Reactivity And Orientation Flashcards
What effects do substituents already on the ring have?
The substituents determines whether the next electrophilic substitution is easier or harder than the first by making the ring more or less reactive
It also controls the location at where the next substitution occurs
What is an activating group?
These are substituents that make the ring more reactive by donating electrons to the ring
They also stabilise the carbocation which results in a faster rate of reaction
What is a deactivating group?
These are substituents that make the ring less reactive by withdrawing electrons from the ring
These also destabilise the carbocation and so result in a slower rate of reaction
Why is phenol more reactive than benzene?
The OH group is electron donating and so is an activating group
This makes the ring more reactive and so does not require a catalyst for bromination unlike benzene
Apart from controlling regiochemistry, what else do substituents on the ring control?
They also control the rate at which the reaction takes place
Describe the reaction of methyl benzene with NO2 and subsequent substitutions (TNT)
The methyl group activates the pi system so the first nitration is faster than that of benzene
The nitro group then deactivates the pi system and so the second and third nitrations become slow and need higher temperature
How can the effect of substituents be explained?
With inductive and mesomeric effects
What causes inductive effects?
Inductive effects are due to the arrangement of electron density in sigma bonds and are determined by electronegativity
How do alkyl groups donate electron density?
Via hyperconjugation which is the donation of electron density from the C-H or C-C sigma bond to the pi antibonding orbital of the benzene ring
What causes mesomeric effects?
Mesomeric effects are due to the arrangement of electron density in pi bonds and lone pairs
They are determined by resonance/ conjugation
What is the strongest effect in the halogens?
The -I effect is the strongest
Although they have a +M effect due to a lone pair of electrons, they are highly electronegative and so want to hold onto their electrons
Which effect is normally stronger M or I
Mesomeric effects are stronger than inductive effects
+M effects also stabilise the cation more effectively than +I effects
What effects do activating groups have ?
They have +I or +M effects and so donate electron density into the ring
What effect do activating groups have on the rate of reaction??
They react faster with electrophiles than an unsubstituted benzene ring
They also stabilise the cation which increases rate
What effects do deactivating groups have?
They have -I or -M effects and so withdraw electron density from the ring
What effect do deactivating groups have on the rate of reaction?
They react slower with electrophiles than an unsubstituted benzene ring
They also destabilise the cation which decreases rate
Which step of the 2 step mechanism needs to be considered when understanding the effect of the substituents?
The first step is the rate determining step (slowest) which is the formation of the wheland intermediate
This step determines the overall rate of reaction
Describe the effects an activating group has on the energy of the reactants and intermediates
What overall effect does this have?
Electron donating substituents raise the energy of the already electron rich starting material (by making it more reactive)
They lower the energy of the wheland intermediate and transition state (by stabilising the cation)
This lowers the Ea of the reaction hence making it go faster
Describe the effects an activating group has on the energy of the reactants and intermediates
What overall effect does this have?
Electron donating substituents lower the energy of electron rich starting material (by making it less reactive)
They raise the energy of the wheland intermediate and transition state (by destabilising the cation)
This increases the Ea of the reaction hence making it go slower
Is N or O a better activating group?
Nitrogen is a better activating group as it donates its electrons more readily
This is because oxygen is more electronegative and so wants to hold onto its electrons
Why does fluorine have a neutral effect? (Neither stabilises or destabilises the ring)
Fluorine is a second row element and so is small
It has a +M effect as its lone pairs are in p orbitals which can overlap well with the benzene 2p orbitals
This means it has a greater +M effect that the other halogens
However, it is the most electronegative so also has a strong -I effect
These effects cancel out making it neutral
Which group do you need to consider when looking at substituents effects
When considered activating and deactivating effects you need to look at the group that is bonded directly to the benzene ring
Why is aniline (NH3 on ring) more reactive than benzene for bromination but less reactive for nitration?
On bromination it shows a strong +M effect which activates the ring as normal
Nitration is carried out under strongly acidic conditions and amines are also bases which can be protonated
The protonated amine has no lone pair so no +M effect
The n+ is more electronegative than C so it becomes a -I group and deactivates the ring
