racemic mixture

Examples of racemic mixture in the following topics:

• Stereogenic Nitrogen

  • It rapidly inverts its configuration (equilibrium arrows) by passing through a planar, sp2-hybridized transition state, leading to a mixture of interconverting R and S configurations.
  • If the nitrogen atom were the only chiral center in the molecule, a 50:50 (racemic) mixture of R and S configurations would exist at equilibrium.
  • If other chiral centers are present, as in the ephedrin isomers, a mixture of diastereomers will result.

• 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).
  • Separation of racemates into their component enantiomers is a process called resolution.
  • Diastereomers, on the other hand, have different physical properties, and this fact is used to achieve resolution of racemates.
  • Reaction of a racemate with an enantiomerically pure chiral reagent gives a mixture of diastereomers, which can be separated.
  • 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.

• Synthesis of Amino Acids

  • If pure L or D enantiomers are desired, it is necessary to resolve these racemic mixtures.
  • A common method of resolving racemates is by diastereomeric salt formation with a pure chiral acid or base.
  • The racemic amino acid is first converted to a benzamide derivative to remove the basic character of the amino group.
  • Since the amino acid moiety is racemic and the base is a single enantiomer (levorotatory in this example), an equimolar mixture of diastereomeric salts is formed (drawn in the green shaded box).
  • If the racemic mixture of amides shown in the green shaded box above is treated with this enzyme, the L-enantiomer (whatever its rotation) will be rapidly converted to its free zwitterionic form, whereas the D-enantiomer will remain largely unchanged.

• Hydroboration

  • When this selectivity results in the formation of an excess of one enantiomer over the other from an achiral or racemic substrate it is sometimes called asymmetric induction.
  • Hydroboration of the prochiral alkene 1-phenylcyclopentene, followed by peroxide oxidation, yields trans-2-phenylcyclopentanol as a racemic mixture.
  • Two new stereogenic centers have been created in this diastereoselective reaction, but since reaction takes place equally at both faces of the double bond, a 50:50 mixture of enantiomers is obtained (equation 1 of the second diagram below).

• Addition Reactions Initiated by Electrophilic Halogen

  • In the absence of chiral catalysts or reagents, reactions of this kind will always give racemic mixtures if the products are enantiomeric.
  • Anti-addition to cis-2-butene gives the racemic product, whereas anti-addition to the trans-isomer gives the meso-diastereomer.

• Anchimeric Assistance

  • For example, ethyl chloride, neopentyl chloride (2,2-dimethylpropyl chloride) and 2,2,2-triphenylethyl chloride are all 1º-alkyl chlorides, which hydrolyze in wet formic acid to mixtures of alcohols and olefins (SN1 & E1 mechanisms).
  • In both cases, the 3º-carbocation intermediate finally disproportionates to a mixture of substitution and elimination products.
  • Furthermore,the erythro compound retains its enantiomeric purity; whereas the threo tosylate gives racemic acetate and is itself racemized during reaction.
  • As a result of equal rates of product formation by acetate bonding to C2 & C3, the achiral threo intermediate yields a 50:50 (racemic) mixture of threo enantiomers: (2R,3S) from the blue arrows and (2S, 3R) from the green arrows.
  • Since the initial ionization to phenonium intermediates is reversible, we are not surprised to find that unreacted erythro tosylate is unchanged; whereas, unreacted threo tosylate is racemized.

• Enantioselective Aldol Reactions

  • Aldol reactions of prochiral donor and acceptor reactants produce racemic mixtures of chiral adducts.
  • Racemization of the α-carbon is possible, but seems to be negligible.
  • Statistically, the reaction of a prochiral enolate with a prochiral aldehyde is likely to produce a mixture of four diastereomeric aldols as their racemates.

• Total Synthesis

  • Thalidomide , a drug used to relieve morning sickness in pregnant women in the 1950s, was initially sold as a racemic mixture of enantiomers.

• Optical Activity

  • A 50:50 mixture of enantiomers has no observable optical activity.
  • Such mixtures are called racemates or racemic modifications, and are designated (±).
  • A racemate may be separated into its component enantiomers.
  • A pure enantiomer may be transformed into its racemate.
  • This process is called racemization.

• Epoxidation

  • Note that all the compounds in this equation are chiral and racemic.
  • For clarity the chiral carbinol group is drawn in its R-configuration, but the racemic alcohol would yield the same mixture of diastereomers as racemates.
  • For the racemic alcohol in which R2 (or R3) is a cyclohexyl group, the S-enantiomer reacts over 100 times faster than the R-enantiomer, presumably due to steric hindrance of R2.