Rank the relative nucleophilicity of halide ions in methanol solution. Most nucleophilic least nucleophilic Br^-, I^-, F^-, Cl^- How is methanol classified? A. a polar protic solvent B. a polar aprotic solvent Nucleophilicity & Effect of Solvent: A nucleophile is an electron-rich species which seeks positive or electron-deficient sites. It either has a lone pair of electrons or has a negative charge. It attacks the electron-deficient sites and donates its lone pair of electrons. The strength of a nucleophile is called its nucleophilicity. The nucleophilicity depends on various factors like the nature of the solvent, electronegativity of the element carrying the lone pair of electrons, etc.
Answer to question 1
(1) Methanol is a polar protic solvent as it has a highly electronegative oxygen atom bonded to a hydrogen atom which can be released.
(2) Methanol being a polar protic solvent, forms hydrogen bonds or have stronger dipole-dipole forces with smaller halogen atoms.
This decreases their ability to donate their lone pairs and decreases their nucleophilicity.
Thus, the order of nucleophilicity is:
Answer to question 2
The order of nucleophilicity :
The order of nucleophilicity can be written by observing the below points,
- Less is the electronegativity of element more is nucleophilicity.
So, as S is less electronegative than O and hence, CH3S– > CH3O–
- The negative charge increases the nucleophilicity,
CH3O– > CH3OH
CH3COO– > CH3COOH
And, CH3COO– > CH3O–
So, the overall order of nucleophilicity is
CH3S– > CH3O– > CH3COO– > CH3OH > CH3COOH
Answer to question 3
decreasing order of nucleophilicity
iodide ion>bromide ion>chloride ion >fluoride ion
because in the polar protic solvent the solvolysis of the fluoride ion will be most resulting in the least nuceophilicity
Answer to question 4
1) Acetonitrile is a polar aprotic solvent because it is a very very weak acid (pKa around 30).The nucleophilicity will be
F- > cl- > br- > i- >
2) a polar aprotic solvent