leaving group

Examples of leaving group in the following topics:

• Elimination

  • Elimination is a type of organic reaction in which a leaving group and vicinal hydrogen are removed, leaving behind a double bond.
  • In unimolecular elimination (E1), a leaving group first detaches itself from a carbon atom, leaving behind a carbocation.
  • In bimolecular elimination (E2), a base deprotonates a carbon vicinal to a leaving group, and the electrons from the C-H bond form a double bond with the adjacent carbon, displacing the leaving group.
  • As in SN2, the leaving group (LG) is "pushed" away by electrons that access the C-LG antibonding orbital.
  • In the case of E2, at the time of deprotonation the hydrogen must be antiperiplanar to the leaving group.

• Nucleophilic Substitution

  • Typically, strength of a leaving group's conjugate acid correlates with its lability.
  • In SN1 reactions, the rate-determining step is the removal of the leaving group.
  • Leaving groups are almost always negative.
  • In SN2, the nucleophile "pushes" the leaving group off the carbon in the R group.
  • Leaving groups for SN2 reactions can be similar to those used in SN1.

• Substitution and Elimination Reactions of Amines

  • Amine functions seldom serve as leaving groups in nucleophilic substitution or base-catalyzed elimination reactions.
  • In the case of alcohols and ethers, a useful technique for enhancing the reactivity of the oxygen function was to modify the leaving group (OH(–) or OR(–)) to improve its stability as an anion (or equivalent).
  • As noted earlier, 1º and 2º-amines are much weaker acids than alcohols, so it is not surprising that it is difficult to force the nitrogen function to assume the role of a nucleophilic leaving group.
  • In this context we note that the acidity of the putative ammonium leaving group is at least ten powers of ten less than that of an analogous oxonium species.
  • The favored anti orientation of the leaving group and beta-hydrogen, noted for dehydrohalogenation, is found for many Hofmann eliminations; but syn-elimination is also common, possibly because the attraction of opposite charges orients the hydroxide base near the 4º-ammonium leaving group.

• Elimination Reactions

  • The use of thionoesters, such as a xanthates, as radical generating functions was described above, and these groups may also serve as excellent radical leaving groups.
  • Once again, the tolerance of radical reactions for a variety of functional groups is demonstrated.

• Rearrangements of Cationic Oxygen

  • Removal of hydroxide anion from a hydroperoxide is energetically unfavorable, unless it is initially converted to a better leaving group in a manner similar to that used to facilitate substitution reactions of alcohols.
  • By protonating the hydroxyl group, the leaving group becomes water, thus generating an oxacation.
  • The migratory aptitude of various substituent groups (e.g. 1R & 2R) is generally: 3º-alkyl > 2º-alkyl ~ benzyl ~ phenyl > 1º-alkyl > methyl.
  • Stereoelectronic factors favor an anti-periplanar orientation of the migrating group to the leaving moiety, and will control the rearrangement in some cases.
  • In example #3 it is interesting that migration of the bridgehead 3º-alkyl group is preferred over a possible phenyl shift.

• Alcohols

  • Alcohols are functional groups characterized by the presence of an -OH group.
  • Alcohols are organic compounds in which the hydroxyl functional group (-OH) is bound to a carbon atom.
  • The presence of the -OH groups allows for hydrogen bonding with other -OH groups, hydrogen atoms, and other molecules.
  • Hydroxyl groups alone are not considered good leaving groups.
  • Often, their participation in nucleophilic substitution reactions is instigated by the protonation of the oxygen atom, leading to the formation a water moiety—a better leaving group.

• Gaps in the Fossil Record

  • Because of the specialized and rare conditions required for a biological structure to fossilize, many important species or groups may never leave fossils at all.
  • Even if they do leave fossils, humans may never find them—for example, if they are buried under hundreds of feet of ice in Antarctica.
  • The fossil record is very uneven and is mostly comprised of fossils of organisms with hard body parts, leaving most groups of soft-bodied organisms with little to no fossil record.
  • Groups considered to have a good fossil record, including transitional fossils between these groups, are the vertebrates, the echinoderms, the brachiopods, and some groups of arthropods.

• Diversity of Gymnosperms

  • Gymnosperms are a diverse group of plants the protect their seeds with cones and do not produce flowers or fruits.
  • The fourth phylum (the Gnetophyta) are considered the closest group to angiosperms because they produce true xylem tissue.
  • The single surviving species of the gingkophytes group is the Gingko biloba .
  • Like angiosperms, they have broad leaves.
  • Notice the yellow leaves of the tamarack.

• Molecularity

  • For example, if a crowd is leaving a theater through a single exit door, the time it takes to empty the building is a function of the number of people who can move through the door per second.
  • Once a group gathers at the door, the speed at which other people leave their seats and move along the aisles has no influence on the overall exit rate.

• Formal Structure

  • Formal structure of an organization or group includes a fixed set of rules for intra-organization procedures and structures.
  • The 1924 Hawthorne studies led to the Human Relations Movement—the researchers of organizational development who study the behavior of people in groups, in particular workplace groups.
  • The formal structure of a group or organization includes a fixed set of rules of procedures and structures, usually set out in writing, with a language of rules that ostensibly leave little discretion for interpretation.
  • Numerous empirical studies in sociological organization research followed, particularly during the Human Relations Movement—the researchers of organizational development who study the behavior of people in groups, in particular workplace groups.
  • As such, it is usually set out in writing, with a language of rules that ostensibly leave little discretion for interpretation.