Why E2 Favors Secondary Halides: Exploring 2-Chlorobutane Reactivity in Ethoxide

[silversurf]

Homework Statement

Why would this reaction favor E2 over SN2? Since its a secondary halide reacting with a strong base/nucleophile, and all other conditions being equal isn't SN2 favored? Or is ethoxide somehow a stronger base than nucleophile? If so, how can you tell that it's a better base than nucleophile? Below is the question.

What is the major product formed from the reaction of 2-chlorobutane in aqueous sodium ethoxide?
A.

2-ethoxybutane
B.

1-butene
C.

cis-2-butene
D.

trans-2-butene

Correct Answer
Explanation:
D. 2-Chlorobutane, a secondary alkyl halide, will react with a strong base, such as ethoxide, via an E2 mechanism. Choice A can be eliminated because it is the result of a substitution reaction. Elimination reactions generally follow Zaitsev's rule, which states that the product with the most substituted double bond is formed (provided an unhindered base is used). 1-Butene has a terminal double bond, which is less substituted than the double bond in 2-butene, so choice B can be eliminated. The most stable geometric isomer will have the two largest groups on opposite sides of the double bond, so choice C can be eliminated.

Homework Equations

The Attempt at a Solution

 

[chemisttree]

For secondary alkyl halides, any nucleophile that is a base stronger than acetate (pKa 4.8) will deprotonate rather than undergo substitution. What do you think this says about the acidity of secondary alkyl halides with alpha protons?

 

[silversurf]

Hi Chemisttree,
I think I understand but I'm not sure. I'm confused on why you used pka of acetate, for comparison, to determine basicity. Are you just using this as a starting point converting to pkb and then looking at basicity? If so couldn't we just look at each pkb and pick the one with the smallest pkb to determine the stronger base. Is there a way to determine basicity, relative to nucleophilicity without using pkb/pka values? I read there's a trend where basicity is highest going towards the upper left hand side of the periodic table and a trend where nucleophilicity is highest going towards the bottom left hand side of the periodic table but they aren't explained and I can't find information on what these trends are based on, relative to each other. Would you happen to know what these trends are based on? About your question, this basically tells us that the alpha proton in this case is not very acidic so we need a strong base right?

 

[chemisttree]

Hi Chemisttree,
I think I understand but I'm not sure. I'm confused on why you used pka of acetate, for comparison, to determine basicity. Are you just using this as a starting point converting to pkb and then looking at basicity? If so couldn't we just look at each pkb and pick the one with the smallest pkb to determine the stronger base. Is there a way to determine basicity, relative to nucleophilicity without using pkb/pka values?

We use either pKa or pKb as a measure of basisity. Of course when we use pKa, we assume the protonated form. pKa of acetate being 4.8 is actually that of the acid form... acetic acid. Kind of confusing to speak of base strength in terms of acidity but you will note that they are related by the formula pKa +pKb = 14.

Is there a way to determine basisity relative to nucleophilicity without using pKa/pKb? Not really. You see, even a fairly weak base like acetate is a pretty good nucleophile. Pretty good towards the right alkyl halide, that is. Good nucleophile for primary alkyl halides without beta carbon branching, not so good toward secondary, tertiary or primary with beta carbon branching.

About your question, this basically tells us that the alpha proton in this case is not very acidic so we need a strong base right?

Actually it was a bit of a trick question. If a base only a little stronger than acetate is capable of deprotonating alpha to the C-X bond you might be tempted to assume that something like chloroethane is a fairly strong acid and might exist in equilibrium as:
CH₃-CH₂-X <------> CH^-₂-CH₂-X + H⁺

which doesn't happen... hence the "2" subscript for an E2 elimination. Plucking off that proton doesn't happen without the acetate and a particular arrangement of the C-Cl bond anti to the proton being removed. It is more than an acid/base thing.

 

[silversurf]

Is there a way to determine basisity relative to nucleophilicity without using pKa/pKb? Not really. You see, even a fairly weak base like acetate is a pretty good nucleophile. Pretty good towards the right alkyl halide, that is. Good nucleophile for primary alkyl halides without beta carbon branching, not so good toward secondary, tertiary or primary with beta carbon branching.

@hat about KCN reacting with a secondary carbon, in aprotic solution, this results in a nucleophilic attack sn2 major product, how would we know that the major product is SN2 in this case? since aprotic conditions favor sn2 and e2 and secondary carbons are in the middle favor ability for both too.

 

[silversurf]

Actually never mind, I just realized the reason why is because it's a weak base.