Which of the alcohols pictured in Problem 14.48 are chiral? In...

Question

Which of the alcohols pictured in Problem 14.48 are chiral? Indicate the chiral carbons for those that are chiral.

a.

b.

c. 2,3-Pentanediol

d.

e.

f.

Accepted Answer

Hi, everybody. Here's our next problem is the alcohol shown below chiral. If yes, what are its chiral carbon centers? And we have an alcohol molecule here that consists of 1234 carbons with an alcohol group at the end and it has three or two, excuse me, methyl groups attached to it. Three of the carbons are labeled as 12 and three that we'll need to evaluate to see if they are chiral centers. And our answer choices are choice. A no, it is not a chiral molecule. B yes, it is one and three are its chiral centers. C Yes, it is one and two. Are its chiral centers or D yes, it is one and three are chiral centers. So first we want to determine if the molecule is chiral and looking at the chirality of the entire molecule, we look at its overall geometry. So it's not the same thing as chiral centers. A molecule can have multiple chiral centers, it can have carbons that are not chiral centers. But is the molecule chiral? We look at its overall geometry. Is there a plane of symmetry? Always being careful that we recognize that molecules are three dimensional shapes and our drawings are two dimensional. So don't let something that looks symmetrical in two dimensions fool you when it is not symmetrical in three dimensions. So if we look at this molecule here, is there any way we can draw a plane of symmetry to it such that it will be same on either side? Well, no, there's not. We might see is there could we divide it with the methyl group on either side? But then we have alcohol on one side and not the other. When you have just one substituent group like oxygen, you might see kind of plane pass through this group, but it's definitely different on the two different sides. If you put the plane through alcohol, you could see can you draw something through the plane of the molecule? But then we need to remember this molecule is not all in one plane, each of these carbons has a tetrahedral geometry. These substituent groups are projecting up out of the plane and down below the plane. So since we've got that lone alcohol group, it's just never going to be symmetrical. So it is, it is a chiral molecule. So, but yes, I'm circling the word. Yes. So we'll eliminate choice a which is no, it is not a chiral molecule. So our answer is yes. And we just need to evaluate which of the carbons is a chiral center. So let's start with number one. So number one with the Roman numeral is carbon one in our structure. We number the carbons in our parent chain or actually, I should say it's carbon two, excuse me, because then this isn't a methyl group on it. This would be the beginning of my parent chain. So let me number it, we always number it in ways such as to give the substituents the lowest numbers. So the way our structure is drawn, carbon one actually looks like a methyl group sticking up. But we can call that the beginning of our chain. And then carbon two has the oh group, carbon three has a methyl group and then carbon four, carbon five at the end of the chain. So this way of numbering gives our methyl group and our alcohol group the lowest substituent numbers. So number one in Roman numerals is carbon two in my drawing. Let's look at what the substituents are or let's look at what the bonds are around it. Well, on our skeletal structure, we have the alcohol group, the terminal carbon one which is ach three group, a methyl group and then the remainder of the molecule that's only three bonds. So we know there must be a hydrogen also on that carbon. So this carbon most definitely has four different bonds. It has hydrogen, it has alcohol, oh it has a methyl group, which is that carbon one and then I'll just write rest of molecule. We don't need to name it, we just know the rest of the chain. These other three carbons and methyl group are there. So number one is definitely a chiral center with that carbon bonded to four different groups or atoms. So let's look at our answer choices. Do they all include number one, the remaining answer choices. No choice. D says two and three are chiral centers. So let's eliminate choice. DB and C do include number one. So let's keep going. Now let's look at number two Roman numerals, that's carbon three. It has a methyl group and then two bonds to two parts of the chain, that's three bonds. So we know it must also have a hydrogen on it. So it's four bonds are two hydrogen to ch three. That's its methyl group. Now, if we go over to carbons four and five, that's an ethyl group and then we'll just write rest of molecule rather than worrying about its name. It's clearly different from the other three groups. So number two is also a chiral center. Looking back at our answer choices, we see that choice C correctly has number two in it. While choice B does not have number two. So we eliminate choice B and we're down to answer choice C by elimination. So we know according to just eliminating our other answers, that number three here must not be a chiral center, but to be thorough, let's just go through it. So we have number three, Roman numeral three which is carbon four, but carbon four has no substituents. It has the last C five methyl group has the rest of the molecule and it must have two hydrogens as it doesn't have any other bonds. So right away with no substituent, we know it has two hydrogen atoms. So it is not a Cairo center. So we've confirmed that our correct answer is choice. C Yes. Our alcohol is shown as a chiral molecule. One and two arts chiral centers. See you on the next video.

Which of the alcohols pictured in Problem 14.48 are chiral? Indicate the chiral carbons for those that are chiral.
a.
b.
c. 2,3-Pentanediol
d.
e.
f.

Key Concepts

Chirality

Chiral Carbon

Stereochemistry

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