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Ethene and ethyne can be distinguished by employing the test

a. $\ce{Br2}$ in organic solvent
b. Baeyer's test
c. Phenyl hydrazine
d. Tollen's reagant

According to me it must be both a and b, but there's no such option.

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Maryam Mumtaz
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  • None of these techniques will distinguish between alkene and alkyne. Both will react with a and b, neither will react with c and d. See http://academics.wellesley.edu/Chemistry/chem211lab/Orgo_Lab_Manual/Appendix/ClassificationTests/unsaturation.html – Waylander Jul 12 '19 at 06:53
  • But it's a mcq of a test – Maryam Mumtaz Jul 12 '19 at 06:58
  • And I've to choose one answer as a right one – Maryam Mumtaz Jul 12 '19 at 06:59
  • It's like that Higher the unsaturation greater would be the extent of decolorization in Bromination..So can I take "a" as a right option on the basis of extent of decolorization – Maryam Mumtaz Jul 12 '19 at 07:04
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    Terminal alkynes could react with tollens reagent.The following link from stack exchange could help you https://chemistry.stackexchange.com/questions/33854/color-of-precipitate-formed-on-reaction-of-acetylene-with-tollens-reagent&ved=2ahUKEwj5p7WF6K7jAhWSeisKHaHhDBYQjjgwA3oECAoQAg&usg=AOvVaw1RWjIx1ZvYmoHKTEqEUJPz – Chakravarthy Kalyan Jul 12 '19 at 07:06
  • Please choose a or b,just forget c and d – Maryam Mumtaz Jul 12 '19 at 07:26
  • @ChakravarthyKalyan good spot. You should make this an answer – Waylander Jul 12 '19 at 07:38
  • @MaryamMumtaz I think you should answer d to this question. See the line just below the equation for the reaction with aldehydes in the Wiki articl: https://en.wikipedia.org/wiki/Tollens%27_reagent – Waylander Jul 12 '19 at 07:38
  • Okay...but key to this test has marked "a" as the right option to this question,so please give your five minutes to this question and think that why it's "a" and not "b"...there must be some reason which we all are neglecting...So please do think about it. – Maryam Mumtaz Jul 12 '19 at 07:54
  • @MaryamMumtaz Given answers can be wrong. Don't always trust answer keys. – William R. Ebenezer Jul 12 '19 at 08:21
  • I have two guide books by two different authors,and both have same answer(option a) to this mcq.But you're also right. – Maryam Mumtaz Jul 12 '19 at 08:24
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    Neither of the authors has adequately researched the question - possibly one copied the other. Don't worry about it, nobody in the real world uses these tests, I had to look up Baeyer's test as it is not something I'm familiar with. – Waylander Jul 12 '19 at 08:59

1 Answers1

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I see two possible answers. Not (A) and (B) but rather (A) and (D).

Chakravarthy Kalyan is responsible for reminding us that (D) can be an answer too (refer to comments).

For option (D):

$\ce{HC ≡ CH + 2AgNO3 + 2NH4OH→ AgC ≡ CAg↓ + 2NH4NO3 + 2H2O}$

The greyish precipitate of silver acetylide will readily form when ethyne is passed through freshly prepared tollen's reagent.

You may read more from this answer by @ron: Does acetylene give a positive Tollens' test? If not, why is this so?.

For option (A):

While $\ce{Br2}$ looks like a seemingly unsuitable candidate, it can actually be used as a crude way to differentiate them.

Take the same volumes of equal strength of bromine water in two vessels and allow equal amounts of ethyne in one and ethane in the other to pass through them (this shouldn't be difficult, just passing equal volumes at STP will do the trick).

Carefully observing which vessel has the more intense color will help you identify which one is ethene, which would consume half the amount of bromine consumed by ethyne.

If you're not convinced, put the final solutions in a cuvette each and measure the absorption or transmittance in a spectrophotometer at 530 nm.

P.S. Make sure that you have a sufficient amount of bromine water, or you'll end up comparing two colorless solutions.

P.P.S. You could also use titration to find which one reaches the end point first, which would be ethene.

William R. Ebenezer
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