limiting reagent

Examples of limiting reagent in the following topics:

• Limiting Reagents

  • The reagent that limits how much product is produced (the reactant that runs out first) is known as the limiting reagent.
  • If the amount of B present is less than is required, then B is the limiting reagent.
  • Since there is only 0.28 mol C2H3Br3 present, C2H3Br3 is the limiting reagent.
  • The reactant that produces the least amount of product is the limiting reagent.
  • Determine the limiting reagent and the amount of a product formed in a given reavion

• Calculating Theoretical and Percent Yield

  • A reaction should theoretically produce as much of the product as the stoichiometric ratio of product to the limiting reagent suggests.
  • The theoretical yield of a reaction is 100 percent, but this value becomes nearly impossible to achieve due to limitations.
  • Next, identify the limiting reagent.
  • This video explains the concept of a limiting reactant (or a limiting reagent) in a chemical reaction.
  • It also shows how to calculate the limiting reactant and the percent yield in a chemical reaction.

• Functionalized Organometallic Reagents

  • However, the reactivity of these powerful nucleophiles limits the functional groups that are tolerated in their preparation and use.
  • The first example is a conjugate addition of the mixed zinc-copper reagent to an unsaturated nitro compound.
  • Finally, the allylzinc reagent in equation #(ii) rearranges in the course of the coupling.
  • This brief discussion of functionalized organometallic reagents would not be complete without illustrating the synthetic utility of low temperature magnesium-halogen exchange reactions involving simple Grignard reagents.
  • The second example is more complex, and over a sequence of half a dozen steps, both aryl iodides are converted to Grignard reagents which are then converted to copper reagents prior to coupling reactions with alkyl halides.

• Organometallic Reagents from Geminal Dihalides

  • Lombardo's reagent, and several similar organotitanium compounds (e.g.
  • Tebbe's reagent) act to methylenate carbonyl groups.
  • In this sense they mimic Wittig reagents.
  • Because they are less basic than the alkylidenenephosphorane Wittig reagents, Lombardo reagents do not epimerize sensitive ketones, such as used in reaction (ii).
  • By converting these lithium reagents to Gilman (cuprate) reagents, conjugate addition to enones may be accomplished.

• Reactions with Organometallic Reagents

  • The facile addition of alkyl lithium reagents and Grignard reagents to aldehydes and ketones has been described.
  • The aldehyde or ketone product of this elimination then adds a second equivalent of the reagent.
  • The acidity of carboxylic acids and 1º & 2º-amides acts to convert Grignard and alkyl lithium reagents to hydrocarbons (see equations), so these functional groups should be avoided when these reagents are used.
  • Two such modifications that have proven effective are the Gilman reagent (R2CuLi) and organocadmium reagents (prepared in the manner shown).
  • Imines themselves do not react with Grignard reagents.

• Reactants and Reagents

  • In contrast, both alkyl bromides form Grignard reagents (RMgBr) on reaction with magnesium.
  • Apparently minor changes in a reagent may lead to a significant change in the course of a reaction.

• Important Reagent Bases

  • The significance of all these acid-base relationships to practical organic chemistry lies in the need for organic bases of varying strength, as reagents tailored to the requirements of specific reactions.
  • The common base sodium hydroxide is not soluble in many organic solvents, and is therefore not widely used as a reagent in organic reactions.
  • Most base reagents are alkoxide salts, amines or amide salts.

• Reactions of Substituent Groups

  • The reaction of alkyl and aryl halides with reactive metals (usually Li & Mg) to give nucleophilic reagents has been noted.
  • Many reactions of these aryl lithium and Grignard reagents will be discussed in later sections, and the following equations provide typical examples of carboxylation, protonation and Gilman coupling.
  • Just as an expert carpenter must understand the characteristics and limitations of his/her tools, chemists must appreciate the nature of their "tools" when applying them to a specific synthesis.

• Resolution of Racemates

  • As noted earlier, chiral compounds synthesized from achiral starting materials and reagents are generally racemic (i.e. a 50:50 mixture of enantiomers).
  • Reaction of a racemate with an enantiomerically pure chiral reagent gives a mixture of diastereomers, which can be separated.
  • Reversing the first reaction then leads to the separated enantiomers plus the recovered reagent.
  • The following diagram illustrates this general principle by showing how a nut having a right-handed thread (R) could serve as a "reagent" to discriminate and separate a mixture of right- and left-handed bolts of identical size and weight.

• Irreversible Addition Reactions

  • The following table summarizes some important characteristics of these useful reagents.
  • This equation is typical in not being balanced (i.e. it does not specify the stoichiometry of the reagent).
  • In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to reduce alkenes.
  • The two most commonly used compounds of this kind are alkyl lithium reagents and Grignard reagents.
  • Two additional examples of the addition of organometallic reagents to carbonyl compounds are informative.