Stereoisomers resulting from hindered rotation about solitary bonds where the steric strain barrier to rotation is high enough to enable for the isolation that the conformers.

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From: considerable Organic synthetic (Second Edition), 2014

Related terms:


Atropisomer

Stereoisomers that result from hindered rotation around a solitary bond.

Chiral derivatizing agent

Chiral reagent the reacts with a compound to form a covalent bond.

Chiral solvating agent

Chiral reagent the associates through a compound v non-covalent interactions.

Diastereomers

Stereoisomers that space not enantiomers or mirror photos of each other and have various reactivity and physical properties.

Diastereotopic groups

Two teams in a molecule that are different and, if replaced, generate compounds that are stereoisomers.

Dipolar coupling

A magnetic communication that arises between two corpuscle such as hydrogen nuclei v non-zero spin.

Kinetic resolution

When two enantiomers have various rates that reaction.

Prochiral

Refers to molecule that can be converted from achiral to chiral in a solitary step.

Quadrupolar coupling

An interaction that wake up in nuclei with more than two various spin says (I>½).

Residual dipolar coupling

A weak type of dipolar coupling that occurs in partly oriented media.


Josef Michl, in Encyclopedia the Physical science and an innovation (Third Edition), 2003

I.D.2 Stereoisomers

Stereoisomers have the very same connectivity but differ in the method in i m sorry the constituent atoms are oriented in space. They deserve to be divided into configurational stereoisomers and conformational stereoisomers. The an accurate specification the the spatial setup of the teams in a configurational isomer is called its configuration, and also in a conformational isomer, its conformation.

I.D.2.a Configurational stereoisomers

These stereoisomers cannot be make superimposable by any kind of rotations about single bonds. In order to do them superimposable, rotation around a double bond or a dissociation that one or much more single bonds, or both, is essential (e.g.,6 and also 8). Because these processes normally require significant energy, castle usually carry out not occur at a measurable price at room temperature. Configurational stereoisomers can usually be isolated from one another and also stored basically indefinitely in ~ room temperature.

I.D.2.b Conformational stereoisomers

Often called conformers for short, this stereoisomers have the right to be do superimposable by rotations about single bonds. Examples are the axial (21) and equatorial(22) conformers the a monosubstituted


21
*

22
*

cyclohexane. Since rotations about solitary bonds are normally an extremely facile, the is usually difficult to separate conformers native one another and also to handle them individually at room temperature.


I.D.2.c Chirality

Another important classification of stereoisomers right into two teams is pertained to optical activity, materialized by the rotation of a aircraft of polarized irradiate on passage v a sample.


A pair that stereoisomers the are related to one another in the same way as an object and the mirror picture are referred to as enantiomers (e.g.,23 and 24):


23
*

24
*

Any pair that stereoisomers that room not associated in this means are referred to as diastereomers (e.g.,6 and 8 or21 and also 22).


A molecule that is not the same with that is mirror image is referred to as chiral and also occurs together a pair the enantiomers. The necessary and sufficient condition for chirality is the absence of an wrong rotational axis of the contrary in the molecule (this consists of a center of inversion and also mirror plane symmetry elements).

A mixture of equal quantities of two enantiomers is well-known as a racemic modification and is optically inactive. A pure enantiomer or one unbalanced mixture of 2 enantiomers is optically active; the 2 enantiomers have actually opposite handedness and cause the aircraft of polarization to turn in the opposite directions.


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Stereoisomers having actually instability at a stereogenic center can reaction to produce stereochemically amendment forms.15–18 This can be prompted by an nearby group the decreases stability at the chiral center.15 an enantiomer have the right to irreversibly ‘racemize’ and also reach the racemic mixture in ~ equilibrium. It can likewise ‘enantiomerize,’ or reversibly interconvert come the various other enantiomer. Diastereomers deserve to ‘diastereomerize’ or ‘epimerize’ by conversion of an turbulent stereogenic center. For example, paclitaxel (I) epimerizes in organic options under an easy conditions in ~ the 7 place (II) in addition to other decomposition reactions (Figure 6).19 Thalidomide enantiomerizes at an easy pHs and also in solution with person serum albumin (likely due to catalysis by Arg and also Lys residues).18


*

C.M. Galea, ... D. Mangelings, in Supercritical fluid Chromatography, 2017

Abstract

Stereoisomers space isomeric molecules that have the same molecular constitution, but a various three-dimensional spatial setup of the atoms. These could be extensively classified as enantiomers and also diastereomers. Various enantiomers bind to different receptors and also possibly bring about different reactions. Because that this reason, stereoselective separation approaches are an useful tools in the pharmaceutical, food, and agricultural fields. Liquid chromatography and also supercritical liquid chromatography room the most popular techniques provided for chiral separations. This chapter presents review of the different chromatographic parameters the are crucial when arising a chiral method in SFC. Various stationary phases and also their properties room described and also a comparison of SFC with other techniques, also suitable because that enantioselective separations, is made. Review of applications in the pharmaceutical, agricultural, and food industries, using SFC, native the last 10 years is presented. Diastereomers can be separated using timeless separation methods, and also a short mention of these applications is also made.


Robert J. Ouellette, J. David Rawn, in organic Chemistry research Guide, 2015

Keys come the Chapter

8.1 configuration of Molecules

Stereoisomers have different configurations. The hatchet “configuration” refers to the arrangement of atom in space. Geometric isomers, i m sorry we previously studied in cycloalkanes and alkenes, are additionally stereoisomers.

8.2 winter Images and Chirality

Some molecules have actually mirror photos that room not superimposable. Together molecules are chiral. Molecule that have actually a plane of symmetry are achiral; they room superimposable on their mirror image.

A stereogenic center in an essential molecule is a carbon atom bonded to four different atoms or teams of atoms. The is also called a chiral center. By inspecting the atom or teams of atoms bonded to each carbon atom in a molecule, us can conveniently identify any chiral centers. If a carbon atom is external inspection to two or an ext identical atoms or groups, such as two hydrogen atom or two methyl groups, it is no a chiral center. If a carbon atom is bonded to four various atoms or groups, it is a chiral center, and the molecule has actually a nonsuperimposable winter image. The two feasible isomers having different configurations at a chiral facility are enantiomers.

Another way to determine a molecule together chiral or achiral is come look because that a aircraft of symmetry. A airplane of symmetry have the right to bisect atoms, groups of atoms, and also bonds between atoms. In a molecule v a airplane of symmetry, one next of the molecule is the mirror picture of the other side. Thus, a molecule through a airplane of the opposite is achiral. If a molecule contains two or much more chiral centers and also does not have actually a airplane of symmetry, the is chiral. If is has actually a airplane of symmetry, that is an achiral meso compound.

Pairs that enantiomers have actually the exact same physical properties but behave in different ways in a chiral atmosphere such together a chiral binding site in one enzyme. Most of the molecules isolated from living organisms space chiral. They create a chiral environment that enables distinctions to be made between enantiomers.

8.3 Optical Activity

Each member of a pair that enantiomers rotates the airplane of polarized light in one instrument called a polarimeter. This phenomenon is referred to as optical activity. The rotation observed for one enantiomer is equal in magnitude but opposite in direction for the other enantiomer. A chiral substance the rotates plane-polarized irradiate clockwise is dextrorotatory; a chiral substance the rotates plane-polarized irradiate counterclockwise is levorotatory. The quantity of rotation under characterized standard experimental conditions is the details rotation. Optical purity is a measure up of the overfill of one enantiomer over another in a mixture.

8.4 Fischer forecast Formulas

Enantiomers in a Fischer projection are attracted according come the following conventions:

1.

Arrange the carbon chain vertically v the most oxidized team (—CHO in glyceraldehyde) at the “top.”

2.

Place the carbon atom in ~ the chiral center in the aircraft of the paper. The is C-2 in glyceraldehyde.

3.

C-2 is bonded to four groups, the CHO group and the CH2OH group prolong behind the plane of the page, and also the hydrogen atom and the hydroxyl group extend up and out of the plane.

4.

Project these four groups top top a plane. The carbon atom at the chiral center usually not presented in this convention. The is situated at the point where the shortcut lines cross. The upright lines job away from the viewer. The horizontal lines project toward the viewer.

8.5 pure Configuration

The Kahn–Ingold–Prelog configurational nomenclature system, i m sorry is the same for both E,Z geometric isomers and chiral molecules, offers an unambiguous summary of the absolute configuration the a molecule.

Priority is assigned come atoms based upon the atomic numbers of straight bonded atoms. Atom farther under the chain are ignored also though castle may have still higher atomic numbers. Thus, a fluorine atom has a greater priority than a carbon atom even if the carbon atom is bonded to three chlorine atoms; because that example, F— > —CCl3. The chlorine atoms in this situation are irrelevant since the to compare is between the atom numbers the fluorine and carbon.

Once the priority order of the atoms or teams of atom bonded come the chiral carbon atom has been determined, the molecule is regarded through the bond come the lowest priority group. The various other three groups then lied on a circle. If the movement from concerns 1 → 2 → 3 is clockwise, the molecule is R; if the activity is counterclockwise, the configuration is S.

The assignment of R or S configuration to a link does not determine its optical rotation as being either (+) or (−). The direction that optical rotation is experimentally established with a polarimeter. The absolute configuration is experimentally figured out by X-ray crystallography.

8.6 Molecules with Two or much more Stereogenic Centers

Some molecules have two or more stereogenic centers. The result stereochemistry depends on whether those centers are equivalent or nonequivalent. Equivalent sterogenic centers have actually identical to adjust of substituents. Because that n nonequivalent centers, there space 2n stereoisomers. Some of those isomers room pairs that enantiomers. This stereoisomers have actually opposite configurations at every center and are for this reason mirror images. All other stereoisomers room termed diastereomers.

The configuration of every stereogenic facility is determined independently. Then, the construction of each facility is created as R or S. Because that example, the enantiomer the a molecule with a stereogenic facility 2S,3R is 2R,3S. Any other combination—2S,3S or 2R,3R— is a diastereomer.

Compounds through two or more equivalent stereogenic centers have actually fewer stereoisomers than predicted by the 2n formula. Several of the stereoisomers have a airplane of symmetry and also are not optically active; they are meso compounds. For two chiral centers, the configurations room R,S, i m sorry is the exact same as S,R because of the airplane of symmetry. The isomers R,R and also S,S are optically active and room enantiomers.

8.7 Cyclic Compounds v Stereogenic Centers

Cyclic compounds can have stereogenic centers. We apply the exact same rules to assign configuration to cyclic compounds and also acyclic compounds. The only distinction is that we at some point return to the stereogenic facility as we move around the ring. However, in a chiral compound, the suggest of very first difference is reached before that time.

Cyclic compounds having actually two nonequivalent stereogenic centers can exist in four stereoisomeric forms. An interesting feature of this molecules is seen as soon as there are tantamount stereogenic centers. In those cases, there is at the very least one aircraft of symmetry. The plane, in some cases, bisects bonds, and also in other instances bisects the atom of the ring. In this last case, it also bisects the atoms bonded to the stereogenic centers.

8.8 Separation that Enantiomers

Enantiomers have the same physical properties and, therefore, can not be be separated by physics methods. However, diastereomers have various physical properties and can be separated. Figure 8.18 illustrates the counter of a mixture that enantiomers right into a mixture the diastereomers. The diastereomers space separated, ~ which they are broken down to achieve one enantiomer indigenous one diastereomer and also the other enantiomer from the second diastereomer. Chiral chromatography provides a method to separate enantiomers based upon your diastereomeric interactions through a chiral obelisk support.

8.9 reactions at Stereogenic Centers

If a reaction at a stereogenic center does not change the bonds to the stereogenic center, then the configuration at that center is unchanged.

Reactions in ~ the stereogenic center impact the configuration of the molecule. If the product has actually a configuration opposite the of the reactant, us postulate a shift state in i beg your pardon the nucleophile attacks opposite the bond come the leave group and inverts the configuration together the reaction occurs.

Radical reactions proceed through a planar intermediate, i m sorry is achiral. Thus, succeeding reaction with one more radical can occur with equal probability from either next of the aircraft of the molecule. The result is a racemic mixture.

8.10 formation of Compounds v Stereogenic Centers

Formation of compounds through one stereogenic facility from achiral compounds making use of achiral reagents cannot yield a single stereoisomer. However, in one enzyme catalyzed process, the reaction of an achiral compound generates a single stereoisomer. Such reactions room stereospecific.

In some reactions where 2 stereogenic centers are generated from one achiral substrate, part mechanistic info is obtained based on the diastereomers formed. Because that example, the development of two identical centers can give a mixture of the R,R compound and also the S,S compound. Although not optically active, that result is various than a procedure that offers the R,S (meso) compound.

8.11 reactions that form Diastereomers

If a brand-new stereogenic center is generated in a reaction that a substrate that currently has a stereogenic center, climate a mixture of diastereomers results. The amounts of this isomers space not equal due to the fact that the new center is created in a chiral environment. The overabundance of one diastereomer over one more is referred to as the stereoselectivity of the reaction.

8.12 Prochiral Centers

In a chiral environment, two apparently equivalent groups can it is in distinguished, and also the result product the a reaction including those teams is chiral. The atomic center at i m sorry optical task may an outcome is prochiral. The equivalent teams bonded to the prochiral facility are enantiotopic and also are designated pro-R or pro-S to suggest the potential configuration, R or S, if the group is replaced.

Groups in ~ a prochiral facility in a molecule that consists of a chiral center are diastereotopic. The “faces” the a planar site or functional team that has a facility that can be converted into a stereogenic facility are designated together re or si relying on the priority ranking that the three groups and their plan using the R,S rules.


Harry Salem, Frederick R. Sidell, in Encyclopedia that Toxicology (Second Edition), 2005

Animal Toxicity

The stereoisomers of soman have various median lethal doses. The C(+)P(+) soman and the C(−)P(+) soman are the least toxic and subcutaneous LD50 values ⩾5000 and also ⩾2000 μg kg−1, respectively. The much more toxic stereoisomers, C(−)P(−) soman and also C(+)P(−) soman, have subcutaneous LD50 values of 38 and 99 μg kg−1, respectively. The racemic mixture that soman has a subcutaneous LD50 of 156 μg kg−1 in mice.

The reason of fatality is attributed to anoxia result from a mix of central respiratory paralysis, major bronchoconstriction, and weakness or paralysis the the accessory muscles for respiration.

Signs that nerve certified dealer toxicity vary in rapidity of onset, severity, and also duration the exposure. These room dependent ~ above the particular agent, route of exposure, and also dose. In ~ the greater doses, convulsions and seizures suggest CNS toxicity.

Following nerve certified dealer exposure, pets exhibit hypothermia result from the cholinergic activation the the hypothalamic thermoregulatory center. In addition, plasma concentration of pituitary, gonadal, thyroid, and also adrenal hormones space increased during organophosphate intoxication.


An LCt50 the 30 mg min m−3 to be reported in rats complying with a 30-min inhalation exposure to soman. The acute toxicities by various other routes that exposure in assorted animal species are gift in Table 1.

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Table 1. Acute toxicities the soman in various varieties by various routes of exposure


Route the exposure/speciesLD50 (μg kg−1)
Percutaneous
Rat7800
Subcutaneous
Chicken50
Dog12
Guinea pig24
Monkey13
Rabbit20
Mouse10
Rat71
Intramuscular
Monkey9.5
Mouse89
Rat62
Intraperitoneal
Chicken71
Frog251
Mouse393
Rat98
Intravenous
Cat15
Rat44.5
Mouse35

Two stereoisomers related as nonsuperimposable mirror images are called enantiomers (Greek enantios, opposite + meros, part). We deserve to tell the a substance is chiral and also predict that two enantiomers exist by identifying the substituents on every carbon atom. A carbon atom v four different substituents is a stereogenic center, and a molecule with a stereogenic center is chiral. It can exist as either of a pair the enantiomers. For example, 2-bromobutane is chiral due to the fact that C-2 is attached come four different groups (CH3–, CH3CH2–, Br–, and H–). In contrast, no carbon in 2-bromopropane is external inspection to four various groups; C-2 is bonded to 2 methyl groups. Thus, 2-bromopropane is no chiral.