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Melting and boiling points of benzene and fluorobenzene
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Melting and boiling points of benzene and fluorobenzene
The answer has everything to do with fluorine. Dunitz, in his early studies on fluorine interactions found that intermolecular $\ce{C-H..F}$ interactions can be solid-state structure (e.g., crystal packing) directing. In the case of fluorobenzene, the molecule adopts a solid-state structure that sacrifices long-range order so that these short-range $\ce{C-H..F}$ interactions can be preserved (also see here, and here where it is stated that, "Its [fluorobenzene] melting point is $\pu{-44 ^\circ C}$, which is lower than that of benzene, indicative of the remarkable effect of fluorination on the intermolecular interactions as seen throughout organofluorine chemistry"). The loss of long-range order in the solid-state, as compared to benzene, produces a lower melting point in fluorobenzene $(\pu{-44 ^\circ C})$ than in benzene ($\pu{5.5 ^\circ C}$). Note too that the "size" of fluorine in organic molecules is very similar to that of hydrogen (that is why it has found such widespread use in pharmaceuticals) - it is often referred to as a hydrogen "mimic" (see here). So steric (or symmetry) arguments cannot be used to explain the packing disruption with fluorobenzene.
Once the molecule is in the liquid state the intermolecular $\ce{C-H..F}$ short-range interactions are also diminished due to increased molecular motion; as a result, fluorbenzene and benzene boil relatively close to one another ($\pu{84 ^\circ C}$ and $\pu{80 ^\circ C}$, respectively).
The answer has everything to do with fluorine. Dunitz, in his early studies on fluorine interactions found that intermolecular $\ce{C-H..F}$ interactions can be solid-state structure (e.g., crystal packing) directing. In the case of fluorobenzene, the molecule adopts a solid-state structure that sacrifices long-range order so that these short-range $\ce{C-H..F}$ interactions can be preserved (also see here, and here where it is stated that, "Its [fluorobenzene] melting point is $\pu{-44 ^\circ C}$, which is lower than that of benzene, indicative of the remarkable effect of fluorination on the intermolecular interactions as seen throughout organofluorine chemistry"). The loss of long-range order in the solid-state, as compared to benzene, produces a lower melting point in fluorobenzene $(\pu{-44 ^\circ C})$ than in benzene ($\pu{5.5 ^\circ C}$). Note too that the "size" of fluorine in organic molecules is very similar to that of hydrogen (that is why it has found such widespread use in pharmaceuticals) - it is often referred to as a hydrogen "mimic" (see here). So steric (or symmetry) arguments cannot be used to explain the packing disruption with fluorobenzene.
Once the molecule is in the liquid state the intermolecular $\ce{C-H..F}$ short-range interactions are also diminished due to increased molecular motion; as a result, fluorbenzene and benzene boil relatively close to one another ($\pu{84 ^\circ C}$ and $\pu{80 ^\circ C}$, respectively).
Its not about fluorine as many of the specific examples show. Toluene also has a much lower melting point than benzene and it is all to do with crystal packing not the specific substituent added to the ring. Most non-substituted benzenes have More
@Dissenter Some describe it as the postive electrostatic potential of the ring edge interacting with the negative electrostatic potential of the ring face, while others describe it as a hydrogen bond interaction. Even the gas phase dimer has the edge-face T-shape geometry as lowest energy by 2 kcal/mol.More
@ron Steric arguments arent going to explain disruption of packing, but charge arguments may. Benzene has + on the Hs and - charge in the ring and the + and - favorably interact, perfluorbenzene has - on the Fs and + in the ring, and the + and - favorably interact. Substituting one F for an H would decrease such favorable charge interaction. The hydrogen bonding effect may compensate for some of the lost interaction. Also, there are the residual entropy considerations that make the more symmetric structure higher melting point More
The answer has everything to do with fluorine. Dunitz, in his early studies on fluorine interactions found that intermolecular $\ce{C-H..F}$ interactions can be solid-state structure (e.g., crystal packing) directing. In the case of fluorobenzene, the molecule adopts a solid-state structure that sacrifices long-range order so that these short-range $\ce{C-H..F}$ interactions can be preserved (also see here, and here where it is stated that, "Its [fluorobenzene] melting point is $\pu{-44 ^\circ C}$, which is lower than that of benzene, indicative of the remarkable effect of fluorination on the intermolecular interactions as seen throughout organofluorine chemistry"). The loss of long-range order in the solid-state, as compared to benzene, produces a lower melting point in fluorobenzene $(\pu{-44 ^\circ C})$ than in benzene ($\pu{5.5 ^\circ C}$). Note too that the "size" of fluorine in organic molecules is very similar to that of hydrogen (that is why it has found such widespread use in pharmaceuticals) - it is often referred to as a hydrogen "mimic" (see here). So steric (or symmetry) arguments cannot be used to explain the packing disruption with fluorobenzene.
Once the molecule is in the liquid state the intermolecular $\ce{C-H..F}$ short-range interactions are also diminished due to increased molecular motion; as a result, fluorbenzene and benzene boil relatively close to one another ($\pu{84 ^\circ C}$ and $\pu{80 ^\circ C}$, respectively).
The answer has everything to do with fluorine. Dunitz, in his early studies on fluorine interactions found that intermolecular $\ce{C-H..F}$ interactions can be solid-state structure (e.g., crystal packing) directing. In the case of fluorobenzene, the molecule adopts a solid-state structure that sacrifices long-range order so that these short-range $\ce{C-H..F}$ interactions can be preserved (also see here, and here where it is stated that, "Its [fluorobenzene] melting point is $\pu{-44 ^\circ C}$, which is lower than that of benzene, indicative of the remarkable effect of fluorination on the intermolecular interactions as seen throughout organofluorine chemistry"). The loss of long-range order in the solid-state, as compared to benzene, produces a lower melting point in fluorobenzene $(\pu{-44 ^\circ C})$ than in benzene ($\pu{5.5 ^\circ C}$). Note too that the "size" of fluorine in organic molecules is very similar to that of hydrogen (that is why it has found such widespread use in pharmaceuticals) - it is often referred to as a hydrogen "mimic" (see here). So steric (or symmetry) arguments cannot be used to explain the packing disruption with fluorobenzene.
Once the molecule is in the liquid state the intermolecular $\ce{C-H..F}$ short-range interactions are also diminished due to increased molecular motion; as a result, fluorbenzene and benzene boil relatively close to one another ($\pu{84 ^\circ C}$ and $\pu{80 ^\circ C}$, respectively).
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