Better nucleophile
Substitution reactions at saturated carbon involve displacement of a group bonded to that carbon by a nucleophile.
The group that is displaced is referred to as the leaving group.
What is a nucleophile?
A nucleophile (or nucleophilic reagent) is a species that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons.
The electrophile in nucleophilic substitution reactions is the species containing the carbon bearing a leaving group.
Nucleophiles that react by substitution at saturated carbons in electrophiles have an atom that has a nonbonding electron pair.
Substitution reactions involving better nucleophiles are faster.
The group that is displaced is referred to as the leaving group.
CH3Br + –OH 
CH3OH + Br–
The nucleophile in this reaction is hydroxide ion; the leaving group is bromide ion.
The atom in the leaving group that was originally bonded to carbon is one unit more negative in the product.
The atom in the nucleophile that becomes bonded to carbon is one unit more positive in the product.
CH3OH + Br–The nucleophile in this reaction is hydroxide ion; the leaving group is bromide ion.
The atom in the leaving group that was originally bonded to carbon is one unit more negative in the product.
The atom in the nucleophile that becomes bonded to carbon is one unit more positive in the product.
What is a nucleophile?
A nucleophile (or nucleophilic reagent) is a species that forms a bond to its reaction partner (the electrophile) by donating both bonding electrons.
The electrophile in nucleophilic substitution reactions is the species containing the carbon bearing a leaving group.
Nucleophiles that react by substitution at saturated carbons in electrophiles have an atom that has a nonbonding electron pair.
As shown below, nucleophiles may be negatively charged, or they may have no charge.
Common nucleophiles for substitution reactions:
water alcohols ROH | hydroxide ions alkoxide ions RO– (like CH3O–) |
amines | halide ions (X = Cl, Br, I) | cyanide ion –:C≡N: |
Substitution reactions involving better nucleophiles are faster.
The better nucleophile
when comparing species which have the same nucleophilic atom, has a negative charge at that atom (–OH > OH2).
has a less electronegative and/or larger atom bearing the non-bonding pair (Br– > Cl–) (NH3 > OH2).
has a less electronegative and/or larger atom bearing the non-bonding pair (Br– > Cl–) (NH3 > OH2).