Topic 18
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Describe bonding and structure of a benzene molecule
3 sigma bonds between each carbon and 2 adjacent carbon and 2 hydrogen The lone electron in the p orbitals combine to form a delocalised ring overlaps sideways to form pi bonds C-C bonds are all the same length Trigonal planar Arrangement of atoms around each carbon
Explain why there is resonance in a benzene ring
The lone electrons in the p orbital combine and overlap and the pi bonds overlap causing resonance. Due to this all the C-C bonds are the same length
What are the evidence for the delocalised model of bonding in benzene instead of kekule
Data from enthalpy of hydrogenation Carbon carbon bond lengths
Explain evidence for benzene being delocalised via enthalpy of hydrogenation
The enthalpy of hydrogenation of benzene(-208KJ) is higher than the expected which was -360KJ This means more energy is needed to break bonds in benzene suggesting more stable due to resonance
What is an arene
Aromatic compound and are a class of organic compounds characterised by having the benzene ring
Why is benzene resistant to bromination and what reactions do arenes undergo
Arenes undergo electrophillic substitution Benzene has a high electron density due to delocalised ring this attracts electrophiles They cannot be brominayed as they are stable as this disrupts the ring of electrons
What are halogen carriers and why are they used in electrophillic substitution reactions of benzene
Benzene is very stable so reactions require a VERY STRONG electrophile. These can be generated via halogen carriers such as AlCl3
Explain alkylation reaction
React an acyl chloride or haloalkane with AlCl3 to create a strong electrophile
Nitration of benzene: generating the electrophile What two acids are used
Equation for Formation of electrophile: (Learn!) HNO3 + 2H2SO4 →NO2+ + 2HSO4+ H3O+ Acid base reaction
Nitration of benzene conditions
NO2+ electrophile by forming it from 2 reactions H2SO4 catalyst Temperature below 55
Why are electrophiles attracted to react with benzene
Benzene has a high electron density as it has a delocalised ring of electrons This is attractive to electrophiles
Does benzene go through electrophillic addition or electrophillic substiution
Electrophillic substitution
What type of reaction is Fields Craft Acylation and Fields Craft Acylation
Electrophillic aromatic substitution
Fields Craft Alkylation reaction conditions,reagents And give an example: Benzene → alkylbenzene
Reagents: Chloroalkane in AlCl3 catalyst Conditions: Heat under reflux Mechanism: Electrophillic substitution
Formation of CH3CH2+ electrophile which will react with benzene to form alkylbenzene
Formation of the electrophile. AlCl3 + CH3CH2Cl → CH3CH2 + AlCl4-
What is the purpose of Fields Craft ALKYLATION
To form an alkyl Arene(alkylbenzene)
What reaction will help form Phenyl Ketone from a benzene
Fields Craft ACYLATION
Conditions reagents of going from benzene to Phenyl Ketone
Acyl Chloride in presence of AlCl3 catalyst Heat under reflux Electrophillic substitution
Formation of electrophile for Fields Craft Acylation
Equation for formation of the electrophile. AlCl3 + CH3COCl → CH3CO+ AlCl4
Explain the reactivity of phenols
In a phenol the lone pair of electrons on the oxygen is delocalised with the electron charge cloud of the arene ring. The delocalised bonding changes the reactivity of the OH group and the arene ring.
Uses of nitrating benzene
Dyes for clothing and explosives
Describe how orbitals overlap to form pi and sigma bonds in a benzene ring
Sigma bond: The s orbitals directly overlap from neighbouring carbon atoms Pi bonds: Perpendicular p orbitals overlap sideways So p orbitals form a delocalised pi system
How to make aspirin Is aspirin an ester
Ethanoic anhydride + salicylic acid —> aspirin + ethanoic acid
Why is ethanoic anhydride used instead of ethanoyl chloride
Less corrosive Cheaper Does not produce HCl gas Does not react vigorously with water