How to find out whether a given compound is a good nucleophile or a good base.....?

Question

yarr...how to find out whether a given compound is a good nucleophile or a good base.....?

for example which of these is a good base and a good nucelophile

CH3-CH2-O^- AND CH3-CH2-S^-

THERE IS NEGATIVE CHARGE ON O AND S.....

plzz explain answer also....

Answer 1

Good nucleophile or a good base is a compound that has either negative charge(anion) or has extra electrons.

OH^- is a good nucleophile, it is an anion

NH3 and H2O are good nucleophiles because contain extra electrones on N one pair and O two paires.

On oldgreek language phileo means want(like) so, the nucleophiles wants particles that are with positive charges like cations or with less electrones.

Both of CH3-CH2-O^- and CH3-CH2-S^- are nucleophiles, that with O is stronger because the electronegativity of O is bigger than that one of S

Answer 2

Ethoxide is the conjugate base of ethanol as an acid .As an acid ethanol is weaker than water which means ethoxide is a stronger base than OH-1. Thioethoxide CH3CH2S^-1 is the conjugate base of thioethanol .S has a lot lower electronegativity than oxygen and thus will not hold on to electrons as firmly as O.This means that thioethanol will lose a proton easier than ethanol and therefore the conjugate base is weaker than ethoxide. However because S is a bigger atom with less electronegativity...the electrons around R-S^- will interact with a positive center easier and from greater distance than those of R-O^-. So although the S anlogue is a weaker base ( ability to abstract an H+ from another molecule ) it is a better nucleophile as it can interact with a positive center and share its electrons easier.  Being a strong base does NOT mean a compound is a good nucleophile. As an example the strained strong base DBU( diaazabicycloundecene) can easily abstract a proton from a compound but because of steric hindrance cannot approach a positive center in a molecule that has any , even moderately bulky substituents . So being a strong base does NOT make a compound a strong nucleophile. Most S analogues of oxygen compounds are better nucleophiles.....exp...  cysteine ( with a side chain SH is a far better nucleophile than serine with a side chain OH )