Which one of the following could NOT be the formula of an aldehyde?
A. {C_6H_{12}O_2}
B. {C_5H_{10}O_2}
C. {C_5H_{10}O}
D. {C_5H_{12}O}
E. {C_6H_{12}O}
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Here it is beneficial to know the general formula for the Aldehyde group; C_nH_{2n}O.
Now all we have to do is choose which option does not satisfy our formula.
A and B are most easily eliminated because they could contain hydroxyl groups to explain for the oxygen- this does not, however, mean that they are not Aldehydes.
C is a perfectly accurate Aldehyde according to our general formula, therefore it is also not our answer.
We can see our answer becomes D because of the incorrect amount of Hydrogen for that number of Carbons.
E is also a perfectly accurate Aldehyde according to our general formula, therefore it is also not our answer.
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Having a carboxylic group of A doens’t mean it has a aldehyde group, isn’t it?