[moe6141]

n-Octadecanol has a molar mass of 270.5 g/mol and a melting point of 57-59 °C
whereas urea has a molar mass of 60.1 and a melting point of 133-135 °C.
Explain why n-octadecanol, which is 4.5 times heavier than urea, has a much
lower melting point.

please help...

[Arkcon]

How is melting point connected to molecular mass, and how could the differences in the structures of urea and octadecanol give you a hint?

[PhDoc]

What intermolecular forces are present in octadecanol? What forces are present in urea? How do they differ? What effects do you believe they have upon melting point?

What is the effect of carbon/heteroatom ratio on melting point?

Learning Organic Chemistry has nothing to do with knowing the answers. You'll learn more and perform better if you recognize patterns and know how to ask yourself the right questions. This is what it's all about.

Good luck!

[Babcock_Hall]

I would think about hydrogen bonding.

[IsotopeBill]

A question often asked, and one you've probably already discussed in class, is "why does water have such a  high boiling point, considering it's such a small molecule?"  The answers are similar, and comparing them will help you recognize the patterns mentioned by LennoxTutoring