Sterically Hindered Base
Sterically Hindered Base - Related Videos
01:10
Radical Reactivity: Steric Effects
0 Views •
2023
The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified. Along with electronic factors, steric factors also account...
01:18
Directing and Steric Effects in Disubstituted Benzene Derivatives
0 Views •
2023
When disubstituted benzenes undergo electrophilic substitution, the product distribution depends on the directing effect of both substituents. When the directing effects of both substituents reinforce each other, a single product is obtained. For example, bromination of p-nitrotoluene occurs ortho to the methyl group and meta to the nitro group, which is the same position, resulting in a single product. However, if the directing effects of the two groups oppose each other, the more strongly...
02:27
DNA Base Pairing
0 Views •
2020
Erwin Chargaff’s rules on DNA equivalence paved the way for the discovery of base pairing in DNA. Chargaff’s rules state that in a double-stranded DNA molecule, the amount of adenine (A) is equal to the amount of thymine (T); the amount of guanine (G) is equal to the amount of cytosine (C); and the sum of purines, A and G, is equal to the sum of pyrimidines, C and T (i.e., A+G = C+T). Later work by Watson and Crick revealed that in double-stranded DNA, A always forms two hydrogen bonds...
06:19
An Inexpensive Adaptation of a Commercial Microwave Reactor for Solid Phase Peptide Synthesis
0 Views •
2024
A simple, inexpensive, and novel apparatus for performing solid phase peptide syntheses in a commercial microwave reactor is presented.
09:00
Lewis Acid-Base Interaction in Ph3P-BH3
0 Views •
2023
Source: Tamara M. Powers, Department of Chemistry, Texas A&M University One of the goals of chemistry is to use models that account for trends and provide insights into the properties of reactants that contribute to reactivity. Substances have been classified as acids and bases since the time of the ancient Greeks, but the definition of acids and bases has been modified and expanded over the years.1 The ancient Greeks would characterize substances by taste, and defined acids as those that...
FAQs
Steric crowding changes which proton is easiest to remove. When the base approaches a substrate, bulky substituents hinder access to crowded electrophilic centers, so deprotonation can occur preferentially at a more accessible site. This accessibility-based preference helps explain why hindered bases can influence site selectivity and, in suitable substrates, favor formation of particular alkenes.
Solvent, temperature, and substrate structure can all shift the outcome. A base that favors one pathway under one set of conditions may produce a different balance of deprotonation, elimination, or substitution when these variables change. Consequently, steric bulk should not be treated as the sole predictor; the reaction environment and substrate structure must be considered together.
Bulky bases have difficulty approaching crowded electrophilic centers for nucleophilic addition, but they can still remove an accessible proton. This difference reduces the relative importance of competing substitution and can favor E2 elimination, which forms an alkene. The preference is not absolute, because substrate structure, solvent, and temperature also influence the final reaction pathway.
Selection should match the desired balance between proton removal, elimination, and limited nucleophilic behavior. Potassium tert-butoxide is useful when promoting elimination or enolate formation is important, whereas diisopropylethylamine can maintain basic conditions with limited nucleophilic addition. Comparing these roles with the substrate structure and reaction conditions helps improve chemoselectivity and alkene control.
Potassium tert-butoxide is associated with promoting eliminations and forming enolates, making it useful when those transformations are desired. Diisopropylethylamine instead maintains basic conditions while limiting nucleophilic addition. Their different behavior allows chemists to choose between encouraging a specific reaction pathway and preserving basic conditions without strongly promoting nucleophilic attack.
The base can favor deprotonation at an accessible site while reducing competing nucleophilic substitution. That combination may direct the reaction toward elimination and improve control over which alkene forms. Because the result also depends on solvent, temperature, and substrate structure, evaluating those variables alongside base choice is essential when interpreting alkene formation.