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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: appleseed on March 01, 2009, 10:16:32 AM

Title: Hydrogen bonds. What are the rules /exceptions?
Post by: appleseed on March 01, 2009, 10:16:32 AM
Hydrogen bonding is when H is bonded to NOF only right ? and the electronegative H is attracted to lone e- pairs right
so....

Which species is most likely to form hydrogen bonds?
a.
   
LiH
   
c.
   
HCl     
b.
   
H2
   
d.
   
CH3OH
         

ANSWER:
C




the answer is C, how come?
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: nj_bartel on March 01, 2009, 10:59:25 AM
hydrogen attached to a very electronegative and unsterically hindered atom.
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: Vidya on March 01, 2009, 11:26:41 PM
CH3OH
This should be the answer.
HCl  has no Hydrogen bonds.
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: nj_bartel on March 01, 2009, 11:30:47 PM
Sorry, yes - hydrogen bonding is only between hydrogen and nitrogen, fluorine, and oxygen.
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: cliverlong on March 02, 2009, 04:11:22 AM
Quote from: vidya kaushik on March 01, 2009, 11:26:41 PM
HCl  has no Hydrogen bonds.
Can you explain why HCl does not experience hydrogen bonding? I always thought that due to the electronegativity difference between hydrogen and chlorine that the molecule exhibits a permanent dipole where chlorine is d- and hydrogen is d+ in the molecule. This dipole generates the hydrogen bonding. Why is my understanding incorrect?

Thanks

Clive
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: AWK on March 02, 2009, 07:35:30 AM
Quote from: vidya kaushik on March 01, 2009, 11:26:41 PM
CH3OH
This should be the answer.
HCl  has no Hydrogen bonds.
???
CsHCl2 is known over 50 years
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: Vidya on March 02, 2009, 08:17:28 AM
I am talking about intermolecular hydrogen bonding.The question was based on IMF .And I will again say H-Cl has no intermolecular hydrogen bonding .
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: AWK on March 02, 2009, 09:39:19 AM
Quote from: vidya kaushik on March 02, 2009, 08:17:28 AM
I am talking about intermolecular hydrogen bonding.The question was based on IMF .And I will again say H-Cl has no intermolecular hydrogen bonding .
HCl dimer is at least considered by quantum chemists
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: cliverlong on March 02, 2009, 05:06:12 PM
Quote from: vidya kaushik on March 02, 2009, 08:17:28 AM
I am talking about intermolecular hydrogen bonding.The question was based on IMF .And I will again say H-Cl has no intermolecular hydrogen bonding .
You are probably right but my question is still, "why?".

If my analysis is correct then HCl should exhibit hydrogen bonding - but it doesn't.

Thanks,

Clive
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: Astrokel on March 03, 2009, 12:55:55 AM
Hey Clive, if you were to look up on the definition of hydrogen bonding, it says the molecule needs to have a hydrogen atom bonded to a very electronegativity atom(F,O or N) and not with Cl. This is very clearly seen in the boiling point of hydrogen halides. The adnormally high(and also it doesn't follow the general increase) B.p of HF is due to hydrogen bonding.
Title: Re: Hydrogen bonds. What are the rules /exceptions?
Post by: cliverlong on March 03, 2009, 11:23:00 AM
Quote from: Astrokel on March 03, 2009, 12:55:55 AM
Hey Clive, if you were to look up on the definition of hydrogen bonding, it says the molecule needs to have a hydrogen atom bonded to a very electronegativity atom(F,O or N) and not with Cl. This is very clearly seen in the boiling point of hydrogen halides. The abnormally high(and also it doesn't follow the general increase) B.p of HF is due to hydrogen bonding.
I have found the following article

http://www.chemguide.co.uk/atoms/bonding/hbond.html

This restates Astrokel's point that the b.pt. of HF is anomalous in relation to the other hydrogen halides. H2O and NH3 both exhibit the same anomalous behaviour of b.pt. The explanation given for this anomalous behaviour is hydrogen bonding. The mechanism of hydrogen bonding is explained as due to both:
(quote)

* The hydrogen is attached directly to one of the most electronegative elements, causing the hydrogen to acquire a significant amount of positive charge.

and

*   Each of the elements to which the hydrogen is attached is not only significantly negative, but also has at least one "active" lone pair.

      Lone pairs at the electron level 2 (CRL: period 2 elements: N, O and F) have the electrons contained in a relatively small volume of space which therefore has a high density of negative charge. Lone pairs at higher energy levels  (CRL: for example chlorine)  are more diffuse and not so attractive to positive things (CRL: such as H+).

(\quote)

Hence, only N, O and F have both properties therefore they are the only elements that can accept hydrogen bonds.

The article goes on to argue that Cl- can be considered to accept a "weak form" of hydrogen bonding when Cl-  is hydrated in aqueous solution.


Clive