Organic Chemistry : Stereochemistry
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Example Question #91 : Stereochemistry
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.
equatorial up . . . axial down . . . equatorial up
equatorial down . . . equatorial up . . . axial down
axial up . . . equatorial down . . . equatorial up
axial down . . . equatorial up . . . equatorial down
equatorial up . . . equatorial down . . . axial up
equatorial up . . . equatorial down . . . axial up
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 #2 : Help With Comformers
What is the relationship between these compounds?
Enantiomers
Conformational isomers
Meso
Diastereomers
Constitutional isomers
Conformational isomers
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 #1 : Help With Comformers
Determine the most stable chair conformation corresponding to the given Haworth projection.
More than one of these
IV
II
I
III
I
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 #2 : Help With Comformers
How many constitutional isomers exist for the general molecular formula 
Six
Five
Four
Three
Four
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:
Example Question #3 : Help With Comformers
In general, what is the most stable orientation in a Newman projection?
Eclipsed
Anti
Gauche
They are all equally stable
Totally eclipsed
Anti
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 #92 : Stereochemistry
In regards to the Fischer projection shown, how are the hydroxide groups oriented?
Both are going into the screen
The left one is going into the screen and the right one is coming out
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 coming out of the screen
Both are coming out of the screen
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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