I would have thought adding a fluorine to a solvent would withdraw electrons away from the electron donating oxygen (or other electron donating part of the solvent) and thus give it a lower dielectric constant. Instead, fluoro ethylene carbonate (FEC) has a higher dielectric constant than ethylene carbonate (EC).
Can anyone explain this phenomena, and would it hold when fluorinating other solvents like ethers or acetates?
Jacob Johnston When you fluorinate a solvent, the dielectric constant will be changed and it may increase or decrease and so change the strength of solvation. I would like to say it is either increasing or decreasing the dielectric constant and this depends on the chemical environment. Generally, using flouro solvents increase the ability of the solvent to be a good hydrogen bonding donor (or electron acceptor from other electron donating group).
For example, hexaflouroisopropanol has around ϵ=16.7 whereas isopropanol has ϵ = 19.92. So, it may increase or decrease and that depends on the environment. Importantly, the inductive effect will play important role here, especially in your case.
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