This is important throughout organic chemistry, but will be especially important when trying to determine the products of elimination and substitution (E1, E2, SN1, SN2)reactions.  In fact, there is not a more important part of an organic chemistry reaction than the nucleophile and the electrophile.   So, let’s look at what makes a good nucleophile. 

There are generally three trends to remember when discussing how nucleophilic a reactant is:

1)      Size – Generally, the more linear and/or smaller the nucleophile, the more nucleophilic it will be.  This is because it can react at more sites and will not be sterically hindered if it is smaller or linear.

2)       Electronegativity– The more electronegative an atom is, the less nucleophilic it will be.   This is because more electronegative atoms will hold electron density closer, and therefore will be less likely to let that electron density participate in a reaction.  We see this in calculations and experiments that show nucleophilicity decreases as you get closer to fluorine on the periodic table (C > N > O > F)

3)      Polarizability- The more polarizable an atom is, the more nucleophilic it will be.   Polarizability is defined as the ability to distort the electron cloud of an atom, which allows it interact with a reaction site more easily.  Generally, polarizability increases as you travel down a column of the periodic table (I > Br > Cl > F)

 Below is a table of relative nucleophilic strength.  This is relative because nucleophilic strength is also dependant on other factors in the reaction, such as solvent. 

VERY Good nucleophiles

HS, I, RS

Good nucleophiles

Br, HO, RO, CN, N3

Fair nucleophiles

NH3, Cl, F, RCO2

Weak nucleophiles

H2O, ROH

VERY weak nucleophiles

RCO2H

 

As shown above, as a general rule, the anion of a reactant will be a better nucleophile than the neutral form.  (i.e. RCO2 is a better nucleophile than RCO2H)

Step 2 is learning about the electrophiles.  Please visit our recent post on this topic –> electrophiles

For more information on this and other topics of organic chemistry interest, please visit organic chemistry

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16 Thoughts on “Know your strong nucleophiles

  1. Andres on January 21, 2009 at 7:08 am said:

    CzvcsTIIX3uW0

  2. gftws on May 2, 2009 at 3:42 pm said:

    thank you so much for the informations
    they’ve been so useful

  3. felix on May 3, 2009 at 3:27 pm said:

    The poor nucleophiles is more favor to Sn1 reaction than Sn2 reaction. Is my statement correct?

  4. how does base strength correlate with nucleophile strength?

  5. k.murugavel on March 23, 2010 at 11:05 am said:

    all ready use to website more

  6. Thank you so much for this!!!!!

  7. Robert on October 5, 2010 at 9:11 pm said:

    So, would R-O-NH2 be a fair nucleophile or a weak nucleophile? I’m thinking it would be weaker than NH3 because of the oxygen, but I’m not sure.

  8. brittany on October 28, 2010 at 5:08 pm said:

    this is about to help me on my orgo exam yesss

  9. Haashir on March 30, 2011 at 2:40 am said:

    If the iodide ion is a stronger nucleophile than the hydroxide ion, why does the latter displace the former in a reaction involving aqueous Sodium hydroxide and alkyl iodide?

  10. Pingback: Electrophiles and Electrophilic Reactions: What makes a good electrophile? « Organic Chemistry made easy

  11. olivia on January 16, 2012 at 2:53 am said:

    Hi,
    I am quite confused I ampretty sure in an SN2reaction I- would be a good electrophile not nucelophile?

  12. size and polarizable effects are contracdictory,if size of the atom is larger more polarizablity is increases, therefore larger the size nucleophilicity increases. for (CH3)3C- > (CH3)2N->CH3O-
    and also C->N->O->F- C size is larger than N,O and F.
    I->Br->Cl->F- I- is larger in size than Br-, Cl- and F-

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