Question 5

[phyziksdoo2053]

5. Consider the following compound: a cyclohexane with a secondary alcohol substituent as well as a methyl ketone in the 3rd carbon position. What group in Figure 1 does the preceding molecule belong to?

1. IV [x]
2. V [x]
3. VI [x]
4. VII [x]

INCORRECT:
Your Answer: C
Correct Answer: B
Explanation
Firstly, one must notice that there are 2 functional groups present on the ring. There is a secondary alcohol as well as a methyl ketone in the para position (ORG 6.1 and 7.1). Therefore, according to Figure 1 and Table 1, the compound should be insoluble in NaHCO3, give a positive 2,4-DNP test, a negative Fehling's test, and a positive iodoform test resulting in group V.


I'm missing why the compound gives a positive iodoform test. According to the table, the following gives a positive test...

Acetaldehydes and ketones with the
CH3-CH-CO- group
Alcohols with the CH3CH(OH)- as a structural feature

A cyclohexane with a methyl ketone does not have CH3-CH-CO: there's an extra carbon b/w the methyl and the C=O.
Nor does it have an alcohol with an adjacent methyl group...

What am I missing???

[admin]

Your missing the second part: the secondary alcohol.

When you draw cyclohexane with a "secondary alcohol substituent", I guess that you drew cyclohexanol (which is a secondary alcohol). However, the question stem says that the substituent is a secondary alcohol thus the ligand is of the form CH3CH(OH)- (not just -OH) which in turn gives a positive iodoform test.

[jvuofm1709]

why is the methyl ketone in the para position? it is on the 3rd carbon position .... it is meta. para would be the 4th carbon relative to the secondary alcohol.

[calena7178147]

As the passage indicates a cyclohexane this is a cyclic alkane and the ortho, meta and para designations are really used for cyclic alkenes. Moreover, benzene (ALKENES) substituents use the respective designation. Thus, as the passage is not referring to any substituted alkene or benzene, these terms should not be used all together. If it were referring to a disubstituted cyclohexene (or benzene) then to my knowledge, it would be a meta as the prefix ortho is used for adjacent substituents and meta is used for substituents separated by one carbon (as per the passage) and para would be used for substituents located opposite one another. So as stated, as the passage refers to a cyclohexane (or an ALKANE), then the respective terminology is not really correctly used.

Having said the above this should not change your answer choice. The correct answer choice is B.

The explanation has been modified.

[andrewtfai3609]

i feel like the wording of this question is the difficult part. A secondary alcohol substituent could be oh-ch-ch2-ch2-ch3 (in this case, it would not give a positive test).

Is it not correct to say that if you have cyclohexanol, the oh group is considered a substituent on the ring? In this case as well, a positive iodoform test would not be the result.