What's the Point of the Solvent?

[MissionStanford]

I know in some organic reactions the solvent is needed to act as a nucleophile, but what about in a reaction such as halogenation, when you just want to add Br2 across a double bond in an alkene? Why do we use the inert solvent CH2Cl2? What does it do?

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[MedPR]

I know in some organic reactions the solvent is needed to act as a nucleophile, but what about in a reaction such as halogenation, when you just want to add Br2 across a double bond in an alkene? Why do we use the inert solvent CH2Cl2? What does it do?

inert solvents reduce the likelihood of undesirable side reactions. A lot of chemistry revolves around purity so you need to consider which solvent will make the major product form quickly, but also in the highest yield.

 

[akim6890]

Yup. Think about the intermediates. In that reaction, you have a unstable bromonium ion so say you had water as your solvent you would end up with a bromoalcohol instead of a dibromoalkane.

If the solvent is aprotic, it is less likely to go via a 2 step mechanism (E1 or E2). Also, protic vs aprotic solvents change nucleophilicities of anions.

 

[heartsavior]

Although the practical explanation is indeed as MedPR stated above, physically speaking an aprotic solvent such as dichloromethane is polarized with an electron rich (negative) end near the chloride atoms. You could argue that it can stabilize the bromonium ion (positively charged) intermediate, and according to the hammond postulate, any process that stabilizes the reaction intermediate effectively stabilizes the transition state; ie lower the activation energy and hence speeding up the overall reaction.