Organic Chemistry : Help with Comformers

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #1 : Help With Comformers

The molecules shown below are best described as __________.

Molecules

Possible Answers:

isomers

diastereomers

epimers

enantiomers

Correct answer:

isomers

Explanation:

The molecules in this problem are isomers because they each have unique configurations and do not share the same funcitonal groups at the same carbon positions. Enantiomers are reflections of each other. Diastereomers are stereoisomers that differ at one or more stereocenters, while epimers are stereoisomers that differ at only one stereocenter.

Example Question #2 : Help With Comformers

Screen shot 2015 07 08 at 8.37.35 am

In the most stable chair conformation of the following Haworth projection, the bromine at carbon 1, the hydroxide at carbon 2, and the hydroxide at carbon 4 are __________, __________, and __________, respectively.

 

Possible Answers:

axial up . . . equatorial down . . . equatorial up

equatorial up . . . equatorial down . . . axial up

equatorial down . . . equatorial up . . . axial down

equatorial up . . . axial down . . . equatorial up

axial down . . . equatorial up . . . equatorial down

Correct answer:

equatorial up . . . equatorial down . . . axial up

Explanation:

Based on the Haworth projection given, the relative orientations of the three substituents are bromine up, hydroxide: down, hydroxide: up. In the most stable chair conformation, the largest substituents are oriented equatorial. If bromine is oriented equatorial up, the hydroxide at carbon 2 is equatorial down and the hydroxide at carbon 4 is axial up.

Example Question #3 : Help With Comformers

Screen shot 2015 07 07 at 8.05.03 pm

What is the relationship between these compounds?

Possible Answers:

Meso

Diastereomers

Conformational isomers

Constitutional isomers

Enantiomers

Correct answer:

Conformational isomers

Explanation:

The two given molecules are conformational isomers. The molecules are superimposable by horizontally rotating either structure 180 degrees and rotating the bond between carbons 2 and 3. Both enantiomers and diastereomers are non-superimposable and constitutional isomers must have non-configurational differences in structure. There is no internal plane of symmetry, thus neither compound can be meso.

Example Question #4 : Help With Comformers

Screen shot 2015 07 02 at 8.08.20 pm

Determine the most stable chair conformation corresponding to the given Haworth projection.

Possible Answers:

IV

More than one of these

II

III

I

Correct answer:

I

Explanation:

The most favorable chair conformation of a six-member ring is that in which the greatest number of large substituents are oriented equatorially. Conformation I corresponds to the given Haworth projection and orients all three large substituents in equatorial positions, making it the least energetic (most favorable) conformation.

Example Question #5 : Help With Comformers

How many constitutional isomers exist for the general molecular formula ?

Possible Answers:

Four

Five

Six

Three

Correct answer:

Four

Explanation:

There are four unique constitutional isomers possible for the given formula; meaning that they may not be made identical by conformational changes. They are illustrated as follows:Question 12 image

Example Question #6 : Help With Comformers

In general, what is the most stable orientation in a Newman projection?

Possible Answers:

Gauche

Anti

Eclipsed

Totally eclipsed

They are all equally stable

Correct answer:

Anti

Explanation:

The anti conformation is most stable in a Newman projection because this is the form where the two largest functional groups are facing opposite directions and are furthest away from each other. However, note that in rare cases, such as in 1,2-ethandiol, the gauche conformation is more stable due to intramolecular hydrogen bonding. All other conformations have the functional groups closer to each other causing some repulsion and instability. The least stable of them all is the eclipsed conformation because this is the form where the functional groups are closest to each other.

Example Question #7 : Help With Comformers

In regards to the Fischer projection shown, how are the hydroxide groups oriented? 

Fischer projection

Possible Answers:

Both are lying flat in the plane of the screen

The right one is going into the screen and the left one is coming out

Both are going into the screen

The left one is going into the screen and the right one is coming out

Both are coming out of the screen

Correct answer:

Both are coming out of the screen

Explanation:

Fischer projection

In a Fischer projection, the groups on the left and right are coming out of the screen and towards the viewer. On the other hand, the top and bottom groups are going into the screen away from the viewer. One way to help you remember this is to think of the Fischer projection as a skeleton wearing one or more bowties (the skeleton is vertical and its bonds are drawn as dashes, which are going into the plane of the page/screen, and the horizontal bonds are drawn as wedges, which are coming out of the plane of the page/screen).

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