[nirhananya]

Hello. Is it possible to make electrophilic addition of HBr to ethene, although it involves a primary carbocation intermediate?
Any references?

[OrgXemProf]

This, from Wikipedia (Reference: http://en.wikipedia.org/wiki/Bromoethane)

"The preparation of EtBr stands as a model for the synthesis of bromoalkanes in general. It is usually prepared by the addition of HBr to ethene:

H₂C=CH₂ + HBr → H₃C-CH₂Br
Bromoethane is inexpensive and would rarely be prepared in the laboratory. A laboratory synthesis includes reacting ethanol with a mixture of hydrobromic and sulfuric acids. An alternate route involves refluxing ethanol with phosphorus and bromine; phosphorus tribromide is generated in situ.[2]"

Reference [2]: Oliver Kamm and C. S. Marvel (1941), "Alkyl and alkylene bromides", Org. Synth.; Coll. Vol. 1: 25

[AromaticAcrobatic]

Hello. Is it possible to make electrophilic addition of HBr to ethene, although it involves a primary carbocation intermediate?
Any references?

Your getting your definitions mixed up. Electrophilic reactions require electrophiles. In this example, I only see nucleophiles.
Because your asking about a primary carbocation, I think your asking can you unimolecularly add to an Alkene.
Which, because this requires a primary carbocation, with no opportunity for significant rearrangement, I'm going to say this can't be done. At least I've never seen it happen or have never seen any examples of this happening.

 

[clarkstill]

I don't think he was getting his definitions mixed up... HBr is the electrophile. OrgXemProf (pronunciation??) gave a good example.

Hello. Is it possible to make electrophilic addition of HBr to ethene, although it involves a primary carbocation intermediate?
Any references?

Your getting your definitions mixed up. Electrophilic reactions require electrophiles. In this example, I only see nucleophiles.
Because your asking about a primary carbocation, I think your asking can you unimolecularly add to an Alkene.
Which, because this requires a primary carbocation, with no opportunity for significant rearrangement, I'm going to say this can't be done. At least I've never seen it happen or have never seen any examples of this happening.

 

[Irlanur]

Your getting your definitions mixed up. Electrophilic reactions require electrophiles. In this example, I only see nucleophiles.
Because your asking about a primary carbocation, I think your asking can you unimolecularly add to an Alkene.
Which, because this requires a primary carbocation, with no opportunity for significant rearrangement, I'm going to say this can't be done. 

I afraid that this is all wrong... it is an electrophilic addition. but a unimolecular addition does not make sense at all, not even semantically.

[nirhananya]

Thank you OrgXemProf.

[AromaticAcrobatic]

Your getting your definitions mixed up. Electrophilic reactions require electrophiles. In this example, I only see nucleophiles.
Because your asking about a primary carbocation, I think your asking can you unimolecularly add to an Alkene.
Which, because this requires a primary carbocation, with no opportunity for significant rearrangement, I'm going to say this can't be done. 

I afraid that this is all wrong... it is an electrophilic addition. but a unimolecular addition does not make sense at all, not even semantically.

Okay, I just rechecked the book and not once does it mention these type of alkene reactions as electrophilic reactions, nor does that wiki page call the reaction electrophilic. I've never heard of them being called an electrophilic addition reaction. I have heard of aromatic electrophilic reactions and diene electrophilic reactions.
Lets talk about the unimolecular thing. I meant in this specific example a unimolecular reaction is unlikely because of the primary carbocation, although I do know unimolecular alkene addition reactions do happen. Hence, carbocation rearrangements.

 

[Irlanur]

nor does that wiki page call the reaction electrophilic.

it says:
"Typical electrophilic additions to alkenes with reagents are:

    -dihalo addition reactions: X2
    -Hydrohalogenations: HX
   - Hydration reactions: H2O"

HX can for example be HBr.

I meant in this specific example a unimolecular reaction is unlikely because of the primary carbocation

When we're already using wiki:

"An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one."

how can that be unimolecular?

[AromaticAcrobatic]

nor does that wiki page call the reaction electrophilic.

it says:
"Typical electrophilic additions to alkenes with reagents are:

    -dihalo addition reactions: X2
    -Hydrohalogenations: HX
   - Hydration reactions: H2O"

HX can for example be HBr.

yes, I am aware of hydrohalogenations, etc. I have just never heard of them being called electrophilic reactions. Usually, in the electrophilic reactions I'm used to, their is an electrophile stronger then a proton. But if some books call it electrophilic reactions and some don't then its a simple case of tomato vs tomaato(tried to emphasize a difference).

nor does that wiki page call the reaction electrophilic.

When we're already using wiki:

"An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one."

how can that be unimolecular?

Hydrohalogenations are unimolecular because carbocations are observed. Also, thinking about the mechanism the reaction proceeds proton transfer, formation of carbocation, rearrangement when neccassary, followed by nucleophilic attack. Which is all step wise.
 

[Irlanur]

Hydrohalogenations are unimolecular because carbocations are observed. Also, thinking about the mechanism the reaction proceeds proton transfer, formation of carbocation, rearrangement when neccassary, followed by nucleophilic attack. Which is all step wise.

you probably mean first order(http://goldbook.iupac.org/M03989.html) and not unimolecular(http://goldbook.iupac.org/M03989.html).

[AromaticAcrobatic]

Hydrohalogenations are unimolecular because carbocations are observed. Also, thinking about the mechanism the reaction proceeds proton transfer, formation of carbocation, rearrangement when neccassary, followed by nucleophilic attack. Which is all step wise.

you probably mean first order(http://goldbook.iupac.org/M03989.html) and not unimolecular(http://goldbook.iupac.org/M03989.html).

Interesting links, thanks for sharing them! 
But what I meant by unimolecular is step wise, as compared to bimolecular everything at once. Using the ethene example this would be SN1 mechanism (unimolecular) compared to an SN2 mechanism(bimolecular).
 

[Irlanur]

But what I meant by unimolecular is step wise, as compared to bimolecular everything at once. Using the ethene example this would be SN1 mechanism (unimolecular) compared to an SN2 mechanism(bimolecular).

These are just not the proper terms in this context. "molecularity" describes how many molecules are involved in an elementary reaction, e.g. the formation of a Carbocation from ONE Molecule with a leaving group. the SN1 Mechanism involves more than one elementary reaction, the first one being the formation of  a carbocation (a unimolecular elementary reaction) the second one being the attack of a nucleophile (usually bimolecular but it can be unimolecular in the case of an intramolecular reaction). Since the the first step in SN1 is the rate determining step the order of the reaction is one (in the usual range of concentrations).

[AromaticAcrobatic]

But what I meant by unimolecular is step wise, as compared to bimolecular everything at once. Using the ethene example this would be SN1 mechanism (unimolecular) compared to an SN2 mechanism(bimolecular).

These are just not the proper terms in this context. "molecularity" describes how many molecules are involved in an elementary reaction, e.g. the formation of a Carbocation from ONE Molecule with a leaving group. the SN1 Mechanism involves more than one elementary reaction, the first one being the formation of  a carbocation (a unimolecular elementary reaction) the second one being the attack of a nucleophile (usually bimolecular but it can be unimolecular in the case of an intramolecular reaction). Since the the first step in SN1 is the rate determining step the order of the reaction is one (in the usual range of concentrations).

Good call man!
I'm glad you caught that because I completely forgot it. I was just associating step wise/SN1 reactions with unimolecular reactions, but after checking the book you are 100% right.
+1 for you 

 

[Irlanur]

Good call man!
I'm glad you caught that because I completely forgot it. I was just associating step wise/SN1 reactions with unimolecular reactions, but after checking the book you are 100% right.
+1 for you 

That's an unexpected and very satisfying answer:) usually in internetforums this would have ended somewhere between useless discussions and insults. it's these associations we all have in a different way that make precise discussions so difficult.

[AromaticAcrobatic]

Good call man!
I'm glad you caught that because I completely forgot it. I was just associating step wise/SN1 reactions with unimolecular reactions, but after checking the book you are 100% right.
+1 for you 

That's an unexpected and very satisfying answer:) usually in internetforums this would have ended somewhere between useless discussions and insults. it's these associations we all have in a different way that make precise discussions so difficult.

I agree, unfortunately in internet forums a disagreement usually does end in useless discussion and insult. 
I think people forget that some of the most exciting times is when you realize your wrong. Although I was wrong, I am happy we had this discussion because I remembered things that I have forgotten. I should have used and now will continue to use concerted vs step wise processes.