Introduction

The reactivity of organic compounds refers to their ability to undergo chemical reactions. This reactivity is determined by several factors, including the presence of functional groups, the size and shape of the Molecule, and the types of bonds present.

Functional groups, such as alcohols, aldehydes, ketones, and carboxylic acids, are groups of atoms within a molecule that are responsible for its chemical properties. Each functional group has a characteristic set of reactions that it can undergo, which affects the overall reactivity of the molecule.

The size and shape of the molecule also affect its reactivity. Larger molecules tend to be less reactive than smaller ones, as the increased distance between atoms in larger molecules makes it more difficult for chemical reactions to occur. Similarly, complex molecules with many branches and substituents can be more difficult to react due to steric hindrance.

The types of bonds present in a molecule also influence its reactivity. Carbon-carbon double and triple bonds are more reactive than single bonds, as they are more polar and have a higher degree of unsaturation. Similarly, polar bonds such as carbon-oxygen and carbon-nitrogen bonds are more reactive than nonpolar carbon-carbon bonds.

Overall, the reactivity of organic compounds depends on a complex interplay of these factors, as well as the conditions under which the reactions take place. Understanding the reactivity of organic compounds is essential for designing and optimizing chemical reactions in organic synthesis.

Inductive effect and Mesomeric effect

The inductive effect and mesomeric effect are two important concepts in organic chemistry that help to explain the behavior of organic compounds.

The inductive effect is the polarization of a chemical bond due to the presence of electronegative or electropositive atoms or groups in the molecule. This polarization creates a partial positive or negative charge on the atom, which can influence the reactivity of the molecule. The inductive effect is transmitted through sigma bonds and decreases in strength as the distance from the polarized atom increases.

For example, consider the following molecule:

CH3-CH2-CH2-Cl

The chlorine atom is more electronegative than the carbon atoms, so it attracts Electrons towards itself, creating a partial negative charge on the carbon atom closest to it. This inductive effect makes the carbon atom more susceptible to nucleophilic attack, as it has a partial positive charge.

The mesomeric effect, also known as resonance effect, is the distribution of electrons through a pi bond or a conjugated system of pi bonds in a molecule. This effect can stabilize or destabilize the molecule, depending on the direction of electron flow. The mesomeric effect is transmitted through pi bonds and can affect the reactivity of the molecule.

For example, consider the following molecule:

Phenol

In the phenol molecule, the oxygen atom has a lone pair of electrons that can delocalize into the aromatic ring through a pi bond. This creates a resonance structure where the electrons are shared between the oxygen and the aromatic ring. This mesomeric effect stabilizes the molecule, making it less reactive towards electrophiles.

Resonance is a concept in chemistry that describes the delocalization of electrons in certain molecules, particularly those with conjugated pi systems. Benzene is a classic example of a molecule that exhibits resonance.

Benzene is a six-membered ring molecule with alternating double and single bonds between its carbon atoms. The Lewis structure of benzene shows three double bonds and three single bonds, but this does not accurately represent the electronic structure of the molecule.

Instead, benzene is better described as a resonance hybrid of two Lewis structures that differ only in the placement of the double bonds. In the first structure, the double bonds are between carbons 1 and 2, 3 and 4, and 5 and 6. In the second structure, the double bonds are between carbons 2 and 3, 4 and 5, and 6 and 1.

In reality, the electrons in the pi bonds of benzene are not confined to one specific set of double bonds, but rather are delocalized over the entire ring. This delocalization results in a more stable molecule with a lower energy than would be expected based on the individual Lewis structures alone.

The resonance hybrid of benzene can be represented by a series of resonance structures, each of which shows the delocalization of electrons across the ring. For example, one resonance structure shows a single bond between carbons 1 and 2, and a double bond between carbons 2 and 3, while another shows a double bond between carbons 1 and 2 and a single bond between carbons 2 and 3.

Overall, resonance in benzene and other molecules with conjugated pi systems plays an important role in determining their stability, reactivity, and other chemical properties.

Chemical terms explained

1) Nucleophiles and Electrophiles

Nucleophiles are chemical species that contain one or more pairs of electrons and are attracted to positively charged centers or atoms that have vacant orbitals. They are electron-rich and have a tendency to donate their electrons to form a new bond. Examples of nucleophiles include anions (e.g. Cl-, Br-, OH-), neutral molecules with lone pairs (e.g. H2O, NH3), and radicals (e.g. CH3•).

On the other hand, electrophiles are species that are electron-deficient and have a tendency to accept a pair of electrons to form a new bond. They are attracted to negatively charged centers or atoms with high electron density. Examples of electrophiles include cations (e.g. H+, Fe3+), neutral molecules with an incomplete octet (e.g. BF3, AlCl3), and radicals (e.g. Cl•, CH3+).

2) Free Radicals: Free radicals are chemical species that have one or more unpaired electrons in their outermost shell. These unpaired electrons make them highly reactive and unstable, and they can cause damage to biological molecules such as DNA and proteins. Free radicals can be formed by homolytic fission of a covalent bond, which results in the separation of a pair of electrons into two unpaired electrons. Examples of free radicals include oxygen-centered radicals such as superoxide (O2•-) and hydroxyl (•OH) radicals, and carbon-centered radicals such as methyl (CH3•) and ethyl (C2H5•) radicals.

3) Ions: Ions are atoms or molecules that have a net electrical charge due to the gain or loss of one or more electrons. Cations are positively charged ions that have lost one or more electrons, while anions are negatively charged ions that have gained one or more electrons. Examples of ions include sodium cation (Na+), chloride anion (Cl-), and nitrate anion (NO3-).

4) Homolytic Fission and Heterolytic Fission

Homolytic fission is the cleavage of a covalent bond in which each of the atoms involved takes one of the bonded electrons, resulting in the formation of two free radicals. Homolytic fission is typically initiated by heat or light and is common in reactions involving free radicals. For example, the homolytic fission of a chlorine molecule (Cl2) can produce two chlorine radicals (Cl•).

Cl2 → 2Cl•