Question

A ketone has molar mass 86. Which of the following cannot be IUPAC name for this ketone?
A.2-pentanone
B. 2-methyl butanone
C. 3-methyl butanone
D. 3-pentanone

Answer 1

Hint: The concept of IUPAC nomenclature along with the calculation of molar mass is to be used in this question. Try to find out molar masses of all compounds and check which one does not result in the required molar mass.

Complete answer:
In order to answer the question, we need to learn about the nomenclature and structure of carbonyl groups. The respective rules are:
Aldehyde: The common names of most aldehydes are derived by replacing suffix 'ic' of acid from the name of carboxylic acid by "aldehyde". Location of substituents on the carbon chain is indicated by Greek symbols i.e., $\alpha ,\beta ,etc$,. -CHO is considered functional group and carbon atom adjacent to this is taken as $\alpha $carbon and so on. IUPAC names of aldehydes are given as alkanals and their names are written by replacing suffix 'e' of alkane by 'al'. In case of aldehydes the longest carbon chain is numbered starting from carbon of the aldehyde group.
Ketone: Common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group. Location of substituents always starts from the adjacent carbon of carbonyl group i.e., $\alpha ,\beta $ and so on. Some of the ketones have common popular names. In the IUPAC system ketones are named as "alkanones" and their name is formed by replacing suffix e' of alkane by another suffix 'one'.
Now, let us come to our question. The chemical formulas of the organic compounds respectively are $C{{H}_{3\,}}C{{H}_{2}}C{{H}_{2}}C=O-C{{H}_{3}},C{{H}_{3}}CC{{H}_{3}}C=O-C{{H}_{3}},C{{H}_{2}}CC{{H}_{3}}C=OC{{H}_{3}}\,and\,C{{H}_{3}}C{{H}_{2}}C=OC{{H}_{2}}C{{H}_{3}}$Their molar masses respectively are 86, 76, 86 and 86 grams per mole respectively. So, the chemical formula of 2 methyl butanone does not match with the molar mass.

So, it is incorrect, so our correct answer is option B.

NOTE: It is to be noted that in order to calculate the molar mass of the compound, then we have to add the molar masses of each of the elements. Also in case of diatomic molecule ${{O}_{2}}$, the molar mass is twice of that of oxygen, that is $2\times 16=32g\,mo{{l}^{-1}}$.