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Author Topic: SN1 and SN2 reactions.  (Read 105064 times)

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SN1 and SN2 reactions.
« on: February 06, 2010, 04:21:41 PM »

I have some homework for my CHM lab about SN1/SN2 reactions. However I am quite rusty about them since I've been 1 year since I took CHM 1 where we reviewed those reactions (can't figure out why the SN1/SN2 lab is with the CHM 2 course.)
Anyhow I was able to answer all the questions but I am not sure if they are right or wrong.

1.   In both the sodium iodide test and the silver nitrate test, why does 2-bromobutane react faster than 2-chlorobutane?
Bromine is a better leaving group since it is a weaker base than chlorine is.

2.   a. Why does benzyl chloride react under both SN1 and SN2 conditions?
            Benzyl chloride is a primary alkyl halide, hence reactive under SN2 conditions.
            The primary carbocation formed due to the departure of Cl- is stabilized by the pi electrons in the benzene ring.
        b. Why is bromobenzene non-reactive under both SN1 and SN2 conditions?   
            For an SN2 reaction to occur, the nucleophile must attack from behind the carbon which is hindered by the bulky ring.
            An SN1 reaction cannot occur since the all the carbons in the benzene ring have sp2 hybridized orbitals.

3.      Benzyl chloride and 1-chlorobutane are both primary alkyl chlorides, but in the sodium iodine test, benzyl chloride reacts much faster. Why?
             Benzyl chloride reacts faster because its transition state is more stable than the transition state of 1-chlorobutane.

4.   You wish to substitute the bromine on the following molecules with a nucleophile. Explain whether the given molecule would react by the SN1 or SN2 mechanism and explain why.
        1-methyl-1bromocyclohexane:  SN1 since a tertiary carbocation is relatively stable. SN2 reaction is highly unfavorable at a tertiary carbocation.
        1-bromopropane: SN2 since the nucleophile can readily attack the primary alkyl group. The formation of a primary carbocation intermediate for an SN1 reaction to occur is energetically unfavorable.
        2-bromohexane:  Both SN1 and SN2. The secondary carbocation of the intermediate is relatively stable therefore SN1 reaction is possible. The secondary carbon is also relatively accessible for a back side attack of a nucleophile therefore SN2 reaction is also possible.

5.   The structures below react via the SN1 mechanism. Which would you expect to react slowest and which faster? Explain your prediction.
        5-bromo-1,3-pentadiene: Fastest. Transition state is stabilized by electron delocalization.
        Bromocyclopentane:  Slower.  Transition state  contains a secondary carbocation which is less table than tertiary (c) and electron delocalization stabilized carbocation (a)
        (R)-2-bromo-2-methylhexane:  The transition state is relatively stable due to a tertiary carbocation, therefore (R)-2-bromo-2-methylhexane reacts faster than bromocyclopentane but not as fast as 5-bromo-1,3-pentadiene.
             5-bromo-1,3-pentadiene > (R)-2-bromo-2-methylhexane> Bromocyclopentane

6.   Suppose you find that bromocyclohexane reacts faster than chlorocyclohexane in an SN2 reaction. What reason can you give for this observation?
        Bromine is a better leaving group since it is a weaker base than chlorine is.

7.   1-Chlorobutane (2.5 mL, d=0.886) in 20 mL of acetone is reacted with 90 mL of a 15 wt% solution of NaI in acetone. After workup, you obtain 1.3 g of 1-iodobutane. What is the limiting reagent? What is your % yield?
   g (Acetone)=90mL*d(Acetone)=90mL*0.7925 g/mL=71.325g
   15g(NaI)/(100g (Acetone) )=g(NaI)/(71.325g (Acetone) )→g(NaI)=(15g(NaI)*71.325g(Acetone))/100g(Acetone) = 10.69875g
   g (C_4 H_9 Cl)=V*d=2.5mL*0.886g/mL=2.215g
   2.215g (C_4 H_9 Cl)=0.024mol (C_4 H_9 Cl); 10.7g (NaI)=0.071mol (NaI)
   1-Chlorobutane reacts with one mole of Sodium Iodine to form one mole of 1-iodobutane and one mole of Sodium Chloride. Therefore 0.024mol of 1-Chlorobutane reacts with 0.024mol of Sodium Iodine to form 0.024mol of 1-iodobutane and 0.024mol of sodium chloride. 1-Chlorobutane is the limiting reagent.
   % yield=1.3g/4.4g*100= 29.5%
8.      True or False: To promote the SN1 mechanism, you used AgNO3 in a polar, protic solvent. Why is this statement true or false?
        True. Protic solvent is used in SN1 reaction to help in the dissociation of the halogen by stabilization (solvation) of the resulting ions.

9.   True or False: The rate of reaction for the SN2 mechanism is dependent on the concentration of both nucleophile and the electrophile. Justify your answer.
        True. SN2 reaction follows second order kinetics. The reaction occurs when the nucleophile collides (effectively) with the alkyl halide, this collision rate is increased if either one (or both) of the reactants is increased.



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Re: SN1 and SN2 reactions.
« Reply #1 on: March 24, 2014, 07:58:50 AM »

Ok, so there is a lot going on here so Ill try to address it in the order its presented. 

1: Yes.  Generally I>Br>Cl>F as far as leaving group ability.

2: A: correct
    B: Yes, but the electronics of the benzene ring also play an important part in blocking SN2 reactions.

3:  Perhaps, though I think of it in terms of electronics.  The benzyl CH2 is more electrophillic than the CH2 attached to an alkyl chain.

4: A: Yes and no, a SN2 reaction does not involve a carbocation (I think you ment to say something else). 
    B: Yes
    C: Yes

5:  Yes, looks reasonable.

6:  Yes, but again I tend to think of it in terms of electronics.  Br is much larger than Cl and is more able to displace/delocalize a negative charge since it is further away from the nucleus.  The technical term for this is charge density.

7: Yes.

8: Yes

9: Yes
When in doubt, avoid the Stille coupling.

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