C14 Arenes Flashcards
What are arenes?
Arenes, also referred to as aromatic hydrocarbons are hydrocarbons based on the benzene ring as a structural ring.
What are aromatic compounds?
‘aromatic’ indicates certain chemical characteristics rather than odorous properties. An aromatic compound has special stability. They are compounds possessing the ring structure or benzene or other molecular structures that resemble benzene in structure and chemical behaviour.
Describe the nomenclature of monosubstituted, disubstituted, polysubstituted benzene derivates.
Pg 2 to 4 of notes
The benzene ring is treated as a substituent and called a phenyl group when:
- the substituent on the benzene ring has more than 6/at least 7 carbon atoms
- the highest priority functional group is not a substituent on the benzene ring
Note: Phenyl and substituted phenyl groups are called aryl groups.
Describe the physical properties of benzene.
- Benzene is a colourless liquid with a characteristic odour.
- It is non-polar, insoluble in water and less dense than water.
- It is soluble in all organic solvents and is a good solvent for organic compounds.
- It freezes at 5.5 and boils at 80 degree celsius.
- Like all aromatic compounds, it burns with a smoky and luminous flame, owing to its relatively high carbon content (tends to result in incomplete combustion).
Describe and explain the resonance structure of benzene.
To minimise electronic repulsion, the three regions of electron density about each carbon atom in a benzene molecule (C6H6) adopt a trigonal planar molecule. Hence, all bond angles in benzene are 120 degrees and it is a planar molecule.
Each of the carbon atoms in benzene is sp2 hybridised, comprising of three sp2 hybrid orbitals that are arranged in a trigonal planar manner and one unhybridised p orbital which is occupied by a single electron. Each singly-filled p-orbital overlaps side-on with the adjacent p orbital on either side.
This continuous side-on overlap of the p-orbitals results in a delocalised pi electron cloud above and below the plane of the ring. Thus, resonance is present.
Explain how various experimental evidence shows resonance in benzene.
As a result of the aromatic character of benzene,
(a) All carbon-carbon bonds in benzene are equal in length and intermediate in length between that of the C-C and C=C bond lengths.
(b) The enthalpy change of hydrogenation of benzene is less exothermic than expected.
(c) Benzene undergoes substitution reactions rather than addition reactions to retain its aromaticity.
Explain how experimental evidence of the carbon-carbon bond lengths shows resonance in benzene.
(a) Carbon-carbon bond lengths and energies
If benzene were to have two kinds of carbon bonds i.e. C-C and C=C bonds, the carbon-carbon bonds in benzene should be of different lengths. However, X-ray diffraction studies show that all the carbon-carbon bonds in benzene are identical and equal in length. The measured carbon bond length in benzene is intermediate between the length of the C-C bond and that of the C=C bond. This agrees with the description of benzene as a resonance hybrid of two Kekule structures. It also indicates that the carbon-carbon bonds have a partial double bond character.
Explain how experimental evidence of enthalpy change of hydrogenation of benzene shows resonance in benzene.
(b) Enthalpy change of hydrogenation of benzene
The enthalpy change of hydrogenation (reduction by H2) of benzene is less exothermic than the expected value (enthalpy change of hydrogenation of cyclohexa-1,3,5-triene structure). This shows that benzenes do not contain localised pi bonds. Instead, it has a structure in which six pi electrons are delocalised and the delocalisation confers extra stability on benzene - reflected by the resonance energy (difference between the expected and actual enthalpy change of hydrogenation of benzene)
Explain how experimental evidence of the reactivity of benzene towards addition reactions shows resonance in benzene.
(c) Benzene undergoes substitution reactions instead of addition reactions
If benzene were to have the cyclohexa-1,3,5-triene structure, it is expected to undergo addition reactions which are characteristic of alkenes. However, under conditions that would cause an alkene to under rapid addition, benzene does not react or react very slowly. This is because, if benzene undergoes addition reactions, its overall aromatic character is destroyed. The extra stability associated with the delocalisation of the six pi electrons is lost. Hence, the majority of reactions that benzene undergoes involve substitution in the ring. The delocalisation of the six pi electrons in the continuously overlapping p-orbitals, hence its aromatic character, is retained.
Explain why benzene undergoes electrophilic substitution reactions.
Benzene undergoes substitution reactions instead of addition reactions because, if benzene undergoes addition reactions, its overall aromatic character is destroyed. The extra stability associated with the delocalisation of the six pi electrons is lost. Hence, the majority of reactions that benzene undergoes involve substitution in the ring. The delocalisation of the six pi electrons in the continuously overlapping p-orbitals, hence its aromatic character, is retained.
The availability of six pi electrons in benzene makes it electron-rich. Hence, benzene attracts electrophiles and undergoes electrophilic substitution reactions.
Name the four electrophilic substitution reactions that benzene (or other related aromatic compounds) undergo.
When asked for the name of the mechanism, do not give the names below. Instead, only state ‘electrophilic substitution’.
1) Nitration
2) Halogenation (e.g. chlorination)
3) Friedel-Crafts alkylation
4) Friedel-Crafts acylation
