Difference Between Syn and Anti Addition

Main Difference – Syn vs Anti Addition

The terms syn addition and anti addition describe different ways from which substituents can be added to a reactant molecule containing either a double bond or a triple bond. Syn addition is the addition of two substituents to the same side of the unsaturated molecule. Anti addition, on the other hand, is the addition of two substituents in opposite directions. This is the main difference between syn and anti addition. These addition reactions alter the bond order of the reactant molecule as well as the number of substituents.

Key Areas Covered

1. What is Syn Addition
     – Definition, Effects, Example
2. What is Anti Addition
    – Definition, Effect, Example
3. What are the Similarities Between Syn and Anti Addition
     – Outline of Common Features
4. What is the Difference Between Syn and Anti Addition
     – Comparison of Key Differences

Key Terms: Addition, Alkene, Alkyne, Anti Addition, Bond Order, Diol, Substituents, Syn Addition

What is Syn Addition

Syn addition is the addition of two substituents to the same side of an unsaturated molecule. An unsaturated molecule is a molecule composed of pi bonds along with sigma bonds. The unsaturated molecule may contain double bonds or triple bonds. Then the substituents are added to the same side of the double bond or the triple bond of the molecule in syn addition.

Syn addition decreases the bond order, which is the number of bonds present between two atoms. The bond order between two carbon atoms of a double bond is two while the bond order of a triple bond is three. But after a syn addition, the bond order between two carbon atoms is decreased to one. However, the number of substituents around each carbon atom is increased after the addition.

Figure 1: Oxidation of alkenes to diols involves a syn addition.

A good example of the syn addition is the oxidation of alkenes to diols in the presence of OsO₄ (Osmium tetroxide). Here, both hydroxyl group (-OH) are attached to the same side of the alkene double bond.

What is Anti Addition

Anti addition is the addition of two substituents to the opposite sides of an unsaturated molecule. Unsaturated molecules are generally either alkenes containing double bonds or alkynes containing triple bonds. In anti addition, the two substituents are added to the double bond or the triple bond in opposite sides.

Anti addition decreases the bond order of the carbon atoms having the unsaturation. The bond order of double bond is 2 and bond order of triple bond is 3. But when two substituents are added to these carbon atoms, the bond order is decreased to 1. But the number of substituents around the carbon atoms is increased.

Figure 2: Bromine addition to an alkene in the form of anti addition.

A good example for anti addition is halogenation of alkenes. The above image shows bromine addition to an alkene. Here, the two bromine atoms are added to the double bonds in opposite sides.

Similarities Between Syn and Anti Addition

• Both are types of addition reactions.
• Both reactions decrease the bond order around carbon atoms having a double or triple bond.
• Both types increase the number of substituents around carbon atoms having a double or triple bond.

Difference Between Syn and Anti Addition    

Definition

Syn Addition: Syn addition is the addition of two substituents to the same side of an unsaturated molecule.

Anti Addition: Anti addition is the addition of two substituents to the opposite sides of an unsaturated molecule.

Addition

Syn Addition: Syn addition involves the addition of substituents to the same side of the double bond or the triple bond.

Anti Addition: Anti addition involves the addition of substituents to the opposite sides of the double bond or the triple bond.

Examples

Syn Addition: A good example of syn addition is the oxidation of alkenes to diols in the presence of OsO₄.

Anti Addition: A good example of anti addition is halogenation of alkenes.

Conclusion

Syn addition and anti addition are two types of addition reactions that give different products. The main difference between syn and anti addition is that syn addition involves the addition of substitutes to the same side of the double bond or triple bond whereas anti addition involves the addition of substitutes to the opposite sides of the double bond or triple bond.

Reference: 

1. “Syn and anti addition.” Wikipedia, Wikimedia Foundation, 15 Jan. 2018, Available here.
2. “Syn and anti addition.” StudyBoard, Available here.

Image Courtesy:

1. “Lemieux–Johnson oxidation” By Project Osprey – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Alkene-bromine-addition-2D-skeletal” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia