Reactivity And Orientation

Question 1

What effects do substituents already on the ring have?

Answer 1

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

Question 2

What is an activating group?

Answer 2

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

Question 3

What is a deactivating group?

Answer 3

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

Question 4

Why is phenol more reactive than benzene?

Answer 4

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

Question 5

Apart from controlling regiochemistry, what else do substituents on the ring control?

Answer 5

They also control the rate at which the reaction takes place

Question 6

Describe the reaction of methyl benzene with NO2 and subsequent substitutions (TNT)

Answer 6

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

Question 7

How can the effect of substituents be explained?

Answer 7

With inductive and mesomeric effects

Question 8

What causes inductive effects?

Answer 8

Inductive effects are due to the arrangement of electron density in sigma bonds and are determined by electronegativity

Question 9

How do alkyl groups donate electron density?

Answer 9

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

Question 10

What causes mesomeric effects?

Answer 10

Mesomeric effects are due to the arrangement of electron density in pi bonds and lone pairs
They are determined by resonance/ conjugation

Question 11

What is the strongest effect in the halogens?

Answer 11

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

Question 12

Which effect is normally stronger M or I

Answer 12

Mesomeric effects are stronger than inductive effects

+M effects also stabilise the cation more effectively than +I effects

Question 13

What effects do activating groups have ?

Answer 13

They have +I or +M effects and so donate electron density into the ring

Question 14

What effect do activating groups have on the rate of reaction??

Answer 14

They react faster with electrophiles than an unsubstituted benzene ring
They also stabilise the cation which increases rate

Question 15

What effects do deactivating groups have?

Answer 15

They have -I or -M effects and so withdraw electron density from the ring

Question 16

What effect do deactivating groups have on the rate of reaction?

Answer 16

They react slower with electrophiles than an unsubstituted benzene ring
They also destabilise the cation which decreases rate

Question 17

Which step of the 2 step mechanism needs to be considered when understanding the effect of the substituents?

Answer 17

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

Question 18

Describe the effects an activating group has on the energy of the reactants and intermediates
What overall effect does this have?

Answer 18

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

Question 19

Describe the effects an activating group has on the energy of the reactants and intermediates
What overall effect does this have?

Answer 19

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

Question 20

Is N or O a better activating group?

Answer 20

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

Question 21

Why does fluorine have a neutral effect? (Neither stabilises or destabilises the ring)

Answer 21

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

Question 22

Which group do you need to consider when looking at substituents effects

Answer 22

When considered activating and deactivating effects you need to look at the group that is bonded directly to the benzene ring

Question 23

Why is aniline (NH3 on ring) more reactive than benzene for bromination but less reactive for nitration?

Answer 23

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