Why would chloroethane have a lower boiling point than ethane?

Question

koikkara · Answer

Molecular formula : C2H5Cl
MW : 64.52


nature : under normal temperature and pressure of gas, or compressed at low temperature to the low viscosity colorless volatile liquid. Burns easily. Similar ether odor. Dry Chloroethane stable, non-corrosive, but in the water and the presence of alkaline hydrolysis will become alcohol, prone chlorine atom substitution reaction, good thermal stability, similar to methyl chloride. Compatibility with ether, dissolved in ethanol (48.3g/100ml), micro-soluble in water (20 ° C solubility 0.574g/100ml) -138.7 ° C melting point, boiling point of 12.3 ° C, the relative density of liquid 0.9214, the relative density of steam 2.22 (Air = 1), 187.2 ° C critical temperature, critical pressure 5.2689MPa, vapor pressure 134.788kPa (20 ° C), 1.1179 refractive index (0 ° C), a flash point of -43 ° C (open cup), - 50 ° C (closed cup), the self-ignition point 519 ° C Fuels have some smoke green flame and generate hydrogen chloride gas. Commodities easy with the air to form explosive mixtures explosion limits of 3.16-15% (volume)....

matt101 · Answer

Compared to hydrogen, chlorine is a much larger atom. Think of ethane as a straight stick, and chloroethane as a stick that has a branch at one end. You can see then that you can pack many molecules of ethane together tightly, meaning more dispersion forces are possible, and consequently more energy (i.e. a higher boiling point) is needed to change its state. Chloroethane, on the other hand, has a chlorine "branch" that prevents it from packing together quite as tightly, so fewer dispersion forces occur and less energy is required to overcome them and produce a state change (i.e. a lower boiling point).

aaronq · Answer

"Why would chloroethane have a lower boiling point than ethane?"

it doesn't, chloroethane has a HIGHER BP than ethane due to the dipole moment.

aaronq · Answer

Chloroethane's BP = 12.3 Celsius, Ethane's BP = -89 Celsius.

matt101 · Answer

Hmm you're right...is the Cl-C bond really that polar?

aaronq · Answer

yep C and Cl have very different electronegativities

matt101 · Answer

Good to know! Just did a little more research and the electronegativity difference between carbon and chlorine is about 0.6. If I remember correctly, a difference of 0.5 or less is considered nonpolar. It looks like this small degree of polarity is pretty significant.