Principles and Processes of Chemical Reactions

Chemical reactions are processes that transform chemical substances to create new substances with different identities. Traditionally, these processes involve solely the movement of electrons in forming and breaking bonds between atoms; processes that change the composition of nuclei are nuclear reactions. Equations are often used to represent the path from starting materials to end products, and provide details such as reaction conditions, stoichiometry, and intermediates. In a chemical equation, the number of atoms for each species must be equal on either side of the arrow, which indicates the direction in which the reaction proceeds. Reversible reactions are denoted by double arrows, which means the forward and reverse reactions will compete with each other until the system reaches equilibrium. The simplest form of a chemical reaction―in which reactants are converted into products in a single step with a single transition state―are known as elementary reactions. Although more commonly, reactions take place in a single phase, in certain cases, they occur at the interface between two different states (e.g. solid/gas interface). Depending on the specific conditions, a chemical reaction can proceed under thermodynamic or kinetic control. A thermodynamically favored reaction is one where the products are more stable than the reactants (i.e. energy state of products is lower than that of reactants), resulting in the release of energy or heat (exothermic). On the other hand, a reaction is kinetically favored when the activation energy is low (i.e. low energy transition state), leading to faster reaction rates. Catalysts can lower the intrinsic activation energy of a reaction and thus speed it up. Generally speaking, thermodynamic and kinetic variables both need to be taken into account in determining whether or not a chemical reaction is favorable. There are four basic reaction types: synthesis, decomposition, single replacement, and double replacement. Synthesis refers to bond-forming reactions in which two or more simple substances combine into a more complex substance. Decomposition is the reverse of a synthesis reaction, involving the breakage of bonds in a complex substance to form simpler ones. Single replacement reactions involve the exchange of a single, uncombined element for another within a compound, while double replacement reactions involve a switch between two elements that are both within compounds. Reactions that lead to a change in oxidation states of atoms are classified as either oxidation or reduction reactions (“redox”). Photochemical reactions are ones that are catalyzed by light energy. Acid-base reactions involve the transfer of protons from the acidic species to the basic species. In organic chemistry, reactions are generally classified according to their mechanism (movement of electrons). In substitution reactions, a functional group is replaced by another group (the “nucleophile”) in a compound. Elimination reactions lead to the loss of a functional group from a compound via the formation of a C=C double bond. By contrast, rearrangement reactions give rise to changes in the carbon skeleton, often resulting in structural isomers. Other organic reactions include radical reactions, oxidations/reductions, condensations, hydrolysis, polymerizations, among many others. Many molecules will have more than one reactive site – much research effort in synthetic chemistry has been put into developing conditions and catalysts that will selectively or specifically functionalize one site over another. Chemical reactions lie at the core of biological, industrial, and many other important processes. This book aims to provide a general overview of the type, mechanism, and application of chemical reactions.