Question

How many stable conformers of ethanol are possible? (A) 4 (B) 3 (C) 2 (D) 1

Short answer

There are \(2\) stable conformers of ethanol possible - the gauche conformer and the anti-conformer.

Step-by-step solution

Draw the ethanol molecule

Draw the ethanol molecule (C2H5OH), so that we have a clear understanding of the molecule. Ethanol is composed of two carbon atoms (C), six hydrogen atoms (H), and one oxygen atom (O). The structure is as follows: H H H | | | H - C - C - O - H | | | H H H

Identify the rotatable bonds

Now, let's identify the bonds in the ethanol molecule around which rotations can occur to form conformers. In ethanol, there is only one rotatable carbon-carbon (C-C) single bond between the two carbon atoms that can generate conformers.

Determine all possible conformers

Let's determine all the possible conformers by rotating the carbon-carbon (C-C) bond. There are three rotation possibilities: 1. Eclipsed conformer: Hydrogens on adjacent carbons are overlapping 2. Gauche conformer: A hydrogen on one carbon is eclipsing the oxygen on the adjacent carbon 3. Anti-conformer: Hydrogens on adjacent carbons are farthest apart (anti to each other) We can draw these conformers as follows (keep in mind that this is a simplification to demonstrate the concept): Eclipsed conformer: H - C - C - O - H | | | H H H | H Gauche conformer: H - C - C - O - H | | H H | | H H Anti-conformer: H - C - C - O - H | | | H H H | H

Identify stable conformers

To identify which of these conformers are stable, we need to consider steric hindrance and repulsion between atoms. The eclipsed conformer is the least stable due to strong steric hindrance between the hydrogen atoms. The gauche conformer and the anti-conformer are relatively more stable due to lesser steric hindrance between atoms. The anti-conformer is considered the most stable conformer due to the least steric repulsion between atoms.

Count the stable conformers and find the answer

We have identified two stable conformers of ethanol - the gauche conformer and the anti-conformer. Therefore, the correct answer is (C) 2 stable conformers.

Key concepts

Conformational Isomerism

Conformational isomerism is a form of stereoisomerism where isomers can be interconverted just by rotations around single (sigma) bonds. This phenomenon is crucial to understand when studying the structure and reactivity of organic molecules such as ethanol. In simpler terms, think of each bond as a tiny axis around which the attached atoms can twist and turn. Unlike other types of isomerism, conformations are not separable: they can convert to one another simply through thermal motion at room temperature.

For ethanol, this type of isomerism is significant because a change in the rotation around the central C-C bond leads to different spatial arrangements of the atoms, which can influence the molecule's physical and chemical properties. Despite these changes, the molecular formula remains unchanged, and the connectivity of atoms does not alter, differentiating conformers from other isomers.

Rotatable Bonds

Rotatable bonds in organic molecules are single covalent bonds between two atoms that allow the parts of the molecule on either side to rotate relative to each other. These rotations result in different spatial orientations of the atoms, known as conformers. In the case of ethanol (C2H5OH), the rotatable bond is the C-C bond between the two carbon atoms. The rotation around this bond is not completely free, as it's influenced by a variety of factors, including the presence of electron clouds and steric effects from neighbouring atoms or groups.

Understanding which bonds are rotatable and how rotations can affect the molecule's shape provides insight into the molecule's potential interactions with other molecules and its overall stability. When working with complex organic molecules, recognizing rotatable bonds is key to envisioning their three-dimensional structure and dynamics.

Steric Hindrance

Steric hindrance occurs when atoms within a molecule are so close to each other that their electron clouds experience repulsion. This repulsion can hinder the chemical reactions or rotations that usually occur. In ethanol, certain conformers are less favorable due to the increased repulsion between atoms that are adjacent to each other. For example, the eclipsed conformer of ethanol experiences significant steric hindrance because the hydrogen atoms on one carbon are directly behind the hydrogen atoms on the adjacent carbon, resulting in a higher energy configuration.

The concept of steric hindrance helps explain why certain shapes or conformations of molecules are more stable than others. By minimizing these close contacts, molecules can achieve lower energy, more stable conformers, like the gauche and anti conformers in ethanol, which are spaced in such a way that minimizes this steric strain. In summary, analyzing steric hindrance is essential for predicting the most stable conformations of a molecule.