Understanding Alkenes & Alkynes: Bromine Water Test

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Discussion Overview

The discussion focuses on methods to distinguish between alkenes and alkynes, particularly in the context of chemical tests and spectroscopy. Participants explore various techniques to identify these compounds, including bromination and spectral analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that bromine water discolors in both alkenes and alkynes, suggesting this method is not effective for differentiation.
  • Another participant proposes bromination as a distinguishing method, stating that excess Br2 will lead to tetrabromination in alkynes and dibromination in alkenes, allowing for identification based on the number of bromines added.
  • A different participant suggests using infrared (IR) spectroscopy, indicating that the stretching frequencies for alkynes and alkenes differ significantly, particularly if the alkyne is not symmetrical.
  • Another participant mentions that nuclear magnetic resonance (NMR) could also be used to identify the compound by analyzing different splitting patterns.

Areas of Agreement / Disagreement

Participants present multiple methods for distinguishing alkenes from alkynes, but there is no consensus on a single best approach. Each method has its proponents, and the discussion remains open to various techniques.

Contextual Notes

Some methods depend on specific conditions, such as the symmetry of the alkyne for IR spectroscopy, and the effectiveness of bromination may vary based on the compound's structure.

Bladibla
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Hey all

How do you distinguish between and Alkene and Alkyne?

Bromine water is discoloured in both, so that rules out BW.
 
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you could try to take the compound and try to brominate the bond twice. excess Br2+alkyne---> tetrabromination excess Br2+alkene----->dibromination. So if your compound is an alkyne you can add on 4 bromines, if it is an alkene you can add on 2 bromines. After you are done reacting you can figure out how many bromines were added by finding mass or doing spectroscopy thus you can figure out if the compound is an alkyne or alkene
 
How about just getting an IR spectrum? If the alkyne isn't symmetrical, that is.

The alkyne stretching frequency should be significantly different from the alkene stretching frequency.
 
NMR would tell you what your compound is as well, you would just have to look for different splitting patters.
 

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