Because of this, if an Sn1 reaction is desired, a bulky molecule may be used to make sure the Sn2 reaction does not occur. Both of these reactions can be performed with simple molecules however, Sn1 can occur with bulky molecules, while Sn2 cannot because it is too congested for the mechanism known as a ‘backside attack’. STP - commonly used in the Imperial and USA system of units - as air at 60oF (520oR, 15.6oC) and 14.696 psia (1 atm, 1.01325 bara) At these conditions, the volume of 1 mol of. For example, there are two types of substitution reactions, Sn1 and Sn2. STP - Standard Temperature and Pressure - is defined by IUPAC (International Union of Pure and Applied Chemistry) as air at 0 oC (273.15 K, 32 oF) and 105 pascals (1 bar). Reaction SelectivityĪ molecule’s steric hindrance can be used to favor a specific reaction. Energy is being used to force the bond angles to stay a certain way even though the electrons of the groups are repelling each other. But steric strain can occur when there are multiple bulky groups near each other in a molecule. The lowest energy form of a molecule is usually favored because it is the most natural structure, meaning there is little to no repulsion within groups and angle strain between bonds. The Effects of Steric Hindrance Steric Strain Therefore, as the steric bulk increases, a molecule can be hindered from performing different reactions. 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. We can use the same nucleophile, HO –, and change the bulk of the electrophile by adding more methyls, or CH 3 groups. In these contexts, the capital letters and the small letters represent distinct and unrelated entities. Let’s go through an example below! Example of Steric HindranceĪ simple way to see the effects of steric hindrance is in a reaction between a nucleophile and an electrophile. Greek letters are used in mathematics, science, engineering, and other areas where mathematical notation is used as symbols for constants, special functions, and also conventionally for variables representing certain quantities. ![]() 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. Steric hindrance is a phrase used in organic chemistry to describe how a molecule’s physical structure can affect its ability to react.
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