UMass Amherst CHEM 261 - Organic Chemistry Exam 2 Practice Answers 261 (10 pages)

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Organic Chemistry Exam 2 Practice Answers 261



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Organic Chemistry Exam 2 Practice Answers 261

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10
School:
University of Massachusetts Amherst
Course:
Chem 261 - Organic Chemistry
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1 Rank the following substituents from lowest to highest priority according to the Cahn Ingold Prelog rules CIP rules rank groups based on their atomic number The atom with the lowest atomic number is the lowest group and the atom with the highest number is the highest priority C 6 O 8 H 1 N 7 I 53 O 8 CHI3 53 6 7 166 OH 8 1 NH2 7 1 1 9 BEDA 2 Which of the following structures are meso compounds A and E only A meso compound is a stereoisomer and thus by default must have a stereocenter Meso compounds require at least two stereocenters A compound with only one stereocenter cannot be considered a meso compound as it would not be achiral and would not have another stereocenter to oppose its optical behavior See if the molecule has an internal mirror plane Any molecule can have stereoisomers but not all have internal mirror planes The mirror plane should bisect the molecule Ensure that both halves are in fact mirror images of each other The meso compound must be symmetric about the internal mirror plane Each opposing stereocenter must have differing absolute configurations and the attachments on each stereocenter must be the same If the opposing stereocenters have the same absolute configurations or the attachments on either side are different then one half will not be a mirror image of the other half 3 Specify the number of Chiral Centers in the molecule below Chiral molecules usually contain at least one carbon atom with four nonidentical substituents 3 4 What is the relationship between molecules A and B They re Constitutional isomers Diastereomers sometimes called diastereoisomers are a type of a stereoisomer Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more but not all of the equivalent related stereocenters and are not mirror images of each other Enantiomers are chiral molecules that are mirror images of one another Furthermore the molecules are non superimposable on one another This means that the molecules cannot be placed on top of one another and give the same molecule Constitutional isomers are compounds that have the same molecular formula and different connectivity To determine whether two molecules are constitutional isomers just count the number of each atom in both molecules and see how the atoms are arranged 5 Which of the following is the definition of a pair of diastereomers A pair of stereoisomers that do not mirror images of one another 5 Assign an R S designation to each of the molecules below A R B S Number each of the substituents on the chiral center carbon using the Cahn Ingold Prelog system According to the Cahn Ingold Prelog prioritizing scheme the highest priority goes to the substituent whose first atom has the highest atomic number For example Br would be a higher priority than Cl because Br has a larger atomic number If the first atoms on two substituents are the same you keep going down the chain until you reach the first higher priority atom and the tie is broken After you ve assigned priorities to each of the substituents rotate the molecule so that the number four priority substituent is oriented in the back Draw a curve from the first priority substituent through the second priority substituent and then through the third If the curve goes clockwise the chiral center is designated R if the curve goes counterclockwise the chiral center is designated S Here s an example Try to determine the R S stereochemistry of the chiral center in the molecule shown in the figure STEP 1 PRIORITIZING THE SUBSTITUENTSThe first step is to prioritize all the substituents from one to four Bromine is the atom with the largest atomic number so this substituent is given the highest priority hydrogen has the smallest atomic number so it s given the lowest priority Chlorine gets the number two priority because it has a higher atomic number than fluorine which is given priority three The priorities are shown here 7 What movement of electrons accounts for the formation of the product in the following mechanistic step 2 8 Which of the following represent s an elimination reaction A only An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two step mechanism The one step mechanism is known as the E2 reaction and the two step mechanism is known as the E1reaction 9 Determine whether each indicated position in the molecule below would behave as a nucleophile or an electrophile B oth are nucleophiles A nucleophile is an electron rich like an alkene or amine and an electrophile is electron poor i e a carbocation with an empty p orbital N H is an Amine and C C is an alkene 10 Which of the following statements is false regarding the following energy diagram The reaction shown in the following energy diagram cannot possibly occur Although the reaction shown above is an endergonic reaction which means that heat must be absorbed for the reaction to proceed if one applies enough heat to overcome the activation barriers the reaction will proceed Another way to get the reaction to proceed is to drive the reaction forward by removing one of the products as it is formed thus using LeChatlier s Principle to make the reaction go to completion The intermediate transition states and product are indicated below 11 Which of the Neither 12 Which of the following following reactions carbocation would have rearrangements a positive is S exothermic 4 13 What is the IUPAC name of the following compound E 3 tert butyl 4 ethyl 2 hexene Although carbon 3 possesses a very large group pointing towards the left of the screen size has no bearing on the E Z System of nomenclature Recall that the first point of difference is used to prioritize groups extending from the same point Thus the molecule should be named E 3 tert butyl 4 ethyl 2 hexene This point of difference is highlighted below 14 Assign an E Z designation to each of the molecules below A Z B Z 15 Which of the following products would be obtained in the highest yield from the reaction below A 16 Which of the following explains the observation that th e tert butyl carbocation is more stable than the 1 butyl carbocation Hyperconjugation is the source of the enhanced stability of m ore highly substituted carbocations 17 What is the best choice of reagent s to perform the following transformation OsO 4 followed by NaHSO 3 H 2 O 18 What is the major organic product obtained from the following reaction 3 Reaction of


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