Hydrohalogenation

This is the most simple addition reaction, mostly because this is the example that we use to teach the mechanism in the first place. You should already be familiar with this reaction.

Concept: General properties of hydrohalogenation.   

5m
Video Transcript

All right, so this brings us to our first named reaction of the section and it turns out that it's a really easy reaction because it's just the one that we've already been practicing with. It's called hydrohalogenation. Just like the name says all we're going to be doing is we're going to be adding a hydrogen and a halogen as sigma bonds. That's it.
So we just want to fill out a few facts before we begin. Let's talk about the intermediate because all these addition reactions, you're going to have intermediates. So either intermediates or transition states. So let's go ahead and talk about what this is. The intermediate is going to be a carbocation. So we know that we're going to have a carbocation intermediate.
And the stereochemistry is going to be unknown. The reason is because whenever you have a carbocation in place and you're attacking it, remember that carbocations are trigonal planar so what that means is they have a front side and a back side that are both easy to attack. So what that means is that when something attacks it, I'm not going to know exactly where it's coming from. Is it coming from the front, back? We don't really know so actually you're just going to get a mixture.
So the product here is going to be alkyl halides. So what that means is I'm going to go from double bonds to alkyl halides using hydrohalogenation.
Now let's talk about rearrangements. This is basically a checkbox. Could we expect carbocation rearrangements or not? The answer is that any time you have a carbocation intermediate in your mechanism, you always need to be thinking about rearrangements. So this is going to get a huge checkmark because of the fact that we have carbocation intermediate so we'd expect that you have to be able to shift if it wants to.
Then finally, we've got what we call the regiochemistry. I'm just going to go ahead and define this word really quick. Regiochemistry. What regiochem has to do with is actually where the reactive site is. Where does the electrophile add? So I'm just going to put where does electrophile add. That's the regiochemistry. The stereochemistry says what's the shape of the end product. Is it cis? Is it trans? Stuff like that.
Regiochem says where does the electrophile add? Does it add to a specific atom? And the answer is yes. The regiochemistry is going to be Markovnikov because Markovnikov's rule says that your carbocation is going to form in the most stable place and that your electrophile attacks your carbocation. So that's the regiochemistry.
Let's go ahead and look at the general reaction. The general reaction is simply going to be this, a double bond reacts with an HX, hydrogen halide. And what we're going to wind up getting is that if it's asymmetrical, your halogen is going to attach to the more substituted side or the Markovnikov side. So that would be this right here. And then your hydrogen is going to attack to the less substituted side. So that would be this hydrogen right there.
Now you might be wondering what is the squiggly line. We've never seen that before. Maybe you have, maybe you haven't. Squiggly line just has to do with the unknown stereochem. Basically, all that's saying is that I don't know if the hydrogen is facing towards the front. I don't know if it's facing towards the back because when it attacked it was trigonal planar. So I don't really know exactly where it is.
Now notice that if that H is there, there's actually no chiral center on this molecule at all. So what that means is that this X that I just drew here, I could have drawn it on the front, I could have drawn it in the back. It doesn't really matter. All I'm trying to show you is that honestly, we're not going to know the stereochemistry of the end product.
So there you go. Pretty easy. Let's go ahead and do an example of this. I want you guys to try it yourself and then I'll go ahead and give you guys the answer. 

General Reaction:

Note: The squiggly line on the product just means “indeterminate stereochemistry”. We aren’t sure where that –H will add, so we’ll just draw it on a squiggly line. 

Example: Provide the mechanism of the following addition reaction.    

3m

Hydrohalogenation Additional Practice Problems

Fill in the box with the product(s) that are missing from the chemical reaction equation. Draw only the predominant regioisomer product or products (i.e. Markovnikov or non-Markovnikov products) and please remember that you must draw the structures of all the product stereoisomers using wedges and dashes to indicate stereochemistry. When a racemic mixture is formed, you must write "racemic" under both structures EVEN THOUGH YOU DREW BOTH STRUCTURES.

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Provide the products for the following reaction.

a) clearly label each drawing with the correct regio- and stereochemistry.

b) clearly label pairs of enantiomers & diastereomers, if any.

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Propose a mechanism for the following reaction.

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Propose a mechanism for the following reaction.

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Propose a mechanism for the following reaction.

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Consider the strucutres below and answer the following questions. 

h. Which compound(s) would be most likely to react with HCl to form alkyl chloride with a rearranged reaction? 

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Give the product, or products, including stereochemistry of the reaction of (Z)-3-methyl-2-pentene with the reagent below. If the products are a pair of enantiomers, you need to draw only one and state that the other enantiomer is formed. 

I/ H2O

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Provide the major product for each of the following alkene reactions. 

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Predict the product(s) or reagents of the following reactions.

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Predict the product(s) or reagents of the following reactions.

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How many atoms and electrons are directly involved in the bond-making and bond-breaking in the first step of HBr addition in the absence of peroxides?

(A) four atoms, four electrons

(B) four atoms, three electrons

(C) four atoms, two electrons 

(D) three atoms, four electrons

(E) three atoms, three electrons

(F) three atoms, two electrons 

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Which of the alkenes shown below would produce a chirality center upon Markovnikov hydrohalogenation?

 

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What is the product of the following reaction (encompasses 2 steps)? (Hint: Styrene is a π-nucleopile; also use the concept of resonance to understand the reactivity of the reaction intermediate) 

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Predict the major organic product for the following reaction:

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The reaction shown below would be expected to produce as major products which of the following compounds?

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Which of the given reaction schemes would produce the molecule shown below?

 

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Determine the mechanism and predict the product of the reaction:

 

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