Syn & Anti Addition in Stereochemistry: Mechanism, Reactions & Examples

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  • 0:04 Addition Reactions Overview
  • 0:50 Syn Addition Reaction
  • 1:53 Anti-Addition Reaction
  • 2:41 Enantiomer Reactions
  • 3:30 Lesson Summary
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Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Addition reactions that don't include stereochemistry are oversimplified. In this lesson, we learn about syn and anti addition reactions and how they occur. We also look at enantiomer reactions.

Addition Reactions Overview

While studying chemistry, you see reactions written out all the time. Sometimes, reactions are shown without including the stereochemistry, or three-dimensional arrangement, like this one:

The oversimplified addition reaction with no stereochemistry
Basic reaction

But this sort of presentation oversimplifies what is really happening. In addition reactions two atoms are added to a double or triple bond, reducing it to a single or double bond. The two atoms can either be added to the same side or to opposite sides of the molecule.

When the two atoms are added to the same side, this is called a syn relationship, while the two atoms added to opposite sides is called an anti relationship. These reactions can be stereospecific or create enantiomers, which are two molecules with the same chemical formula but different 3D orientation.

Syn Addition Reaction

Syn addition occurs when H2 reacts with a double bond. In this type of a reaction, both hydrogen atoms are added to the same side. The product that puts the hydrogen atoms on opposite sides doesn't form, like you can see here:

Only the syn product is formed
Syn reaction

The reason has to do with the mechanism for adding hydrogen to an alkene. We're actually first plating hydrogens onto a thin metal sheet (the Pd-C). This plating keeps the hydrogen atoms from freely rotating around the molecule, so they must be added to the same side of the molecule.

Since the hydrogen atoms are not free to rotate around the molecule they must be both added to the same side
Syn mechanism

Note: this is an extremely oversimplified version of the mechanism for adding hydrogen to an alkene; the entire mechanism is complicated and beyond the scope of this lesson. Just remember that the hydrogen atoms are unable to rotate and need to be added to the same side since they're bound to a metal plate, thus forming the syn product.

Think of the molecule like a house with double doors in the front and back. Normally, two people could enter from either side. However, if the two people were attached by a rope, then they would need to enter through the same door.

Anti-Addition Reaction

Anti-addition occurs when a halogen (X2, such as Cl2 or Br2), is added to an alkene. Each halogen is added to opposite sides of the molecule, like you can see on your screen now:

When a halogen such as bromine is added to an alkene, only the anti product forms
Anti reaction

Let's look at the house with double doors in the front and back again. Normally, these double doors are big enough for two people to walk through at the same time, but if someone comes and blocks both double doors in the front, then one of the people will need to come in from the back.

When we look at the mechanism of the reaction we can see that this is what is occurring in this reaction. The first halogen is added, and forms a bond with both carbon atoms.

The halogen forms a bond with both carbon atoms
Anti mechanism 1

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