Syn and anti addition

Inorganic chemistry,syn-andanti-additionare different ways in whichsubstituentmolecules can be added to analkene(R₂C=CR₂) oralkyne(RC≡CR). The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by reflecting thestereochemistryof theproductsin a reaction.

The type of addition that occurs depends on multiple different factors of a reaction, and is defined by the final orientation of the substituents on theparent molecule.Syn and anti addition are related toMarkovnikov's rulefor the orientation of a reaction, which refers to the bonding preference of different substituents for different carbons on an alkene or alkyne.In order for a reaction to follow Markovnikov's rule, the intermediatecarbocationof themechanism of a reactionmust be on the more-substituted carbon, allowing the substituent to bond to the more-stable carbocation and the more-substituted carbon.

1,2-disubstitutedCycloalkeneundergoing syn andanti addition

Syn addition is the addition of two substituents to the same side (orface) of adouble bondortriple bond,resulting in a decrease inbond orderbut an increase in number of substituents.Generally thesubstratewill be an alkene oralkyne.An example of syn addition would be the oxidation of analkeneto a diol by way of a suitable oxidizing agent such asosmium tetroxide,OsO₄,orpotassium permanganate,KMnO₄.

Anti addition is in direct contrast to syn addition. In anti addition, two substituents are added to opposite sides (or faces) of a double bond or triple bond, once again resulting in a decrease in bond order and increase in number of substituents. The classical example of this isbromination(anyhalogenation) of alkenes.An anti addition reaction results in a trans-isomer of the products, as the substituents are on opposite faces of the bond.

Depending on the substrate double bond, addition can have different effects on the molecule. After addition to astraight-chainalkene such asethene(C₂H₄), the resulting alkane will rapidly and freely rotate around its singlesigma bondunder normal conditions (i.e.room temperature). Thus whether substituents are added to the same side (syn) or opposite sides (anti) of a double can usually be ignored due to free rotation. However, ifchiralityor the specific absolute orientation of the substituents needs to be taken into account, knowing the type of addition is significant. Unlike straight-chain alkenes,cycloalkenesyn addition allows stable addition of substituents to the same side of the ring, where they remain together. The cyclic lockedring structureprevents free rotation.

Syn eliminationandanti eliminationare the reverse processes of syn and anti addition. These result in a new double bond, such as inE_ielimination.

Reactions and their addition type

Common reactions and respective addition types

Name	Example	Classification notes (type of addition, molecule location, regiochemistry, stereochemistry, etc.)
Hydrohalogenation		Can occur either in syn or anti addition fashion, depending on the solution it is in; 50% of each orientation. This reaction is considered Markovnikov because the halogen substituent attaches to the more substituted carbon.
Hydration		Can occur either in syn or anti addition fashion, depending on the solution it is in; 50% of each orientation. This reaction is considered Markovnikov because the hydroxyl group attaches to the more substituted carbon.
Alkeneoxymercuration-demercuration		Stereospecific: Can only be anti addition – water kicks out the mercury from underneath the intermediate three-membered ring. The reaction is considered Markovnikov as it results in water addition with same regiospecificity as a direct hydration reaction.
Alkenehydroboration-oxidation		Stereospecific: Can only be syn addition – hydrogen and hydroxyl (-OH) are added to the same face. The reaction is anti-Markovnikov. Hydroxyl attaches to the less substituted carbon.
Halogenation		Stereospecific: Can only be anti-addition – both halogen molecules are on different planes. Neither Markovnikov or anti-Markovnikov because the substituents are the same.
Dihydroxylation		Can occur either in syn or anti addition fashion depending on the specific mechanism followed. If osmium tetroxide is used, hydroxide groups are added in syn fashion. If anepoxidemechanism is followed, hydroxide groups are added in an anti fashion. Neither Markovnikov or anti-Markovnikov because the substituents are the same.
Hydrobromination		Stereospecific: Can be syn or anti addition, depending on situation. When alkenes undergo hydrobromination, the alkyl bromides are formed Markovnikov.
Alkyneoxymercuration-demercuration		In this reaction,HgSO4reacts with an alkyne in a Markovnikov regioselective manner to form an enol that istautomerizedinto a ketone. This process utilizes anti addition of an OH group to the more substituted carbon, making this reaction a Markovnikov reaction.
Alkynehydroboration-oxidation		In this reaction, adisiamylboranereaction, disiamylborane is added to one face of the triple bond. The boron atom attaches to the less substituted carbon, and later forms a bond between the more substituted carbon and the OH group. This reaction utilizes syn addition.

External links

• IUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "endo, exo, syn, anti".doi:10.1351/goldbook.E02094