1,2 conjugate addition versus 1,4 conjugate addition- Help needed

[rskhan29]

Hey guys,
I'm having some trouble recognizing the difference between a 1,2 nucleophillic addition and a 1,4 nucleophillic/conjugate addition. From what I know, a 1,2 nucleophillic addition is when a Nuc: attackes the C of the carbonyl. The 1,4/conjugate addition occurs when the Nuc: attacks the Beta carbon in an alpha-beta unsaturated carbonyl compound.

But this practice question in Examkrackers has thrown me off:

According to the answer passage, this is a 1,2 conjugate addition. I don't understand how this is possible. If it is an electrophillic 1,2 addition, then the CN should have attached to the middle carbon. If it was a 1,2 nucleophillic addition, shouldn't the CN have attacked the carbonyl?

Any help would be appreciated. Thanks in advance.

---

Members don't see this ad.

 

[cavalier329]

answer to follow

 

[cavalier329]

the problem you put up tells me its a 1,4 addition because the C=O is retained in the solution. The product you give looks like a 1,4. If the C=C was retained, it would be a 1,2.

Check the answer again and re-post the question if it is indeed 1,2. Ill take a look again.

 

[sleepy425]

Hey guys,
I'm having some trouble recognizing the difference between a 1,2 nucleophillic addition and a 1,4 nucleophillic/conjugate addition. From what I know, a 1,2 nucleophillic addition is when a Nuc: attackes the C of the carbonyl. The 1,4/conjugate addition occurs when the Nuc: attacks the Beta carbon in an alpha-beta unsaturated carbonyl compound.

But this practice question in Examkrackers has thrown me off:

According to the answer passage, this is a 1,2 conjugate addition. I don't understand how this is possible. If it is an electrophillic 1,2 addition, then the CN should have attached to the middle carbon. If it was a 1,2 nucleophillic addition, shouldn't the CN have attacked the carbonyl?

Any help would be appreciated. Thanks in advance.

no, this is definitely a 1,4-addition (a michael addition). cyanide is a nucleophile. it attacks the 4 position (the beta position) and kicks the electrons up to make an enolate. then you protonate to regenerate the carbonyl

 

[Steveoph]

I barely remembered this, but in the PR section on B-unsaturated ketones, it talked about how the beta carbon has a partial positive charge (look at resonance structures) and it will be preferentially nucleophilically attacked over the carbonyl. The above poster laid it out.

 

[tastybiscuit]

Perhaps it would throw you off then that a Grignard Reagent would add 1,2 but is nucleophilic, eh? This question is probably too particular for the MCAT. Strong Nucleophiles that are also strong bases attack 1,2....weaker Nucleophiles and bases go 1,4. Grignards are both, so they go 1,2 while KCN and organocuprates (two oft-used examples) are weaker nucleophile/bases (I know KCN is thought to be a strong nucleophile but in this case think about it being a weak base). The diagram is right but the explanation is wrong. I wouldn't worry that much about this...just know about the partial positive on the Beta Carbon and know that there are two sites for attack. I don't know how this is covered in review texts but it is standard class textbooks for Orgo 1/2.

 

[sp0988]

Bump. I had the same question and have trouble understanding the distinction between 1,2 and 1,4 addition.

 

[orgohacks]

no, this is definitely a 1,4-addition (a michael addition). cyanide is a nucleophile. it attacks the 4 position (the beta position) and kicks the electrons up to make an enolate. then you protonate to regenerate the carbonyl

This is 100% correct, just in case you want a second or third opinion.

 

[cta]

Had the same problem in EK problem stating it is a 1,2-addition in the answer key but it is a 1,4-addition