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My chemistry teacher today told that only two mono chloro substituted products are possible for naphthalene. He explained this by saying since the naphthalene molecule is symmetrical about the axis passing through the C-C bond where the benzene rings are fused, the corresponding positions should be identical & hence only two such products are possible. I personally don't find this convincing & hope to look for a genuine answer. I will be extremely thankful to anyone who helps.

If someone is unable to understand my teacher's explanation, just ignore it. I myself wasn't able to understand it properly so could have mistaken it for something else.

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  • $\begingroup$ :/ Well, you better try to understand symmetry, because it's one of most important things in chemistry. Hmm, imagine cutting naphtalene like a cake. If you cut it into four parts, their are gonna be identical. $\endgroup$ – Mithoron Jul 24 '19 at 20:38
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    $\begingroup$ Naphtalene has ten hydrogen atoms. Draw ten naphtalenes, each monochlorinated, and let the Cl wander around the double ring. Now check how many of them are actually different. (And don't draw double bonds, naphtalene is aromatic! Two six-rings, each with a circle in it to represent the delocalised pi-electrons.) $\endgroup$ – Karl Jul 24 '19 at 20:51
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    $\begingroup$ Alternatively, use models (or even cut a sheet of paper into 10 naphthalenes and mark the position of the chlorine by a paper clip). Superimpose the models, turn them, rotate them, flip them and see how quite some of them are in fact identical. $\endgroup$ – Buttonwood Jul 24 '19 at 23:38
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Your chemistry teacher is partially correct by saying that naphthalene molecule is symmetrical about the axis passing through the $\ce{C-C}$ bond where the benzene rings are fused (going through the common carbons to both rings). Your chemistry teacher has forgot to mention that naphthalene molecule is also symmetrical about the axis perpendicular to the axis passing through the $\ce{C-C}$ bond where the benzene rings are fused (See the top structure of the diagram below). Thus, each quarter of the naphthalene molecule is equal and each quarter contains only two carbon atoms (disregarding common $\ce{C-C}$ bond). I named them as 1 and 2, because they are symmetrical. Therefore, only two different mono-substituted derivatives are possible from naphthalene. If substituent is chloro, then only 1-chloronaphthalene and 2-chloronaphthalene are possible, and so on.

Chloronaphthalenes

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    $\begingroup$ I had this notion that for molecules to be identical they ought to have similar iupac naming. Never knew symmetry had a role to play. Thanks for your answer. $\endgroup$ – Swapnil Gangwar Jul 25 '19 at 5:39

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