Melting point lab, TILT style PDF

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Summary

This is a lab handout for the melting point experiment. This is the first lab in Organic Chemistry I Lab. The author attempted TILT format (https://www.unlv.edu/provost/teachingandlearning) to make the purpose and expectation clear. ...

Description

Experiment 2

Organic Chemistry I Molecular modeling

Attention! 

  

Bring the Organic Chemistry Model Kit (HGS Maruzen 1002Alpha/Organic Chemistry Introductory Set, sold at the bookstore), a required course material for CHEM2411L. The kit is used for two labs (including this one), and is useful for your lecture study as well- make a good use of it! It is recommended that you bring the lecture textbook and handouts (if any). You will complete the in-class Worksheet during the lab period and will turn it in by the end of the class period. You may work with one lab partner for this lab.

Due dates:  

Online Prelab Quiz (CourseDen): Due 2 hours before the start of this lab class. In-class Worksheet: Complete during this lab class, and turn in by the end of this lab.

Purpose: The purpose of this lab is to supplement your learning of the lecture content: conformation of alkanes. You will make and examine 3D molecular models of different conformers of organic compounds, and to practice drawing them accurately in appropriate formats. Recognizing different conformations of organic compounds is an important skill to fully understand organic compounds and their reactions. Being able to accurately draw the conformers on a 2D surface is an essential skill in organic chemistry to communicate the structures of organic substances in writing. This assignment will help you practice the following skills:  Converting the views of a 3D model into Newman projections (butanes) or chair forms (cyclohexanes)  Recognizing staggered/eclipsed, gauche, anti, and what interaction adds to the overall potential energy of a conformer  Drawing chair forms cyclohexanes with accurate axial and equatorial angles This assignment will help you gain the following knowledge:  How Newman projections help distinguishing different conformers of alkanes  How terms axis, equatorial, cis, and trans apply to cyclohexanes  How to recognize the relative stability of cyclohexane conformers (chair vs. boat, before or after flipping cyclohexane chair)

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Experimental Procedure Follow the procedure below, and write all your answers and drawings directly in the In-class Worksheet, using a pencil (so you can erase and rewrite if needed). The procedure step numbers should be matching the answer step numbers. Answer and address all questions. The drawings must be as comprehensive and accurate as possible. 1. Butane 1.1)

Build a molecule of butane by connecting four carbon atoms (black) with the white “bonds” as C-C bonds, in a straight chain. Add H atoms (small white balls) using the short pink “bonds” as C-H bonds, to complete the CH3-CH2-CH2-CH3.

1.2)

Adjust the conformation indicated in the first box of the Worksheet (under “1. Butane Conformations” and under 0°). Two terminal methyl groups are completely eclipsed. Examine the proximity of the two methyl groups.

1.3)

Rotate the back carbon (C3) clockwise by 60° and sketch the staggered conformation in a Newman projection under 60°. Keep the front C2 unmoved.

1.4)

Rotate the back carbon (C3) by 60° clockwise again, and sketch the Newman projection under “120°” position (this would be eclipsed again). Repeat the back carbon clockwise 60° rotation (it will be 180° staggered  240° eclipsed  300° staggered…) and sketch the total of six conformers.

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1.5)

1.6)

Using Table 1, calculate the potential energy of each conformer in the space below each Newman projection. Show clearly what numbers are added to give the total.

Table 1. Energy diagram for different conformers of alkanes

Type of interaction H / H eclipsed H / CH3 eclipsed CH3 / CH3 eclipsed CH3 / CH3 gauche

Energy (kJ/mol) 4.0 6.0 11.0 3.8

Plot the calculated potential energies in a diagram as a function of the angle of rotation. Give a sensible Y-axis scale. Connect the points with a smooth curvy line, making the points either as minimum or maximum points.

2. 2-Methylbutane 2.1)

Replace a –H group on C-2 of the butane molecule with a methyl (-CH3) group. This is 2methylbutane.

2.2)

Hold the molecule so you are seeing it through C-2 to C-3, and adjust the conformation to match the Newman projection shown in the first box under “2. 2-methylbutane Conformations,” under 0°.

2.3)

Rotate the C3 (back carbon) by 60° clockwise and sketch the conformer in Newman projecton under 60°. Repeat this to sketch the total of six conformers.

2.4)

Using Table 1, calculate the potential energy for each Neweman projection (show work) and plot the potential energy diagram as a function of the angle of rotation.

3. Cyclohexane Whenever short blank is indicated, write a brief answer in the Worksheet (E.g. Yes, No, Done or �, two, six, staggered, eclipsed, etc). For a longer blank, write a short answer. 3.1)

Build a molecule of cyclohexane by connecting six carbon atoms (black) in a ring, and adding C-H bonds (pink bonds with white balls). i) Make a chair conformation. ____ ii) Do you see axial bonds (either straight up or straight down if you see the ring from the side)? _____ Are they alternating like up, down, up…? ____ iii) Do you see equatorial bonds (they are slightly down or slightly up)? _____ iv) If axial is straight up, then the equatorial bond on the same C is slightly ____. If axial is straight down, then the equatorial bond on the same C is slightly _____. v) Hold one pair of opposite C-C bonds. Are they parallel to each other? ____ Imagine a rectangle containing the same set of opposite C-C bonds. See that the other C atoms are forming triangle tips. Is one tip pointing down, and the other pointing up? _____

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vi) Examine each of the three sets of two opposite C-C bonds. Are they all parallel to each other? ____ From each set of opposite C-C bond pair, can you see up-tip and down-tip? ____ vii) Hold two opposite carbons again. One tip is pointing down, and the other is pointing up. Bring the down-tip up, and bring the up-tip down. Repeat. Are you going from one chair conformation to the other? _____ viii) By flipping the chair conformation, pay attention to what is happening to the axial and equatorial positions. Take one H atom (white ball) off, and see if it is in axial or equatorial position; then do the chair flip. What is happening to the axial/equatorial position? _____________________ 3.2)

Change the cyclehexane to a boat conformation. i)

Do you see “two tips up” as opposed to “one up, one down”? _____

ii) Which conformation has a more steric interaction: chair or boat? ______ iii) In a boat conformation, how many sets of opposite C-C bonds are parallel? ____ 3.3)

Chair vs. boat: staggered or eclipsed C-C bonds i) Make a chair form again. ____ ii) Mark one C atom as “C1” by taking a white ball off from that C atom. Seeing the ring from the top, name the rest of the C atoms as C2, C3…. Clockwise. ____ iii) See the C-1 through C-2 bond as if you are drawing a Newman projection. Are the bonds staggered or eclipsed? ____ Do the same examination for all six C-C bonds (C2-C3, C3-C4…. ). _____________ iv) Make a boat form again. ____ v) Similar to what you did for the chair form, examine all six C-C bonds to see if they are staggered or eclipsed: C1-C2, C2-C3…. _________________ vi) Based on the counting of the staggered and eclipsed C-C bonds, what can be said about the relative stability of the chair and boat conformations? _________________________________

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Tip: Newman can be “Y” or “Inverted Y” The Newman projections you draw in Part 1 was in “Inverted Y” form. One bond was straight up, and the other two are slightly down-left, and slightly down-right.

But in the activity below, “Y” would be appropriate, as the front carbons have one bond straight down, and the other two are slightly up left, and slight up-right.

3.4)

3.5)

3.6)

Adjust the cyclohexane ring to the angle similar to the diagram on the right. Draw a Newman projection sighting along the C1-C2 bond. Draw substituents on both carbons (either H atom or “-CH2- ” groups).: Draw a Newman projection sighting along the C5-C4 bond. (Optional: you may connect this drawing to the previous drawing by bridging with the C6 and C3 atoms). Draw substituents on both carbons (either H atom or “CH2- ” groups).

4

6

5 3

2

1

4

6

5 3

2 1

Hold the chair-form cyclohexame model so that your view matches one of the chair templates below.

 Decide on which way to hold the model to sketch, then start with a chair ring skeleton.  Then draw the axial C-H bonds from each ring C. *Tip 1: Axial straight up if the tip is upwards; axial straight down if the tip is downwards.

*Tip2: Front axial bond should cross over the back ring C-C, and the back axial bond should cross under the front C-C bond  Then add all equatorial C–H bonds.

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*Tip 1: If axial is up, equatorial is slightly down. If axial is down, equatorial is slightly up.

*Tip 2: Pay close attention to the direction of “middle” equatorials

 Once you have completed the sketch of cyclohexane, hold the model to see it just like your sketch.  Double-check the following on your sketch. Check 1: Does your ring have three sets of parallel lines? ___ Are the “side” C-C bonds tilted enough, rather than straight vertical? ____ Check 2: Are axial bonds are alternating up  down  up… ? ____ Are they corresponding the direction of the tip? ___ Check 3: Double-check the accuracy of the equatorial bond angles. Do all equatorial bonds follow the Rule of Z (see below)? ____ Rule of Z: Also remember that each equatorial line has a parallel line in the ring. You should be able to find a nice “Z” as below. Note that some “Z” shapes are flatter.

 Fix your sketch as needed. 3.7)

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Now view your model so that it matches the chair template that you did not use in the previous step (e.g. if you used “left down, right up” then this time use “right down, left up” template). Then draw the chair-form cyclohexane from this angle, including axial and equatorial C-H bonds.

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4. Methylcyclohexane 4.1) Place a methyl (-CH3) group at an axial position of the chair-shaped cyclohexane model. This is methylcyclohexane. Sketch this model, including all axial and equatorial groups. Double-check the accuracy of all the bond angles. 4.2) The ring carbon atom that holds the methyl group is Carbon 1 (C1). Draw a Newman projection through C1-C2 of the molecule with methyl at the equatorial position. Write “CH2” for the ring carbons. Count how many gauche interactions the methyl group has with the ring CH2 group(s). 4.3) Now wiggle/flip the ring to another chair conformation with the methyl group at an axial position. Draw this conformer with all axial and equatorial bonds with accurate angles. 4.4) Draw a Newman projection through C1-C2 of the molecule with methyl at the axial position. Count how many gauche interactions the methyl group has with the ring CH2 group(s). 4.5) Based on the examination above, explain which conformation, equatorial or axial, is the most stable (lowest in potential energy), and why. 5. 1,2-Dimethylcyclohexane 5.1) To your axial methylcyclohexane model in a chair conformation, add another sp3 tetrahedral carbon to the 2-axial position. This is 1,2-dimethylcyclohexane, with methyl groups should be both axial. Draw this conformer. Is this cis or trans? 5.2) Wiggle/flip the ring to the other chair conformation. Now the two methyl groups are equatorial. Draw this conformer. Is this cis or trans? (Should cis/trans be the same or different from the previous conformer?) 5.3) Now from ring C1, detach both –CH3 and –H groups, and reattach the –CH3 group at the axial position, and –H at the equatorial position. You have now changed the stereochemistry. Draw this conformer. Is this cis or trans? 5.4) Wiggle/flip the ring to the other chair conformation. Draw this conformer. Is this cis or trans? 5.5) Among the four conformers you sketched (5-1, 5-2, 5-3 and 5-4), which one is the lowest in energy? Explain. 6. 1,3-DiMethylcyclohexane 6.1)

Build a 1,3-dimethylcyclohexane, with both methyl groups at axial positions in a chaircyclohexane. Draw this conformer. Is this isomer cis or trans?

6.2)

Wiggle/flip the ring to the other chair conformation. Now the two methyl groups are equatorial. Draw this conformer. Is this cis or trans? (Should cis/trans be the same or different from the previous conformer?)

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6.3)

Now from ring C1, detach both –CH3 and –H groups, and reattach the –CH3 group at the equatorial position, and –H at the equatorial position. You have now changed the stereochemistry. Draw this conformer. Is this cis or trans?

6.4)

Wiggle/flip the ring to the other chair conformation. Draw this conformer. Is this cis or trans?

6.5)

Among the four conformers you sketched (6-1, 6-2, 6-3 and 6-4), which one is the lowest in energy? Explain.

7. Finish-up 7.1) 7.2)

Turn in the completed Worksheet by the end of the lab period. Put the kit back to the bag. The –H unit (short pink bond and small white ball) can stay together. (The model will be useful for your lecture study. We’ll use the model again in a later lab as well.)

------------------------------------------End of activities------------------------------------------------

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______________________________ _______________________ Name

______________

Lab Partner

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In-class Worksheet

Date

Organic Chemistry I

Sketch (Newman)

Potential Energy (kJ/mol)

1. Butane Conformations

0°

H

angle

240°

300°

Complete the structure

CH3 CH3

CH3

H H

H

H

# Interactions and calc.

H

120° Dihedral 180°

60°

2. 2-Methylbutane Conformations

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Sketch (Newman) H

# Interactions and calc.

Potential Energy (kJ/mol)

H

0°

CH3 CH3

H CH3

60° angle

120° Dihedral 180°

28

240° 300°

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3. Cyclohexane 3-1) i) ______ ii) _______ , ________ iii) ________ iv) ________ , ________ v) _______ vi) _______ , ________ vii) _______ viii) __________________________________________________________________ 3-2) i) ______ ii) _______ iii) _______ 3-3) Chair i) ______ ii) _______ iii) C1-C2 __________ C2-C3 ____________ C3-C4 ____________ C4-C5 __________ C5-C6 ____________ C6-C1____________ Boat iv) ______ v) C1-C2 __________ C2-C3 ____________ C3-C4 ____________ C4-C5 __________ C5-C6 ____________ C6-C1____________ vi) _______________________________________________________________________ 3-4)

3-5)

3-6)

3-7)

Check 1: ______, _______ Check 2: ______, _______ Check 3: ______ Exp. 2

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4. Methylcyclohexane (Chair conformations) 4-1) Chair, -CH3 as equatorial

4-2) Newman C1-C2

How many CH3/CH2 gauche?______

4-3) Chair, Me as axial

4-4) Newman C1-C2

How many CH3/CH2 gauche?______

4-5)_________________________________________________________________________

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5. 1,2-Dimethylcyclohexane (Chair conformations) 5-1)

5-2)

cis or trans? ________

cis or trans? ________

5-3)

5-4)

cis or trans? ________

cis or trans? ________

5-5) ____________________________________________________________________________ 6. 1,3-Dimethylcyclohexane (Chair conformations) 6-1)

6-2)

cis or trans? ________

cis or trans? ________

6-3)

6-4)

cis or trans? ________

cis or trans? ________

6-5) ____________________________________________________________________________ Exp. 2

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