Question

I did an organic chemistry lab where I purified a sample of crude biphenyl. The following questions are in regard to the lab.

a. More polar solvents move molecules more rapidly than less polar solvents. If you used a 1:1 hexanes: methanol instead of the 4:1 hexanes:ethyl acetate, would the products elute fater or slower? is the mixture of 1:1 hexanes:methanol a good choice as an eluent with biphenyl?

b. Why do impure compounds have lower melting points than the pure samples? Explan in molecular terms using biphenyl as an example. (we removed methyl orange from biphenyl)

c. Naphthalene has a melting point of 80.3 degrees Celcius. Is that higher or lower than the melting point of biphenyl? Explain the answer in molecular terms (biphenyl has 2 rings connected with a bond and Napthalene is 2 fused rings)

Answer 1

a. More polar solvents move molecules more rapidly than less polar solvents. If you used a 1:1 hexanes: methanol instead of the 4:1 hexanes:ethyl acetate,

yes the products elute fater.

the mixture of 1:1 hexanes:methanol is NOT a good choice as an eluent with biphenyl because it is a nonpolar molecule.

b. impure compounds have lower melting points than the pure samples because of the vapor pressure depression of the compound in presence of the impurity.

in presence of methyl orange , biphenyl has a lower melting point because when heating takes place , molecules of both compounds evaporates and there are vapor molecules of both and therefore melting poind depression takes place.

c. Naphthalene has a melting point of 80.3 degrees Celcius. that is higher or than the melting point of biphenyl. biphenyl has 2 rings connected with a bond and Napthalene is 2 fused rings therefore in biphenyl both ring can rotate and therefore there will be less interacion b/w two biphenyl molecules

but in case of naphthalene , the two rings are fused so the other molecule interacts with each other via pi-pi stacking and so have larger melting point