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ORGANIC Chemistry: Nucleophilic Substitution

  1. Oct 26, 2011 #1
    1. The problem statement, all variables and given/known data

    8) How might you prepare each of the following molecules using a nucleophilic substitution at some step?
    a) (CH3)3-O-CH3

    b)CH3CH2CH2CH2CN

    2. Relevant equations
    none


    3. The attempt at a solution

    I'm really not sure what's being asked.

    I'm going to assume that I need to pick a leaving group and substitute it with some kind of nucleophile...

    So...for part a I chose O for the leaving group and part b I chose CN as the leaving group.

    However, I have no clue where to go from there...what am I trying to create?

    Thanks for any help!!
     
  2. jcsd
  3. Oct 26, 2011 #2
    You have it backwards. . . they are giving you the products, and asking how you would make them from (insert your own reactants) using substitution.

    I am not sure if I am supposed to help you any further, so I will refrain from more details at this point. . .

    The idea is being able to identify parts of a product that would make a good electrophile, or in this case, have already made a good electrophile; as well as knowing what are some GOOD/GREAT nucleophiles, and then use this knowledge to think about how a nucleophile would attack an electrophile to give the desired product.

    Others: please feel free to correct me with any terminology or other mistakes I have made, my o-chem is rusty, but hopefully with the more time I spend here it will start to come back to me :P
     
    Last edited: Oct 26, 2011
  4. Oct 27, 2011 #3
    Ohh, that makes more sense...

    Though, I'm still not completely sure how to go about doing this.

    We've worked problems like this in class, but the other way. We usually push Br out of the system and then the nucleophile takes its place.

    Could I pick Br to replace CN in part b?

    In part a, I'm not sure which molecule came in...

    EDIT:
    I think I figured it out....

    (CH3)3C-Br + NaOCH3 ----> (CH3)3C-OCH3 + NaBr

    CH3CH2CH2CH2I + NaCN ----> CH3CH2CH2CH2CN + NaI
     
    Last edited: Oct 27, 2011
  5. Oct 27, 2011 #4
    I have no idea what the first reagent is.

    For the second one which contains cyanide, usually we use halogenoalkanes. Halogens usually contains very polarised bonds, since halogens have high electronegativity usually. The polarised carbon would attract a nucleophile which is CN- ions in this case.

    You could use for example CH3CH2CH2CH2Cl and acidified KCN solution as reagents. Then CN- ions are likely to attack the C-Cl bond which has positive charge on the carbon atom.

    Just giving some ideas
     
  6. Oct 27, 2011 #5
    Both of your answers look correct to me, but it would be best if someone else could double check. . .

    Another thing of importance to note is that using CN as a nucleophile adds one more carbon to the chain. . . there will be problems later on (in either ochem 1 or 2) where you will have to add carbons, so this is something good to remember.

    :)
     
  7. Oct 27, 2011 #6
    Yes, CN group could be hydrolysed in to carboxylic group and ammonia by either acid or base.
     
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