Hydrogenation of a Diene

[Machine33]

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So I always thought Alkenes have higher melting points than alkanes (of similar structures) but I came across a question that asked, "How does the hydrogenation of an eight carbon diene affect the melting point?" The answer is that it increases it, but I don't get why. What am I missing here?

 

[mcloaf]

So I always thought Alkenes have higher melting points than alkanes (of similar structures) but I came across a question that asked, "How does the hydrogenation of an eight carbon diene affect the melting point?" The answer is that it increases it, but I don't get why. What am I missing here?

For purely hydrocarbon compounds melting point depends on Van der Waals forces. These in turn are dependent on the size of the electron cloud of a molecule. An alkane is more reduced than an alkene, so it will have a slightly larger electron cloud and slightly stronger Van der Waals interactions.

 

[MedPR]

So I always thought Alkenes have higher melting points than alkanes (of similar structures) but I came across a question that asked, "How does the hydrogenation of an eight carbon diene affect the melting point?" The answer is that it increases it, but I don't get why. What am I missing here?

Alkanes can rotate and are less rigid than alkenes, thus allowing them to have more interactions with molecules around them. More interactions = higher melting point.

A good MCAT Bio example is saturated and unsaturated fatty acids. Saturated fatty acids have no double bonds, while unsaturated fatty acids do. Unsaturated fatty acids are more likely to be liquid at room temperature because they have a lower melting point than the saturated fatty acids.

 

[SaintJude]

Alkene molecules cannot form compact arrangement as alkanes can, so alkanes have a higher melting point.

Edit: Sorry, didn't see that MedPr already responded before I hit the refresh button.