Viscosity Question?

Question

Both 2-methylheptane and 2,2,3,3-tetramethylbutane have C8H18. how can you tell which has a higher viscosity? They both have London force and I would assume it is based off of the shape. But I need 2-3 reasons why one has a higher viscosity than the other and im unsure how to go about it. hydrogen bonding can not be one of the answers

Answer 1

However, the useful point here is probably *why* all side chains per se have the enormous effect on viscosity -- for a molecule this compact, the side chains of adjacent molecules aren't tangling much around each other (as would much longer chains), there's no steric room for them to do so! A better argument would be made that these very small, blunt molecules can be stacked efficiently together, achieving very high simultaneous near-contact London forces; this raises both viscosity and melting points.

There's some interesting points to consider here, on the relative attraction of C-H, C-F, and C-OH groups. Would you imagine that C2F6, when liquefied, has a higher or lower viscosity than comparable-temperature liquid C2H6? Why?

Answer 2

There are two factors here. The first is that viscosity increases with the number of side chains, and clearly, TMB has more side chains than does MH.

 

Secondly, the melting point of TMB is about 98^oC, so, at ordinary temperatures, it is at least a semisolid, and will not flow.

 

The melting point of MH is -112^oC, and is isometric with octane, so its viscosity will be low.

 

Speaking in common terms, TMB is more viscous than MH.