Relationship Between Structure And Properties Of Organic Compounds

Resonance is a fundamental concept in chemistry. It helps us understand how electrons are distributed within a molecule or ion. In chemistry, we use Lewis Structures to represent the arrangement of electrons around atoms. For example, the structure of benzene is usually indicated as a single Lewis Structure. This Lewis Structure contains alternating single and double bonds. The Lewis Structure of benzene says that carbon carbon bond lengths in benzene are not equal. Meanwhile we have already studied that all carbon carbon bond lengths in benzene are equal.

We can conclude that a single Lewis Structure is not sufficient to accurately describe the electronic structure of a molecule. This is where resonance becomes relevant. Resonance occurs when a molecule can be depicted by multiple valid Lewis Structures called Resonance Structures. These Resonance Structures differ only in the arrangement of electrons. The positions of atoms remain the same. The molecule or ion is the hybrid of its Resonance Structures.

For example, two possible Lewis Structures of benzene are illustrated here. These two Lewis Structures of benzene are called Resonance Structures. They only differ in position of pi bonds. Pi electrons are delocalized all over the benzene ring. The original molecule of benzene is hybrid of these two Resonance Structures.

Now let us take an example of acetate ion. There are two possible Lewis Structures of acetate ions. These structures are illustrated here. The electron on the oxygen atom can delocalize between two oxygen atoms that are attached to carbon. These two Lewis Structures are also called Resonance Structures.

Phenol is an alcohol in which hydroxyl group is attached to carbon atom of benzene ring. Molecular formula of phenol is C₆H₅OH. Phenol is more acidic than other alcohols. It can dissociate to phenoxide ion and hydrogen ion. Now that we have studied the concept of resonance, can you explain why phenols are more acidic? Well, the answer to this question also lies in resonance.

Phenols are more acidic due to stability of phenoxide ion. As we know, phenoxide ion is formed by the dissociation of phenol. This phenoxide ion is very stable due to resonance. The electron on the oxygen atom of phenoxide ion is delocalized all over the benzene ring. Due to this delocalization of electron, phenoxide ion has multiple Resonance Structures. Due to delocalization of the electron, the electron is not readily available on oxygen atom of phenoxide ion. This makes phenoxide ion less reactive; thus more stable.

The shifting of electron density between atoms or group of atoms is called Inductive Effect. It describes electron-withdrawing or electron-donating influence. It does so, for atoms or groups of atoms, on nearby atoms or group of atoms, within a molecule. For example, in alkyl halides, electron density is shifted from carbon atoms of alkyl group towards halogen atom. This is due to high electronegativity of halogen atom as compared to carbon atom. Halogen atom withdraws electron density towards itself.

The Negative Inductive Effect occurs when an atom or group of atoms withdraws electron density from the rest of the molecule. This leads to a decrease in electron density in rest of the molecule. For example, in chlorobutane, the chlorine atom is more electronegative than carbon and hydrogen. It exhibits a negative Inductive Effect. It decreases electron density at the nearby carbon atom.

Atoms or group of atoms that indicate negative Inductive Effect are called Electron Withdrawing Groups. Electron Withdrawing Groups have high electronegativity. These groups have a tendency towithdraw electron density from the rest of the molecule. Examples of Electron Withdrawing Groups include halogens, carbonyl group, cyano group and nitro group.

The positive Inductive Effect occurs when an atom or group of atoms donates electron density to the rest of the molecule. This results in a rise in electron density on rest of the molecule. For example, in toluene, methyl group increases the electron density in the benzene ring. Methyl group exhibits positive Inductive Effect.

Electron donating groups are the atoms or group of atoms that exhibit Positive Inductive Effect. Electron donating groups usually have low electronegativity. Some examples of electron donating groups are alkyl groups, amino group and hydroxyl group. Alkyl groups can be methyl group or ethyl group.

When an electron donating group is attached to a benzene ring it will give rise to the electron density at ortho and para position of the benzene ring. Such a group is called Ortho and Para Directing Group. For example in Resonance Structures of toluene, we can see methyl group is increasing electron density at ortho and para position.

During Electrophilic Substitution Reactions, ortho and para directing groups direct the incoming electrophile at ortho and para positions. This is because electron density is rises at ortho and para position. For example, nitration of toluene results in the formation of ortho nitrotoluene and para nitrotoluene. Methyl group in toluene increments the electron density at ortho and para position. As a result, the nitro group is attached at ortho and para position.

Electron Withdrawing Groups decrease the electron density at ortho and para positions in benzene ring. Electron density at meta position remains same. Such groups are called Meta Directing Groups. Ortho and para positions becomes partially positive. Meta position becomes partially negative. Nitro group in nitrobenzene acts as meta directing group.

In nitrobenzene, nitro group withdraws electron density from ortho and para positions. Now the meta position has more electron density as compared to ortho and para position. As a result, the incoming electrophile gets attached to meta position. For example, during chlorination of nitrobenzene Cl⁺ ion acts as electrophile. It attaches to the meta position of nitrobenzene. meta chloronitrobenzene is formed as a product.