The Handy Chemistry Answer Book. Justin P. Lomont

stability of the carbocation. In the previous example, if bromide ion had dissociated first, a primary carbocation would have formed:

      This is a much more difficult reaction than the bimolecular process where the elimination of HBr takes place in a single step.

      But if an alkyl halide can form a stable carbocation, the unimolecular elimination reaction is faster. It’s referred to as “unimolecular” because the slow step has only one molecule in the transition state, just like substitution reactions.

       Who were Diels and Alder?

      The Diels-Alder reaction, like many reactions in organic chemistry, is named after the chemists who discovered it—in this case Otto Paul Hermann Diels (1876–1954) and Kurt Alder (1902–1958). Kurt Alder was actually Diels’ student at the University of Kiel, and Alder was awarded a PhD in 1926. Alder and his advisor Diels jointly received the Nobel Prize in Chemistry in 1950.

      What’s a cycloaddition reaction?

      In the most general terms, a cycloaddition reaction is one that forms a ring from multiple π-bonds. To illustrate some of the concepts and ways that these reactions are classified, let’s look at the cycloaddition reaction shown below. This reaction, known as the Diels-Alder reaction, is between a conjugated diene (double alkene) and a another alkene that likes to react with dienes (hence dienophile, or diene lover). It is classified as a [4 + 2] cycloaddition, referring to the number of atoms directly involved in bond-forming events.

      (There’s a second system for classifying cycloaddition reactions that uses the number of π-electrons involved in ring formation, but let’s just leave that aside.)

      What’s a pericyclic reaction?

      A cycloaddition reaction is actually a type of pericyclic reaction, but the term “pericyclic” includes other types of reactions. The textbook definition of pericyclic is a reaction whose transition state has a cyclic structure (i.e., the electrons are flowing in a closed loop). In addition to cycloaddition reactions (which exchange two π-bonds for two σ-bonds, or vice versa), pericyclic reactions include sigmatropic reactions, electrocyclic reactions, and cheletropic reactions (and a few others which we’ll ignore).

      What’s a sigmatropic reaction?

      While a cycloaddition reaction uses two π-bonds to make two new σ-bonds, a sigmatropic reaction takes one σ-bond and makes…well…one σ-bond. One of the most well-known sigmatropic rearrangements is known as the Cope rearrangement. Using the system we talked about earlier, this reaction is classified as a [3,3]-sigmatropic reaction (the methyl substituent is not directly involved, so we don’t count it).

      What’s an electrocyclic reaction?

      Okay, so a cycloaddition reaction was 2π2σ, and we just saw that sigmatropic reactions are 1σ1σ, so what about a pericyclic reaction that is 1π1σ? That’s an electrocyclic reaction. Like most pericylic reactions, electrocyclic reactions can either make or break a σ-bond. If a σ-bond is made in the process, it’s an electrocyclic ring-closing reaction. If the σ-bond is consumed in the process, the process is named an electrocyclic ring-opening reaction, but it’s the same process.

      One noteworthy example of an electrocyclic ring-closing reaction is the Nazarov cyclization, which converts divinyl ketones into cyclopentenes, usually in the presence of an acid.

      What’s a cheletropic reaction?

      Finally, there’s a second type of reaction that involves two π-bonds being transformed into two σ-bonds. The difference here is that two bonds are formed (or broken) at a single atom, while in cycloaddition reactions, only one bond is made at each reactive atom. An example involving SO₂ is shown below (surprisingly this reaction isn’t named after anyone!). In this reaction both σ-bonds are made (or broken) at the sulfur atom. Compare this with the Diels-Alder reaction mentioned above, where the two new σ-bonds are connected to the two different ends of the dieneophile.

      What is a tautomerization reaction?

      A tautomerization reaction interconverts two constitutional isomers of an organic molecule, referred to as tautomers, which differ only by the positions of hydrogen atoms. The scheme below shows how the reaction typically takes place at the ketone functional group of an organic molecule. The ketone form of the molecule is commonly referred to as the “keto” form of the molecule, while the alcohol form is commonly referred to as the “enol” form of the molecule.

INORGANIC CHEMISTRY

STRUCTURE AND BONDING

      Which molecules are considered inorganic?

      Any molecule without a carbon atom is technically considered inorganic, but in practice, there are some exceptions. Many salts (like carbonates, CO₃²⁻, or cyanides, CN⁻) are thought of as inorganic, even though they do contain carbon.

      What elements are metals?

      Metals are elements that easily conduct electricity. There are a number of sets of metals in the periodic table. Instead of just listing the metallic elements, let’s take a look at those sets one at a time in the next few questions.

      What are the alkali metals and alkaline earth metals?

      The first two columns of the periodic table are known as the alkali (Group 1) and alkaline earth (Group 2) metals. The elements in both groups have very low ionization energies, which means they readily give up one or two electrons to reach a Noble gas electron configuration (e.g., Na⁺, Mg²⁺). In their elemental forms, these elements are soft, silver-colored substances. These elements are almost never found in their pure elemental state in nature because they quickly react with air or moisture, sometimes violently.

      What are the transition metals?

      Elements in the d-block (Groups 3–12) of the periodic table are referred to as transition metals. (The lanthanides and actinides are usually excluded.) With the exception of Group 12, the transition metal elements have an incomplete d-shell of electrons. The chemistry of these elements depends strongly on these d-electrons, and most transition metals are stable in multiple different oxidation states. If the metal has any unpaired d-electrons, it can have magnetic properties. Many transition metals have uses as catalysts in bond-forming reactions, and we’ll talk about a few later in this chapter.

      What is a metalloid?

      Metalloid