[intrusionv2]

When given an alkyl halide, depending on the reagent (strong nucleophile, weak nucleophile, strong base, or weak base), the reaction can undergo SN1, SN2, E1, or E2 mechanisms, right? I know SN1 and SN2 reactions require nucleophiles, while E1 and E2 reactions requires bases. My question:

If given an alkyl halide and a reagent, how do you determine whether the reagent is a nucleophile or a base?

This is confusing to me because the reagent can be both a nucleophile and a base at the same time.

[Jorriss]

You don't have bases and then you have another class that is nucleophiles, the two groups overlap - there is just a question of strength.

Generally, a strong base is a strong nucleophile IF it is not hindered (one important exception if you're still on E1,E2, etc is hydride, hydride, such as NaH, is basic, not nucleophilic). For example, Tertbutoxide is very basic, but it is so bulky it is not nucleophilic. Methoxide is 'nearly' as basic, but it is nucleophilic because it is not hindered.


There are other systems, such as hard and soft theory of nucleophiles/electrophiles and there are more accurate ways to classify a nucleophile such as by the ease of polarizing the molecule or atom, but for now, that should help.

If you already knew that though, let me know, we can go over more.

[intrusionv2]

Yeah, I knew most of that, but it still helped.

[Jorriss]

Alright, we can move onto something else then.

If you compare atoms in the same group, as you move down a row nucleophilicity increases. This may seem counter intuitive as basicity decreases but the reason is because it is easier to polarize the molecules.

For example, fluoride is immensely more basic than iodide, yet iodide is a stronger nucleophile.

Hydroxide is a weaker nucleophile than HS- and so fourth.




One point that may help in picking between E1,2, Sn1 or 2 is this: If the atom of molecule is a strong base, it will undergo E2 or Sn2. Then it is a matter of deciding if it is hindered, polarizable, etc.

[Jorriss]

Bah, that should say as you move down a column.

[intrusionv2]

Awesome, that helps a lot. Thanks!