# How does selectivity work in chemical reactions again?

• physicsforums9
In summary, the conversation centers around understanding basic selectivity in organic chemistry, specifically in relation to a given reaction involving isopropyl alcohol. The textbook provides the necessary information and equations, but the speaker is still unsure of how the side reactions affect the overall conversion and selectivity. They suggest checking notes or the textbook for a commonly accepted definition of selectivity that may answer their question.
physicsforums9
I got a bit of a question that needs answering.
See, I'm still not understanding how basic selectivity in organic chemistry works. I'm not looking to know the mechanism or anything. Just one simple thing. I'll give the description given from some textbook first:

## Homework Statement

The feed to the reactor contains 88wt% isopropyl alcohol and the rest water (at 1 atm, 25degC). Three reactions take place; conversion to acetone, some other reaction forming di-isopropyl ether, and dehydration of IPA into propene. Outlets are at the temperature of the reactor. The relation when taking place at 300degC is given as:

Acetone conversion: 46.3%
Selectivity (mol di-iso-ether/ mol Ac): 0.01
Selectivity (mol propene/ mol Ac): 0.08

## The Attempt at a Solution

Okay, so let's establish a basis. Let's put in, for simplicity's sake, one mol of pure IPA in the reactor, after heating it up to 300C of course. So, 46.3% is converted, leaving 53.7% of the IPA as unreacted.

But how does it get affected by the side reactions? Does that mean 0.01 x 46.3% becomes di-iso-ether and 0.08 x 46.3% becomes propene, while the rest is the actual acetone (that is to say, 0.91 x 46.3% becomes acetone)?

It depends on how the selectivity is defined - please check your notes or the textbook.

Could be there is a commonly accepted definition that I am not aware off, and the answer is obvious - but even if so, this commonly accepted definition must be in your book. Check it, and it should answer your question.

## 1. How does selectivity differ from specificity in chemical reactions?

Selectivity refers to the ability of a reactant or catalyst to preferentially produce a specific product in a reaction. Specificity, on the other hand, refers to the ability of a reactant or catalyst to only react with a specific substrate or produce a specific product. In other words, selectivity is a measure of how well a reaction can control the outcome, while specificity is a measure of how limited the reaction is to certain substrates or products.

## 2. What factors influence selectivity in chemical reactions?

Selectivity can be influenced by a variety of factors such as the nature of the reactants, the temperature and pressure of the reaction, the presence of a catalyst, and the solvent used. Additionally, the reaction conditions, such as the concentration of reactants and the reaction time, can also impact selectivity. These factors can affect the stability and reactivity of different intermediates, leading to different product distributions.

## 3. How can selectivity be controlled in chemical reactions?

Selectivity can be controlled through the choice of reactants, reaction conditions, and the use of catalysts. For example, using a specific catalyst can promote the formation of a desired product by stabilizing certain intermediates or by lowering the energy barrier for a specific reaction pathway. Additionally, changing the temperature or pressure of the reaction can also influence the selectivity by altering the concentration of different intermediates.

## 4. What are the limitations of selectivity in chemical reactions?

While selectivity is important in controlling the outcome of a reaction, it is not always possible to achieve 100% selectivity. In some cases, competing reactions may occur, leading to the formation of unwanted byproducts. Additionally, selectivity can also be affected by the purity of reactants, the presence of impurities, and the stability of intermediates. These limitations must be considered when designing and optimizing a chemical reaction.

## 5. How does selectivity impact industrial chemical processes?

Selectivity is a crucial factor in industrial chemical processes as it directly affects the yield and purity of the desired product. Achieving high selectivity can increase the efficiency of a process and lower production costs. Furthermore, controlling the selectivity of a reaction can also reduce waste and decrease the environmental impact of the process. Therefore, understanding and optimizing selectivity is essential in the development of sustainable and cost-effective industrial chemical processes.

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