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Hydrogen bonding questions

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Tokspor

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Hi guys, I have a couple of questions about hydrogen bonding.

1. Aside from solely considering the identity of the atom (i.e. F, O, and N) for hydrogen bonding, is a compound such as NH4+ capable of forming hydrogen bonds if the nitrogen does not have a lone pair?

2. The text I am using states that HCF3 is capable of hydrogen bonding while HCN is not. Their reasoning is that HCN cannot form hydrogen bonds because the hydrogen is not bound to an electronegative atom. Isn't that the same case with HCF3? In both compounds, the hydrogen is attached to a carbon.
 

PiBond

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Hi guys, I have a couple of questions about hydrogen bonding.

1. Aside from solely considering the identity of the atom (i.e. F, O, and N) for hydrogen bonding, is a compound such as NH4+ capable of forming hydrogen bonds if the nitrogen does not have a lone pair?

2. The text I am using states that HCF3 is capable of hydrogen bonding while HCN is not. Their reasoning is that HCN cannot form hydrogen bonds because the hydrogen is not bound to an electronegative atom. Isn't that the same case with HCF3? In both compounds, the hydrogen is attached to a carbon.

NH4+ cannot accept a hydrogen because of the lack of lone pairs, yet it can donate a hydrogen to form a h-bond with, for example, H20

i believe that HCF3 may be an exception due to the electronegativity of the fluorines pulling electron density away from the carbon. therefore, the hydrogen has a partial positive charge on it that allows some weak h-bonding. HCN will definitely not h-bond with itself.

edit: is this a kaplan test question? I'm not quite sure about CHF3, although my reasoning makes sense
 
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Tokspor

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NH4+ cannot accept a hydrogen because of the lack of lone pairs, yet it can donate a hydrogen to form a h-bond with, for example, H20

i believe that HCF3 may be an exception due to the electronegativity of the fluorines pulling electron density away from the carbon. therefore, the hydrogen has a partial positive charge on it that allows some weak h-bonding. HCN will definitely not h-bond with itself.

edit: is this a kaplan test question? I'm not quite sure about CHF3, although my reasoning makes sense

Thanks for the response. Yes, this is from Kaplan. I understand your reasoning, but doesn't HCN also have a partial positive charge since it is polar? I thought the polarity of HCN was greater than that of HCF3 since HCN is known to be weakly acidic.
 

PiBond

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Thanks for the response. Yes, this is from Kaplan. I understand your reasoning, but doesn't HCN also have a partial positive charge since it is polar? I thought the polarity of HCN was greater than that of HCF3 since HCN is known to be weakly acidic.

HCN doesn't have a significant partial positive charge on the H because nitrogen is the least electronegative out of the potential hydrogen bonding atoms (N, O, F). The reason HCN is weakly acidic has to do with the hybridization of the carbon (which is sp). Because of the more s-character the electrons are held tighter by carbon and result in a decently stable anion.
 
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boaz

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Hi guys, I have a couple of questions about hydrogen bonding.
1. Aside from solely considering the identity of the atom (i.e. F, O, and N) for hydrogen bonding, is a compound such as NH4+ capable of forming hydrogen bonds if the nitrogen does not have a lone pair?
Like Pibond says, NH4+ can be an H-bond DONOR.

2. The text I am using states that HCF3 is capable of hydrogen bonding while HCN is not. Their reasoning is that HCN cannot form hydrogen bonds because the hydrogen is not bound to an electronegative atom. Isn't that the same case with HCF3? In both compounds, the hydrogen is attached to a carbon.

Do they mean H-bond donor or acceptor?

Both of them can be acceptors: there is a lone pair on the N of HCN, and a lone pair on the three F's of HCF3.

Neither of them are donors, because neither of the protons are bonded to F, O, or N. The proton on HCN is bonded to C.

When thinking about H-bonding it's always best to think in terms of the atoms involved rather that the molecule as a whole.

I'd love to see the wording of the question as well as explanation, because the way I see it now just doesn't make sense to me.
 
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