Steric strain

In the conformations are different arrangements of atoms originating as a result of rotation(s) around a single bond(s). This means that the ring flip is a reversible process which at some point reaches an equilibrium. Let’s discuss methylcyclohexane as an example: What is interesting here is that this equilibrium is not a 50% : 50% mixture of the conformations. It is shifted to the more stable chair conformation. The ratio of two chair conformations in the following mixture is 95 : 5 in favor of the one where the methyl group is equatorial. What is the reason for this? 1, 3-Diaxial Interactions To understand this preference of chair conformations, let’s draw out the hydrogens of the methyl group and the ring: What we notice is that the methyl at the equatorial position is pointing away from the ring and has plenty of space. On the other hand, when the methyl is axial, it lacks space (steric interaction) mainly due to the hydrogens that are also on axial pistons. Notice also that, these groups (the axial methyl and the hydrogens) are one-carbon apart and if we number, we can see that their relative position is 1,3 and that is why it is called 1,3-diaxial interaction: Now, one question for you: do you see anything in common between the 1,3-diaxial interactions and gauche conformation ( ? Or, even better – can you see that they are the same thing?! Let’s see how that happens. If we convert the chair cyclohexane to its 2 group just like in butane: The second gauche interactio...

Core Concepts In this tutorial, you will learn about how physical structure can affect the reactivity of organic molecules through steric hindrance. Additionally, you will be able to visualize this concept by walking through an example. Topics Covered in Other Articles • • • • Substitution Reactions Vocabulary • Electrophile- electron-rich molecule • Nucleophile- electron-poor molecule • Steric Strain- increase in potential energy of a molecule due to electron repulsion of large side groups Definition of Steric Hindrance Steric hindrance is a phrase used in organic chemistry to describe how a molecule’s physical structure can affect its ability to react. When a molecule is bulky, meaning it has multiple bonds to compounds or groups other than hydrogen, it can slow down or even prevent another molecule from efficiently finding the desired bond site in a reaction. Let’s go through an example below! Example of Steric Hindrance A simple way to see the effects of steric hindrance is in a reaction between a –, and change the bulk of the electrophile by adding more methyls, or CH 3 groups. As shown in the diagram below, as more methyl groups are added to the molecule, there is less space for the covalent bond to the electrophile to form. Therefore, as the steric bulk increases, a molecule can be hindered from performing different reactions. The Effects of Steric Hindrance Steric Strain The lowest energy form of a molecule is usually favored because it is the most natural structur...

Steric hinderance is a major component in determining the feasibility and the rate of a chemical reaction. Wouldn't it be useful to measure it quantitatively then? This would make it easier to compare the property of two molecules. Are there currently ways to measure steric hindrance, or is it not possible for some reason? $\begingroup$ Steric hindrance is as much a made up concept as ring strain. You can try to quantify it based on model systems and reference states, but it will never be more than a simple guideline. Even quantum chemistry is not sophisticated enough to correctly account for all effects and then it is almost impossible to separate them. $\endgroup$ There are two ways I know of to measure steric hindrance. Once such quantitative measurement are Additionally, the $$$ $\begingroup$ This shows both the power of organic chemistry and its weakness. The power as it enables empirical estimates to be made by comparison with other data, the weakness in that one measurement is analysed by four empirical parameters. However, there is no alternative, as experimental data on a fundamental level, not rate constants, but observation of transition states is almost entirely lacking, and is so hard to obtain it may never be. Hard because its difficult to observe one successful event that lasts only a picosecond, or so, in a million failed ones possibly spread over a microsecond. $\endgroup$ There is a full way but it's not easy to calculate. Steric hindrance is indeed a gen...

3.4. Types of Strain in Molecules In any given system most things like to rest in the lowest possible energy state. This is not always possible. For example, there may be geometric or physical constraints on what angles are required, or how close two substituents have to be to each other. When molecules are forced to adopt some condition that is not ‘ideal’, they experience strain. Experiencing strain raises the amount of energy needed to exist. There are several common types of strain that molecules may experience. Angle Strain arises from bond angles deviating from their ideal values. It costs energy to do this because the orbitals can not overlap as well, which leads to weaker bonds. There are several reasons why angle strain may occur, but the most common is a geometric constraint (Figure 3.5). For example, in cyclopropane the three carbons form a triangle. The internal angles of an equilateral triangle are 60°. The carbons are all sp 3 hybridized. Ideally, they would have angles of 109° between their substituents. Since this is not possible, they experience angle strain. The energy of the system (the energy required to exist) increases and the bonds become weaker compared to ‘normal’ C-C σ bonds. An accurate definition of torsional strain requires knowledge beyond the scope of an introductory course. Instead, a (highly) simplified explanation is more useful. Recall that electrons have small negative charges associated with them. A negative charge repulses other negati...

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