Is hydrogenation of alkenes considered to be electrophilic addition?

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

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.

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.

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!

Is this nucelophilic addition-elimination in this case?