substitution effect

Examples of substitution effect in the following topics:

• Applications of Principles on Consumer Choices

  • The income effect and substitution effect combine to create a labor supply curve to represent the consumer trade-off of leisure and work.
  • Central principles to analyzing consumer actions and choices are income effect and the substitution effect, which ultimately generate a labor supply to illustrate the labor-leisure trade-off for consumers.
  • The substitution effect is closely related to that of the income effect, where the price of goods and a consumers income will play a role in the decision-making process.
  • In the substitution effect, a lower purchasing power will generally result in a shift towards more affordable goods (substituting cheaper in place of more expensive goods) while a higher purchasing power often results in substituting more expensive goods for cheaper ones.
  • Explain the labor-leisure tradeoff in terms of income and substitution effects

• Electrophilic Substitution of Disubstituted Benzene Rings

  • When a benzene ring has two substituent groups, each exerts an influence on subsequent substitution reactions.
  • The activation or deactivation of the ring can be predicted more or less by the sum of the individual effects of these substituents.
  • The site at which a new substituent is introduced depends on the orientation of the existing groups and their individual directing effects.
  • Symmetry, as in the first two cases, makes it easy to predict the site at which substitution is likely to occur.
  • The major products of electrophilic substitution, as shown, are the sum of the individual group effects.

• 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.

• Substitution Reactions of Benzene Derivatives

  • The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects:
  • The first is the inductive effect of the substituent.
  • The second effect is the result of conjugation of a substituent function with the aromatic ring.
  • In the case of the nitrogen and oxygen activating groups displayed in the top row of the previous diagram, electron donation by resonance dominates the inductive effect and these compounds show exceptional reactivity in electrophilic substitution reactions.
  • We have already analyzed the activating or deactivating properties of substituents in terms of inductive and resonance effects, and these same factors may be used to rationalize their influence on substitution orientation.

• Nucleophilic Substitution of the Hydroxyl Group

  • In fact ethyl alcohol is often used as a solvent for alkyl halide substitution reactions such as this.
  • The only problem with this strategy is that many nucleophiles, including cyanide, are deactivated by protonation in strong acid, effectively removing the nucleophilic co-reactant needed for the substitution.
  • The following equations illustrate some substitution reactions of alcohols that may be effected by these acids.
  • The following diagram shows some modifications that have proven effective.
  • The importance of sulfonate esters as intermediates in many substitution reactions cannot be overstated.

• Artificial Blood Substitutes

  • The benefits to having a blood substitute are substantial.
  • Blood substitutes are useful for many reasons.
  • Synthetic oxygen carriers may also show potential for cancer treatment, as their reduced size allows them to diffuse more effectively through poorly-vasculated tumor tissue, increasing the effectiveness of treatments like photodynamic therapy and chemotherapy.
  • Transfused blood is currently more cost effective, but there are reasons to believe this may change.
  • There are also risks associated with blood substitutes.

• The Alkyl Moiety

  • If we examine a series of alkyl bromide substitution reactions with the strong nucleophile thiocyanide (SCN) in ethanol solvent, we find large decreases in the rates of reaction as alkyl substitution of the alpha-carbon increases.
  • Furthermore, β-alkyl substitution also decreases the rate of substitution, as witnessed by the failure of neopentyl bromide, (CH3)3CCH2-Br (a 1º-bromide), to react.
  • Inversion of configuration during nucleophilic substitution has also been confirmed for chiral 1º-halides of the type RCDH-X, where the chirality is due to isotopic substitution.
  • The following models clearly show this "steric hindrance" effect.
  • The stereoselectivity of SN2 reactions is in large part due to a stereoelectronic effect.

• Substitution of the Hydroxyl Hydrogen

  • Likewise, the phenolate anion is an effective nucleophile in SN2 reactions, as in the second example below.

• Nucleophilic Substitution

  • Generally, the form of a nucleophilic substitution reaction can be expressed as:
  • In unimolecular nucleophilic substitution (SN1), a leaving group is replaced by a nucleophile in a two-step process.
  • Halide anions are effective in SN1, and even water can replace a leaving group (the water's O atom bonds to the carbon, and one of the H atoms is removed to form an alcohol).
  • Bimolecular nucleophilic substitution (SN2) occurs in a one-step process that can be modeled as:
  • Lesser substitution on the carbon (more hydrogen and fewer carbon atoms attached) favors SN2.

• Substitution and Elimination

  • Replacement or substitution of the halogen on the α-carbon (colored maroon) by a nucleophilic reagent is a commonly observed reaction, as shown in equations 1, 2, 5, 6 & 7 below.
  • It is also worth noting that sp2 hybridized C–X compounds, such as the three on the right, do not normally undergo nucleophilic substitution reactions, unless other functional groups perturb the double bond(s).
  • When several reaction variables may be changed, it is important to isolate the effects of each during the course of study.
  • For example, we can examine the effect of changing the halogen substituent from Cl to Br to I, using ethyl as a common R–group, cyanide anion as a common nucleophile, and ethanol as a common solvent.
  • We would find a common substitution product, C2H5–CN, in all cases, but the speed or rate of the reaction would increase in the order: Cl < Br < I.