Main Difference – Addition vs Substitution Reactions
Addition reactions, substitution reactions, and elimination reactions are fundamental reactions in organic chemistry. Most chemical synthesis and identifications are based on these reactions. These reactions can occur in either one step or two steps. The main difference between addition and substitution reactions is that addition reactions involve the combination of two or more atoms or functional groups whereas substitution reactions involve the displacement of an atom or a functional group by another functional group.
Key Areas Covered
1. What is Addition Reaction
– Definition, Classification, Characteristics, Examples
2. What is Substitution Reaction
– Definition, Classification, Characteristics, Examples
3. What is the difference between Addition and Substitution Reactions
– Comparison of Key Differences
Key Terms: Addition Reaction, Adduct, Cyclo-Addition, Electrophile, Electrophilic Addition, Electrophilic Substitution, Free Radical Addition, Leaving Group, Non-Polar Addition Reaction, Nucleophile, Nucleophilic Addition, Nucleophilic Substitution, Polar Addition Reaction, Radical Substitution, Substitution Reaction, Substrate
What is Addition Reaction
The addition reaction is the combination of two or more atoms or molecules in order to form a large molecule. This large molecule is known as an adduct. Most addition reactions are limited to molecules with unsaturation that have either double bonds or triple bonds. These addition reactions can be classified as follows.
Classification of Addition Reaction
- Polar Addition Reactions
- Electrophilic Addition
- Nucleophilic Addition
- Non-Polar Addition Reactions
- Free Radical Addition
An electrophilic addition is the combination of an electrophile with a molecule. An electrophile is an atom or a molecule that can accept an electron pair from an electron rich species and form a covalent bond. In order to accept more electrons, electrophiles are positively charged or neutral charged and have free orbitals for incoming electrons. A byproduct is not given from addition reaction.
In the above example, H+ acts as the electrophile. It is positively charged. The pi-bond of the double bond is rich with electrons. Therefore, the electrophile (H+) attacks the double bond and obtains electrons to neutralize its charge. In the above example, the newly formed molecule is again an electrophile. Therefore, it also can undergo electrophilic addition reactions.
Nucleophilic addition is a combination of a nucleophile with a molecule. A nucleophile is an atom or molecule which can donate electron pairs. Nucleophiles can donate electrons to electrophiles. Molecules having pi bonds, atoms or molecules having free electron pairs, act as nucleophiles.
In the above image, “H2O“ is a nucleophile and it has lone electron pairs on the oxygen atom. It can be attached to the central carbon atom since C atom has a partial positive charge due to the polarity of –C=O bond.
Free Radical Addition
Free radical addition may occur between two radicals or between a radical and a non-radical. However, the free radical addition occurs through three steps:
- Initiation – formation of a radical
- Propagation – radical reacts with non-radicals to form new radicals
- Termination – two radicals combine and formation of new radicals is terminated
Formation of a cyclic molecule from the combination of two cyclic or non-cyclic molecules is known as cyclo-addition. The Diels-Alder reaction is a good example for cyclo-addition.
The above image shows the addition of carboxylic compounds with alkenes. These additions have resulted in the formation of a cyclic compound.
What is Substitution Reaction
A substitution reaction is a reaction that involves the replacement of an atom or a group of atoms by another atom or a group of atoms. This results in a byproduct named as the leaving group. The general classification of substitution reactions (depending on the type of substituent) is as below.
- Electrophilic Substitution
- Nucleophilic Substitution
- Radical Substitution
Electrophilic substitution is the replacement of an atom or a functional group by an electrophile. Here also, electrophile is an atom or a molecule that can accept an electron pair from an electron rich species and bears either a positive charge or neutral charge.
In the above example, one hydrogen atom of the benzene ring is displaced by the NO2+. Here, NO2+ group acts as an electrophile which is positively charged. The hydrogen atom is the leaving group.
Nucleophilic substitution is the replacement of an atom or a functional group by a Nucleophile. Here also, a nucleophile is an atom or molecule which can donate electron pairs and has a negative charge or is neutrally charged.
In the above image, “Nu” indicates a nucleophile and it replaces the “X” atom of the aromatic molecule. “X” atom is the leaving group.
Radical substitution includes the reactions of radicals with substrates. Radical substitution also contains at least two steps (same as in radical addition reaction) for the completion of the reaction. Most of the times, three steps are involved.
- Initiation- formation of a radical
- Propagation- radical reacts with non-radicals to form new radicals
- Termination- two radicals combine and formation of new radicals is terminated
In the above example, a hydrogen atom of methane is replaced by “.Cl” radicals. The hydrogen atom is the leaving group.
Difference Between Addition and Substitution Reactions
Addition Reaction: Addition reaction is the combination of two or more atoms or molecules in order to form a large molecule.
Substitution Reaction: A substitution reaction is a reaction that involves the replacement of an atom or a group of atoms by another atom or a group of atoms.
Addition Reaction: The large molecule formed after the addition reaction is called the adduct.
Substitution Reaction: The part of the molecule excluding the electrophile or the leaving group is called the substrate.
Addition Reaction: A by-product is not formed in addition reactions.
Substitution Reaction: A by-product is formed in substitution reactions. The by-product is the leaving group.
Molar Mass of the Substrate or Adduct
Addition Reaction: The molar mass of the adduct in addition reaction always increases than that of the initial molecule due to the combination of a new atom or a group.
Substitution Reaction: The molar mass of the substrate in substitution reaction can either be increased or decreased than that of the initial molecule depending on the substituted group.
Addition and substitution reactions are used to explain the reaction mechanisms in organic chemistry. The main difference between addition and substitution reactions is that addition reactions involve the combination of two or more atoms or functional groups whereas substitution reactions involve the displacement of an atom or a functional group by another functional group.
1. “Electrophilic addition hydron mechanism” By Omegakent – Own work (Public Domain) via Commons Wikimedia
2. “Aldehyde hydrate formation” By Sponk (talk) – Own work (Public Domain) via Commons Wikimedia
3. “Benzene hydroxyl reaction” By DMacks (talk) – Own work (Public Domain) via Commons Wikimedia
4. “KetGen” By OrganicReactions – Own work (CC BY 3.0) via Commons Wikimedia
5. “Benzene-nitration-mechanism” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
6. “Aromatic nucleophilic substitution” Public Domain) via Commons Wikimedia
7. “MethaneChlorinationPropagationStep” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
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