Question about single bond rotation

[Mantis Toboggin]

TBR asked the question: Which of the following compounds contains a single bond about which complete rotation is not possible?

a.) Dipropyl ether
b.) 2-butanone
c.) 2,3-butanediol
d.) methylcyclopentane

The answer key states D, given that rotation isn't possible between two atoms of a cyclic compound. But isn't this exactly what happens during ring flipping in cyclohexane?

Also, can somoene explain to me why 2-butanone isn't the answer? For example why is rotation allowed between C1 and C2 of this compound, does the presence of the =O affect rotation?

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[AureatEagle]

TBR asked the question: Which of the following compounds contains a single bond about which complete rotation is not possible?

a.) Dipropyl ether
b.) 2-butanone
c.) 2,3-butanediol
d.) methylcyclopentane

The answer key states D, given that rotation isn't possible between two atoms of a cyclic compound. But isn't this exactly what happens during ring flipping in cyclohexane?

Also, can somoene explain to me why 2-butanone isn't the answer? For example why is rotation allowed between C1 and C2 of this compound, does the presence of the =O affect rotation?

Rotation isn't possible between two atoms that are both located within the ring of a cyclic compound. When cyclohexane ring flips, the positions of the atoms are moving relative to each other, but none of the carbons can actually rotate in place. The easiest way to think about this is to consider the direct consequence, which is that substituents connected to the atoms in the ring cannot swap places (which should be intuitive). Consider 1-bromo-1-chlorocyclohexane. The bromine and chlorine can never rotate and switch positions (make a model if you can't see it). A crappy real-life analogy is sort of like how how you can rotate your wrist (think turning a doorknob), but you can't rotate it anymore once you link your two hands together and create a completely cyclic system.

2-butanone isn't the answer because the C1-C2 bond is still a single bond in which neither of the two atoms are both part of a ring. The fact that C2 is involved in a pi bond has no effect on the C1-C2 bond-- it would only affect the C2-O bond (which can no longer freely rotate).

 

[sakabato93]

TBR asked the question: Which of the following compounds contains a single bond about which complete rotation is not possible?

a.) Dipropyl ether
b.) 2-butanone
c.) 2,3-butanediol
d.) methylcyclopentane

The answer key states D, given that rotation isn't possible between two atoms of a cyclic compound. But isn't this exactly what happens during ring flipping in cyclohexane?

Also, can somoene explain to me why 2-butanone isn't the answer? For example why is rotation allowed between C1 and C2 of this compound, does the presence of the =O affect rotation?

Cyclohexane's geometry affects the rotation. It has four planar carbons that cannot rotate due to torsional strain. The strain prevents flipping.