Having trouble deducing 4 organic structures (High school exam qn)

In summary: No, the mechanism is not taught. We are only taught that the only way it works is if it has an alpha methyl group.The hydrogens are substituted by I (hence the loss of the colour of I2). But unlike CI3-, RCI2- is not a good enough leaving group, so the reaction stops there.
  • #1
phantomvommand
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Summary:: Please see picture below

Screenshot 2021-10-17 at 3.11.41 AM.png

The dotted line at Test 3 means to continue using the above solution, and add NaOH with the teat pipette. The dotted line separating tests 3 and 4 should be a hard line, which means to use a fresh sample of FA10 and perform the test. It is also known that FA10 is different from FA11.

I do not have the experimental results, which you are supposed to obtain and fill in the blank boxes. However, one can still deduce the most likely structures given the highly restrictive molecular formula; not that many structures even fit the molecular formula.

I have deduced FA8 to be CH3COOH. FA9 can be propan-1-ol or propan-2-ol, depending on whether the iodoform test (test3) gives the yellow ppt of CHI3.

For test 3, I am not sure why they want us to add I2 until the solution turns orange/red. Does this mean adding I2 in excess?

What might FA10 and FA11 be? Could someone please explain all reactions happening in the tests, especially for FA11?

Thanks so much.
 
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  • #2
These are all standard tests you should have learned about before being given a question like this. If you know about the iodoform test, you ought to know about the DNPH and silver mirror tests. Do you know what functional groups they are testing for?
 
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  • #3
mjc123 said:
These are all standard tests you should have learned about before being given a question like this. If you know about the iodoform test, you ought to know about the DNPH and silver mirror tests. Do you know what functional groups they are testing for?
Yes, FA11 and FA10 have carbonyl functional groups. It is likely FA11 is an aldehyde (hence the silver mirror), and FA10 should be a ketone (it is not FA11). However, what is happening in test 3 for FA11, which should be propanal?
 
  • #4
Do you know the mechanism for the iodoform reaction? What would happen if you have hydrogens α to the carbonyl, but not an α methyl group?
 
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  • #5
mjc123 said:
Do you know the mechanism for the iodoform reaction? What would happen if you have hydrogens α to the carbonyl, but not an α methyl group?
No, the mechanism is not taught. We are only taught that the only way it works is if it has an alpha methyl group.
 
  • #6
The hydrogens are substituted by I (hence the loss of the colour of I2). But unlike CI3-, RCI2- is not a good enough leaving group, so the reaction stops there.
(Note: I there is capital I, not small L as I'm sure you realize. Can't we have a font that distinguishes them?)
 
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1. How can I improve my ability to deduce organic structures?

One way to improve your ability to deduce organic structures is to practice regularly. This can include solving practice problems, reviewing examples, and participating in group discussions or study groups. Additionally, it can be helpful to familiarize yourself with common functional groups and their properties.

2. What are some common mistakes to avoid when deducing organic structures?

Some common mistakes to avoid when deducing organic structures include overlooking important functional groups, misinterpreting spectroscopic data, and not considering all possible isomers. It's also important to carefully check your work and make sure all parts of the structure fit together logically.

3. How can I use spectroscopic data to help me deduce organic structures?

Spectroscopic data, such as IR and NMR spectra, can provide valuable information about the functional groups present in a molecule. By analyzing the peaks and patterns in the spectra, you can narrow down the possible structures and make more accurate deductions.

4. What resources can I use to help me deduce organic structures?

There are many resources available to help you deduce organic structures, including textbooks, online tutorials, and study guides. Your teacher or classmates can also be valuable resources, as they may have different perspectives and approaches to problem-solving.

5. What strategies can I use to approach difficult organic structure deduction problems?

One strategy is to break down the problem into smaller parts and focus on one functional group or section at a time. Another approach is to use trial and error, starting with a possible structure and adjusting it based on the given information. It can also be helpful to make a list of all the information given and refer back to it as you work through the problem.

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