25.1 - 25.4 Flashcards
What is benzene
- a cyclic, planar molecule
What is the general formula for benzene
- C6H6
Describe the bonding in benzene
- each carbon is bonded to 2 other carbons and 1 hydrogen
- the final lone election is in a p-orbital which sticks out about the and below the planar ring
- the lone electrons in the p-orbitals combine to form a delocalised ring of electrons
What causes all the bond lengths in benzene to be the same
- because of the delocalised electron structures
How is benzene normal drawn
- skeletal formula
What did Kekulè think about the structure of benzene
- that there was alternating double single bonds
What is the stability of benzene and how do we find it
- more stable then Kekulè’s initial compound of benzene ( lower value=> predicted 360kJmol^-1 but it is 208kJmol^-1)
- we cab measure the stability by comparing the enthalpy change of hydrogenation
What is benzene’s stability due to
- the delocalised electron structure
What are arenes
- another word to describe a substance with a benzene ring in it
What is a NH2 group attached to a benzene ring called
- phenylamine
What type of reactions do benzenes undergo
- electrophilic substitution
Complete the sentence:
Benzenes don’t react with bromine water..
- like alkenes meaning that they don’t have a double bond at Kekulé proposed
Complete the sentence:
Once you have added an acyl group….
- the benzene structure is weaker and it makes it easier to modify it further to make useful products
In order to add onto the benzene ring what does the electrophile need to have
- a VERY STRONG positive charge as benzenes don’t react readily
As acyl groups aren’t positive enough what do we use
- halogen carries to act as a catalyst (e.g. AlCL3)
- they provide a much stringer electrophile with a stronger positive charge
What is an acylation reaction between
- AlCl3 (halogen carrier) and the electrophile
What are the products of this reaction and what does this create
- a STRONGER ELECTROPHILE
Draw a mechanism for acylation of benzene after the acyl group has reacted with he halogen carrier
How is nitration of benzene useful
- use it for dyes for clothing
- used for explosives
What are the 3 steps in nitration of benzene
1) generation of the catalyst with H2SO4
2) mechanism
3) regeneration of catalyst
Draw the mechanism for the nitration of benzene
What functional groups do phenols have
- hydroxyl group (-OH)
What is skeletal formula for salicylic acid
Describe the reactivity of phenols compared to benzenes
- phenols are more reactive than benzene due to the electron density in the ring being higher
Why are electrophilic substitutions more likely to happen with phenol than with benzene
- because of the -OH group and orbital overlap
(Oxygen overlaps with delocalised ring structure) - so phenols are more susceptible to attack from electrophiles
Why does the position of functional groups on a benzene ring affect the reactivity with electrophiles
- because the substituted benzene rings distort the electron density in the ring
What do electron withdrawing groups affect
- they affect the substitution reactions of carbon 3 and 5
Where are electronegative groups more likely to withdraw electron density from and what does this mean
- electronegative groups (like NO2) are more likely to withdraw electron density from carbon 2,4 and 6
- this means that electrophiles are more likely to attack carbons 3 and 5
What do electron donating groups affect
- affect substitution on carbons 2,4 and 6
Where are electronegative groups more likely to withdraw electron density from and what does this mean
- donating groups like (NH2 and OH) donate electrons and increase density in the ring
- specifically to carbon 2,4 and 6
- this means electrophiles are more likely to attack carbons 2,4 and 6 and substitution is more likely to happen at these points
When reacting phenol and chloroethane using AlCL3 as a catalyst where is substitution. More likely to happen
- electron donating group so substation is most likely at carbons 2,4 and 6
When reacting nitrobenzene and chloroethane using AlCL3 as a catalyst where is substitution more likely to happen
- electron withdrawing group so most likely to get substitution at carbons 3 and 5
Why are phenols weak acids
- as it dissociates weakly
What do phenols dissociate to form
- phenoxide ion and H+ ion
What does phenol and alkalis form
- a salt and water
Why can phenols react with bromine water
- because they are much more reactive than benzene because of the overlap of the oxygen molecule in -OH and the p orbital
How does phenol react with bromine water
- phenol decolourises bromine water
- producing 2,4,6-tribromophenol (as OH is a electron donating group)and 3HBr
How do phenols react with dilute nitric acid
- produces 2-nitrophenols or 4-nitrophenol
- as OH is an electron donating group
