10.8 Determining the Function of the Reagent

10.8 Determining the Function of the Reagent

  • The last three sections help you become competent in performing all three steps.
    • Step 1 is determining the function of the reagent.
  • The function of the reagent is the main difference between substitution and elimination.
    • An elimination reaction occurs when the reagent functions as a base.
    • The first step in any specific case is to determine whether the reagent is a strong or weak base.
    • Students assume that a strong base must also be a strong nucleophile, but this is not always the case.
    • It is possible for a reagent to be weak.
  • It is possible for a reagent to be strong and weak at the same time.
    • Basicity and nucleophilicity do not always match up.
  • Let's compare H N- and HO-.
  • When we saw the factors determining charge stability in Chapter 3, we realized that oxygen is more stable than nitrogen.
  • Let's compare the two.
    • In Chapter 3, we saw that sulfur, being larger than oxygen, can be better at stabilizing a charge than oxygen can be.
    • HO- will be a stronger base because it will be more stable.
    • HO- is a better nucleophile than HS-.
  • Basicity and nucleophilicity are different concepts.
    • Basicity and nucleophilicity are measures of how stable the charge is.
    • An interesting effect comes into play when you have a large atom.
    • The electron density within the sulfur atom can move around as the sulfur atom approaches a compound with d+).
    • The rate of attack is very fast because of this effect.
    • The same is true for most of the halides.
    • The halides are too weak to be bases.
    • You don't need to worry about elimination reactions when you see one of these nucleophiles.
    • It is helpful to know that you don't need to worry about elimination reactions when you see a halide as a reagent.
  • Let's look at each category quickly.
    • The first category has reagents that only function as nucleophiles.
    • They are strong nucleophiles, but they are weak bases.
    • You should only look at substitution reactions when you see a reagent from this category.
    • This category does not include sulfuric acid.
    • The sulfur atom in sulfuric acid doesn't have a single pair, so it can't function as a nucleophile.
    • The name implies that sulfuric acid only works as an acid, so it is not listed in any of the other categories.
  • The second category contains only bases and not nucleophiles.
    • The hydride ion is the first reagent on the list.
    • Despite the presence of a negative charge, the hydride ion of NaH is not a good nucleophilic.
    • The hydride ion is a strong base.
  • The use of a hydride ion as a reagent shows that elimination will occur rather than substitution.
  • The second and third categories have butoxide in them.
    • It's a strong base and a strong nucleophile, so it's in the third category.
    • The sterically hindered butoxide prevents it from functioning as a nucleophile in most cases.
    • It is often used as a base to favor E2 over S 2.
  • There are reagents in the third category that are strong.
    • These reagents are used for bimolecular processes.
  • There are reagents in the fourth and final category that are weak.
  • The first step in predicting the products of a reaction is determining the nature and identity of the reagent.
    • To determine the category to which it belongs, you must analyze the reagent.
  • Let's practice with this skill.
  • The function of each of the reagents can be identified.
  • There are three main steps for predicting substitution and elimination reactions.
    • The first step is determining the function of the reagent.
    • The second step of the process in which we analyze the substrate and identify which mechanism operates is explored in this section.
  • There are four different categories of reagents.
    • The expected outcome must be explored with a primary, secondary, or tertiary substrate.
    • The flow chart summarizes all of the relevant information.
    • It is important to know the flow chart, but not to memorize it.
    • Understanding the reasons for all of these outcomes is more important.
    • A proper understanding on an exam will prove to be more useful than a set of rules.
  • The mechanism that operates is determined by the substrate.
  • For primary, secondary, and tertiary substrates, buOK can occur.
  • Bimolecular reactions are preferred when the reagent is strong and the base is strong.
  • The reactions are too slow.
  • Unimolecular reactions are favored under these conditions.
    • A tertiary alcohol will have an E1 reaction when treated with acid and heat.
  • The flow chart can be used to figure out which mechanism operates in a specific case.
    • Let's practice.
  • Identifying the function of the reagent is the first step.
  • The next step is to identify the material.
  • Secondary substrates give N to steric hindrance.
  • Each of the following cases is expected to have a mechanism.
    • Don't worry about drawing the products yet.
    • In the next section, we will do that.
    • Pick out which mechanisms are operating.