Animated Solution for Chemistry - Organic Chemistry: Among the following, the conformation that corresponds to the most stable conformation of meso–butane–2,3–diol is –

Sterics vs. Hydrogen Bonding

When we talk about the stability of different conformations in organic chemistry, the general rule of thumb is simple: keep the bulky groups as far apart as possible. This is why the anti conformation is usually the undisputed champion of stability. But chemistry is full of beautiful exceptions, and the molecule $meso$-butane-2,3-diol provides one of the most classic examples where the rules are rewritten by a special intermolecular force.

Before we can determine the most stable conformation, we must first ensure we are looking at the correct molecule. The question specifically asks for the conformation of the $meso$ isomer. A $meso$ compound is a molecule that contains multiple chiral centers but is overall achiral because it possesses an internal plane of symmetry.

When looking at Newman projections, the easiest way to check for a plane of symmetry is to rotate the back carbon until the molecule is in an eclipsed conformation. If the front groups perfectly align with identical back groups (e.g., $-C{H}_3$ on $-C{H}_3$, $-OH$ on $-OH$, and $-H$ on $-H$), the molecule has a plane of symmetry and is therefore $meso$.

If we apply this rotation test to the given options: - Option A: Rotating the back carbon by $180^{\circ }$ perfectly eclipses the groups. It is a $meso$ isomer. - Option B: Rotating the back carbon by $60^{\circ }$ perfectly eclipses the groups. It is also a $meso$ isomer. - Options C and D: No matter how you rotate them, they will never perfectly eclipse. These represent the optically active enantiomers (the $(2R,3R)$ and $(2S,3S)$ forms), so we can immediately eliminate them.

Gauche Debate

Now the battle is between Option A and Option B.

In Option A, the two bulky $-OH$ groups are positioned $180^{\circ }$ apart. This is the anti conformation. Steric hindrance is minimized here, which usually makes it the most stable form.

In Option B, the two $-OH$ groups are positioned $60^{\circ }$ apart. This is the gauche conformation. Normally, placing two electronegative and relatively bulky groups this close together would cause steric and electrostatic repulsion, destabilizing the molecule.

However, there is a catch! Because the two $-OH$ groups in Option B are in close proximity (gauche), they are perfectly positioned to form an intramolecular hydrogen bond.

The oxygen atom of one hydroxyl group shares its lone pair with the partially positive hydrogen atom of the other hydroxyl group. The energy released by the formation of this hydrogen bond is highly stabilizing. In fact, this stabilization energy is significantly greater than the energy cost of the steric repulsion between the gauche groups.

As a result, the intramolecular hydrogen bond acts like a molecular "glue," locking the molecule into the gauche conformation and making Option B the most stable conformation of $meso$-butane-2,3-diol.