Difference Between Addition and Substitution Reactions

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
  • Cyclo-Addition

Electrophilic 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.

Figure 01: Electrophilic Addition

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

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.

Figure 02: Nucleophilic Addition

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:

1. Initiation – formation of a radical
2. Propagation – radical reacts with non-radicals to form new radicals
3. Termination – two radicals combine and formation of new radicals is terminated

Figure 03: Reaction of “.OH” radical with Benzene forms a new radical.

Cyclo-Addition

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.

Figure 4: Example of 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

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.

Figure 05: Electrophilic substitution of NO2+ to benzene

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

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.

Figure 06: Aromatic nucleophilic substitution

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

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.

1. Initiation- formation of a radical
2. Propagation- radical reacts with non-radicals to form new radicals
3. Termination- two radicals combine and formation of new radicals is terminated

Figure 7: Radical Substitution of methane

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

Definition

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.

Final Molecule

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.

By-product

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.

Conclusion

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. 

Image Courtesy:

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

References:

1.”Substitution Reaction | Types – Nucleophilic & Electrophilic.” Chemistry. Byjus Classes, 09 Nov. 2016. Web. Available here. 28 June 2017. 
2.”Substitution reaction.” Encyclopædia Britannica. Encyclopædia Britannica, inc., 05 Feb. 2009. Web. Available here .28 June 2017. 
3.”Addition Reactions – Boundless Open Textbook.” Boundless. Boundless, 08 Aug. 2016. Web. Available here. 28 June 2017.