F324 - Module 1 Flashcards
What is an aromatic compound?
Any compound which contains a benzene ring is aromatic.
State some uses of benzene
Phenol
Detergents
Explosives
Dyes
What is the kekule structure of benzene
A 6 membered carbon ring with alternating double and single bonds.
What are arenes?
Aromatic hydrocarbons containing one or more benzene rings.
Evaluate the evidence for a delocalised model of benzene
Kekulé’s structure failed to explain the low reactivity of benzene. With double bonds, benzene would be expected to react like an Alkene and undergo electrophilic addition eg: de colourise bromine water. This is not the case.
1) bond lengths
Kekulé presented a symmetrical structure. However, C=C and C-C bond have different lengths. X-Ray imaging found that all the bonds in the benzene ring are 0.134nm( bond length between the single and double bond lengths)
2) hydrogenation enthalpy
The hydrogenation enthalpy of cyclohexene is -120 kJ/mol. Therefore , you would expect the hydrogenation enthalpy of benzene to be -360 kJ/mol. However the actual value is much less exothermic , suggesting that the benzene structure is more stable than originally thought.
Describe the structure of benzene
Benzene is a cyclic hydrocarbon with the formula C6H6. The carbon are bonded to two carbon atoms and a hydrogen atom. The shape around each carbon atom is trigonal planar and the bond angle is 120 degrees. The carbon ring is hexagonal and planar.
Benzene has a delocalised structure. The fourth p electron is not bonded to anything. The p orbitals there overlap sideways above an below the ring. This forms a ring of electron density and a system of pi bonds. The electrons are delocalised over the whole ring.
Explain why benzene does not take part in addition reactions.
In an addition reaction, electrons from the delocalised system would need to bond with the atoms being added. The product would therefore be less stable as the delocalisation is lost. This is energetically unfavourable.
Benzene will therefore take part in substitution reactions, which replaces one the H atoms in the ring. This means the delocalisation and stability of the carbon ring is retained.
What are the conditions for the nitration of benzene?
Electrophilic substitution
50 degrees
Concentrated HNO3
Concentrated H2SO4 catalyst
To form nitrobenzene and water
What is required for halogenation of benzene ?
A halogen carrier : AlCl3 or FeBr3
Explain the relative resistance to bromination of benzene compared with Alkenes
Cyclohexene will behave like a typical Alkene. Bromine will therefore react more readily with cyclohexene because electrons in the pi bond are localised. The pi bonds induce a stinger dipole in the Br2 molecule.
In benzene , the electrons are delocalised and therefore electron density of benzen is lower. The halogen cannot be polarised sufficiently and the electrophile cannot be generated. This makes benzene resistant to reactions with non polar halogens.
A halogen carrier is therefore needed to generate the electrophile Br+.
What are phenols?
A group of organic compounds in which a hydroxyl group is attached directly to the benzene ring.
Why is phenol soluble in water ?
The hydroxyl group forms hydrogen bonds with water molecules.
Why are phenols less soluble than alcohols?
The presence of the benzene ring which cannot take part in hydrogen bonding makes phenols only slightly soluble. Alcohols are aliphatic and are therefore more soluble.
What are some uses of phenol?
Antiseptic
Mouthwash
Detergent
Paint
What is the reaction of phenol with bromine?
Electrophilic substitution. No halogen carrier is required and the reaction takes place at room temperature.
The bromine is decolorised and a white ppt of 2,4,6 tribromophenol is formed.
