Organic Mechanism & Analysis of Carbocations Report #13 PDF

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Lab report #13 for organic mechanisms...

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Chem 223 Lab Report #13

Organic Mechanism & Analysis Of Carbocations Objective Observation of SN1 and SN2 conditions through organic mechanisms. Introduction Carbocations are molecules in which a carbon atom has a positive charge and three bonds. SN1 has a carbocation intermediate and results in a mixture of stereoisomers if a chiral center is reacting while the SN2 mechanism is a concerted process and gives inverted stereochemistry at a reacting chiral center. A factor that could decide whether to do SN1 or SN2 is the structure of the substrate. Substrates with leaving groups on primary carbons use SN2, substrates with leaving groups on secondary carbons use SN1 or SN2, and substrates with leaving groups on tertiary carbons use SN1. Discussion 1. What structural factors present in these test compounds in this experiment contribute to SN1 reactivity? Which ones would NOT react by SN1? If they would react under forcing conditions, what product would they mostly form and by what mechanism. Which compounds have the choice between SN1 and SN2, and which would it do under these conditions? For an SN1 reactivity a tertiary structure would contribute to it because it would give the most stable carbocation intermediate and the structure of the intermediate carbocation is the most important to an SN1 reactivity. A primary carbocation formation within an SN1 reactivity is unlikely to happen and that means n-butyl bromide and bromobenzene would not be able to react by SN1. It is also because it is too unstable to exist as a lone intermediate. 2. One of these compounds would not react at all under these reaction conditions, even with more reagent and higher temperatures. Which one and why in terms of the carbocation intermediate?

Bromobenzene would not react at all under these conditions, even with more reagent and higher temperatures because bromobenzene is aromatic and this means that it will not undergo in SN1 or SN2. In SN1, bromobenzene requires an unstable carbocation intermediate and due to a positive charge not being present on the sp2 hybridized carbon it becomes unreactive. In SN2, the carbocation is too unstable for it to form with the benzene ring. 3. One of these compounds is capable of reacting, but you may not have seen it because it is the least reactive one (except for the compound mentioned in Question 2). Which compound is it, and why would it not react by an SN1 mechanism as implied on p 152 of Pavia? When it does react, what possible mechanisms could it undergo and what products would it make? Within an SN1 mechanism, n-butyl bromide is the least reactive. N-butyl bromide is unable to react by an SN1 mechanism because of an unstable carbocation intermediate. Within forcing conditions it would cause a carbocation rearrangement to react in the SN1 mechanism. When the reaction occurs it could undergo an SN2 mechanism which would produce primary alkyl products. 4. Rank the compounds that are capable of reacting to make an ether according to the yield of the ether that would be formed. Explain, including what the likely side-products would be. The compounds that are capable of reacting to make an ether are bromobenzene, n-butyl bromide, tert-butyl bromide, sec-butyl bromide, and benzyl chloride. The likely side products would be dibenzyl ether which would come from the reaction of benzyl chloride with aqueous sodium hydroxide. 5. 5. If isobutyl bromide were forced to react under these conditions, give all the products that might reasonably be formed, and explain which one is likely to be the major product. Note that this reaction may do something the other alkyl halides are unlikely to do.

The products that might reasonably be formed if isobutyl bromide were forced to react would be tert-butyl iodide or isobutylene. The one that would most likely be the major product would be tert-butyl iodide. 6. 6. Pavia indicated, again on p 152, that the alkyl halides make a carbocation and the halide, before the halide reacts with the silver. Explain why this is wrong and the actual role of the Ag+ ion as a Lewis acid. It is wrong because the halide will react with the silver because the Ag + will react with the alkyl halide. When tert-butyl chloride and Ag + react, Ag + is a Lewis acid where it will then take the electron lone pair from the chlorine atom since it will associate with an alkyl halide. This will cause the halide to form a carbocation with the silver which would cause a precipitate to form and AgCl. Pavia, p 171 (4th ed.) "4-Methylcyclohexene" Some reactions can be directed towards SN1/SN2, or they can be directed towards E1/E2 reactions. This is done partly by the choice of reagent, but reaction conditions, such as solvent and temperature, can be important. Review the 4-methylcyclohexene lab, paying attention to the reaction conditions and reagents. Then answer the following questions. Also review any parts of Klein, Chapter 7 & 8 that you may need to. 1.

What is the mechanism of this reaction? Show it in detail. You may not have learned it, but you learned the mechanism for the addition of water to alkenes, and it is the same mechanism backwards. (Principle of Microscopic Reversibility, which applies to any reversible reaction)

2. Why is only one major product formed in this reaction? Only one major product is formed in this reaction because it is a substitution reaction and since 4-methylcyclohexene is highly symmetrical that means that when the elimination occurs it does not matter which side it happens because it will form the same product. 3. If the intermediate carbocation in this elimination reaction were to rearrange by a hydride shift, it would lead to an additional product. Why is this rearrangement not seen?

Rearrangement is not seen since there is not a stable carbocation by doing a rearrangement since it will have the same secondary carbocation. The secondary carbocation is formed when the leaving group is lost because of a potential hydride shift which would only result in another secondary carbocation. 4. What factors favor elimination as this reaction is done? The factors that favor elimination as this reaction is done would be the presence of an electrophile and the formation of a stable carbocation that favors the reaction. 5.

If 2-methylcyclohexanol were the starting material, what would the results be?

6. Show the mechanism for the Br2 test for alkenes with 4-methylcyclohexene. Explain why the intermediate can be considered a carbocation, and why it is different.

Pavia, p 431, Synthesis of β-Benzopinacolone (The Pinacol Rearrangement) Read through this reaction in Pavia, and also Pinacol Rearrangement

1. In this virtual lab, we are exploring the possible fates of intermediate carbocations. The reaction in this section represents a third possibility, which also alluded to briefly. What are the three possible carbocation fates we have examined? Three possible carbocation fates are capturization by a nucleophile, loss of a neighboring hydrogen, and rearrangement. 2) Show a mechanism for this reaction (ignore the I2). You may abbreviate the aromatic rings as "Ph" for phenyl in your structures. What is the driving force for this mechanism?

Using the acid catalyst would be the driving force of the rearrangement.

3) Show the mechanism and give the product of the pinacol rearrangement for the diol shown to the right. For the key rearrangement step, which group migrates and why?

The Phenol group migrates because the ring shift would make cyclopentane which has more of a ring strain. Read (study) Cationic Polymerization and Biosynthesis of Steroids

Give a mechanism for the reaction shown on the right. Show any possible alternative reaction pathways and why they are much less likely. Hint: Two C=C double bonds, four possible carbocations. Would all four form and what might they do if they would form?

Final Question

List the four possible ways that carbocations react, and indicate what factors play a role in deciding what a specific cation does. Use the reactions we have seen in this assignment as examples. Four possible ways that carbocations react is through SN1 which would depend on the behavior of the carbocation on whether it would be primary, secondary, or tertiary. It could happen through an E1 reaction which would also depend on the stability of the carbocation intermediate. Rearrangement could be used to help determine the stability of the carbocation and whether it significantly increased. Finally, the addition of a carbocation if intramolecular and favorable. Conclusive Remarks It could be concluded that SN1 and SN2 reactions undergo different organic mechanisms....