g. Consider the neutralisation reaction between phenol and sodium hydroxide - it forms the salt sodium phenoxide, C6H5O-Na+:
C6H5OH + NaOH → C6H5O-Na+ + H2O
In contrast, it wouldn’t react with sodium carbonate (a weak base).
6.1.1 i) i) What product is formed in the reaction between phenol and bromine?
Phenol is more reactive than benzene: it undergoes an electrophilic substitution reaction, decolourising the orange bromine water and forming a white precipitate of 2,4,6-tribromophenol.:
C6H5OH + 3Br2 → C6H2Br3OH + 3HBr
This triple substitution can be carried out at room temperature, without a halogen carrier (this is because phenol is able to induce a dipole in the non-polar bromine molecule).
6.1.1 i) ii) What product is formed in the reaction between phenol and nitric acid?
Phenol will undergo a single substiution with diute nitric acid at room temperature. This reaction forms a mixture of 2-nitrophenol and 4-nitrophenol:
C6H5OH + HNO3 → C6H4(NO2)OH + H2O
Unlike nitration wih benzene, this reaction does not require concentrated nitric acid or a sulfuric acid catalyst. If concentrated nitric acid is used, a triple substitution reaction occurs, forming 2,4,6-trinitrophenol.
6.1.1 j) Explain the relative ease of electrophilic substitution in phenol, when compared with benzene.
The hydroxyl group means that phenol is more likely to undergo electrophilic substitution than benzene. A lone pair of electrons from the oxygen p-orbital in phenol is donated to the π-system. They are said to be partially delocalised. This increases the electron density of the aromatic ring, making phenol more susceptible to electrophilic attack, and enabling it to induce dipoles in non-polar molecules.
6.1.1 k) How can functional groups affect the position of substitution on benzene derivatives?
If you have a substituted benzene ring, such as phenol or nitrobenzene, the functional group can change the electron density at certain carbon atoms - this changes the rate of substitution, and makes it more or less likely to react.
1) Electron-donating groups include -OH and -NH2
- they have electrons in orbitals that overlap with the delocalised ring, and donate a lone pair of electrons to the π-system. This increases the electron density at carbons 2-, 4- and 6-, so electrophiles are more likely to react at these positions [the 2- and 4- directing effect].
2) Electron-withdrawing groups include -NO2
- there are no orbitals that overlap with the delocalised ring; instead it’s electronegative, so electrons are withdrawn from the π-system. In particular, electrons are withdrawn from carbons 2-, 4- and 6- (resulting in a decrease in electron density), so electrophiles are unlikely to react at these positions. This has the effect of directing electrophilic substitution to the 3- and 5- positions [the 3- directing effect].
6.1.1 l) Consider the importance of the directing effect in organic synthesis.
The directing effect allows you to predict the substitution products of aromatic compounds. For organic synthesis, it is important that a reaction pathway can be designed to maximise the desired product.
6.1.2 a) Explain the difference between an aldehyde and a ketone.
Aldehydes and ketones are organic compounds that contain the carbonyl functional group, C=O.
- In aldehydes, the carbonyl functional group is found at the end of a carbon chain. The carbon atom in C=O is attached to at least one hydrogen atom.
- In ketones, the carbonyl functional group is joined to two other carbon atoms.
