Why does 2-butene react differently than 1-butene with hydrogen chloride gas?

  • Thread starter walker
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In summary, when 2-butene reacts with hydrogen chloride gas, only one product is detected. However, when 1-butene reacts similarly, two products are usually found. The difference in position of the C=C bond between the 2-butene and 1-butene alkenes is responsible for the difference in reaction results.
  • #1
walker
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Yeah I'm pretty much stumped on this one...

Basically I'm supposed to explain the following:

When 2-butene reacts with hydrogen chloride gas, only one product is detected, whereas when 1-butene reacts similarly two products are usually found.

My best guess is the difference in position of the C=C bond between the 2-butene and 1-butene alkenes. But I have no idea how to explain it. If anyone wants to give me a hint or something that would be great.

Thanks
 
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  • #2
Would that be cis- or or trans- 2-butene ?
 
  • #3
I thought it might be, but I don't think the number of isomers has anything to do with the number of products 2-butene creates when it is combined with hydrogen chlroide. Plus it is already stated that 2-butene only forms one product (2-chlorobutane) when it is combined with hydrogen chloride. So I really am stumped on this one. I believe that 1-butene might also form 2-chlorobutane but I'm not 100% sure and I have no idea what other product it would form... frustrating!
 
  • #4
OK, a little hint : what intermediate is formed after the first step in the reaction for each reaction ?
 
  • #5
2-butene

CH3CH=CHCH3 + HCl -> CH3CH2-CHCH3+Cl

1-butene

CH2=CHCH2CH3 +HCl -> CH3-CHCH2CH3+Cl

is that correct?
 
  • #6
Well, 2-butene has two same carbanion intermediate, either [itex]H_3C-CH^--CH^+-CH_3[/itex] or [itex]H_3C-CH^+-CH^--CH_3[/itex]. In each case, the result does not change; the product will contain [itex]Cl^-[/itex] where the [itex]CH^+[/itex] lies, and [itex]H^+[/itex] at the position of [itex]CH^-[/itex].

The case for 1-butene is different. Here, two kinds of carbanion may be produced; the one with higher yield is [itex]H_2C^--CH^+-CH_2-CH_3[/itex] and the other, low-yield-one is [itex]H_2C^+-CH^--CH_2-CH_3[/itex]. The products after reacting with HCl may be written by referring to the paragraph above. Do you have an idea why I wrote "high-yield" and "low-yield" for these two compounds? If so, you have understood the phenomenon, aka "stable carbocation".
 
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  • #7
I wish I understood what chem_tr was talking about

walker said:
Basically I'm supposed to explain the following:

When 2-butene reacts with hydrogen chloride gas, only one product is detected, whereas when 1-butene reacts similarly two products are usually found.

Each addition reaction has 2 possibilities

2-butene CH3-CH=CH-CH3
It's important to notice that if you draw it like a gimp it looks symmetrical. In one case, the chloride goes to carbon 2 and the hydrogen goes to carbon 3. In the other case, chloride goes to carbon 3 and hydrogen goes to carbon 2. Just rotate those products and you'll see they are the same thing, so that's why there's only 1 product. Both are 2-chlorobutane.

1-butene CH2=CH-CH2-CH3
In one case, chloride goes to carbon 1 and hydrogen goes to carbon 2; 1-chlorobutane. In the other case, chloride goes to carbon 2 and hydrogen goes to carbon 1; 2-chlorobutane.
Following markovnikov's rule, the 2-chlorobutane will be the major product and 1-chlorobutane will be the minor product.
 
  • #8
Alright that makes sense. So can it be said that the location of the C=C bond results in 1-butene creating two products and 2-butene only creating one?

What you said makes perfect sense. I'm just trying to figure out a way of applying it within the scope of the lesson material.
 
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  • #9
walker said:
Alright that makes sense. So can it be said that the location of the C=C bond results in 1-butene creating two products and 2-butene only creating one?
Yes, it can.
 
  • #10
The mechanism is bimolecular in its rate limiting step. It occurs when the pi electrons of the C=C bond are attracted to the polarized hydrogen atom. The intermediate I was referring to was the carbocation formed when the R-C=CH2 bond is protnated to become [tex]CH_3-CH_2-CH^+-CH_3[/tex]. As ShawnD noted, this follows Markonikov's rule because it will be the more stable carbocation (secondary as opposed to primary)

Now the Cl- can attack the trigonal planar(flat) carbocation intermediate. It can attack from either direction, which yields a pair of enantiomers 2R chlorobutane and 2S chlorobutane. 1-chlorobutane should be a very minor product.

chem_tr: this reaction does not involve carbanions

Edit: now, actually that I think about it, you get a pair of enantiomers with the 2-butene as well, so the question has not been very well though out! It must be the 1-chlorobutane minor product as others have mentioned.
 
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Related to Why does 2-butene react differently than 1-butene with hydrogen chloride gas?

1. What are alkenes?

Alkenes are unsaturated hydrocarbons that contain at least one double bond between two carbon atoms. They are also known as olefins and have the general formula of CnH2n, where n is the number of carbon atoms.

2. What are the potential problems associated with alkenes?

One potential problem with alkenes is their reactivity due to the presence of a double bond. This makes them prone to addition reactions and can lead to the formation of unwanted byproducts. Additionally, alkenes can also undergo isomerization, which can impact their properties and usefulness.

3. How are alkenes formed?

Alkenes can be formed through various methods, including catalytic cracking of hydrocarbons, dehydration of alcohols, and elimination reactions of alkyl halides. They can also be produced through natural processes, such as the decomposition of organic matter.

4. What are some uses of alkenes?

Alkenes have a wide range of applications in industries such as plastics, pharmaceuticals, and agriculture. They are used as raw materials for the production of various chemicals, such as ethylene for the production of polyethylene plastic. They are also used in the synthesis of important compounds like alcohols, aldehydes, and carboxylic acids.

5. What are the risks associated with handling alkenes?

Some alkenes, such as ethylene, are flammable and can pose a fire hazard. They can also be harmful if inhaled or ingested, and proper precautions should be taken when handling them. Reactions involving alkenes should also be carefully monitored and controlled to prevent the formation of hazardous byproducts.

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