[Q]Destroyer orgo #187#188

[hunterpostbacst]

#187, I answered a), but it said b) because b) is fulfilling the octet rule. But, whenver I bumped into this kind of question, I am always wondering. So, octet has to be checked prior to activating group directly attached to the + charge?

#188, I got confused because....remember more double bonds makes a compound higher have higher melting point and boiling point? But, does it not work for fatty acid? The fatty acid with double bonds have a kind(cis or trans) that makes it not packed well(loose the compound), so which makes the lower melting point, the solution said. Are they different?

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[pbure9]

Well yes, if the opportunity presents itself than try to make sure everything has an octet as that is MOST stable. If you look at 185, there is no resonance we can make and choice A is correct bc like you were saying that is most stable (having an electron donating group attached to a cation in this case).

As for 188 check out number 85 in the bio. It's sorta similar.

Anyway, saturation insures that your fatty acids are going to stack on top of each other, kinda what happens in plague buildup in arteries. If you think about melting point, remember that the more branched something is/packed tightly, the higher the melting point. So same thing here, saturated fats are able to pack really tightly and that gives them a high melting point. Thats why something like butter is a solid at room temp, whereas something like oil is a liquid since it has a lower melting point.

And trans also would be have a higher melting point than cis fats since they can (edit) pack more tightly.

Hope that helps!

 

[vhlam]

don't you mean cis will have a dipole, while trans will not?

 

[pbure9]

don't you mean cis will have a dipole, while trans will not?

oops.. well i got that half right. It is trans. It's not due to dipole that was my bad... its because trans can pack in much more tightly.
Thus the whole trans-fat "must go" craze

 

[hunterpostbacst]

Isn't it the more branched one distupts the spatial packing and then it's easy to break, so the lower boiling point/melting point?

I got confused,,,please make sure what I am missing...


"remember that the more branched something is/packed tightly, the higher the melting point. So same thing here, saturated fats are able to pack really tightly and that gives them a high melting point. Thats why something like butter is a solid at room temp, whereas something like oil is a liquid since it has a lower melting point. "

 

[pbure9]

Sorry if I confused you... I was just trying to make a connection where it wasnt really necessary.

But the more branched a molecule is, the more easily it can be packed into a crystal, thus a higher melting point.
As for boiling point, a larger molecule will have a higher boiling point (ex. pentane > butane)