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Orbital Help Please/ Bonding

Discussion in 'MCAT Study Question Q&A' started by labqi, 05.14.14.

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  1. labqi

    labqi 2+ Year Member

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    I hope everyones studying is going well. I was doing Kaplan FL #1, and I think I may have misunderstood the concept or question. In any regards, my concern is the following:
    I know that the s orbital is lower in energy than the p-orbital. Thus the more the s character, the lower the energy. Thus, sp would be considered the lowest energy. This is where I become confused. I always thought that the sp bond would be the highest energy, and strongest. That the triple bond would be the highest energy.

    I dont understand the connection, if someone could please clear this up for me. Also, how do the C-H energy levels compare in sp3, sp2, and sp. For those that have kaplan, its question:

    Molecular orbitals in hydrocarbons are formed between the 1s atomic orbital of H and sp, spy, and sp3 orbitals of C. Which correctly lists the energy levels of C-H bonds.

    Benzene, ethyne, and methane.

    Thanks in advance.
     
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  3. labqi

    labqi 2+ Year Member

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    P.S. An alkyne releases more energy than an alkene. With the above logic, how does that work?
     
  4. Czarcasm

    Czarcasm Hakuna matata, no worries. 2+ Year Member

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    So, if you recall s orbitals are the lowest in energy (most stable); the other subshells are increasing energy levels (p, d, f, g, etc). For hybrid orbitals, orbitals with the most s-character are therefore the lowest in energy; The actual bond length is due to the fact that maximal bond overlap occurs between s orbitals (a stabilizing characteristic); The more s characteristic, the more overlap occurs.

    This is why sp-sp bonds are shorter and stronger than sp2-sp2 bonds. For the sigma bond, the bonding electrons are drawn in closer to the positive nuclei of both bonding atoms. (For comparison: an s-s orbital bond is stronger than a p-p orbital bond bc s orbitals are smaller and closer to the nucleus). Sigma bond (Single Bonds) are very difficult to break because they are very stable. Bonding electrons are in between the nuclei of the bonding atoms, which is very favorable for electrons. In this instance, carbon carbon triple bonds, having the most s characteristic, will have the strongest sigma bond and consequently, the shortest and strongest bond. It's bond strength and length is due entirely to the sigma bond (pi bonds are weak and easy to break).

    When we consider the reactivity of each alkynes, alkenes, and alkanes, it has to do with the electrons that reside in the p-orbitals (pi bonds). Pi bonds are weak bonds and fairly reactive. They are free to donate their electrons and act as lewis bases. This is why the heat released is greatest during the hydrogenation of a c-c triple bond than the energy released for a c-c double bond (you can even compare trans/cis/geminal double bonds), etc and their stabilities. If you recall a c-c triple bond has 1 sigma and 2 pi bonds: 2 pairs of p orbitals (orthogonally to each other), a c-c double bond has 1 sigma and 1 pi bond: 1 pair of p orbitals above and below the plane.

    So in a nutshell, bond strength is due to the stability of sigma bond; reactivity is due to the unstability of pi bonds.

    We often simplify this by just considering the sigma bond between a hyrbidized carbon and hydrogen (C-H), because by comparing the energy required to break these bonds, we can confirm that alkynes have the strongest sigma bond, then alkenes, then alkanes (without regard for the pi bonds themselves, which are fairly weak).

    EDIT: Revised for clarity; sorry, took me a moment to recall this info.
     
    Last edited: 05.14.14
    labqi likes this.
  5. Chrisz

    Chrisz 2+ Year Member

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    Your major confusion when dealing with orbital is that you are mess up the concept of energy level and stability. The lower the energy level a molecule is, the more stable it is. So, SP has 50% s, SP2 has 33.33%, SP3 has only 25%. Therefore, an SP orbital is lower in energy than SP2, which in turn lower than SP3. Remember electrons prefer lower energy levels, because they always seek the most stable state. An analogy would be two magnets with two opposite poles facing toward each other. When they are forcibly moved apart, potential energy is stored in the system. A lot of energy means not stable, that is why the two magnets tends to move toward each other. Once the attach to each other, they are now in a stable state. They would not move apart unless energy is provided. I think you should have learned columb's law. PE=kq1q2/r . For a simplified case, in which there is only one proton and electron, the smaller the r is, the more negative the potential energy becomes, the more stable they system becomes. I think, based on this information, you be able to figure out the order of Benzene, ethyne, and methane. Just try to ask your self which C-H bond is lower in energy, which orbitals are hybridized in each carbon.
     
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  6. Chrisz

    Chrisz 2+ Year Member

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    A little bit more information I want to add to make you better understand the concept. You know alkyne is more acidic than alkene, and alkene is more acidic than alkane. The relative acidity is related to the orbital theory. You know once a proton is removed, the lone pair resides in the corresponding orbitals. The more stable the corresponding the orbital, the more acidic it is. Alkane sp3, alkene sp2, alkyne sp. See the relationship now.
     
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