MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › Haloalkanes and HaloarenesConceptualNumerical
Benzene has six hydrogen atoms, and any of them can be replaced by chlorine…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Benzene has six hydrogen atoms, and any of them can be replaced by chlorine. The compounds are the chlorobenzenes with from 1 to 6. Two arrangements are the same compound if turning or flipping the ring makes one into the other. Count the isomers with 2 chlorine atoms, with 3 and with 4. Count all the chlorobenzenes, from one chlorine atom to six. Then count the compounds of formula that contain a benzene ring.
Need a reminder? The key ideaShow
A haloarene is made by ring halogenation, or from a diazonium salt: diazotisation with and HCl at to , then CuCl or CuBr (Sandmeyer), KI, or and heat. Isomers of a ring are counted by checking which patterns turn into each other by rotating or flipping the ring. Percent yield is .
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.
Step 1. Dichloro. Two chlorine atoms can be at positions 1,2 (ortho), 1,3 (meta) or 1,4 (para). Every other pair is the same as one of these by turning the ring. So 3.
Step 2. Trichloro. 1,2,3-; 1,2,4-; and 1,3,5-trichlorobenzene. So 3.
Step 3. Tetrachloro. Take away the two hydrogen atoms that are left. They can also be ortho, meta or para, so 3: 1,2,3,4-; 1,2,3,5-; and 1,2,4,5-tetrachlorobenzene.
Step 4. All. One chlorine atom: 1. Two: 3. Three: 3. Four: 3. Five: 1. Six: 1. The total is .
Step 5. C7H7Cl. The ring holds a methyl group and a chlorine atom in the ortho, meta or para position, which is 3 compounds. Or the chlorine is on the methyl group, which gives benzyl chloride. So .
Answer: 3, 3, 12 and 4.
Check. The counts for 4 chlorine atoms and 2 chlorine atoms are equal because the two missing hydrogen atoms can be arranged in the same ways as two chlorine atoms.
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