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Thread: addition of unsaturated hydrocarbons

  1. #1 addition of unsaturated hydrocarbons 
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    Hello, my book is very bad at explaining the additionreactions of unsaturated hydrocarbons. ( halogenation, hydration & halogenation). My book does have "unsaturated hydrocarbons" as a title, so I would think it also applies to both alkynes & alkenes, but the book doesnt even give examples of alkynes. It only discusses alkynes & this gives me the impression that alkynes do not undergo addition.
    They have only given one example for alkynes and thats for the hydrogenation of ethyne for example. They didnt say anything on the halogenation of alkynes or the hydration of alkynes.

    So I went on youtube and found this:


    Its the same there ( they only give an example for the hydrogenation of alkynes, the rest is all alkenes). So this got me wondering: does the addition for the halogenation & hydration apply to alkynes?

    If yes, then I assume that I can write the following reactions (for example):

    Halogenation of alkynes:


    Hydration of alkynes:



    Thanks in advance!


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    Yes alkynes undergo these reactions, the halogenation will proceed to the unsaturated CHBr2-CHBr2 but if I recall correctly in the hydration reaction the product is the unsaturated alcohol CH(OH)=CH2. This is unreactive with respect to further hydration due to a resonance form which is an aldehyde (O=CH-CH3).

    You might find this page useful:

    Addition Reactions of Alkynes


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  4. #3  
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    Thank you for the help, youre really helping me out with this.
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  5. #4  
    Bullshit Intolerant PhDemon's Avatar
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    No problem, it's good for me to try and remember this stuff that I haven't thought about for years. It's good practice as well, I'm going to start my teacher training to be a chemistry and physics teacher later this year
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  6. #5  
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    I certainly do appreciate the help. Chemistry can be the easiest thing in the world according to what Ive experienced if you have access to the right information/source. If you dont, then it becomes the most difficult thing youve ever seen. Thank god Ive got the internet. With google you can look up almost everything. Really I wouldnt be studying if the internet was not invented.
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  7. #6  
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    Quote Originally Posted by PhDemon View Post
    Yes alkynes undergo these reactions, the halogenation will proceed to the unsaturated CHBr2-CHBr2 but if I recall correctly in the hydration reaction the product is the unsaturated alcohol CH(OH)=CH2. This is unreactive with respect to further hydration due to a resonance form which is an aldehyde (O=CH-CH3).

    You might find this page useful:

    Addition Reactions of Alkynes
    Er, "resonance"?

    Isn't this keto-enol tautomerism, rather than resonance?

    Keto
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  8. #7  
    Bullshit Intolerant PhDemon's Avatar
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    Yes, maybe sloppy terminology on my part but I've always thought of tautomers as being "resonance forms"...(but I haven't done any real organic chemistry since 1997!)
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  9. #8  
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    Quote Originally Posted by PhDemon View Post
    Yes, maybe sloppy terminology on my part but I've always thought of tautomers as being "resonance forms"...(but I haven't done any real organic chemistry since 1997!)
    I think the difference is these are distinct chemical forms, with both bonds and actual atoms in different places (i.e. different molecular geometry), whereas "resonance" hybrids are just ways of expressing molecular orbitals in terms of mixtures of conventional 2 centre bonds.
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  10. #9  
    Bullshit Intolerant PhDemon's Avatar
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    You are correct but whenever I've come across the phenomenon in my research, we call them resonance forms, a bad habit but a deeply entrenched one...
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  11. #10  
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    Quote Originally Posted by PhDemon View Post
    You are correct but whenever I've come across the phenomenon in my research, we call them resonance forms, a bad habit but a deeply entrenched one...
    How shocking!

    But now that you are going to teach, you can enjoy correcting pupils who conflate the two.
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  12. #11  
    Bullshit Intolerant PhDemon's Avatar
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    True

    I'm spending this week in a primary school as part of my pre-training. No chemistry today, this morning I was trying to teach a German boy whose English isn't great to read English and teaching long division. This afternoon it was helping a kid with Aspergers read his book and answering his questions and then some basic English comprehension reading exercises. Let's see what I get given for the rest of the week (frankly anything except PE would be fine, I can imagine that turning into a riot!)
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  13. #12  
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    Quote Originally Posted by PhDemon View Post
    True

    I'm spending this week in a primary school as part of my pre-training. No chemistry today, this morning I was trying to teach a German boy whose English isn't great to read English and teaching long division. This afternoon it was helping a kid with Aspergers read his book and answering his questions and then some basic English comprehension reading exercises. Let's see what I get given for the rest of the week (frankly anything except PE would be fine, I can imagine that turning into a riot!)
    Well it's crash helmets on, then. You won't be arguing the toss about tautomerism and mesomerism this week, clearly.
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