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MolarBear541

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In regards to nucleophilic addition "...steric crowding causes the product to be relatively less stable, thus decreasing reactivity" I'm a bit confused about this explanation, doesn't steric crowding make a compound more stable and less reactive? (Ketone vs aldehyde)
 

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In regards to nucleophilic addition "...steric crowding causes the product to be relatively less stable, thus decreasing reactivity" I'm a bit confused about this explanation, doesn't steric crowding make a compound more stable and less reactive? (Ketone vs aldehyde)
When you have a nucleophilic addition, do not focus on the product, but focus on the reacting specie. An aldehyde has a greater reactivity . One reason is steric. The nucleophile must approach at 107 degrees. In advanced organic chemistry, we call this the Burgi -Dunitz angle. Any groups that will get in the way will effectively reduce the rate of addition. This is one reason as to why aldehydes are more reactive than ketones. Hopefully as you can see, nucleophiles do not simply attack the carbon, but approach from a particular angle. 2 R groups as in a ketone, make this approach quite difficult. I hope this allows you to see one of the main reasons as to why aldehydes are more reactive than ketone. Another reason is electronic, which we need not go into here. Remember.....focus on the reactant molecule, not on the final product when doing carbonyl chemistry. If you need more clarity on this, the text written by Jonathan Clayden and Stuart Warren of the University of Manchester and Cambridge University do a fantastic job in discussing the Burgi-Dunitz trajectory.

Hope this helps.

Dr. Romano
 
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MolarBear541

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Dec 5, 2016
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When you have a nucleophilic addition, do not focus on the product, but focus on the reacting specie. An aldehyde has a greater reactivity . One reason is steric. The nucleophile must approach at 107 degrees. In advanced organic chemistry, we call this the Burgi -Dunitz angle. Any groups that will get in the way will effectively reduce the rate of addition. This is one reason as to why aldehydes are more reactive than ketones. Hopefully as you can see, nucleophiles do not simply attack the carbon, but approach from a particular angle. 2 R groups as in a ketone, make this approach quite difficult. I hope this allows you to see one of the main reasons as to why aldehydes are more reactive than ketone. Another reason is electronic, which we need not go into here. Remember.....focus on the reactant molecule, not on the final product when doing carbonyl chemistry. If you need more clarity on this, the text written by Jonathan Clayden and Stuart Warren of the University of Manchester and Cambridge University do a fantastic job in discussing the Burgi-Dunitz trajectory.

Hope this helps.

Dr. Romano
Thank you so much for your explanation!
 
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