hydrogen bonding question

[jamesq]

Hi,

I just took Kaplan Fulllength 8 and question 5 is troubling me. Im not sure if I can post the question, but it boils down to which one cannot experience hydrogen bonding. Their answer is HCN, however, how can H3BO3 experience hydrogen bonding? The hydrogen is not bonded to an F O or N, its bonded to a B? Furthermore, how can HCF3 undergro hydrogen bonding, once again the rules dont seem to apply, thanks

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[sicboy188]

Hi,

I just took Kaplan Fulllength 8 and question 5 is troubling me. Im not sure if I can post the question, but it boils down to which one cannot experience hydrogen bonding. Their answer is HCN, however, how can H3BO3 experience hydrogen bonding? The hydrogen is not bonded to an F O or N, its bonded to a B? Furthermore, how can HCF3 undergro hydrogen bonding, once again the rules dont seem to apply, thanks

why cant HCN participate in h-bonding...? are you sure its an EXCEPT question?

 

[jamesq]

why cant HCN participate in h-bonding...? are you sure its an EXCEPT question?

Here is the Q:

The passage lists two mechanisms for hydrogen bonding to increase the heat of vaporization of pure solutions . Which of the following molecules will be least affected by by hydrogen bondin?

 

[chemnerd31]

The nitrogen containing compond is the lest affected by h-bonding because it is the least electronegative out of N,O and F. Also in BH3O3 the H's are bound to an O if you draw out the lewis structure the B has 3 OH groups on it.

 

[jamesq]

thanks your explanation makes sense, but they say that HCN would be totally not be able to hydrogen bond, whereas you seem to suggest it still can, but weakly. which one is it? Thanks!

 

[Character]

thanks your explanation makes sense, but they say that HCN would be totally not be able to hydrogen bond, whereas you seem to suggest it still can, but weakly. which one is it? Thanks!

only fon can h-bond. HCN is Hydrogen cyanide.....The H is bonded to the C...Not the N...The N is triple bonded to the C..There is no h bonding in Hydrogen Cyanide...

 

[BlackSails]

only fon can h-bond. HCN is Hydrogen cyanide.....The H is bonded to the C...Not the N...The N is triple bonded to the C..There is no h bonding in Hydrogen Cyanide...

Sulfur and Selenium can also hydrogen bond, iirc.

Also, depending on solvent polarity, the HCN might just dissociate into ions. pka is about 9.

 

[sicboy188]

only fon can h-bond. HCN is Hydrogen cyanide.....The H is bonded to the C...Not the N...The N is triple bonded to the C..There is no h bonding in Hydrogen Cyanide...

N can be a H-bond acceptor in the HCN configuration (not a donor).

the question is worded badly IMO.

 

[BlackSails]

N can be a H-bond acceptor in the HCN configuration (not a donor).

the question is worded badly IMO.

The actual question was though: .

Which of the following molecules will be least affected by by hydrogen bonding?

B(OH)3 will definitely have more hydrogen bonding than HCN. Oxygen is more electronegative than Nitrogen and there are three times as many per mole.

 

[BloodySurgeon]

HCN will not part-take in ANY hydrogen bonding with itself. A hydrogen must be connected to Oxygen, Nitrogen, or Fluorine and it could only connect to O,N, or F. Second the O, N, or F must be connected to a less Electronegative atom or again hydrogen bonding is not possible because it would not gain a partial negative charge. In HCN, the hydrogen is connected to the carbon not O, N ,or F. Therefore the answer much be H3BO3. Remember Boron can only have three sigma bonds with no lone electrons. The hydrogen is connected to oxygen in a manner like so, B(OH)3. The hydrogen is connected to Oxygen and the oxygen is connected to a lower EN atom, boron. That is why the answer is so.

 

[BlackSails]

HCN will not part-take in ANY hydrogen bonding with itself.

Whats the pkb of HCN/-CN at nitrogen? You might very well end up protonating some molecules at nitrogen, which could then hydrogen bond to other molecules.

 

[BloodySurgeon]

Read the question wrong... the answer is H3BO3 because it can form more H-bonds. Yea HCN can form hydrogen bonds with for example H2O but it will be much less significant than H3BO3 which can form 3 H-Bonds per molecule.

And there is nothing wrong with my last statement because HCN cannot H-bond with itself (other HCN molecules).