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Lab Stereochemistry Purpose In this exercise we will investigate stereochemistry, a branch of chemistry that deals with the spatial arrangement of atoms and groups in molecules. Students will learn...

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Lab Stereochemistry
Purpose
In this exercise we will investigate stereochemistry, a
anch of chemistry that deals with the spatial a
angement of atoms and groups in molecules. Students will learn how to differentiate between different types of isomers including constitutional isomers and stereoisomers including both enantiomers and diastereomers. This will prepare students to better understand why molecules that are structurally similar will behave differently based on their 3D a
angement.
Learning objectives are:
· Identify constitutional isomers.
· Identify enantiomers.
· Identify diastereomers.
· Differentiate between identical compounds and isomers.
· Develop additional skills in visualizing 3D models of molecules.
· Identify the presence or absence of chirality centers.
Knowledge
This assignment will help you review & strengthen knowledge on the following content:
· Constitutional isomers
· VSEPR (valence shell electron pair repulsion) theory and how it governs shapes of molecules
· Rotational limitations for bonds in double bonded, triple bonded and cyclic chemical compounds
Skills
After this assignment you should be able to:
a. Identify and distinguish between identical compounds, isomers, enantiomers and diastereomers.
. Identify and describe chirality centers for a given molecule.
Criteria for Passing Grade
In order to achieve a passing grade student must do the following as a minimum:
· Answer all of the questions using detailed, complete answers with clear reasoning and evidence used as necessary to support your answer.
· It is recommended to compare your lab results with other lab teams. Just be sure all your answers are your own original thoughts and in your own words.
Introduction
Stereochemistry involves the three-dimensional aspects of structures and reactions. The prefix stereo- means three dimensional, as in stereophonic sound, which is sound that seems to come from all sides. Some of the terms that are associated with the study of stereochemistry seem as though they come from a foreign language. Indeed, many of these terms are derived from foreign words. You will become comfortable with these terms as you use them. This laboratory period focuses on the three-dimensional nature of organic molecules. You will examine organic structures in three dimensions with the aid of a model kit. Use a model kit or use the online modeling tools introduced in Lab 1 to build models of the compounds in ths lab.
Isomers
Iso- means the same, and -mer means part. Thus, isomers have the same parts or the same atomic composition. Thus, all structurally unique compounds that have the formula C4H8ClBr are isomers. They are isomers because they have the same parts. In this case, the parts are atoms. Since the number and kind of atoms determines the molecular formula, isomers have the same molecular formula. Thus, two structurally different compounds that have the same molecular formula are isomers. If two compounds have different molecular formulas, they are not isomers. If two structures are identical, they have the same molecular formula. Given two different molecular formulas such as C4H8ClBr and C5H10ClBr, we can tell immediately that they represent different molecules—hence different compounds.
Sometimes, we are given two structural formulas to compare. Two structures may represent the same compound, two different compounds, or two isomers. If the structures differ by one or more atoms, they represent different compounds. If two structures match atom-per-atom in three dimensions, they represent the same compound. Structures that match atom-per-atom in three dimensions are said to be superposable. If two structures with the same molecular formula are not superposable, they are isomers. As a means of classification of organic compounds, we want to determine whether two compounds with identical molecular formulas are identical structurally or are isomers. If they are isomers, we want to determine what kind of isomers they are. They may be constitutional isomers or they may be stereoisomers. If they are stereoisomers, we want to know whether they are enantiomers, diastereomers, or are a special kind called meso forms. Thus, we will always compare two compounds to determine their structural relationship. If we are asked to compare three or more compounds, we will compare them two at a time.
I. Isomers
Question 1: Build models of 1-
omobutane and 1-
omopropane. (Note: If you have questions or doubts about what you are doing during this lab, ask the instructor for assistance.)
a. In the space below, draw skeletal formulas of these compounds. Then, write the molecular formula beneath each structure.
. Are the molecular formulas identical?______
c. Are these compounds isomers or different compounds?_______________
Note: When we compare two compounds and they are not isomers, we are finished with their classification. They are different compounds, not isomers.
Question 2: Build models of 1-
omobutane and 2-
omobutane. (Note: You can use the model of 1-
omobutane from question 1.)
a. In the space below, draw structural formulas for these compounds and write the molecular formula beneath each structure.
. Are the molecular formulas identical?______
c. Are these compounds isomers or different compounds?_______________
Question 3: Build two models of 1-
omobutane. These models are identical.
However, if you align them as BrCH2CH2CH2CH3 and as CH3CH2CH2CH2Br, they look different. Sometimes, we are given two identical structural formulas that are drawn differently, and we must recognize they are identical and classify them accordingly. In other words, two compounds with the same molecular formula may not be isomers; they may be the same compound. Thus, when we encounter two compounds with the same molecular formula, we must determine if the molecules represent the same compound or isomers. Consider 1-
omobutane written as BrCH2CH2CH2CH3 and as CH3CH2CH2CH2Br. If you rotate one of the molecules by 180o, it will match atom-per-atom with the other molecule. Two compounds are identical only if their molecules match atom-per-atom in three dimensions. When two molecules with the same molecular formula match atom-per-atom in three dimensions, they are superposable. Superposable means that, when one molecule is placed on top of another molecule, the atoms match perfectly. If the atoms do not match perfectly, they are isomers.
a. Are the 2 models of 1-
omobutane that you were asked to build above identical?____
. Do they represent identical or different compounds?________________
Let’s review our thought processes. We are given two compounds to classify.
1. First, we determine whether the two structures have identical molecular formulas.
2. If they do not have the same molecular formula, they represent different compounds. If they have the same molecular formulas (the exact same number and kind of atoms), they may be identical compounds or isomers.
3. If the structures of two molecules with the same molecular formulas are superposable, they represent the same compound.
4. If they do not match atom-per-atom, they represent isomers. See Table I.
Question 4: Classify the structures in the following Table as different, identical or isomeric. You may use models.
II. Constitutional Isomers vs Stereoisomers
When we conclude that two compounds are isomers, we are not finished with the classification. We must next determine what kind of isomers they are (i.e., whether they are constitutional isomers or stereoisomers). Constitutional isomers differ in connectivity. Stereoisomers have the same connectivity but differ in three-dimensional space.
Question 5: Build models of 1-chloropropane and 2-chloropropane.
a. These models represent isomers, are they constitutional isomers or stereoisomers?
. Draw skeletal structures for these molecules below.
XXXXXXXXXX-chloropropane XXXXXXXXXX2-chloropropane
These skeletal structures clearly show the connectivity of the ca
on atoms and the chlorine atom. You can see that the chlorine atom is bonded to the end ca
on in 1-chloropropane and to the middle ca
on in 2-chloropropane. The connectivity for 1-chloropropane is Cl-C-C-C and for 2-chloropropane is C-C-(Cl)-C.
c. Are the connectivities identical?________
d. Compounds that differ in connectivity are called constitutional isomers. Are 1-choropropane and 2-chloropropane constitutional isomers? _____
e. Another way to consider connectivity is to consider the skeletal structure as a molecular backbone. Are the two skeletons or molecular backbones identical for 1-chloropropane and 2-chloropropane?____
Question 6: Build models of the compounds shown in the structures below.
a. The heavy wedge points upward; the dashed wedge points downward. Are these structures identical? ___
(If you are unsure about your answer, refer to Table 1 for help.)
If they are isomers, we must determine whether they are constitutional isomers or stereoisomers. Look at the bond-line structures above. Connectivity only considers bonding, not geometry. Thus, the connectivities of these two structures are their skeletons without wedges, as shown below.
. Are the connectivities of these two structures the same or different? _________________
c. Are they constitutional isomers? _______
d. If they are not constitutional isomers, they are stereoisomers. Are they stereoisomers? ___
Let’s review. When we compare two compounds and they are isomers, we must determine what kind of isomers they are. We do this by first examining their constitutions. The constitution of a molecule is its bond-to-bond connectivity or skeleton. A molecule’s constitution is clearly shown in its skeletal structure, drawn without wedges or any other three-dimensional representations. A molecule’s constitution is like a skeleton, it shows only the connectivity or ca
on-heteroatom framework. See Table II.
Determine for each of the following pairs whether they are identical, constitutional isomers or stereoisomers.
III. Classifying Stereoisomers as Enantiomers or Diastereomers
When we classify a pair of compounds as constitutional isomers, we are finished with the classification. However, when we classify compounds as stereoisomers, we must determine what kind of stereoisomers they are. Stereoisomers differ in three-dimensional space. Stereoisomers are either enantiomers or diastereomers. Enantiomers are mi
or-image structures that are not superposable. Diastereomers are stereoisomers that are not enantiomers. Diastereomers are not mi
or images of each other.
A. Stereoisomers Containing Only One Chirality Center—Enantiomers
Many texts still refer to stereogenic center or stereocenter; the modern term is chirality center. For consistency with our text, we shall use chirality center. Should you ever see the expression stereocenter, you should know that it means the same thing as chirality center. The basis of stereoisomerism is the fact that a sp3-hy
idized ca
on atom is tetrahedral. Thus, there are two different ways in which four different groups can bond to a single ca
on atom. These two structures differ in three-dimensional space and are called stereoisomers. The ca
on atom from which the stereochemistry originates is called a chirality center. A three-dimensional structure originates at a chirality center. Consider
omochloroiodomethane. The compound contains ca
on with a
omo group, chloro group, iodo group and hydrogen group bonded to it. Thus, the single ca
on atom is a chirality center, because it has four different groups bonded to it. We signify a chirality center with a star (*). Draw a three-dimensional structure of
omochloroiodomethane and place a star on the ca
on atom. The star means that the ca
on atom has four different groups bonded to it. A compound with a single chirality center (one star) has one stereoisomer called an enantiomer.
Bromochloroiodomethane
Question 7: Build a model of
omochloroiodomethane and a model of its mi
or image. Try to superpose one model on the other model.
a. Are the models superposable? ___
. If they are superposable, they represent the same compound. Do these models represent the same compound? ______
c. If the models are nonsuperposable, they represent enantiomers. Do these models represent enantiomers? _____
You have discovered that a compound with only one star can have only one stereoisomer, and that stereoisomer is its enantiomer.
Let’s review! Suppose we have two structures, which are isomers with identical constitutions. If the two isomers have only one chirality center (ca
on with four different groups) or one sta
ed ca
on, they must be enantiomers. Since many compounds have only one chirality center, it pays dividends to know that compounds with one stereogenic center have a nonsuperposable mi
or image called its enantiomer. A compound with one stereogenic center can be resolved into a pair of enantiomers. See Table III.
Each of the following pairs of compounds contains one chirality center. Determine whether they are the same compound or enantiomers. If they are superposable, they are the same. If they are nonsuperposable mi
or images, they are enantiomers. Note: Me is an a
eviation for a methyl group in this projection.
B. Stereoisomers Containing Two or More Chirality Centers—Enantiomers or Diastereomers
We saw above that after we find we have stereoisomers, we must then determine what kind of stereoisomers we have. Further, if our structure has only one chirality center (one star), then it has only two stereoisomeric forms called enantiomers. This is true because there are only two ways that four different groups can bond to a single ca
on atom.
If we have two chirality centers (two sta
ed ca
on atoms), then the number of possible stereoisomers jumps to four. Every time we add a chirality center, the number of possible stereoisomers doubles because each new center can have two three-dimensional orientations. Thus, the maximum number of possible stereoisomers for a compound with n stars (chirality centers) is 2n. We can also write this expression as 2*, where * is
Answered Same Day Feb 06, 2023

Solution

Baljit answered on Feb 06 2023
37 Votes
Lab Stereochemistry
I. Isomers
Question 1: Build models of 1-
omobutane and 1-
omopropane. (Note: If you have questions or doubts about what you are doing during this lab, ask the instructor for assistance.)
a. In the space below, draw skeletal formulas of these compounds. Then, write the molecular formula beneath each structure.
    Compound
    1-
omobutane
    1-
omopropane
    Skeletal Diagram
    
    
    Molecular Formula
    C4H9B
    C3H7B
. Are the molecular formulas identical?______
No, Both molecular formulas are not same.
1-Bromobutane has one extra ca
on atom than
1-Bromopropane.
c. Are these compounds isomers or different compounds?_______________
These both are different compounds not isomers.
Question 2: Build models of 1-
omobutane and 2-
omobutane. (Note: You can use the model of 1-
omobutane from question 1.)
a. In the space below, draw structural formulas for these compounds and write the molecular formula beneath each structure.
    Compound
    1-
omobutane
    1-
omopropane
    Structural
Formula
    
    
    Molecular Formula
    C4H9B
    C4H9B
. Are the molecular formulas identical?______
Yes both molecular formulas are identical.
c. Are these compounds isomers or different compounds?_______________
Both Compounds are isomers having different position
of Bromine atom.
Question 3: Build two models of 1-
omobutane. These models are identical.
a. Are the 2 models of 1-
omobutane that you were asked to build above identical?____
Yes both are Superposable.
. Do they represent identical or different compounds?________________
Both are identical compounds.
Question 4: Classify the structures in the following Table as different, identical or isomeric. You may use models.
II. Constitutional Isomers vs Stereoisomers
Question 5: Build models of 1-chloropropane and 2-chloropropane.
a. These models represent isomers, are they constitutional isomers or stereoisomers?
Both are constitutional isomers having different bonding a
angement...
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