Which is a better Electrophile

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Dochopeful13

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Can someone please explain why the carbonyl group in the below picture is a better electrophile?

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It's because of the double bond. The oxygen "hogs" more of the electrons via the double bond, making the sp2 carbon more positive than the sp3 carbon.
 
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Can someone please explain why the carbonyl group in the below picture is a better electrophile?

Well... off the top of my head, properties of good electrophiles include... 1) trying to gain electrons 2) they are positively charged/polar. Another factor is steric hindrance. When looking at the 2 molecules you provided... the carbonyl carbon is sp2 hybridized while the hydroxyl carbon is sp3. I feel that an sp2 hybridized electrophile would allow an SN2 reaction to occur more readily.
 
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No. C-OH is more polar than C=O

This is a common misconception. The C-O dipole moment is going to be greater in the ketone than in the alcohol because you have a oxygen pulling strongly via two bonds in a linear fashion. In an alcohol, the oxygen will have both the carbonyl carbon and an extra hydrogen to pull on - and it does so at an angle so the dipole is diluted somewhat. So if you're talking only about dipole moment of a molecule, which is the generally accepted chemical definition of polarity, a ketone is more polar than its analogous alcohol.

However, you may be thinking of elution times on silica columns. This is a special case because not only are you taking into account the dipole moment of the molecule but you also have to take into account H-bonding that is present in the alcohol but isn't present in the ketone. That makes the alcohol get retained more and the ketone less, even though the C-O bond is more polar in a ketone than an alcohol.
 
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Nucleophilicity and electrophilicity are kinetic concepts - so you take into account both sterics and electronics. In terms of electronics, the C=O bond simply has a stronger dipole than the alcohol and thus is better at withdrawing electron density from the carbonyl carbon, making it more electrophilic. Sterically, the ketone is also more electrophilic because it's in a trigonal planar geometry, which creates less steric hindrance to nucleophile attack than a tetrahedral geometry. Also a side note - nucleophilic attack on an sp2-hybridized carbon is no longer an SN1/2 mechanism. SN1/2 are used in relation to sp3 carbons. The correct term is the more general "nucleophilic addition" or in certain cases depending on the nature of your carbonyl, a nucleophilic acyl substitution.
 
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Nucleophilicity and electrophilicity are kinetic concepts - so you take into account both sterics and electronics. In terms of electronics, the C=O bond simply has a stronger dipole than the alcohol and thus is better at withdrawing electron density from the carbonyl carbon, making it more electrophilic. Sterically, the ketone is also more electrophilic because it's in a trigonal planar geometry, which creates less steric hindrance to nucleophile attack than a tetrahedral geometry. Also a side note - nucleophilic attack on an sp2-hybridized carbon is no longer an SN1/2 mechanism. SN1/2 are used in relation to sp3 carbons. The correct term is the more general "nucleophilic addition" or in certain cases depending on the nature of your carbonyl, a nucleophilic acyl substitution.
Your awesome. Thank you very much for the detailed response. May I ask why a double bond creates a stronger dipole than a single bond? I assume it is because sp2 is more electronegative than sp3? Is it safe to say a triple bond has a stronger dipole than a double and single bond?
 
Your awesome. Thank you very much for the detailed response. May I ask why a double bond creates a stronger dipole than a single bond? I assume it is because sp2 is more electronegative than sp3? Is it safe to say a triple bond has a stronger dipole than a double and single bond?

Because the carbon is doubly bonded to an oxygen in a carbonyl- as if it is bonded to 2 oxygens. So the pull is much stronger than in alcohol where the carbon is only bonded to an oxygen. I believe dipoles depend mostly on electronegativity. If you are comparing a double and a triple bond with similar bond connectivity, then I would say yes; the one with the triple bond would be more polar since it could be seen as having three atoms pulling the electron density, whereas the other only has 2.
 
Your awesome. Thank you very much for the detailed response. May I ask why a double bond creates a stronger dipole than a single bond? I assume it is because sp2 is more electronegative than sp3? Is it safe to say a triple bond has a stronger dipole than a double and single bond?

Well in this case, it's more than just a double bond vs. single bond. You have to take geometry into consideration. The hydrogen connected to the oxygen is diluting the oxygen's pull on the carbon. It's like pulling on a rope with both hands versus pulling on a rope with on hand while your other hand is busy with something else that's also pulling back. You're going to get more in the first case.

When comparing double and triple bonds, you can think of each extra bond as a sharing of electrons between carbon and oxygen. A dipole is created when charges are separated - and sharing is a way of separating an electron from carbon's vicinity and putting it in between the carbon and oxygen. Another way of thinking about it is oxidation states. So when we assign oxidation states to carbon, we assume that the carbon has been oxidized, or that the more electronegative element gets both electrons in a bond. So with a C-O single bond, the C is just oxidized by one unit by the O. In a C=O double bond, the C is oxidized two units by the O. In a triple bond, the C is oxidized three units by the O. In other words, the O is stealing more and more electrons from the C with the more bonds there are. And stealing electrons is just another way of saying separation of charge, i.e. an O stealing 2 electrons from C makes the O -2 charged whereas the carbon becomes +2 charged.
 
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