Animated Solution for Chemistry - Organic Chemistry: Increasing order of stability among the three main conformations (i.e., Eclipse, Anti, Gauche) of 2-fluoroethanol is

When Gauche Beats Anti

Welcome to one of the most fascinating exceptions in organic chemistry! When we study conformational isomerism, we are taught a golden rule: bulky groups hate being near each other. They repel, they clash, and they create instability.

But what happens when two groups that are supposed to repel each other actually fall in love? Let's dive into the beautiful molecular architecture of 2-fluoroethanol ($F-C{H}_2-C{H}_2-OH$) and discover why the rules sometimes beg to be broken.

To understand the stability of 2-fluoroethanol, we must look down the carbon-carbon bond using Newman projections. We are primarily concerned with three main conformations: Eclipse, Anti, and Gauche.

In the Eclipse conformation, the bulky Fluorine ($F$) atom and the Hydroxyl ($OH$) group are directly aligned, sitting at a dihedral angle of $0^{\circ }$. Imagine two people trying to occupy the exact same seat on a bus. The steric repulsion is massive, and the torsional strain is at its peak. Without a doubt, the Eclipse conformation is the least stable of the three.

Normally, to achieve maximum peace and stability, a molecule will twist itself into the Anti conformation. Here, the bulky $F$ and $OH$ groups are placed as far apart as geometrically possible—at a dihedral angle of $180^{\circ }$.

In almost every standard alkane (like butane), this Anti position is the undisputed king of stability because it minimizes steric hindrance. If 2-fluoroethanol followed the standard rules, Anti would be the winner.

Hydrogen Bonding

But here is the catch—and it is a favorite concept for JEE examiners! Let's examine the Gauche conformation, where the $F$ and $OH$ groups are positioned at a $60^{\circ }$ dihedral angle.

At first glance, you might think, "Wait, they are closer together than in the Anti form, so there must be more steric repulsion!" And you would be right about the repulsion. However, look closely at the specific atoms involved. We have a highly electronegative Fluorine atom sitting right next to the Hydrogen atom of the Hydroxyl group.

Because they are in such close proximity, a magical electrostatic interaction occurs: Intramolecular Hydrogen Bonding ($F^{\delta -}\cdots H^{\delta +}-O$).

This attractive force acts like a molecular glue. The energy released by forming this hydrogen bond is so significant that it completely overpowers the slight steric repulsion of being at a $60^{\circ }$ angle.

Because of this powerful intramolecular hydrogen bond, the Gauche conformation is stabilized beyond expectations, making it even more stable than the Anti conformation!

Therefore, the increasing order of stability is:

Always remember this crucial takeaway: whenever you have groups capable of hydrogen bonding (like $-OH$, $-N{H}_2$, or $-F$) on adjacent carbons, the Gauche conformation will often trump the Anti conformation. Keep an eye out for this beautiful exception in your exams!