[swmmr1928]

Hello.  I am taking my second semester of Organic Chemistry.  Below, I have tried to summarize vinyl and allyl reactions from my book "Organic Chemistry 5th edition" by Loudon.  Can you help me fill in the blanks and why these react the way that they do?



Allylic and benzylic carbon ions are resonance stabilized so reactions that involve a carbon ion intermediate will occur including sn1.  The book says cites SN1 and dehydrogenation of alcohol as allylic and benzylic reactions.  Sn2 and E2 are possible because the B-protons are highly acidic because of resonance stabilized carbocations.

Vinylic and aryl are impossible to perform nucleophilic substitution because a backside approach is stericlly hindered.  
Are these correct explanations?
How do you explain the rest of the reactions?

Thankyou

[OrgoTutor]

Allylic and benzylic carbon ions are resonance stabilized so reactions that involve a carbon ion intermediate will occur including sn1.  The book says cites SN1 and dehydrogenation of alcohol as allylic and benzylic reactions.  Sn2 and E2 are possible because the B-protons are highly acidic because of resonance stabilized carbocations.

Yes, the carbocation intermediate of an Sn1 rxn will be resonance stabilized, so Sn1 is possible for allylic and benzylic halides (or alcohols for dehydration rxns).

Sn2 and E2 are possible because the B-protons are highly acidic because of resonance stabilized carbocations.

Are carbocations intermediates formed during an Sn2 or E2 mechanism?

Vinylic and aryl are impossible to perform nucleophilic substitution because a backside approach is stericlly hindered.

Vinylic halides: There are instances where nucleophilic substitution will occur for a vinylic halide, though I don't know that you'll cover them in your course. One is an Sn2' (Sn2 "prime") reaction. Another is addition-elimination reaction of a nucleophile with a vinylogous halide (such as 3-chloro-2-cyclohexen-1-one).

Aryl halides: Substitution rxns can occur, with the appropriate substrate. You'll probably learn about SnAr (Substitution nucleophilic aromatic) eventually. It involves a nucleophile undergoing an addition-elimination reaction with an electrophile such as 2,4-dinitrofluorobenzene.

Given these possibilities of reactions of vinyl and aryl halides, it's not steric hindrance which prevents them from occuring in most other normal cases. So why else might substitution reactions not occur for substrates like 1-chloropropene or fluorobenzene, if not steric effects?

[swmmr1928]

Given these possibilities of reactions of vinyl and aryl halides, it's not steric hindrance which prevents them from occuring in most other normal cases. So why else might substitution reactions not occur for substrates like 1-chloropropene or fluorobenzene, if not steric effects?

Fluorine is an absurd leaving group.  

Now I remember that we did say in class that the hydridization of the carbon undergoing substitution is sp because of two bonds: C-H and C=C.  I don't remember what concludion we make from that fact.  I will modify the post when i remember. 

[OrgoTutor]

Fluorine is an absurd leaving group.  

Lol. Yes for and Sn2 reaction. But for an SnAr reaction it works great. Just bear in mind that an SnAr reaction is an addition-elimination reaction: the nucleophile attacks the double bond carbon, thus forming a negatively charged intermediate, after which the leaving group (Fluorine) leaves to give back a neutral complex. So SnAr does differ significantly from Sn2 in the details of the mechanism--but it's still considered a substitution reaction at a double bond.

[swmmr1928]

Given these possibilities of reactions of vinyl and aryl halides, it's not steric hindrance which prevents them from occuring in most other normal cases. So why else might substitution reactions not occur for substrates like 1-chloropropene or fluorobenzene, if not steric effects?  

I have done a internet search and found a relevent paragraph:

In vinylic carbocations, the positive charge is assigned to a carbon with sp hybridization.
...Electrons in an s orbital are closer to the nucleus and therefore more
tightly held than electrons in a p orbital. This can be taken to mean that the
electronegativity of carbon increases with increasing s character...The more
electronegative the atom the less stable the cation. A vinylic carbocation carries the
positive charge on an sp carbon, which is more electronegative than an sp2 carbon of an
alkyl carbocation. Therefore a primary vinylic carbocation is less stable than a primary
alkyl carbocation.

http://no_way/RnvKi

[swmmr1928]

Thankyou for responding to these posts, OrgoTutor. 

I have added color coding to the table from updated knowledge. 



I still left the column of E1 blank.  This would form a cumulene?  Sometimes in these chapters, instead of talking about specifically e1 or e2, Loudon speaks generally about Beta-Elimination.  I wish he would clarify so I know if e1 is performed.  Maybe the forum can help? 

[orgopete]

I don't know how general benzyl, allyl, vinyl, and aryl are in tended to be, but if in the context of normal substitution reactions in which elimination is a reaction that competes with substitution, then vinyl and aryl will not undergo elimination reactions. (They will if very strong bases are used where the objective is to prepare a benzyne or acetylene.) Also, if benzyl and allyl are limited to benzyl and allyl themselves, then they too do not undergo elimination reactions. For an elimination reaction to take place, you need the electrons of a beta hydrogen.

R₂CBr-CH₃ + :B  R₂C=CH₂ + H:B⁺ + Br^-

PhCH₂Br + :Nu^-  PhCH₂Nu + Br^-