HardChemistryHigh School (11–12)Organic Compounds with Functional Groups › Haloalkanes and HaloarenesConceptualNumerical
Chlorine is added to butane and to 2-methylpropane in the presence of light until every possible…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Chlorine is added to butane and to 2-methylpropane in the presence of light until every possible dichloro product has formed. The products are compounds of formula . A chemist wants to count the different compounds, the kinds of dihalide, and the substances she could isolate if she separated every pair of mirror images. A meso compound has stereocentres but is the same as its own mirror image. Remember that 2,3-dichlorobutane has two stereocentres but only three stereoisomers. Count the structural isomers, the gem-dihalides, the vicinal dihalides and the total number of stereoisomers.
Need a reminder? The key ideaShow
A haloalkane is primary, secondary or tertiary by the number of carbon atoms joined to the carbon that holds the halogen. Isomers are counted by listing every carbon skeleton and every place for the halogen, then removing repeats. A carbon with four different groups is a stereocentre, and it doubles the count for its compound.
The worked solution
Have you tried it?
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. List by skeleton. Then classify. Then add the extra stereoisomers.
Step 1. Butane. 1,1-; 1,2-; 1,3-; 1,4-; 2,2-; and 2,3-dichlorobutane. That is 6.
Step 2. 2-Methylpropane. 1,1-dichloro-2-methylpropane; 1,2-dichloro-2-methylpropane; 1,3-dichloro-2-methylpropane. That is 3. Total: .
Step 3. Classes. Gem: 1,1-dichlorobutane, 2,2-dichlorobutane and 1,1-dichloro-2-methylpropane, which is 3. Vicinal: 1,2-dichlorobutane, 2,3-dichlorobutane and 1,2-dichloro-2-methylpropane, which is 3.
Step 4. Stereoisomers. 1,2-Dichlorobutane has one stereocentre, carbon 2: two forms. 1,3-Dichlorobutane has one, carbon 3: two forms. 2,3-Dichlorobutane has two identical ends, so it gives a pair of enantiomers and one meso form: three forms. None of the others has a stereocentre. 1,3-Dichloro-2-methylpropane has two identical groups on carbon 2.
Step 5. Total. Six compounds have no stereocentre. The others add . In all, .
Answer: 9 structural isomers, 3 gem, 3 vicinal and 13 stereoisomers.
Check. The 9 structural isomers split as . Only three of them gain extra forms, and the gain is , so .
Discussion (0)
Log in to join the discussion.
Loading…