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Hydrogen Bonding

ahs4n

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  1. Pre-Medical
    Kaplan Question Full length 8 PS #1

    How many hydrogen bonds is H3BO3 capable of producing.

    Kaplan says the answer is 9 because every hydroxyl group can participate in 3 hydrogen bonds (2 on the oxygen and then 1 from the hydrogen)

    My question is on AAMC tests, is it because of the inherent polarity that they say ethers cannot hydrogen bond nor can an tertiary amine group that has no hydrogen attached to it has no hydrogen bonding or have hydrogen bonding with water??
     

    sleepy425

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    1. Fellow [Any Field]
      Kaplan Question Full length 8 PS #1

      How many hydrogen bonds is H3BO3 capable of producing.

      Kaplan says the answer is 9 because every hydroxyl group can participate in 3 hydrogen bonds (2 on the oxygen and then 1 from the hydrogen)

      My question is on AAMC tests, is it because of the inherent polarity that they say ethers cannot hydrogen bond nor can an tertiary amine group that has no hydrogen attached to it has no hydrogen bonding or have hydrogen bonding with water??

      The last bit of your question was really jumbled, but here's how this all works.

      Ethers cannot self hydrogen bond. In other words, if I have a pure sample of diethyl ether, the ether molecules will not hydrogen bond with other ether molecules because they do not have hydrogens directly bonded to electronegative heteroatoms (most commonly N, O, F). Ethers can act as hydrogen bond acceptors, but not as hydrogen bond donors. The same is true of deprotonated tertiary amines, since they have a heteroatom (nitrogen in this case) with a lone pair to be a hydrogen bond acceptor. They can't act as hydrogen bond donors because they don't have a hydrogen directly attached to an electronegative heteroatom. If you protonate a tertiary amine, it becomes a fantastic hydrogen bond donor, but now it isn't a hydrogen bond acceptor since it doesn't have a free lone pair.

      The point of all of this is that, if a molecule can act as a hydrogen bond acceptor but not a donor, it can participate in hydrogen bonding ONLY if another molecule that can act as a donor is present. A good example of this is from enzymology. Serine proteases catalyze hydrolysis of peptides (amides) to give a free carboxyl terminus and a free amino terminus (so basically it hydrolyzes the amide into the carboxylic acid and the amine). One of the key mechanisms of catalysis is something called an oxyanion hole, which is basically just two good N-H hydrogen bond donors formed by the enzyme's peptide backbone. What happens is that the carbonyl oxygen of the amide, which is a hydrogen bond acceptor but not a hydrogen bond donor, is bound in the oxyanion hole. The first step of the hydrolysis is that water attacks the carbonyl carbon and kicks the electrons in the double bond onto the carbonyl oxygen to make an oxyanion. Oxyanions aren't that stable, but because of the two good hydrogen bond donors in the oxyanion hole, this oxyanion is relatively stable, so the reaction proceeds quickly. There's another similar step, but that should just give you an idea of why hydrogen bonding can be important even if a molecule can't self hydrogen bond.

      On the other hand, if you look at a deprotonated primary amine (R-NH2). It has one lone pair that can act as a hydrogen bond acceptor, and two N-H bonds to act as hydrogen bond donors. So primary amines can hydrogen bond with one another. So this is an example of self hydrogen bonding. Water is the classic example of this
       
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        There is a very simple rule for Hydrogen bonding-
        Molecule must have H bonded to any one of these atoms N,O and F.If this is missing than no Hydrogen bonding.You can apply it on any molecule and is always right.
        Like ethers - R-O-R , here there is no H on O atom so no H bonding
        Tertiary amines (R)3N ,here again no H on N so no H bonding.
        R-OH,RNH2,RCOOH they all will show H bonding because they have H bonded to most electronegative atom N and O
        It also applicable on big molecules like proteins ,DNA etc.
        It is very simple and straight.
         
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