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Kinetic Versus Thermodynamic Control In Addition of HBr to Dienes: 1,2- and 1,4- Addition

In today’s post we’ll discuss 听1,2- and 1,4- addition to dienes – specifically, the addition of strong acid such as HBr.

• When a diene undergoes reaction with a strong acid like HBr, protonation results in a听谤别蝉辞苍补苍肠别-蝉迟补产颈濒颈锄别诲听carbocation
• The resonance-stabilized carbocation can undergo attack at听迟飞辞听possible positions.
• When the reaction is conducted at low temperatures, the reaction is irreversible and the major product will be the one with the lowest-energy transition state, which is the carbon best able to stabilize positive charge.
• This is referred to as running the reaction under听kinetic control.
• When the reaction is conducted at higher temperatures, the reaction is听谤别惫别谤蝉颈产濒别听and the major product will be the one which is most听thermodynamically stable, which is generally the听尘辞蝉迟-蝉耻产蝉迟颈迟耻迟别诲听alkene.
• This is referred to as running the reaction under听thermodynamic control.

Table of Contents

1. Addition To Alkenes, Revisited
2. Reaction Of Butadiene With Acid Gives “1,2-Addition” And “1,4-Addition” Products
3. The “1,4 Addition” Product Of Acids Adding To Butadiene
4. Protonation Of Butadiene Gives A Resonance-Stabilized Carbocation
5. The Effect Of Temperature: Low Temperatures Give “1,2” Addition To Butadiene
6. Higher Temperatures Give More Of The “1,4” Product
7. With Butadiene, The “1,4” Product Is More Stable Because It Has A More Substituted Double Bond
8. “Kinetic Control” vs “Thermodynamic Control”: At Low Temperatures The Reaction Is Irreversible And Products Are Determined By Relative Rates
9. Thermodynamic Control: At Higher Temperatures The Reaction Is Reversible And Product Distribution Is Determined By Stability
10. Using An Energy Diagram To Understand Thermodynamic Versus Kinetic Control
11. A Parting Word Of Warning: The “1,4-Product” Is Not Always The “Thermodynamic” Product
12. Other 188bet金宝博官网登录 Can Give 1,2- and 1,4- Additions As Well
13. Notes
14. Quiz Yourself!
15. (Advanced) References and Further Reading

1. Addition To Alkenes, Revisited

Waaay back in the day we talked about addition reactions of alkenes. Remember听Markovnikov’s rule听in the addition of electrophiles to alkenes?

For example, take an alkene like 1-butene, and add HBr. What happens?

An addition reaction occurs! (Break C-C pi, and form adjacent C-H and C-Br bonds).

Bromine adds to the most substituted carbon of the alkene, and hydrogen adds to the least substituted carbon. That’s due to carbocation stability: the reaction proceeds through the most stable carbocation, which happens to be the most substituted.

2. Reaction Of Butadiene With Acid Gives “1,2-Addition” And “1,4-Addition” Products

All well and good. But since we’ve been discussing conjugation and resonance, let’s throw in an additional wrinkle.

What happens when we try the same reaction on a 诲颈别苍别?听Butadiene, for example.

I’ll tell you. You get two products! (But the second one might not be what you think it is).

Here’s what they look like:

Product #1 is essentially the same product that we saw听in the 1-butene example: 听H and Br are added across two consecutive carbons of a double bond. Note that since butadiene is symmetric, 听the same product is formed regardless of which double bond participates! (we get 3-bromo-1-butene either way).

Since addition occurs across two consecutive carbons, we often call this “1,2 addition”.

3. The “1,4 Addition” Product Of Acids Adding To Butadiene

In contrast, Product #2 shows the result of adding H and Br across听four conjugated carbons. All four carbons participate in the reaction. 听A new C-H single bond has formed on one end of the diene (C-1), and C-Br formed on the other end (C-4). Note that the C₁-C₂ and C₃-C₄ pi bonds are broken, and we’ve formed a new pi bond between C₂ and C₃.

We call this “1,4 addition”.

[Note: to simplify the discussion here, I’m choosing to听ignore double bond isomers (i.e.听E听and听Z) in this analysis.听听In the lab, the 1,4- example above will exist as a mixture of (mostly)听E product with a small amount of the听Z. ]

So why does this “1,4 addition” happen in the first place? What’s different about butadiene, as opposed to 1-butene where only “1,2-addition” was possible?

4. Protonation Of Butadiene Gives A Resonance-Stabilized Carbocation

The first thing to notice is that the initial protonation of butadiene gives a听resonance-stabilized carbocation. See how protonation of C-1 gives a carbocation that has two important resonance forms?

In the case of butadiene,

• the major contributor听to the resonance hybrid will be the resonance form where the carbocation is on the more substituted carbocation (C₂)
• the minor contributor will be the resonance form where the carbocation is on the less substituted carbocation (C₄).

Attack of the nucleophile (Br^鈥�) at the C₂ position of the hybrid will lead to the 1,2-product.

Attack of Br^鈥撎�at the C₄ position of the resonance hybrid will lead to the 1,4-product.

We can draw it up like this:

[Note: never forget听that resonance forms are not in equilibrium with each other.听It’s understandable to听casually say that “Br(鈥�) will attack the top听resonance form with the more stable carbocation” so long as you remember that the true structure of the molecule is a hybrid of the two resonance forms.] In the footnote, I’m including perhaps a more correct way to show formation of the bottom resonance form.

5. The Effect Of Temperature: Low Temperatures Give “1,2” Addition To Butadiene

Here’s the big question: which one of the two products will be major? The 1,2-addition product or the 1,4-addition product?

We’re so used to seeing “Markovnikov addition” in alkenes (where addition occurs to the most stable carbocation) that it seems intuitive that the 1,2-addition product would be dominant.

And indeed, at low temperature, 1,2 addition to butadiene is favoured. Here’s a literature example.

6. Higher Temperatures Give More Of The “1,4” Product

Interestingly, however, as the temperature is increased, the amount of 1,4 product increases.

At room temperature, the ratio of 1,2- and 1,4- addition is 45:55 .

At 40 degrees Celsius the 1,4- product is dominant (about 80%).

7. With Butadiene, The “1,4” Product Is More Stable Because It Has A More Substituted Double Bond

What’s going on here? What structural features are present that could possibly make formation of the 1,4 product more favourable听than the 1,2 product, even though it goes through a “less stable” carbocation?

The answer lies in the substitution pattern of the double bond.

Remember Zaitsev’s rule? Same deal.听The 1,4 product is more stable because it is a more substituted double bond.

The 1,4 product has a听di-substituted double bond, whereas the 1,2-product has a 听mono-substituted double bond. Generally speaking, double bond stability increases as the number of carbons directly attached to the double bond is increased. (See article: Alkene Stability)

So why would 1,4 be more favoured under conditions of higher temperature, and the 1,2 be favoured under conditions of lower temperature?

8. “Kinetic Control” vs “Thermodynamic Control”: At Low Temperatures The Reaction Is Irreversible And Products Are Determined By Relative Rates

At low temperatures, the differentiating factor is the relative energies of the transition states leading to the products. The 1,2 product has a lower-energy transition state, owing to the fact that charge is more stable on the more substituted carbon. The difference between the energies of these transition states will determine the product ratio.

A quick analogy. Imagine you’re hungry, and you only have $5 in your pocket. In a choice between McDonalds and Applebee’s, the only accessible option is McDonalds – even if, for some reason, you like Applebee’s a lot more.

Let’s sketch this out. The carbocation intermediate can pass through the two different transition states 听that lead to the 1,2- and 1,4- products, respectively. If we choose a temperature low enough, then the product distribution will reflect the difference in energy between the two activation energies E_a (1,2) and E_a (1,4).听So long as the reaction is not reversible, the product with the lower energy transition state will dominate. This is called “kinetic control”.

9. Thermodynamic Control: At Higher Temperatures The Reaction Is Reversible And Product Distribution Is Determined By Stability

At higher temperatures, the reaction has the potential to be听reversible. [Note 2]听In this case, this means is that both the 1,2- and 1,4- products can听ionize听(think of the first step in the S_N1 reaction), reforming the carbocation intermediate.

This sets up an听equilibrium.听The product ratio will now reflect the relative stabilities of the 1,2- and 1,4- products, not the transition states leading to their formation. In the case of butadiene, since the 1,4- product is more stable (it has a disubstituted double bond) it will be the dominant product at higher temperatures.

This is referred to as “thermodynamic control“.

Math interlude: 听recall that 螖G = 鈥揜T lnK

A difference of 1 kcal/mol in stability doesn’t sound like much, but it translates into a 83:17听ratio of products at equilibrium at room temperature.

Continuing our analogy: with enough听money in your pocket, the decision where to eat lunch is more a function of how much you like the overall McDonalds vs. Applebee’s experiences, not how much they cost.听

10. Using An Energy Diagram To Understand听Thermodynamic Versus Kinetic Control

Drawing up the reaction energy diagram can be helpful to understand kinetic and thermodynamic control. This post here听goes听into more detail, but听we’ll repeat the basics here.

The reaction energy diagram for the addition of HBr to butadiene looks like this:

Point听A is the starting butadiene, and point听B is the transition state for addition of H to butadiene.

The important part to pay attention to is the “local minimum” C, the resonance-stabilized carbocation.

As we mentioned previously, going from intermediate听C to transition states听D听and听D听represent the energy pathways听for 1,2- and 1,4- addition, respectively.

Hence the听activation energies for the forward reactions is equal to the difference in energy between听D听and C (for 1,2-addition)听and 听D听and听C (for 1,4-addition). We saw that 1,2-addition has a lower activation energy.听

Now let’s look at the reverse reaction.

Points E听and听E听represent the energies of the 1,2- and 1,4- products.

The activation energy for the听reverse听reaction is the difference in energy between听E听and transition state听D, and听E听and transition state听D, respectively.

It should be clear from this diagram that the activation energies for the forward reaction 听(going from C听through transition states D听and听D听give E听and听E, respectively)听听are much lower than the activation energies going in the reverse reaction (i.e. from E听and听E听through transition states听D听and听D听back to carbocation听C听).

• So if we keep the temperature low, we favor the forward reaction and hinder the reverse, and obtain the听kinetic听product.听
• If the temperature is raised, the reverse reaction becomes energetically accessible, and equilibrium is established. We will then obtain the听thermodynamic product.听

11.A Parting Word Of Warning: The “1,4-Product” Is Not Always The “Thermodynamic” Product

Let’s summarize.听听Understanding the following two factors is key to correctly answering exam problems.

1. The听relative importance of the carbocation resonance forms
2. The relative stabilities of the double bond products.

In the case of butadiene, it is true that the听1,2 product was formed through a more stable carbocation (kinetic product), and the听1,4 product had a more stable double bond (thermodynamic product).

But it will not always be true for all dienes!!

Here are three听examples that will help you think through the main issues (answers in the next post).

12. Other 188bet金宝博官网登录 Can Give 1,2- and 1,4- Additions As Well

This post is long enough, but I would be remiss if I failed to note that 1,2 and 1,4 additions to dienes are also possible for a few other classes of reaction.

• Addition of HBr to dienes under free-radical conditions (e.g. HBr + peroxides)
• Addition of Br₂ (and Cl₂) to dienes.

Note that similar issues will arise (i.e. stability of a reactive intermediate versus stability of the final product). We’ll go into more detail when the time comes.

Many thanks to Tom Struble for help with preparing this post.听

Notes

Note 1. All the textbooks I consulted showed diagrams similar to what I drew above, with Br(-) attacking different resonance forms. 听Perhaps a more correct way to draw the formation of the carbocation at the 4-position is to show it like this.

Note 2.听Reversibility

Quiz Yourself!

Become a听 MOC member to see the clickable quiz with answers on the back.

Become a听 MOC member to see the clickable quiz with answers on the back.

Become a听 MOC member to see the clickable quiz with answers on the back.

Become a听 MOC member to see the clickable quiz with answers on the back.

Become a听 MOC member to see the clickable quiz with answers on the back.

Become a听 MOC member to see the clickable quiz with answers on the back.

(Advanced) References and Further Reading

1. THE ADDITION OF HYDROGEN CHLORIDE TO BUTADIENE
   M. S. KHARASCH, J. KRITCHEVSKY, and F. R. MAYO
   The Journal of Organic Chemistry 1937, 02 (5), 489-496
   DOI:
   An early paper by the esteemed chemist M. S. Kharasch on the addition of HCl to butadiene, seeing whether the ratio of 1,2 to 1,4 addition varied with different reaction conditions.
2. THE PEROXIDE EFFECT IN THE ADDITION OF REAGENTS TO UNSATURATED COMPOUNDS. XIII. THE ADDITION OF HYDROGEN BROMIDE TO BUTADIENE
   M. S. KHARASCH, ELLY T. MARGOLIS, and FRANK R. MAYO
   The Journal of Organic Chemistry 1936, 01 (4), 393-404
   DOI:
   Another early study by M. S. Kharasch in which he studies the addition of HBr to butadiene, in which he attempts to rigorously separate the two modes of addition 鈥� electrophilic vs. radical.
3. 鈥擬obile-anion tautomerism. Part I. A preliminary study of the conditions of activation of the three-carbon system, and a discussion of the results in relation to the modes of addition to conjugated systems
   Harold Burton and Christopher Kelk Ingold
   J. Chem. Soc. 1928, 904-921
   DOI:
   A classic paper from a pioneer of physical organic chemistry, Prof. C. K. Ingold. He suggests the general idea that the 1,2 and 1,4 addition products are in equilibrium and that the 1,2-product can reverse reaction and form the 1,4 product. This is the basis of what we now call “kinetic” and “thermodynamic” control.
4. The modes of addition to conjugated unsaturated systems. Part IX. A discussion of mechanism and equilibrium, with a note on three-carbon prototropy
   P. B. D. de la Mare, E. D. Hughes and C. K. Ingold
   J. Chem. Soc. 1948, 17
   DOI:
   Here, C. K. Ingold has reviewed all work (including Kharasch鈥檚) done up to that point in the study of electrophilic additions to conjugated systems.
5. Properties of conjugated compounds. Part XI. Addition of hydrogen bromide to 尾纬- and 伪未-dimethylbutadiene
   Ernest Harold Farmer and Frederick G. B. Marshall
   J. Chem. Soc. 1931, 129
   DOI:
   When methyl groups are added to butadiene, the 1,2 and 1,4 addition ratios can change significantly!听 This is because the stability of the carbocation intermediates will change.