substitution

Examples of substitution in the following topics:

• Substitution Reactions of Benzene and Other Aromatic Compounds

  • The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box).
  • Many other substitution reactions of benzene have been observed, the five most useful are listed below (chlorination and bromination are the most common halogenation reactions).
  • Since the reagents and conditions employed in these reactions are electrophilic, these reactions are commonly referred to as Electrophilic Aromatic Substitution.
  • The catalysts and co-reagents serve to generate the strong electrophilic species needed to effect the initial step of the substitution.

• The Substitution Rule

  • Integration by substitution, also known as $u$-substitution, is a method for finding integrals.
  • When used in the latter manner, it is often known as $u$-substitution (or $w$-substitution).
  • If we make the substitution $u = x^2 + 1$, we obtain $du = 2x dx$  and
  • An antiderivative for the substituted function can hopefully be determined; the original substitution between $u$ and $x$ is then undone.
  • Use $u$-substitution (the substitution rule) to find the antiderivative of more complex functions

• The Substitution Method

  • The substitution method is a way of solving a system of equations by expressing the equations in terms of only one variable.
  • Solve this equation, and then back-substitute until the solution is found.
  • Next, we will substitute our new definition of x into the second equation:
  • To do this, substitute the value of y into the first equation and solve for x.
  • Check the solution by substituting the values into one of the equations.

• Substitution of the Hydroxyl Hydrogen

• Substitution

  • This apparent nucleophilic substitution reaction is surprising, since aryl halides are generally incapable of reacting by either an SN1 or SN2 pathway.
  • To explain this, a third mechanism for nucleophilic substitution has been proposed.
  • Three additional examples of aryl halide nucleophilic substitution are presented on the right.
  • Only the 2- and 4-chloropyridine isomers undergo rapid substitution, the 3-chloro isomer is relatively unreactive.
  • Some distinguishing features of the three common nucleophilic substitution mechanisms are summarized in the following table.

• Reactions of Aromatic Compounds

  • Aromatic compounds can participate in a range of reactions including substitution, coupling, and hydrogenation reactions.
  • An example of an aromatic substitution reaction is shown below.
  • A number of patterns have been observed regarding the reaction of substituted benzene rings.
  • Example of an aromatic substitution reaction.
  • As an exercise, draw out the stabilization of the positive charge when ortho substitution occurs.

• Electrophilic Substitution of the Phenol Aromatic Ring

  • The facility with which the aromatic ring of phenols and phenol ethers undergoes electrophilic substitution has been noted.
  • The sodium salt of salicylic acid is the major product, and the preference for ortho substitution may reflect the influence of the sodium cation.

• Artificial Blood Substitutes

  • The benefits to having a blood substitute are substantial.
  • Blood substitutes are useful for many reasons.
  • For example, the cost of blood substitutes may fall as manufacturing becomes refined.
  • There are also risks associated with blood substitutes.
  • Hemoglobin-based blood substitutes may increase the odds of deaths and heart attacks.

• Trigonometric Substitution

  • Trigonometric functions can be substituted for other expressions to change the form of integrands and simplify the integration.
  • Trigonometric functions can be substituted for other expressions to change the form of integrands.
  • The following are general methods of trigonometric substitution, depending on the form of the function to be integrated.
  • Note that, for a definite integral, one must figure out how the bounds of integration change due to the substitution.
  • In order to better understand these substitutions, let's go over the derivation of some of them.

• Reactions at the α-Carbon

  • Many aldehydes and ketones undergo substitution reactions at an alpha carbon, as shown in the following diagram (alpha-carbon atoms are colored blue).
  • First, these substitutions are limited to carbon atoms alpha to the carbonyl group.
  • Cyclohexanone (the first ketone) has two alpha-carbons and four potential substitutions (the alpha-hydrogens).
  • The second ketone confirms this fact, only the alpha-carbon undergoing substitution, despite the presence of many other sites.
  • Second, the substitutions are limited to hydrogen atoms.