Understanding different ways to represent organic molecules in three dimensions is crucial in chemistry. Projections like Fischer, Newman, and Sawhorse help us visualize molecular structures and their conformations, which are different spatial arrangements of atoms that can interconvert by rotation around single bonds. Let's examine each statement to find the one that is not correct.

Analyzing Molecular Projection Statements

We will go through each statement provided in the options and determine its accuracy regarding molecular projections and conformations.

Statement 1: Fischer projection represents the molecule in an eclipsed conformation.

A Fischer projection is a 2D representation of a 3D molecule, primarily used for carbohydrates and amino acids. It depicts the molecule such that the carbon chain is vertical, with horizontal lines representing bonds coming out of the plane of the page (towards the viewer) and vertical lines representing bonds going into the plane of the page (away from the viewer). When viewed along the carbon chain axis, this arrangement corresponds to an eclipsed conformation, where substituents on adjacent carbons are aligned or nearly aligned. Therefore, this statement is generally considered correct in the context of how Fischer projections are drawn and interpreted.

Statement 2: Newman projection can be represented in eclipsed, staggered and skew conformations.

A Newman projection is a way of viewing a molecule along a specific carbon-carbon bond. It shows the atoms and groups attached to the front carbon (represented by a point) and the back carbon (represented by a circle). Newman projections are explicitly used to illustrate different rotational isomers, or conformations, around the single bond. These conformations include:

• Eclipsed: Substituents on the front carbon directly line up with substituents on the back carbon when viewed down the bond axis. This conformation is typically high in energy due to torsional strain.
• Staggered: Substituents on the front carbon are as far as possible from the substituents on the back carbon. This conformation is typically low in energy and more stable. The most stable staggered conformation is often the anti conformation, where the largest groups on adjacent carbons are 180° apart.
• Skew (or Gauche): Any conformation that is not fully eclipsed or fully staggered. A common skew conformation is Gauche, where two bulky groups on adjacent carbons are staggered but 60° apart.

Thus, Newman projections are indeed used to represent eclipsed, staggered, and skew conformations. This statement is correct.

Statement 3: Fischer projection of the molecule is its most stable conformation.

As discussed under Statement 1, a Fischer projection represents the molecule in an eclipsed conformation. Eclipsed conformations have higher torsional energy due to the close proximity of electron clouds of substituents on adjacent atoms, leading to torsional strain. Conformations with lower energy are more stable. Staggered conformations, particularly the anti-staggered arrangement, are generally the most stable conformations for acyclic molecules because they minimize torsional strain and steric hindrance. Since Fischer projections represent an eclipsed conformation, which is typically the least stable (highest energy) conformation, this statement is incorrect.

Statement 4: In Sawhorse projections, the lines are inclined at an angle of 120° to each other.

A Sawhorse projection views the carbon-carbon bond from an oblique angle. It shows the spatial arrangement of atoms and groups attached to two adjacent carbons. The lines representing bonds to substituents are drawn to show their relative positions in space. While the ideal bond angle around a carbon in a tetrahedral arrangement is approximately \(\text{109.5}^\circ\), the angles depicted in a 2D Sawhorse projection can vary depending on whether an eclipsed or staggered conformation is shown and how the drawing is made. When depicting the staggered conformation, the bonds on each carbon are spread out. If you consider the central C-C bond as a slanted line, the bonds to substituents on each carbon are drawn upwards and downwards. The angles between these lines in the 2D representation are drawn to convey the 3D structure. The statement that lines are inclined at an angle of 120° might be a simplification or specific representation used in some texts to approximate the spread of substituents in a staggered conformation when viewed from the side, although it does not strictly represent the internal tetrahedral angle. However, compared to the clear incorrectness of statement 3, statement 4 is likely considered correct in the context of simplified depictions of Sawhorse projections, aiming to show the spatial separation of groups in a staggered view.

Conclusion on the Incorrect Statement

Based on the analysis, Statement 3, "Fischer projection of the molecule is its most stable conformation," is definitively incorrect. Fischer projections represent the eclipsed conformation, which is typically the least stable due to torsional strain. The most stable conformations are generally staggered conformations.

Revision Table: Key Concepts in Molecular Projections

Additional Information on Molecular Conformations and Stability

The stability of different conformations is primarily determined by two types of strain: torsional strain and steric strain.

• Torsional Strain: This occurs when bonds are eclipsed. It is the resistance to twisting around a single bond. The eclipsed conformation has maximum torsional strain.
• Steric Strain: This occurs when atoms or groups are too close to each other in space, leading to repulsive interactions. This is significant when bulky groups are forced into close proximity, for example, in eclipsed conformations or in gauche interactions in staggered conformations.

Staggered conformations minimize torsional strain. Among staggered conformations, the anti conformation (where the largest groups are 180° apart) typically minimizes steric strain as well, making it the most stable conformation for most simple acyclic molecules. Gauche conformations (staggered, but largest groups 60° apart) are less stable than anti but more stable than eclipsed due to some steric interaction between the gauche groups.

Fischer projections, by depicting the eclipsed-like arrangement, highlight the relative positions of substituents for stereochemical purposes but do not represent the lowest energy state of the molecule, which would be a staggered conformation.