CHEM2020 Lab 4.docx

This is expected as water is a polar and protic

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difference between water and acetone concentrations in the solvent became smaller. This is expected as water is a polar and protic solvent, and so SN1 reactions will occur more easily in its presence. Water solvates the ions in the transition state by forming hydrogen bonds between its hydrogen atoms and the anions, as well as forming a cation complex with its oxygen atom. This allows for water to have a much greater relative ionization rate compared to other solvents. For example, tert-butyl chloride has a relative ionization rate of 8000 in water, whereas in acetone, the tertiary alkyl has a relative ionization rate 1. This means that water is able to effectively stabilize the ions produced in the transition state of the rate-limiting step. Since Experiment 8’s solvent has the greatest proportion of water to acetone, it is expected that more of tert-butyl chlorides ions can be solvated, thus allowing for the SN1 reaction to begin more easily, therefore increasing the rate constant. 6. If tert-butyl bromide was used instead of tert-butyl chloride, it is expected that the reaction would take place much faster. The reason for this is that bromide is a better leaving group than chlorine. Better leaving groups are highly polarizable and are weaker bases, thus being more stable. This becomes important when considering the transition state of the rate-limiting step of SN1 reaction mechanisms. In SN1 reaction mechanisms, the leaving group takes on a negative charge as it stabilizes the carbocation through partial bonding. The leaving group must be stable enough to effectively take on this negative charge and therefore stabilize the carbocation. Bromine has a larger atomic size than chlorine and so bromine is better able to stabilize the negative charge of a molecule. The transition state of this SN1 reaction mechanism is therefore more easily stabilized by bromine, producing a more stable carbocation. This allows for the reaction to start more readily. 7. If tert-butyl bromide was used in place of tert-butyl chloride, the slope of the Arrhenius plot would be steeper. Bromine’s larger atomic size can stabilize the rate-limiting step’s
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transition state to produce a more stable carbocation better than chlorine can. This allows for the reaction to begin faster. This means that the reaction will move to completion in less time, making the rate greater. If the rate increases, so will the rate constant.
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  • Fall '09
  • 20%, 40%, 30%, 5 m/s, 4 m/s

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