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Series of Electronegativity Questions

  1. Nov 19, 2013 #1
    Hi! I would like to know if what I'm thinking is correct. Please help me as to I'm confused :cry:

    1. If atom "X" has a high Electronegativity, then that means that it can take Electrons from atom "Y" with a low Electronegativity easily causing atom "X" to have a more negative charge "-" and atom "Y" a more positive charge "+". Is this statement correct?

    2. Considering statement number 1, does this mean that an atom with a high Electronegativity would most likely have a negative charge "-"?

    3. How about a compound? For example is a Grignard reagent compound "RMgX" where "R" refers to any Hydrocarbon, "Mg" refers to Magnesium, and "X" as a halide. I would like to know how Electronegativity works with a Grignard reaction in forming a new carbon-carbon compound. When carbon dioxide "CO2" is added to the Grignard reagent. Here is what happens as shown in 3.1.

    3.1. RMgX + CO2 -> RCOOMgX

    Then you hydralize it to form the new carbon-carbon compound in 3.2.

    3.2. RCOOMgX + H2O -> RCOOH + Mg(OH)X

    Regarding that reaction, with the bonding of carbon in "R" and carbon "C", how was their Electronegativity affected (about the increase or decrease in Electronegativity or Charge so carbon "R" can form a bond with carbon "C") so they can form a bond as well as it's relationship with magnesium's "Mg" Electronegativity? I don't quite get the explanation in the link http://wiki.answers.com/Q/Why_Magnesium_is_necessary_for_Grignard's_reagent#slide1

    If you could provide a simple and easy to understand explanation in layman's term if possible. I have an admission exam that will include this topic. Thank you. :frown:
  2. jcsd
  3. Nov 19, 2013 #2
    Yeah. Electronegativity just means how strongly an atom attracts electrons. Its more obvious with ionic compounds, when you react sodium metal with chlorine gas, you get sodium chloride. In that case that chlorine literally strips away sodiums single valence electron, so when you dissolve NaCl in water, you get a solution of Cl- anions, and Na+ cations. Sodium has an electronegativity of 0.93 and chlorine is 3.16 so thats a massive difference in electronegativity. Carbon on the other hand has an electronegativity of 2.55, so chlorine can't completely snatch its electrons like it does with sodium, but when chlorine bonds to carbon, chlorine still pulls carbons electron towards it, resulting in carbon being slightly more positively charged and chlorine slightly more negatively charged. The measure of the polarity of a bond is called a dipole moment. This diagram shows the dipole moments of some chlorinated methanes. The arrow points in the direction of the atom which hogs the electrons more.

    Only if it steals an extra electron from somewhere. For example, if it reacts with a low electronegativity element like an alkali metal (i.e. lithium, sodium etc.) and forms an ionic bond. In chloromethane, the chlorine isn't actually negatively charged because its a covalent bond between it and carbon, meaning they share the electrons, chlorine doesn't fully take the electron from carbon. Rather than putting a - on the carbon, you use the delta symbol to indicate that it only partially takes the electrons:

    A Grignard reagent has unusual properties. Usually the halogen (X) would pull electrons away from carbon making the carbon more positively polarised, but in this case the magnesium atom gives the halogen an electron, so the halogen is no longer more electronegative than carbon. Now, carbon can be the one that snatches the electrons. So this is a rare case of carbon being the electron rich part of a molecule. Other examples of this are when carbon bonds to lithium. In a Grignard reagent, you can consider the carbon to be negatively charged.

    Oxygen is a very electronegative element so in carbon dioxide, the oxygen pulls electrons away from carbon, resulting in the carbon being more positively charged. In this case, the carbon needs more electrons. In the Grignard reagent, its the other way around, the carbon has more electrons than it needs, so the CO2 carbon is attracted to it, and grabs its electrons to form a new carbon-carbon bond. Basically what happens there is both the electronegative carbon (from the CO2) and the electropositive carbon (from the Grignard reagent) neutralise each other. Neutral compounds are more stable. Now they both have more similar electronegativities.
    Last edited: Nov 19, 2013
  4. Nov 19, 2013 #3
    Thank you so much for the response CrimpJiggler! I appreciate it a lot that you have not only provided the answers that I was looking for but you also took the time to illustrate it so it will be easier to understand. You saved me. Thanks a lot! =)
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