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One of the special features of organic compounds is that they can be converted from one compound to another through a sequence of chemical reactions. These reactions involving organic compounds are known as organic reactions.
These transformations happen through breaking and forming chemical bonds. Organic conversions are widely used in chemical synthesis to create useful compounds such as pharmaceuticals, agrochemicals, polymers, dyes, etc.
Any reaction involves cleavage and the formation of chemical bonds. According to the symmetry of bond cleavage, there are two types of bond cleavage.
In a covalent bond, there are two electrons. Heterolytic cleavage is breaking the bond asymmetrically. That means both electrons involved in the covalent bond are taken by the most electronegative atom in the bond. This process results in ions.
The atom that has taken both of the electrons will form a negatively charged ion (anion) while the other atom forms a positively charged ion (cation). In reaction mechanisms, heterolytic cleavage is indicated by curved arrows that show the movement of the pair of electrons.
In homolytic cleavage, the covalent bond will be broken down symmetrically. That means the two electrons in the bond are divided equally, as the way each atom gets one electron. This will result in two atoms (or components) with one unpaired electron. These substances are called free radicals.
Free radicals are highly reactive due to this unpaired electron. They tend to react with other species and pair the unpaired electron. The homolytic cleavage is shown in half arrows (fishhooks) in reaction mechanisms. This indicates the movement of a single electron.
Based on how molecules interact and transform during a reaction, there are four main types of reactions in organic chemistry.
In an addition reaction, two reactants react together to result in a single product. Here, two molecules are combined.
When two reactants react, an atom or a functional group in a reactant is replaced by an atom or a functional group. In the following example, the Cl atom in the alkyl halide is replaced by a hydroxyl ion and has resulting in an alcohol.
In elimination reactions, an atom or a group is completely removed, resulting in a new product. Here, double or triple bonds are formed between carbon atoms. As an example, when alcohols are heated up to 350 0C, with anhydrous aluminum oxide, the -OH group will be eliminated and result in an alkene.
In rearrangement reactions, neither an atom is added nor eliminated. The structure rearranges into another isomer of the compound. The number of atoms in the compound remains the same.
In organic chemistry, molecules or ions are categorized into two groups, electrophiles and nucleophiles, depending on their electron deficiency or abundance.
Electrophiles are molecules or ions that have low electron density (electron-deficient). Electrophiles can have a positive charge (cation), a partial positive charge (δ+), or an empty orbital to accept electrons. Electrophiles seek electrons. Therefore, they interact with molecules or atoms with high electron density.
If an electrophile is involved in an addition reaction, it is known as an “Electrophilic addition reaction”. Sometimes, both electrophiles and nucleophiles are involved in the same reaction. In that case, the species that is added first is considered. If the first added species is an electrophile, it is considered an electrophilic addition reaction.
Similarly, if an electrophile is involved in a substitution reaction, it is known as an “Electrophilic substitution reaction”.
Nucleophiles are molecules or ions that have high electron density (electron-rich). Nucleophiles have a negative charge (anion), a lone pair of electrons, or pi bonds. In organic reactions, they tend to donate electrons. Therefore, they interact with molecules or atoms with low electron density.
Examples – H-, -CH3, -OH, -CN, Br-, NH3, H2O
If a nucleophile is involved in an addition reaction, it is known as a “Nucleophilic addition reaction”. Sometimes, both electrophiles and nucleophiles are involved in the same reaction. In that case, the species that is added first is considered. If the first added species is a nucleophile, it is considered a nucleophilic addition reaction.
Similarly, if a nucleophile is involved in a substitution reaction, it is known as a “Nucleophilic substitution reaction”.
Ions that have a positive charge on carbon are called carbonium ions or carbocations. In organic reactions, intermediate carbocations are formed. However, the intermediate carbocation is formed depending on its stability. The reaction tends to happen through the most stable intermediate, carbonation.
The stability of a carbocation depends on the number of carbon atoms that have been attached to the positively charged carbon. Depending on that, there are three types of carbocations.
If the positive charge on the carbon atom is low, the stability increases. Alkyl groups repel electrons. Therefore, when alkyl groups are attached to the positively charged carbon atom, they will repel electrons towards the positively charged carbon. Thus, the positive charge decreases, and it increases the stability.
Thus, the secondary carbocations are more stable than the primary carbocations, and the tertiary carbocations have the highest stability. Also, when the number of carbon atoms in the alkyl group increases, the repulsion of the electrons increases. So, the repulsion of the C2H5 group is higher than the CH3 group.
If there is a pi bond in the neighboring carbon atom to the positively charged carbon, the stability increases further. Because, from the resonance of the molecule, the positive charge is shared between two carbon atoms.
If the positively charged carbon is attached to aryl groups, the stability increases further. Because the positive charge is shared through three carbon atoms in the benzene ring.
