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BR Organic Chapter 1 Passage 1

Discussion in 'MCAT Study Question Q&A' started by avenlea, 05.12.14.

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  1. avenlea

    avenlea 5+ Year Member

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    This is a question from BR's Organic Chemistry, Section I Passage 1:

    Alkene; Heats of hydrogenation (kcal/mole)
    H2C=CH2 ; -32.6
    RHC=CH2 ; -30.2
    cis-RHC=CHR ; -28.5
    R2C=CH2 ; -28.3
    trans-RHC=CHR ; -27.4
    R2C=CHR ; -26.7
    R2C=CR2 ; -26.4

    The greatest amount of energy is released by the oxidative cleavage of an alkene that is
    A. Unsubstituted
    B. Monosubstituted
    C. Disubstituted
    D. Trisubstituted

    A. Everything's awesome and makes sense.

    Next question:

    The greatest amount of energy is required to break which of the following carbon carbon bonds?
    A. H3C-CH3
    B. (H3C)3-C(CH3)3
    C. H2C=CH2
    D. (H3C)2C=C(CH3)2

    Answer is D. Not C. So confused...any insight?
     
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  3. syoung

    syoung MS-3 5+ Year Member

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    I believe you may be confusing energy released by oxidative cleavage with energy required to break the C=C bonds.
     
  4. avenlea

    avenlea 5+ Year Member

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    Okay. I want to verify this then -

    Oxidative cleavage would be this concept we're talking about with bond strength/bond energy - weaker bonds would be substituted bonds. Weaker bonds are also longer bonds

    Heat of hydrogenation would be the concept of amount of heat released when H's are added to carbon - lower heat of hydrogenation, more energy required?
     
  5. SuperSneaky24

    SuperSneaky24

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    Ah I remember running into this problem before. Nah the oxidative cleavage should be referring to the carbon-carbon bonds.

    Think about it like this: the most UNSTABLE molecule RELEASES the most energy when it is broken down, as it is in a very high energy level. This explains the first question, as an unsubstituted alkene is the most unstable.

    Now the most STABLE molecule requires the most energy INPUTTED to break down, as it is in a lower energy state, and needs a lot more energy to bring it to an unstable high energy state where it can break down.

    Hope this helps!
     
  6. avenlea

    avenlea 5+ Year Member

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    Hmmm okay that is really helpful, but I'm confused because the passages in the book talk a lot about how substituted carbon bonds are not as stable as unsubstituted (i.e., example 1.7 says that an sp2 carbon to secondary sp3 carbon is less stable than sp2 carbon to primary sp3 carbon?)
     
  7. SuperSneaky24

    SuperSneaky24

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    I don't have the book on hand with me, so I'm not exactly sure what the example is asking. However, I think you might be confusing single bonds and double bonds?

    Correct me if I'm wrong, but I beleive for alkenes, the substituted groups help stabilize the pi-orbital of the pi bonds, helping increase their stability. For single bonds, there is no pi-orbital involved in the single bonds, so adding substituted groups into the mix (which are usually electron-donating) will add more electron density into single bond, causing it to be less stable. From what I remember, treat substituted groups on double bonds and single bonds as opposite. Substituted for double bonds = good. Substituted for single bonds = bad.
     
  8. avenlea

    avenlea 5+ Year Member

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    MMMM. I gotchu. You're the best, thanks!
     
  9. SuperSneaky24

    SuperSneaky24

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    No prob, good luck studying! I'm taking the MCAT in a month, so all this stuff is super fresh in my mind haha
     
  10. avenlea

    avenlea 5+ Year Member

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    Ok I'm going to revive this because I found another question about a similar concept and I seem to be confused. TBR Ch 1, Phase III, passage 1.

    Question 4 asks:
    The hydrogenation of an eight carbon diene has which of the following effects on the physical properties of the compound?
    A. Both molecular mass and melting point increase
    b. Molecular mass increases and melting point decreases
    C. Molecular mass decreases and melting point increases
    D. Both increase

    The answer is C and I don't get why. Isn't the alkene the more stronger and stable bond? So wouldn't hydrogenation lower the melting point because it's not as strong anymore?

    Question six is a similar concept but has images I can't draw. Essentially it compares a cis chlorinated alkene, trans chlorinated alkene, and a cis chlorinated alkane and says the alkane has a higher boiling point than the trans chlorinated alkene which doesn't make sense to me because while there is the polarity difference, isn't an alkene stronger?!!!

    Thanks


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  11. SuperSneaky24

    SuperSneaky24

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    So make sure you understand the differences between stable/unstable molecules and melting/boiling points. Stability is intramolecular (that is, only the properties of the molecule itself will determine it) while melting/boiling points are intermolecular (that is, due to the interactions between other molecules).

    The reason melting point increases is because hydrogenation will saturate the diene, causing the double bonds to become single bonds. Double bonds are very rigid and don't give room to rotate. On the other hand, single bonds are easily rotatable. Melting point increases because the now single bonds can interact with the single bonds of other molecules via Van Der Waals forces. Whereas before, the double bond didn't give much room for rotation and thus didn't give much rotation and flexibility for the diene to interact with other dienes. More intermolecular interactions = stronger intermolecular forces = higher melting point.

    You also don't need to think about it like the way I explained above. Think about it like this: that double bond causes other similar molecules to pack farther apart from each other. Why do you think unsaturated hydrocarbons are usually liquid at room temperature? That double bond creates the "kink" that prevents the molecules from packing close to one another.
     
  12. SuperSneaky24

    SuperSneaky24

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    And from what I remember, the cis alkene should have the lowest boiling point right? The orientation of the cis alkene would make the molecules difficult to pack together well, leading to increased intermolecular separation
     
  13. avenlea

    avenlea 5+ Year Member

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    This is an amazing answer and cleared up a huge conceptual question! Thank you!

    And no the cis alkene did not. The trans did because it was nonpolar


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  14. SuperSneaky24

    SuperSneaky24

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    Gah I always confuse melting points and boiling points for cis/trans. That makes sense though, as trans will have the lower boiling point but higher melting point. Trans packs better than cis, which makes trans's melting point higher. However, trans is nonpolar while cis has a larger dipole moment due to its polarity, leading to trans's lower boiling point.
     

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