[trainee]

 When I do reactions, I'm often thinking about the pka but as a base, how strong is an alkene? I found myself having a slight crisis. I can look up the pka of the hydrogen of an alkene but what if the carbons are all bonded? The pi electrons can still attack right? How strong/weak is that?

Hopefully my first question wasn't too silly

[pgk]

Nucleophilic strength does not always coincide with the strength of the corresponding base because various other factors are involved therein, such as the molecular size and the base softness. Please, see a previous discussion in this forum:
http://www.chemicalforums.com/index.php?topic=92617.0
Anyway and according to the Evans pka tables, the pka ethylene is 50 in water (= very weak acid / very strong conjugate base):
http://evans.rc.fas.harvard.edu/pdf/evans_pKa_table.pdf
Besides, vinyl anion is a soft base:
http://jurberglab.iqm.unicamp.br/wp-content/uploads/2013/12/HardSoftAcidBases.pdf
Thus, alkene anions are very strong nucleophiles.
PS: This is not a silly question.

[spirochete]

I agree with pgk's points about nucleophilicity, but I think the pKa of ethylene is not the best pKa to look up to see how basic an alkene is. If ethylene has a pKa of 50, that just means it's carbanion conjugate base is extremely strong, and also very nucleophilic because of how basic it is.

But this doesn't tell you how basic the alkene is. The pKa of a carbocation would be a better indication. Carbocations are strong acids which tells you their conjugate bases, alkenes, are very weak bases.

Unfortunately this doesn't completely tell you the alkene is a weak nucleophile. It just tells you it is a weak base. Alkenes I think are more nucleophilic than there basicity suggests, because of their high energy pi molecular HOMO.

Experimentally, you can see that alkenes are fairly weak nucleophiles because they don't do sn2 reactions with alkyl halides usually. They are more likely to react with strong electrophiles like HX, X₂ and carbocations.

[pgk]

Right!
Alkene carbanions are strong nucleophiles. Contrary, non-ionic alkenes are electrophiles. Besides and referring to their hardness, alkene carbocations are borderline acids (See the corresponding link, above).

[rolnor]

pgk "Non-ionic alkenes are electrophiles", how do you mean? I think Mickael-type alkenes like akrylcyanid etc. can react like electrophiles but not those without electron-withdrawing groups?

[pgk]

Meaning just “neutral” alkenes and not alkene carbanions.

[spirochete]

Neutral alkenes are mostly commonly nucleophiles, not electrophiles. See: electrophilic addition to alkenes.

If the alkene is conjugated with an electron withdrawing group it will become electrophilic. See: acrylates

Even electron poor alkenes like acrylates will sometimes react with electrophiles! Somewhat surprisingly. So I think the baseline for an alkene is nucleophile, not electrophile.

[pgk]

It was a wrong expression. Sorry!
The right is: "Contrary, non-ionic alkenes can be electrophiles."
Alkenes are rich in electrons, so and by definition, they might not be considered as electrophiles. Thus, alkenes undergo electrophilic attack, depending on the relative acid hardness of attacking electrophile (e.g. H+, X+, carbocation, etc.). On the other hand, alkenes can also have electrophile properties (e.g. allyl carbanion that is stabilized by conjugation with the double bond). Therefore, their weak nucleophile character is not absolute.

[Vidya]

When I do reactions, I'm often thinking about the pka but as a base, how strong is an alkene? I found myself having a slight crisis. I can look up the pka of the hydrogen of an alkene but what if the carbons are all bonded? The pi electrons can still attack right? How strong/weak is that?

Hopefully my first question wasn't too silly

Nucleophilicity and basicity can or acn not show parallel strength because of different factors.Alekens /alkynes and Aromatic rings full of pi electrons are nucleophiles and show reactions with electrophiles.However they can be considered as moderate/ weak  nucleophiles which reacts  with electrophiles.Remember in most of alkenes reactions we need catalysts.