CHEM S445 Advances in Chemical Synthesis (Free Courseware)

Reactivities of chemical reactions

Organic chemistry belongs to a branch of chemistry that is generally associated with compounds containing carbon. It is so important because millions of compounds that contain carbon are related to our daily lives, so organic chemistry has many close connections with living things. For examples, amino acids can react together to form 1 proteins, which help constitute the essential compounds of muscle, skin and blood. And the DNA in our bodies contain all of our genetic information. A human body is just like a large reaction flask in which many organic reactions occur simultaneously. Those reactions are catalyzed by our enzymes. This is why we refer to carbon-containing compounds organic. Many organic compounds also have a wide range of applications in industry and health care, e.g. fuel, oil, detergents, body lotion and cosmetics. Organic chemistry is therefore an important branch of chemistry.

Advanced organic chemistry involves the strategy of organic synthesis, which is a series of chemical reactions for the construction of large, complex organic compounds from small, simple building blocks of starting materials. An organic chemist must develop advanced knowledge of synthetic tools, and learn how to apply these theoretical strategies to real examples.

Reactivities of chemical reactions

Different organic compounds have different life-times in nature. For example, methane (CH4) has a short life-time in the atmosphere: it is easily converted into carbon dioxide (CO2) via combustion. This makes it useful as an efficient fuel. The lifetime length of a particular compound is closely related to its reactivity. The shorter the life-time, the higher the reactivity. The reactivity of a reaction is related to both kinetic and thermodynamic factors. We therefore need to consider how fast the reaction is, and whether the product is stable to form.

You should note, however, that the term reactivity is sometimes restricted to kinetics. That is, it refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. For example, the reaction rate of hydrolysis of 1-bromohexane is faster than 2-bromohexane in SN2 pathway. Hence, the reactivity of 1-bromohexane is higher than 2-bromohexane. The reaction rate is related to a number of factors, including the reactant’s physical properties, purity, concentration, temperature and pressure:

• Physical properties  The physical properties of a ‘chemical substance’ are quite important to its reaction rate. For example, if a reactant is broken down into small pieces, or ground into powder, this increases the surface area and hence can increase the contact area of that reactant, and thus enhance the reaction rate.
• Purity Reaction rates can also be affected by the purity of the compounds involved. If there are contaminants inside a reactant, the rate of reaction may also be affected. Contaminants can either enhance or reduce a reaction rate, depending on their nature.
• Concentration Reaction rate also depends on the concentration of the reactants. At a high concentration, the number of molecules in a certain area increases. Since these molecules are closer to each other, the chance of their contact increases, which can also increase the frequency of collisions. As a reaction occurs via the collision of molecules, the reaction rate increases if the frequency of collisions increases.
• Temperature Temperature is also an important factor. At higher temperatures, molecules have higher thermal energy, which again increases collision frequency, and thus enhances the reaction rate.
• Pressure Finally, if pressure increases, the reactants are forced into a given area. Again, the collision of molecules can increase and hence increase the reaction rate. This is similar to the effect from concentration.