[Atmosphobe]

Hello forumites,

I am a student and I was wishing the folks here could provide me with some clarification on my doubts regarding Org Chem.

1) Why doesn't Alkenes undergo Electrophilic Substitution, as compared to Arenes?

I know that for arenes, electrophilic sub. is preferred in order to preserve the stable benzene ring and also due to the high stability of the pi bonds.

As both molecules contain regions of high electron density, it would no doubt attract and polarize incoming electrophiles. Textbooks say that alkenes will not undergo E' substitution, but I was hoping I could get a reason for this.

I am thinking about the relative electron densities; arenes have a higher electron density than alkenes, and therefore, as the electrophiles tend to be electron withdrawing (halogens or molecules with multiple polar bonds like NO2), the arenes can "accommadate the electron withdrawing effect much better" *

* I'm not sure if I have phrased this correctly *

2) Moving on, I'm interested to also know why Nucleophilic Addition occurs exclusively for Carbonyl compounds (in the scope of the A Levels H2 syllabus), and why Halogenoalkanes / Alcohols / Carboxylic acids cannot undergo Nucleophilic Addition.

What are the conditions needed for both cases?

I'm thinking about the relative structures; carbonyl compounds form 2 single bonds, and 1 double bond, and thus can accommadate one extra bond (with the repulsion of 1 electron pair to highly EN oxygen), as compared to halogenoalkanes or alcohols which already has 4 bonds, thus being saturated, and in order to have the reaction with a nucleophile, one bond must break.

But then again, following the above logic, carboxylic acid defies that hypothesis because it has generally the same structure as carbonyl compounds.

3) Lastly, for halogenoalkanes, I learnt that they will undergo N' sub reactions in the presence of aqueous group I hydroxides to produce an alcohol. I have also learnt that in the presence of alcoholic group I hydroxides, it will undergo elimination instead.

I would like to ask about the significance of using the alcoholic form of the hydroxide as compared to the aqueous form of it.

Can anyone provide some clarification on the above matters?

Thank you for your kind assistance.

[Honclbrif]

"1) Why doesn't Alkenes undergo Electrophilic Substitution, as compared to Arenes?"

Electrophilic substitution rather than addition is one of the hallmarks of an aromatic compound (along with ring current, etc...). There's a bit of energy tied up in an isolated C=C bond, and turning it into sigma-bonds by addition is usually an energetically favorable process (Add some bromine to cyclohexene and feel the heat if you don't believe me. You can also look up various heats of hydrogenation). However, the stability gained from aromaticity is usually greater than the stability gained from addition. So aromatic systems tend to undergo substitution rather than addition which would, as you said, destroy its aromaticity.

"2) Moving on, I'm interested to also know why Nucleophilic Addition occurs exclusively for Carbonyl compounds (in the scope of the A Levels H2 syllabus), and why Halogenoalkanes / Alcohols / Carboxylic acids cannot undergo Nucleophilic Addition."

Because you can't have supercarbon. You can stick a nucleophile to a carbonyl carbon and push the electrons up onto the oxygen to make an O^- and everyone's valence is satisfied. You can do this with carboxylic acids too (look up orthoacetic acid, or orthoesters), but the product is rather unstable and tends to rapidly decay back to the normal acid/ester if you give it a chance. In an alcohol or haloalkane there's nowhere to stash the electrons and the carbon would have to be pentavalent.

"3) Lastly, for halogenoalkanes, I learnt that they will undergo N' sub reactions in the presence of aqueous group I hydroxides to produce an alcohol. I have also learnt that in the presence of alcoholic group I hydroxides, it will undergo elimination instead."

This is a gross oversimplification of the delicate dance between substitution and elimination. In general: small species (such as hydroxide, methoxide, etc) tend to favor substitution; large bulky species (such as t-butoxide, neopentoxide, etc) tend to favor elimination. However, solvent, substrate, and nucleophile/base all play a role in this dance and you should read up more on it yourself.