TBR Organic Chemistry Example 2.7

[CookieZine]

All of the following are true about geometrical isomers EXCEPT that:

A. the E designation for an alkene refers to the highest priority groups on each alkene carbon in trans orientation
B. in both the E and Z isomers of an alkene, the atoms directly bonded to the alkene carbons are all coplanar.
C. molecules capable of forming geometrical isomers have greater entropy than linear alkanes of equal carbon chain length
D. geometrical isomers have relatively static structural features, such as polarity and solubility

Only by the process of elimination did I arrive at C but I don't understand it.

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[JMMTB]

Entropy is a measure of disorder or number of different microstates. An example I always like to use is a log v. ashes and smoke. The log has very limited microstates in which it exists as that log. It degrees of freedom, as a whole log, are pretty limited - we can rotate it, turn it upright... But if we burn it to ashes and gas, now we could place those molecules in nearly an infinite different microstates.

As far as alkanes v. alkenes an alkane has less order (no double bond) and thus more entropy. One way to think of it is just based on the rotation around the single bond, each angle is another microstate thus meaning it has more entropy. The alkene double bond is fixed and thus has a limited number of mircostates.

 

[Alucard6]

So Answer choice C is basically saying "Double bonds have more entropy than single bonds". They try to confuse you by saying "geometrical isomers", all that means is a lack of free-rotation, which is what we see in double bonds.

Entropy = disorder, or randomness. Think of it like gas particles flying all over the place, there is no ordered structure so lots of entropy. That is similar to what is happening in single bonds. Single bonds are free to rotate wherever they want, so there is disorder and thus lots of entropy.

Double bonds on the other hand, are fixed in their positions and cannot rotate as much. Thus, they are ordered structures and have less entropy.

As a result, the statement "Double bonds have more entropy than single bonds" (answer choice C), is false.