4) Draw all stereoisomers of 2,3,4-tribromopentane (Fisher or perspective 5) In the boxes provided, give the stereochemical relationship between each pair of. Introduction To Stereochemistry. Consider two of the compounds we produced while finding all the isomers of C. 7. H. 2-methylhexame 2-methylpentane. It is easier to compare molecules is you rotate them all into a comparable position . Say 1 with the central C-3 hydroxyl group out of the plane of.
The images on either side of the plan is the same as the other Figure 4. In this case, the molecule is considered 'achiral'. In other words, to distinguish chiral molecule from an achiral molecule, one must search for the existence of the bisecting plane in a molecule.
All chiral molecules are deprive of bisecting plane, whether simple or complex. As a universal rule, no molecule with different surrounding atoms are achiral.
Chirality is a simple but essential idea to support the concept of stereoisomerism, being used to explain one type of its kind. The chemical properties of the chiral molecule differs from its mirror image, and in this lies the significance of chilarity in relation to modern organic chemistry. Compounds with Multiple Chiral Centers We turn our attention next to molecules which have more than one stereocenter.
We will start with a common four-carbon sugar called D-erythrose. A note on sugar nomenclature: You will learn about this system if you take a biochemistry class. As you can see, D-erythrose is a chiral molecule: C2 and C3 are stereocenters, both of which have the R configuration. In addition, you should make a model to convince yourself that it is impossible to find a plane of symmetry through the molecule, regardless of the conformation.
Does D-erythrose have an enantiomer? Of course it does — if it is a chiral molecule, it must. The enantiomer of erythrose is its mirror image, and is named L-erythrose once again, you should use models to convince yourself that these mirror images of erythrose are not superimposable.
Notice that both chiral centers in L-erythrose both have the S configuration.
In a pair of enantiomers, all of the chiral centers are of the opposite configuration. What happens if we draw a stereoisomer of erythrose in which the configuration is S at C2 and R at C3?
Chirality and Stereoisomers - Chemistry LibreTexts
This stereoisomer, which is a sugar called D-threose, is not a mirror image of erythrose. D-threose is a diastereomer of both D-erythrose and L-erythrose.
The definition of diastereomers is simple: In practical terms, this means that at least one - but not all - of the chiral centers are opposite in a pair of diastereomers. But molecule 3 has a plane of symmetry, and so it is achiral and not optically active. It is a meso compound one whose molecule is superimposable on its mirror image even though it contains stereogenic centres. The optical activity due to one stereogenic centre is cancelled by that of the other, which is its mirror image.
Perhaps the best known example of this is tartaric acid, which exists as three stereoisomers two enantiomers and a meso compound. Crystals of potassium hydrogen R,R -tartrate are often to be found in the bottom of wine bottles. As with the CIP system, it is necessary to analyse a structure very carefully in order to classify it. Each stereogenic atom is assigned a descriptor R or S. For revision of this method, see section 2.
In this method the absolute configuration of a molecule is defined as D or L according to its relationship with one or other enantiomers of the simplest carbohydrate molecule, glyceraldehyde.
The glyceraldehyde molecules were originally defined as D and L on the basis of their optical activity one is Dextrorotatory and the other is Laevorotatorysome fifty years before their absolute configurations were actually confirmed. Other carbohydrates and amino acids were assigned by comparison to them. Thus, a sugar, represented in a Fischer projection with the most oxidised carbon at the top, is defined as: The words have a carbohydrate origin, and relate to Fischer projections, thus: When the carbon chain is vertical and like substituents are on the same side of the Fischer projection, the molecule is referred to as an erythro diastereoisomer.
When like substituents are on opposite sides of the Fischer projection, the molecule is a threo diastereoisomer. The 'like' substituents do not have to be the same, and the stereogenic centres do not have to be adjacent, so these terms can seem rather loose. They are only useful in special contexts such as the analysis of stereoselective synthetic reactions.
Compounds are often specified with more than one prefix, e. A sign of rotation in the polarimeter gives no clue as to the configuration of a chiral molecule. R or S are simply to help us when we wish to refer to one particular structure. If it is counterclockwise, it is the S enantiomer. Priority is based upon atomic number, i.
Priority assignment is based upon the four atoms directly attached to the stereogenic center. For example, in 2-butanol, the example we considered previously, the four atoms are H,O, and two C's.
Oxygen gets the first priority, and H the fourth. But the methyl and ethyl groups both are attached through carbonso there is initially a tie for the second and third priorities. In this kind of tie situation, priority assignments proceed outward to the next atoms, which we will call the beta atoms. The directly attached atoms are the alpha atoms.
For the methyl group, the alpha atom is carbon and the beta atoms are three H's, while for the ethyl group the alpha atom is also carbon and the beta atoms are two H's and 1 carbon. This beta C of the ethyl group wins the priority competition because there is no beta atom on the methyl group which has an atomic number greater than 1 all three beta atoms are H.
In general, the competition contines from alpha to beta to gamma to delta atoms until a tie-breaker is found. Some additional conventions are necessary for handling multiple bonds and aromatic bonds, and these are a little tricky to learn. As an example, take the vinyl group. Each carbon of this double bond is considered to have two bonds to carbon, because of the double bond. In the case of a carbonyl group, the carbon is considered to be bonded to two oxygens, and the oxygen is considered to be bonded to two carbons.
For this reason, a vinyl group has priority over an isopropyl group, as shown in the illustration. Thus there are four possible stereoisomers. If we designate one stereocenter as "a" and the other as "b" just for labelling purposes, the four stereoisomers can be designated as RaRb,RaSb,SaRb, and SaSb These designations correspond to the cirucumstance theat stereocenter "a" can have the R or S configuration ,and stereocenter "b" can have either configuration.
In general, if there are n such stereogenic centersthere will be a maximum of 2n stereoisomers.
For example, with three stereogenic centers, there are eight possible stereoisomers. The maximum of 2n occurs when there are all non-equivalent stereocenters. Stereogenic centers are equivalent when all four substituents attached to the center are identical. For example, in 2,3-dibromobutane, both stereogenic carbons have a H, a Br, a methyl, and a 1-bromoethyl substituent.
The maximum of four stereoisomers is not observed here, as we saw before. In fact there are three stereoisomers, including one achiral stereoisomer.
Structural Isomers and Stereoisomers
This is because the 2R,3S molecule is identical to the 2S,3R molecule, since carbons 2 and 3 are equivalent.
On the other hand, 2,3-dibromopentane has two non-equivalent stereogenic centers and there are four stereoisomers, consisting of two pairs of enantiomers. It should be noted that the relationship between one enantiomeric pair and the other pair of enantiomers is that they are diastereoisomers.
There is, first of alla pair of enantiomeers: Note that the mirror image of 2R,3R is 2S,3S i. There is also an achiral stereoisomer. A molecule which has stereocenters but is achiral is called a meso compound. We saw in an earlier diagram that this molecule has a point of symmetry in its most stable conformation. It should be noted carefully that the meso isomer is a diastereoisomer of the two enantiomers. They are essentially like any other pair of isomers e.
Since they have the same functional groups, however, they are usually rather similar to one another in their reactions and properties. Two diastereoisomers can usually be separated from one another bye. Although their chemical properties reactions are similar, the two diastereoisomers will typically react at different rates. Therefore 2 enantiomers have exactly the same energy, solubility in typical achiral solvents, boiling and melting points, NMR and IR spectra, etc. Their chemical properties, including both the qualitative reactions and the quantitative rates of reaction are identical when reacting with achiral chemical species.
In general, then, both chemical and physical properties of 2 enantiomers are exactly identical twoard achiral agents,chemical or physical. Also, one physical property which can distinguish them is "optical activity" see below. Thus, we can easily tell, in using our right hand to shake hands with another person, whether that person is using his left or right hand. There is a better "fit" of the two right hands than there is of right hand to left hand.
Chemically this occurs, as noted above, when enantiomers react with another chiral compound. Both the original enantiomer and its reactant distinguish left from rightso then one of the original enantiomers will find a better energetic fit with the chiral compound than will the other.Stereoisomers, enantiomers, diastereomers, constitutional isomers and meso compounds - Khan Academy
One physical property which distinguishes 2 enantiomers is "optical activity". This term refers to the property of chiral compounds exclusively of rotating the plane of plane-polarized light to the right clockwise or to the left counterclockwise. The two enantiomers have exactly the same ability to rotate this plane, quantitatively, but they rotate it in opposite senses. Thus, if one enantiomer rotates the plane by Since the exact amount of the rotation of the plane by a given enantiomer depends upon how much of that enentiomer the light encounters as it passes through the solution, the measured rotation is divided by the concentration of the enantiomer and by the path length of the polarimeter cell to give a true measure of the inherent ability of the enantiomer to rotate the plane of polarized light.
This number is called the specific rotation.