Activating and Deactivating Groups
Activating and Deactivating Groups
Embedded Files
Resonance Forms
Resonance Forms
Before we can begin discussing the idea of activating and deactivating groups, we need to be familiar with the idea of resonance forms. Resonance forms are a way of depicting the delocalization of electrons within a molecule that cannot be accurately represented by a single Lewis structure. In molecules where electron delocalization occurs, particularly in conjugated systems with alternating double and single bonds (e.g. benzene) or molecules with lone pairs next to double bonds, we can draw resonance forms. We will be exploring this idea using the following examples: benzene, phenol, phenoxide, phenylamine, nitrobenzene and chlorobenzene.
Resonance Forms: Benzene
Resonance Forms: Benzene
Benzene is a classic example of a molecule that exhibits resonance. Although it can be drawn with alternating single and double bonds, these representations do not accurately reflect its structure. Instead, the electrons in the double bonds are delocalized around the ring, leading to equal bond lengths for all carbon-carbon bonds in the molecule.
Resonance Forms: Phenol
Resonance Forms: Phenol
Phenol (C6H5OH) consists of a hydroxyl group (-OH) attached to a benzene ring. The resonance in phenol comes into play due to the interaction between the lone pair of electrons on the oxygen atom and the π electrons of the benzene ring. This delocalization contributes to phenol's acidity by stabilizing the phenoxide ion formed upon deprotonation.
Notice the positions of the negative charges on the ring at positions; they are the the ortho and para positions to the OH. This is important because an electrophile will be attracted to these sites as they are highly negative. The electrophile will bond to the ortho and para positions.
Resonance Forms: Phenoxide
Resonance Forms: Phenoxide
The resonance forms for the phenoxide ion is very similar to that of phenol. (A phenoxide ion is a deprotonated phenol i.e. a phenol with the H in OH removed). This delocalization contributes to phenol's acidity by stabilizing the phenoxide ion formed upon deprotonation.
Notice the positions of the negative charges on the ring at positions; they are the the ortho and para positions relative the O- . This is important because an electrophile will be attracted to these sites as they are highly negative. The electrophile will bond to the ortho and para positions.
Resonance Forms: Benzenamine (Phenylamine)
Resonance Forms: Benzenamine (Phenylamine)
In phenylamine, the lone pair of electrons on the nitrogen atom can delocalize into the benzene ring. This delocalization is facilitated by the overlap of the nitrogen's sp3 hybrid orbital containing the lone pair with the p orbitals of the carbon atoms in the benzene ring. The resonance involves the movement of electron density from the nitrogen atom into the ring, creating several resonance structures.
Phenylamine has multiple resonance forms. The primary structure shows the lone pair on the nitrogen, while additional resonance structures distribute the lone pair electrons around the ring, forming partial double bonds between carbon atoms and introducing positive character on the nitrogen. These structures illustrate how the electron density from the nitrogen can become delocalized over the aromatic system, enhancing the electron density on the ring, particularly at the ortho and para positions relative to the -NH2 group.
Notice the positions of the negative charges on the ring at positions; they are the the ortho and para positions relative to the NH2. This is important because an electrophile will be attracted to these sites as they are highly negative. The electrophile will bond to the ortho and para positions.
Resonance Forms: Chlorobenzene
Resonance Forms: Chlorobenzene
The primary structure of chlorobenzene shows the chlorine atom with its three lone pairs attached to the benzene ring. However, resonance forms can be drawn where a lone pair from the chlorine atom partially contributes to the π-system of the ring. This interaction creates a series of resonance structures in which there's partial π-bond character between the chlorine atom and the carbon to which it's attached, as well as altered electron density distribution around the ring.
Notice the positions of the negative charges on the ring at positions; they are the the ortho and para positions relative to the Cl. This is important because an electrophile will be attracted to these sites as they are highly negative. The electrophile will bond to the ortho and para positions.
Resonance Forms: Nitrobenzene
Resonance Forms: Nitrobenzene
The resonance structures of nitrobenzene illustrate how the electron density can be distributed between the nitrogen and oxygen atoms of the nitro group, and further, how this group can pull electrons away from the benzene ring. In some resonance forms, the nitrogen atom shares its positive charge with the ring, reducing the electron density on the benzene ring.
Notice the positions of the positive charges on the ring at positions; they are the the ortho and para positions relative to the NO2. This is important because if an electrophile attacks the ring, it will be repelled from the ortho and para positions. The electrophile will bond to the meta positions.
Activating vs. Deactivating Groups
Activating vs. Deactivating Groups
Activating and deactivating groups play a pivotal role in electrophilic aromatic substitution reactions by influencing the reactivity of the aromatic ring and directing where the new substituent will add. These effects are crucial for predicting the outcome of reactions involving substituted benzene rings. We shall see that substituents on benzene can be grouped into activators (electron donors), which generally direct a second electrophilic attack to the ortho and para positions, and deactivators (electron acceptors), which generally direct electrophiles to the meta positions.
Understanding the activating or deactivating nature and directing effects of these groups is essential for designing synthetic pathways in organic chemistry. It allows chemists to predict the position where new substituents will be introduced during electrophilic aromatic substitution reactions, facilitating the targeted synthesis of complex aromatic compounds.
Activating Groups
Activating Groups
These groups increase the electron density on the aromatic ring, making it more reactive towards electrophiles. They are typically electron-donating either through resonance or inductive effects. Activating groups also direct electrophilic substitution to the ortho and para positions relative to themselves.
Deactivating Groups
Deactivating Groups
These groups decrease the electron density on the aromatic ring, making it less reactive towards electrophiles. They are typically electron-withdrawing through resonance or inductive effects. Most deactivating groups direct electrophilic substitution to the meta position, except for halogens which are deactivating but ortho/para-directing due to their lone pairs participating in resonance.
Important Functional Groups: Activating or Deactivating?
Important Functional Groups: Activating or Deactivating?
Alkyl ( e.g. CH 3) - Donates electrons through inductive effects, slightly increasing the electron density on the ring and making it more reactive. They are ortho/para directing.
Hydroxyl (OH) and Amine (NH2) - Donate electrons through resonance effects (see resonance of phenol and benzenamine), significantly increasing the electron density on the ring. These groups are strongly activating and ortho/para-directing.
Halogen ( e.g. Cl) - Despite being electron-withdrawing through inductive effects, the lone pairs on halogens can donate electrons into the ring through resonance, which explains their ortho/para-directing nature. However, the overall effect is deactivating because the inductive effect outweighs the resonance donation.
Nitro (NO2), Aldehyde (CHO), and Carboxylic acid (COOH) - Withdraw electrons from the aromatic ring through resonance (and for some, inductive effects), significantly decreasing the electron density on the ring and making it less reactive. These groups are meta-directing.
Here's a table summarizing the effects of the groups that you need to know for MATSEC A-Level Chemistry.
Page updated
Google Sites
Report abuse