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This is a numbering problem.

This is a 1,2-addition to an α,β-unsaturated carbonyl: enter image description here

This is a 1,4-addition to an α,β-unsaturated carbonyl: enter image description here

I don't see why the second reaction would be called 1,4. I'd call it rather 3,4, so obviously I am not getting the numbering rule. Help?

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  • $\begingroup$ start counting from the oxygen. $\endgroup$ – getafix Dec 26 '16 at 12:30
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It is called 1,4 - addition because when you go through the mechanism of this reaction, then actually you will find that the carbonyl bond will break and oxygen will get negative charge. Then nucleophilic will attack on beta carbon, i.e. carbon at 4th position and proton will migrate to negative oxygen which is at first position. Double bond will form between the carbonyl carbon and carbon at 3rd position.

Thus, 1,4-addition takes place. But in the last step, keto-enol tautomerism takes place. And proton transfer takes place from oxygen to the carbon alpha to carbonyl. Therefore, final product is again saturated carbonyl compound.

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Say you were to add a methyl cuprate to acrolein (this example is made-up but the general idea stands). The reaction is given in the scheme below:

$$\ce{H2C=CH-CH=O + MeCuMgBr -> Me-CH2-CH=CH-OCuMgBr}\tag{1}$$

The initial intermediate is not the ketone but the 4-substituted but-1-en-1-olate. The metal has been added to position 1, the organic residue to position 4. Hence 1,4-addition.

Only after aquaeous workup will the but-1-en-1-ol be formed which can tautomerise to give the aldehyde.

You may think of this as a 1,4-addition followed by 1,3-hydrogen rearrangement. Or as a 1,4-addition followed by C-protonation of the resulting enolate.

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1,2-addition happens directly on the carbonyl, so the negative appears on the oxygen and the nucleophile adds to the carbon. If you're number atoms, these are 1 and 2. For 1,4-addition, the oxygen could be labelled 1 and the carbon where the addition happens would be labelled 4.

After the addition, you might have the negative species attack an electrophile, but the attack will not necessarily occur at the 1 position on the oxygen. 1,2- and 1,4- refer to the initial attack.

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