3. An organozine reaction (similar to a Grignard reaction) was done with 3-penta

Question

3. An organozine reaction (similar to a Grignard reaction) was done with 3-pentanone or acetophenone with 3-bromopropane (allyl bromide) and a product isolated. The NMR of the products appeared as expected except for the methylene group of the acetophenone product CHa, marked with a star) next to the double bond. In the product from 3-pentanone, the corresponding methylene group in the NMR displays the expected doublet, but this methylene group in the product from acetophenone displays a much more complex pattern as show below. Explain why there is such a large difference for similar protons in similar compounds (hint: what is the relationship between the 2 methylene protons)? Show how the complex pattern is formed by determining which protons are coupled, and identify where on the NMR spectrum the coupling constants may be found? OH H2C 1 Br R R2 NH,a (aq) Product product 2.2 2.4 2.6 3-

Explanation / Answer

The product formed from 3-pentanone is having an an achiral cabon center (attached to -OH) next to methylene group and thus it shows a simple NMR with a doublet.

Whereas, with acetophenone, the product formed now has a chiral carbon (attached to -OH) next to methylene group and thus it shows a complex NMR. The methylene is coupling with the CH3 in a long range coupling strategy.

The coupling constant is the distance in Hz between two adjacent lines of the splitting in a peak.

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