What is Syn Addition?
The addition of two substituents to the same side (or face) of a double or triple bond reduces the bond order but increases the number of substituents.
The addition of two substituents to the same side of an unsaturated molecule is known as syn addition. The addition of two substituents in opposite directions is known as anti-addition. This is the major distinction between syn addition and anti addition. These addition reactions change the bond order and the number of substituents in the reactant molecule.
Table of Contents
- Syn Addition Reaction
- Stereochemistry of Alkene Addition Reactions
- Difference between Syn and Anti Addition
- Frequently Asked Questions – FAQs
Syn Addition Reaction
A syn addition is an addition reaction of an alkene or alkyne in which the net reaction is the addition of two ligands to the multiple-bonded carbon atoms from the same face of the multiple bond; an anti addition is an addition reaction in which the net reaction is the addition of two ligands from the opposite faces of the multiple bond.
However, because a double or triple bond is frequently broken to generate the requisite single bonds, only chemical compounds with multiple bond characters can conduct an addition reaction. An addition reaction is essentially a reverse decomposition reaction, whereas a decomposition reaction is one in which one or more elements or compounds are compounded. Consider the following reaction (hydrochlorination) of propane (an alkene) as an example of an addition reaction, for which the equation is
Stereochemistry of Alkene Addition Reactions
The stereochemistry of addition to a double bond is explained in terms of the plane in which the double bond and its four associated groups are located. This plane’s sides are referred to as faces. The top face is the side of the plane closest to the observer, while the bottom face is the opposite side.
Two groups add to a double bond from the same face in a syn-addition:
Electrophilic addition has the ability to create two additional chiral centres, depending on the structure of the initial alkene. The reaction between HBr and an alkene is not stereoselective; racemization occurs at both alkene carbons. Consider cis-3,4-dimethyl-3-hexene with HBr added to it. Because the acidic proton might be transferred to either side of the planar alkene carbon with equal chance during the initial proton abstraction stage, the centre could have either S or R configuration.
Difference between Syn and Anti Addition
The difference between syn and anti addition is tabulated below.
| Syn Addition | Anti Addition |
|---|---|
| The addition of two substituents to the same side (or face) of a double or triple bond reduces the bond order but increases the number of substituents. The substrate is usually an alkene or alkyne. | Anti addition is the polar opposite of syn addition. Anti addition involves adding two substituents to opposite sides (or faces) of a double or triple bond, resulting in a drop in bond order and an increase in the number of substituents. Bromination (any halogenation) of alkenes is a classic example of this. |
| The addition of substituents to the same side of a double or triple bond is known as syn addition. | The addition of substituents to the opposite sides of a double or triple bond is known as anti addition. |
| Ex: Oxidation of alkenes to diols in the presence of Os04 | Ex: Halogenation of alkenes |
Frequently Asked Questions on Syn Addition
What are syn additions?
What is an anti-addition in chemistry?
An addition reaction in which two substituents are added to opposite sides (or faces) of a double bond or triple bond.
What are syn and anti-elimination?
In syn elimination, the base attacks the β-hydrogen on the same side as the leaving group. In anti-elimination, the base attacks the β-hydrogen on the opposite side of the leaving group.
What are syn and anti isomers?
The prefixes syn and anti are used to denote geometrical isomerism. Syn: indicates that both H and OH are present on the same side of the double bond. Anti: indicates that H and OH are present on the opposite sides of the double bond.
What do E and Z mean in organic chemistry?
E isomers are alkenes having the substituents with higher priority on the opposite sides of the double bond. Z isomers are alkenes having substituents with higher priority on the same side of the double bond. The letter “E” comes from entgegen in German, which means “opposite”.
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