Relative Rates of Nucleophilic Substitution - Assignment Example

Assignment #10 Draw structural formulas for bromobutane, 2-bromobutane, 2-bromo-2-methylpropane bromo-2-methylpropane, and bromo-2,2-dimethylpropane. Classify each compound as a primary halide, secondary halide, or tertiary halide. 1-bromobutane: primary 2-bromobutane: secondary 2-bromo-2-methylpropane: tertiary 1-bromo-2-methylpropane: primary 1-bromo-2,2-dimethylpropane: primary 2. Draw structural formulas for acetone and ethanol. Which solvent is more polar? Explain. Acetone: Ethanol: more polar Ethanol is more polar than acetone because in ethanol, O is attached to sp3 hybridized C while in acetone, the O is bonded to is sp2 hybridized C. sp3-carbon is more polar than the sp2-carbon.  3. A student wishes to determine the rate law for the reaction A + B giving product C. The student doubles the concentration of A while holding the concentration of B constant, and finds that the rate of the reaction doubles. The student then doubles the concentration of B while holding the concentration of A constant and finds that the rate is unchanged. Write the rate law for this reaction (refer to a general chemistry text, if necessary). In that case, rate is 1st order with A and 0-order with B, so that Rate law = k*[A] 4. Which alkyl bromide reacted fastest with NaI in acetone in part A1? Which alkyl bromide reacted slowest? Explain how the structure of the alkyl halide affects the rate of an SN2 reaction. Fastest SN2 reaction with NaI in acetone – Fastest: 1-bromobutane Slowest: 2-bromo-2-methylpropane The structure of the alkyl group bears impact in two ways: steric hindrance and the ability of the alkyl group to accommodate a positive charge. As the number of alkyl substituents on the substitution center increases, steric hindrance increases. Hence, reactivity for SN2 follows the order: CH3 > 1° > 2° > 3°. 5. Which alkyl bromide reacted fastest with NaI in acetone in part A2? All of these are primary halides. Why was there a difference in reactivity? Fastest: 1-bromobutane Slowest: 1-bromo-2,2-dimethylpropane 6. Which halide reacted faster with NaI in acetone in part A3? Explain how the nature of the leaving group affects the rate of an SN2 reaction. 1-bromobutane reacted faster with NaI in acetone in part A3 (Br is better able to accommodate negative charge). The better the leaving group, the easier it is to break the bond, and the faster the reaction occurs. The more stable the leaving group is, the more likely it takes the two electrons of its carbon-leaving group bond with it when the nucleophile attacks the carbon. 7. How did the following changes affect the rate of the reaction with 1-bromobutane with NaI: doubling the concentration of 1-bromobutane? Doubling the concentration of NaI? Doubling the concentration of 1-bromobutane increased the rate of reaction by a factor of 2. Similarly, doubling the concentration of NaI increased the rate by a factor of 2 as well. 8. Write the generalized rate expression for an SN2 reaction. Following: R – X + Nu -----> RNu + X- Rate expression: rate = k * [R – X] * [Nu-] 9. List the factors studied in this lab that affect the rate of an SN2 reaction. Strength of the nucleophile Solvent effect on the nucleophile Steric effect Leaving group effect 10. Which alkyl bromide reacted fastest with AgNO3 in ethanol for part B1? Which reacted slowest? Explain how the structure of the alkyl halide affects the rate of an SN1 reaction. Fastest: 2-bromo-2-methylpropane Slowest: 1-bromobutane The greater the number of alkyl groups at the carbocation center, the greater the stability of the carbocation, and hence the faster the formation of the carbocation. This implies faster SN1 rate as well. Thus, SN1 reactivity increases in the order: CH3 < 1° < 2° < 3°. 11. Which alkyl halide reacted faster in part B2? Explain how and why the nature of the leaving group affects the rate of an SN1 reaction. Would 2-iodo-2-methylpropane react faster or slower? Explain. 2-bromo-2-methylpropane (t-butyl bromide) reacted faster in part B2. As in SN2 mechanism, the better the leaving group, the easier it is to break the bond, and the faster the reaction occurs. The more stable the leaving group is, the more likely it takes the two electrons of its carbon-leaving group bond with it when the nucleophile attacks the carbon. 2-iodo-2-methylpropane would react slower because a tertiary substrate reacting with a weak nucleophile (which is also the solvent) like an alcohol will react by an SN1 mechanism. 12. Which solvent gave a faster reaction in part B3? How does the polarity of the solvent affect the SN1 reaction? 5% ethanol gave a faster reaction in part B3.  The solvent coordinates to the carbocation and stabilizes it.  The more polar the solvent, the better it is able to stabilize the carbocation.  SN1 reaction occurs faster in more polar solvents. 13. How did changing the concentration of the alkyl halide in part B4 affect the reaction rate? Determine the rate law for the SN1 reaction. In B4, doubling the concentration [R-X] doubles the rate of reaction. The factor by which the alkyl halide concentration is increased is the same factor by which the reaction rate for SN1 increases. The rate law for the SN1 reaction is: rate law = k * [R – X] (first order for the alkyl halide) 14. List the factors that affect the rates of SN1 reactions. Degree of substitution Identity of the leaving group Polarity of the solvent 15. What determines whether 2-bromobutane undergoes SN1 and/or SN2 reactions? Nucleophilicity determines whether 2-bromobutane undergoes SN1 and/or SN2 reactions. (For secondary halides strong nucleophiles in aprotic polar solvents favor SN2, whereas weak nucleophiles in protic solvents favor SN1 reactions.) 16. On the same reaction coordinate diagram, draw the nucleophilic substitution of NaI with 1-bromobutane and 2-bromobutane. Nucleophilic Substitution of NaI with 1-bromobutane: + NaI ---- + NaBr Nucleophilic Substitution of NaI with 2-bromobutane: 17. On the same reaction coordinate diagram, draw the nucleophilic substitution of ethanol with 2-bromobutane and 2-bromo-2-methylpropane. Nucleophilic Substitution of ethanol with 2-bromobutane: + CH3CH2OH → + AgBr (s) + HNO3 (aq) Nucleophilic Substitution of ethanol with 2-bromo-2-methylpropane: + CH3CH2OH → + AgBr (s) + HNO3 (aq) 18. Benzyl bromide reacts rapidly with NaI in acetone, and reacts rapidly with ethanol and AgNO3. Bromobenzene does not react with either. Explain. Bromobenzene does not react with NaI in acetone because for an SN2 reaction, it is necessary to backside attack with the nucleophile and apparently, this is not possible with benzene ring and products could not be inverted in this mechanism. Likewise, bromobenzene does not react with ethanol / AgNO3 because for an SN1 reaction, it is not likely for it to form carbocation in its stable conjugated state, so electron density may not be delocalized into the carbocation p-orbital. 19. What are characteristics of a good leaving group? good leaving groups are weak bases highly stabilized anions (possesses stable energy upon departure) 20. Why doesn’t the nitrate ion react to give SN1 substitution product. Nitrate ion makes a poor nucleophile as it comes from HNO3 (a very strong acid). It would not react to give SN1 substitution product due to the 3 atoms of oxygen that pulls away electron density from the nitrogen whereas the ion’s resonance structure also reduces its nucleophilicity. Bonus Question: 1-Bromoadamantane is a tertiary halide, yet it is 104 times slower than tert-butyl bromide when reacting with silver nitrate in ethanol. Consider the structure of 1-bromoadamantane and explain this observation (Hint: Consider the geometry of a carbocation. It might be helpful to build a model of 1-bromoadamantane). Answer: The crystal structure of 1-bromoadamantange only enables dispersive interactions. Its complex geometry exhibits non-crystallographic mirror symmetry, and compared to tert-butyl bromide, would not be readily flexible to form carbocation (nor be stabilized by the similar hyperconjugative state). Read More