Fichas
Aromatic compounds
Fichas del conjunto (30)
- 1Who proposed the structure of benzene?•August Kekule
- 2Describe the structure of benzene proposed by Kekule?•Molecular formula of C6H6.
•Hexagonal shape.
•Each C is bonded with a sigma bond to the other and pi bonds are formed alternatively. - 3Why was Kekule's approach unsuccessful?•The carbon double and sigma bonds had different lengths. Therefore, Kekule's structure would not be a regular hexagon.
- 4How was it proved that Benzene has a regular hexagonal shape?Through x-ray diffraction.
It was found that the bond lengths were equal and benzene has a planar shape. - 5In terms of energy, why was Kekule wrong?•A broken pi bond released -120kJmol of energy.
•Since benzene contains 3 pi bonds, it should release -360kJmol of energy.
•However, in reality, it only release only -208kJmol - 6How can benzene for form a nitrobenzene?•H2SO4 with HNO3
•Heated under reflux at 55 - 7What are the two friedel craft reactions?•Alkylation
•Acylation - 8How does Alkylation and Acylation differ?•Alkylation requires 50 degrees and reflux.
•Acylation does not. - 9In terms of reaction types why was Kekule wrong?•According to Kekule, Benzene should undergo addition reactions similar to that of ethene.
However, benzene underwent substitution and it did not decolorize bromine. - 10In terms of IR spectra, why was Kekule wrong?•The Carbon carbon double bond shows differences in aliphatic and aromatic compounds.
- 11How did Kekule overcome the problems?•He proposed the idea of resonance structures.
- 12What is a resonance structure?•The actual structure of Benzene lies between two representations.
•Each C-C bond is neither a single or double.
•Instead, it is an intermediate. - 13What can be said about the pi bonds in Kekule's resonance structures?•There were 3 pi bonds which were delocalized between the C atoms.
- 14What issues did Kekule's new structure solve?•The shape and bond length of Benzene.
- 15Describe the molecular orbital explanation of the structure Benzene?•Each of the six atoms is sp2 hybridized.
•Each carbon has 3 sp2 orbitals at 120 degrees.
•Two of the sp2 orbitals are used to bond to two other carbons
•The 3rd is used to form a sigma bond. - 16How does the 3rd sp2 orbital form a sigma bond?•The sp2 orbital overlaps with the s orbital of the Hydrogen atom.
- 17What is significant about the hybridization within the benzene molecule?•It leads to the formation of sigma bonds
•In term, the planar hexogonal structure is formed. - 18What is the importance of the lateral overlapping of the p orbitals in a benzene molecule?•A continous ring shaped, cloud of pi electrons forms.
•The cloud is above and below the benzene ring. - 19What effect does the pi electron cloud have on the naming of benzene?•Benzene cant be named as 1,3, 5 cyclohexatriene as the pi electron cloud is delocalized.
- 20In terms of the bonding of benzene, why is the enthalpy of hydrogenation -280 instead of the expected 360?•Due to the pi electron cloud.
•No pi bonds are formed.
•Instead there is overlapping of the six pi orbitals
•The overlapping causes delocalization which causes benzene to be more stable and more energy to break. - 21Physical properties of benzene?•Non-polar liquid.
•Low boiling point due to london forces only.
•Immiscible in water. - 22Why does benzene undergo electrophilic substitution reactions?•High electron density attracts electrophiles.
•The benzene ring is thermodynamically stable.
•Therefore, it can only undergo addition under forced conditions (uv light and cata) - 23What are the chemical properties of Phenol?•More acidic than alcohols.
•The anion formed (phenate) is quite stable.
•Since its acidic, it cant decompose carbonates or bicarbonates. - 24Physical properties of Phenol?•Solid at rtp
•Strong antiseptic smell
•Forms water with oil
•Burns the skin and highly toxic - 25How does a group attached to benzene influence future electrophilic substitution?1) Rate of the reaction
2) Position of the second electrophile - 26What is an activating group?•A group which donates electrons to the benzene ring and makes it more delocalised.
As a result, electrophiles will attack faster.
•An atom directly attached to benzene ring has lone pair - 27What is a deactivating group?•A group which pulls electrons to the benzene ring and makes it less delocalised.
As a result, electrophiles will attack slower.
•An atom indirectly attached to benzene ring has lone pair - 28Based on the first subsitutent, what are the different positions of the benzene ring?•2, 6 is Ortho (activating)
•3, 5 is Meta (deactivating)
•4 is Para (activating) - 29What are the 4 mechanisms of aromatic substitution?•Bromination
•Alkylation
•Acylation
•Nitration - 30What are the stages of aromatic substitution?1) formation of electrophile and base
2) electrophile reacts with benzene to form a carbocation intermediate
3) base pulls of a proton from carbocation intermediate to form product.
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- A-Level Chemistry Fichas sobre Aromatic compounds, creado por Jio Jenassi el 18/02/2016.
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- Jio Jenassi
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