How do you measure steric hindrance

Steric effect - influence on the course of the reaction

Hello and welcome! This video is about the “steric effect - influence on the course of the reaction”. Understanding chemical reactions is a fundamental concern of organic chemistry. Two important electronic effects are the inductive effect and the mesomeric effect. There is also an effect that is mainly determined by the size of molecular fragments, this effect is called the steric effect. I would like to give three examples in this video that illustrate the steric effect. The SN2 response. It is a two-order nucleophilic reaction. The order of the reaction results from the rate-determining step. The nucleophile and the attacked molecule form a transition state. Consequently, the rate of reaction is determined by the concentration of both components: V = k[Concentration of the nucleophile][Concentration of the molecule]. Let's look at an example of an SN2 reaction: An alkyl bromide reacts with lithium iodide in acetone. An alkyl iodide and lithium bromide are formed. The rate limiting step of the reaction is R-Br + I- → R-I + Br-. Let us consider the relative reaction rates: methyl bromide reacts very quickly with 220,000, the reaction of ethyl bromide is considerably slower with 1350, isopropyl bromide reacts very slowly with 1, and finally tertiary butyl bromide practically does not react. The spatial expansion of the alkyl group hinders the SN2 reaction. The following sequence of reaction rates is thus obtained: methyl >> ethyl >> isopropyl >> tert-butyl. Effect of the steric effect: the spatial expansion, bulkiness, of a fragment hinders the chemical reaction. Ratio of ortho to para orientation in electrophilic substitution. Toluene: The toluene molecule directs the second substituent in the electrophilic substitution preferably in the ortho position or in the para position. First: sulfonation. Toluene reacts with oleum, which is pure sulfuric acid that contains carbon dioxide, SO3. Sulphonic acids are formed, ortho 32%, para 62% and meta 6%. The small proportion of the ortho isomer cannot be explained by electronic effects. The steric effect provides a plausible explanation. The electrophile is the molecule SO3Obviously, the ortho attack on the toluene molecule is limited in its space requirement. As a result, more para isomer is formed. Second: bromination. Bromination takes place using bromine, Br2, and iron tribromide, FeBr3, as a catalyst. The following proportions of isomers are obtained: ortho 33%, para 67% and meta 0%. The proportion of the ortho isomer is relatively low, again the steric effect is responsible. Interesting: it turns out that the small ion, Br +, does not attack the aromatic ring. The data speak for the voluminous complex FeBr3… Br2. Third: nitriding. Nitric acid, a mixture of nitric acid and ENT, is used3, and sulfuric acid, H2SO4. The following isomers are obtained: ortho 58%, para 38% and meta 4%. The steric effect hardly comes into play here. Explanation: The electrophile is the nitronium ion NO2+. It is smaller than the sulfur trioxide molecule SO3, hence the steric effect has no effect. A stable radical. In 1900 the chemist Moses Gomberg reacted triphenylchloromethane (1) with zinc. Its goal was to produce hexaphenylethane (2). It turned out that this is not possible. This is due to the steric effect and the bulky phenyl residues. The reaction initially produced the relatively stable triphenylmethyl radical (3). This then reacts moderately. The depicted reaction product (4) is formed. Summary: The steric effect is an important factor in the course of organic reactions. Basically, large, one also says voluminous or bulky, residues in the vicinity of the reaction center lead to a strong reduction in the reaction rate. That's it again. I wish you all the best and good luck, bye, your André!