Is hydrogenation of alkenes considered to be electrophilic addition?

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Supersniper817
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A level chem
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jack_harrison
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Hydrogenation is usually carried out with a heterogeneous catalytic mechanism. This is usually done with nickel or palladium-based catalysts. What happens is the catalyst will cleave the H-H bond as it adsorbs to its surface, then transfer the hydrogens to an adsorbed alkene. The mechanism is not really electrophilic addition but I can see why you'd think that due to the nature of the product. Hope this helps!
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Supersniper817
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(Original post by jack_harrison)
Hydrogenation is usually carried out with a heterogeneous catalytic mechanism. This is usually done with nickel or palladium-based catalysts. What happens is the catalyst will cleave the H-H bond as it adsorbs to its surface, then transfer the hydrogens to an adsorbed alkene. The mechanism is not really electrophilic addition but I can see why you'd think that due to the nature of the product. Hope this helps!
thank you so much! that explanation really helps.
what about with hydrogen halides, is that electrophilic addition?

thanks
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jack_harrison
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(Original post by Supersniper817)
thank you so much! that explanation really helps.
what about with hydrogen halides, is that electrophilic addition?

thanks
Hydrogen halides absolutely do undergo electrophilic addition; the H-X bond is polar due to the electronegativity difference, so the partially positive hydrogen can be attacked. It can also occur with diatomic halogens because their electron clouds can easily be polarised due to their size. I can only assume these cannot happen with hydrogen molecules because their electron clouds are very small, held strongly to the nucleus, and cannot easily be polarised.
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Supersniper817
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(Original post by jack_harrison)
Hydrogen halides absolutely do undergo electrophilic addition; the H-X bond is polar due to the electronegativity difference, so the partially positive hydrogen can be attacked. It can also occur with diatomic halogens because their electron clouds can easily be polarised due to their size. I can only assume these cannot happen with hydrogen molecules because their electron clouds are very small, held strongly to the nucleus, and cannot easily be polarised.
thank you so so much, it really means a lot, your explanations really help, see the thing is there are people who don't know what they are talking about online saying that hydrogenation is electrophilic addition, but something doesn't feel right with what they are saying. You sound like you actually understand the chemistry hence why I trust your explanations.
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Supersniper817
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Is the reaction between alcohol and PCl5 or HCl to form chloroalkane a substitution reaction?

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(Original post by jack_harrison)
Hydrogen halides absolutely do undergo electrophilic addition; the H-X bond is polar due to the electronegativity difference, so the partially positive hydrogen can be attacked. It can also occur with diatomic halogens because their electron clouds can easily be polarised due to their size. I can only assume these cannot happen with hydrogen molecules because their electron clouds are very small, held strongly to the nucleus, and cannot easily be polarised.
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jack_harrison
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(Original post by Supersniper817)
Is the reaction between alcohol and PCl5 or HCl to form chloroalkane a substitution reaction?

thanks
Any substitution reaction where you're trying to swap out an OH group is slightly more complicated. For reasons it's not necessary to go in to, the OH is a really bad leaving group and requires 'activation' before it can be kicked off. So the mechanism here is slightly more complicated than the usual nucleophilic substitution you see with haloalkanes. Basically what happens is the alcohol and PCl5 form an intermediate by undergoing one nucleophilic substitution, and this intermediate rearranges itself through a number of steps to produce the products you observe, and the haloalkane. This mechanism is by no means as straightforward as a regular nucleophilic substitution. In terms of classifying the type of reaction though, a group of atoms has been swapped for another atom so technically the reaction is a substitution, but the mechanism is not nucleophilic substitution as you may know it.

Hope this helps!
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Ðeggs
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(Original post by jack_harrison)
Any substitution reaction where you're trying to swap out an OH group is slightly more complicated. For reasons it's not necessary to go in to, the OH is a really bad leaving group and requires 'activation' before it can be kicked off. So the mechanism here is slightly more complicated than the usual nucleophilic substitution you see with haloalkanes. Basically what happens is the alcohol and PCl5 form an intermediate by undergoing one nucleophilic substitution, and this intermediate rearranges itself through a number of steps to produce the products you observe, and the haloalkane. This mechanism is by no means as straightforward as a regular nucleophilic substitution. In terms of classifying the type of reaction though, a group of atoms has been swapped for another atom so technically the reaction is a substitution, but the mechanism is not nucleophilic substitution as you may know it.

Hope this helps!
Is this nucelophilic addition-elimination in this case?
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jack_harrison
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(Original post by Deggs_14)
Is this nucelophilic addition-elimination in this case?
I don't think there's any one way to define the mechanism; I believe part of it involves elimination but the whole mechanism is just a big mash of many different steps.
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