MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › Haloalkanes and HaloarenesConceptualNumericalBoard ExamA-LevelIBAEEE (Amrita)AMUEEEAP EAPCET
The alkane C₅H₁₂ has three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane…
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The alkane has three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane. Lucia replaces one hydrogen atom by one chlorine atom, in every possible place, in each isomer. She collects the distinct products. Some of the products have a carbon atom with four different groups. Count the structural isomers of . Count the primary ones, which have chlorine on a carbon with one carbon neighbour. Count the ones that can exist as mirror-image forms. Then count all the stereoisomers.
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The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Go through each carbon skeleton and place the chlorine on every different carbon.
Step 1. Pentane. Three: 1-chloropentane (primary), 2-chloropentane (secondary) and 3-chloropentane (secondary).
Step 2. 2-Methylbutane. Four: 1-chloro-2-methylbutane (primary), 2-chloro-2-methylbutane (tertiary), 2-chloro-3-methylbutane (secondary) and 1-chloro-3-methylbutane (primary).
Step 3. 2,2-Dimethylpropane. One: 1-chloro-2,2-dimethylpropane (primary). All four methyl groups are equal.
Step 4. Count. structural isomers. Primary: 1-chloropentane, 1-chloro-2-methylbutane, 1-chloro-3-methylbutane and 1-chloro-2,2-dimethylpropane, which is 4.
Step 5. Stereocentres. 2-Chloropentane (carbon 2 holds H, Cl, CH3 and a propyl group), 1-chloro-2-methylbutane (carbon 2) and 2-chloro-3-methylbutane (carbon 2) each have one. That is 3 compounds, and each exists as a pair, so stereoisomers.
Answer: 8 structural isomers, 4 primary, 3 with mirror-image forms and 11 stereoisomers in all.
Check. 3-Chloropentane has two identical ethyl groups on its carbon 3, so it is not chiral.
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