Chemistry question help

In which reaction does the inorganic reagent act initially as an electrophile?
[1 mark]
A bromoethane with ethanolic potassium hydroxide
B chloroethane with aqueous sodium hydroxide
C ethane with chlorine
D ethene with concentrated sulfuric acid
The answer is D and I do not understand why can someone please explain

Reply 1

ascetic-keyboard

im in year 13 but I think this is right:

•

H+ disassociates from H2SO4 therefore is an electrophile (it’s attracted to the high electron dense double bond)

•

the double bond ‘breaks’ so the electrons in the pi bond (just one of the bonds in the double bond) are accepted to form the carbocation

•

the HSO4- ion reacts with the carbocation to form ethyl hydrogen sulphate

the answer is D is because the H+ ion that disassociates from the H2SO4 is attracted to the double bond

Reply 2

Emmaclems

Original post by ascetic-keyboard

im in year 13 but I think this is right:

•

H+ disassociates from H2SO4 therefore is an electrophile (it’s attracted to the high electron dense double bond)

•

the double bond ‘breaks’ so the electrons in the pi bond (just one of the bonds in the double bond) are accepted to form the carbocation

•

the HSO4- ion reacts with the carbocation to form ethyl hydrogen sulphate

the answer is D is because the H+ ion that disassociates from the H2SO4 is attracted to the double bond

wow thanks I'm in yr 13 too

Reply 3

TypicalNerd

Original post by Emmaclems

Well, you can always try identifying the type of reaction occurring and see which one is right that way.

A is elimination, so KOH is acting as a base - neither a nucleophile nor an electrophile.

B is nucleophilic substitution and so NaOH is acting as a source of a nucleophile, thus it isn't an electrophile.

C is free radical substitution, so it doesn't involve nucleophiles or electrophiles.

Either by process of elimination or by recognising that ethene is an alkene and so typically undergoes electrophilic addition, it becomes obvious that D must be the right answer. It is a strange one mechanistically, but essentially the reply above is correct when it states "H^+ ions"* from the dissociation of H2SO4 are electrophilic (and of course they have correctly described the rest of the mechanism) and this is why D is correct.

*Although this isn't taught at A level, naked H^+ ions do not exist in practice - the H^+ ion will always preferentially bind to a solvent molecule (if possible) or remain undissociated. In this case, the H^+ ions will be bound to water from the solution in the form of H3O^+ ions. Strictly speaking these ions act as the electrophile, but this is just an extra detail you don't need to know.

(edited 1 month ago)

Reply 4

Emmaclems

Original post by TypicalNerd

Well, you can always try identifying the type of reaction occurring and see which one is right that way.
A is elimination, so KOH is acting as a base - neither a nucleophile nor an electrophile.
B is nucleophilic substitution and so NaOH is acting as a source of a nucleophile, thus it isn't an electrophile.
C is free radical substitution, so it doesn't involve nucleophiles or electrophiles.
Either by process of elimination or by recognising that ethene is an alkene and so typically undergoes electrophilic addition, it becomes obvious that D must be the right answer. It is a strange one mechanistically, but essentially the reply above is correct when it states "H^+ ions"* from the dissociation of H2SO4 are electrophilic (and of course they have correctly described the rest of the mechanism) and this is why D is correct.
*Although this isn't taught at A level, naked H^+ ions do not exist in practice - the H^+ ion will always preferentially bind to a solvent molecule (if possible) or remain undissociated. In this case, the H^+ ions will be bound to water from the solution in the form of H3O^+ ions. Strictly speaking these ions act as the electrophile, but this is just an extra detail you don't need to know.