Textbook Question
For each of the following structures, which staggered Newman projection skeleton from Assessment 3.51 should you draw first to show what is seen when looking down the indicated bond?
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4. Alkanes and Cycloalkanes
Drawing Newman Projections
6:55 minutesProblem 50
Textbook Question
Conformational studies on ethane-1,2-diol (HOCH2–CH2OH) have shown the most stable conformation about the central C―C bond to be the gauche conformation, which is 9.6 kJ/mol (2.3 kcal/mol) more stable than the anti conformation. Draw Newman projections of these conformers, and explain this curious result.
Verified step by step guidance1
Begin by understanding the structure of ethane-1,2-diol (HOCH2—CH2OH). It consists of two carbon atoms connected by a single bond, with each carbon atom also bonded to a hydroxyl group (OH) and hydrogen atoms.
To draw the Newman projection, imagine looking straight down the C—C bond. In the anti conformation, the two hydroxyl groups (OH) are positioned 180 degrees apart, minimizing steric hindrance but not taking advantage of any potential stabilizing interactions.
In the gauche conformation, the hydroxyl groups are positioned 60 degrees apart. This proximity allows for the possibility of intramolecular hydrogen bonding between the OH groups, which can stabilize the molecule.
Draw the Newman projection for the anti conformation: the front carbon will have one OH group pointing up and the other substituents (hydrogens) staggered around it. The back carbon will have its OH group pointing down, directly opposite the front OH group.
Draw the Newman projection for the gauche conformation: the front carbon will have one OH group pointing up and the other substituents staggered. The back carbon will have its OH group positioned 60 degrees from the front OH group, allowing for potential hydrogen bonding, which explains the increased stability of the gauche conformation.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Newman Projection
A Newman projection is a way of visualizing the conformation of a molecule from a specific angle, looking down the axis of a bond. It helps in understanding the spatial arrangement of atoms around a bond, particularly useful for analyzing conformations in alkanes. In the case of ethane-1,2-diol, it allows us to compare the spatial orientation of the hydroxyl groups in different conformations.
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Gauche and Anti Conformations
Gauche and anti are terms used to describe the relative positions of substituents around a single bond in a molecule. In a gauche conformation, substituents are 60 degrees apart, while in an anti conformation, they are 180 degrees apart. For ethane-1,2-diol, the gauche conformation is more stable due to favorable interactions between the hydroxyl groups, despite the typical steric hindrance associated with such proximity.
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Hydrogen Bonding
Hydrogen bonding is a type of dipole-dipole interaction that occurs between a hydrogen atom bonded to a highly electronegative atom (like oxygen) and another electronegative atom. In ethane-1,2-diol, the gauche conformation allows for intramolecular hydrogen bonding between the hydroxyl groups, which stabilizes this conformation over the anti conformation, where such interactions are not possible.
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