Flashcards
Chapter 25 - Benzene
Cards in this deck (19)
- 1What is benzene?- An aromatic arene.
- Colourless, sweet smelling, highly flammable liquid.
- A ring of six carbons with each carbon atom joined to two others and a hydrogen atom. - 2What is the Kekulé model for benzene?- Benzene is a ring of six carbons atoms and six hydrogen atoms.
- They atoms are joined in a hexagonal ring.
- Alternate bonds are double (π) bonds. - 3What evidence disproves Kekulé's model of benzene?- Reactivity: If benzene contained double bonds it would decolourised bromine water.
- Bond length: The bond lengths were found to be somewhere between the length of a single bond and a double bond.
- Hydrogenation: The enthalpy change of hydrogenation was significantly lower than expected? - 4What is the new, accepted model for benzene?- Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms.
- Each carbon uses 3 of its available electrons to bond with 2 carbons and 1 hydrogen.
- The other electron is in a p orbital at right angles to the plane of bonded carbons.
- Adjacent p-orbitals overlap to give a ring of electron density. The electrons are said to be delocalised. - 5How does benzene react with nitric acid?- Benzene reacts slowly with nitric acid to form nitrobenzene.
- The reaction is catalysed with sulfuric acid.
- It takes place at 50'C to give a good rate of reaction but avoid further substitution. - 6How does benzene react with halogens?- Halogens only react with benzene when a halogen carrier catalyst is present.
- 7How is benzene alkylated?- A chloroalkane is mixed with benzene and AlCl3 (halogen carrier).
- This gives benzene with an alkyl group onto it.
- It is known as the Friedel-Crafts alkylation. - 8How is benzene acylated?- Benzene reacts with an acyl chloride in the presence of AlCl3.
- Produces an aromatic ketone. - 9Why does benzene not react with halogens without a halogen carrier present but an alkene does?- Benzene has delocalised electrons.
- The electron density around any two carbons is less than that in a C=C bond in an alkene.
- So when a non polar molecule approaches the benzene ring there is insufficient electron density around any two carbons to polarise the bromine molecules. - 10What is a phenol?- An organic chemical containing a hydroxyl group bonded directly onto a benzene ring.
- 11Which is more soluble, phenol or alcohol?- Alcohol.
- 12Why is phenol classed as a weak acid?- When dissolved in water, phenol partially dissociates into the phenoxide ion and a proton.
- 13Phenol + NaOH →- Salt + Water
- Neutralisation reaction - 14Why do bromine and nitric acid react more readily with phenol than with benzene?- The lone pair of electrons from the oxygen/nitrogen p-orbital is donated into the π- system of phenol.
- This increases the electron density of the benzene ring, so electrophiles are attracted more strongly. - 15What groups are activating?- OH
- NH2 - 16What groups are deactivating?- NO2
- 17How can you tell that nitrobenzene is less reactive than benzene?- It reacts very slowly with bromine.
- Requires a halogen carrier and a high temperature. - 18What substituents are 2/4 directing?- NH2/NHR
- OH/OR
- R
- Halogens - 19What substituents are 3 directing?CN
NO2
CHO
COOH/COOR
SO3H
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- A level Chemistry (Component 2) Flashcards on Chapter 25 - Benzene, created by Beth Williams on 11/06/2018.
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- Beth Williams · Jun 2018
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- Jun 11, 2018
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