# The H-bond's strength

1. Jul 14, 2005

### wangasu

Hi, I have a question in comparing the strength of H-bond. If we mix two H-bond liquids, Propanol (C3H7-OH) and N-Ethylmethylamine (C2H5-NH-CH3), how does the H-bond act in this mixtures? Does the -OH get together with -OH or with -NH? Which pair is stronger among OH...OH, OH...NH and NH...NH? Note that in this liquid mixture, there is no phase separation, and the two liquid molecules have single H-bond. In some references, I could not find relative information. Thanks.

2. Jul 14, 2005

### Gokul43201

Staff Emeritus
The strength of the H-bond goes roughly like the product of the bond-length and the (relative) electronegativity of the atom bonded to H (namely O, N, etc.) since the H-bond is just a special kind of dipole-dipole interaction. I say "roughly" for a few reasons :

(i) This works better for a single component liquid than if you had the kind of mixture you are talking about. In general the hydrogen bonding should be stronger between like molecules, due to geometric factors.

(ii) The charge rearrangement (ie: electron withdrawing = -I, -R or donating = +i, +R) role of the main chain (or whatever the -OH or -NH is sticking to) is not negligible.

(iii) Geometric/steric factors from the rest of the molecule could play a role as well.

I would guess that (in terms of bond strength, for this case) OH - - OH > NH - - OH > NH - - NH. But in terms of what bonds would be prevalent in the mixture (assuming a 1:1 mixture), I would guess that OH - - - OH bonding dominates - and this automatically forces the ethylmethylamine to want to clump together, in pairs, as well. So, I would guess that the propanol and EMA would want to separately dimerize (weakly).

3. Jul 15, 2005

### GCT

I'm not quite sure to what extent N-Ethylmethylamine and propanol are miscible, but I'm guessing it's not efficient as water with ethanol, or water with NaCl...in relevance to the ordered arrangement (entropy considerations). If the two are miscible, forms a relatively ordered structure, then I would imagine that some of the same dyanmics would occur, such as acid base reactions as well as hydrogen bonding interactions (e.g water and ethanol)....I don't know the details though.

4. Jul 15, 2005

### movies

Technically, the term "miscible" only applies to gases and liquids, not to solids. So NaCl isn't "miscible" with water, it's "soluble" in water.

5. Jul 15, 2005

### GCT

6. Jul 15, 2005

### wangasu

I have been searching relative papers about this topic, and found some helpful information. With the inputs, now I seems to reach a somehow different point that is in the binary amines-alcohols systems, the intermoleclar interaction should have the order: NH--OH > OH--OH > NH--NH. The main reason that helps me to get to the conclusion is that when mixing the two kinds of liquids at a fixed temperature, the viscosity has a maximum around 50mol%. this means the formation of stronger interaction in mixtures than the two pure liquids.

7. Jul 15, 2005

### movies

Sorry, I don't mean to be a jerk about it or anything, but I have gotten burned by incorrect terminology before, so I tend to watch that kind of stuff pretty carefully.

8. Jul 15, 2005

### Gokul43201

Staff Emeritus
Might I suggest the possibility that you are misinterpreting the experimental results ?

Here's an alternative explanation of the quoted results : In the pure liquids the molecules virtually dimerize because of strong bonding. These dimers are essentially non-interacting (with other dimers), giving the liquid a low viscosity. In the mixed case, the dimerization is somewhat broken up because of too many molecules of the "other" kind getting in the way. So, in this case, there is bonding between essentially all the molecules forming a weak network with higher viscosity.

I'm not saying my explanation is correct and yours is wrong. I've hardly given this a minute of thought after reading your post, so I could be talking through my hat. But it might nevertheless be instructive to know that you should be very careful when going from experimental results to the underlying theory.

9. Jul 15, 2005

### Nerro

That explanation is a trifle unlikely in my eyes. If larger molecules are more viscous that is most likely a result of their larger mass and thus stronger vanderwaals interaction. Also water is a very small molecule but nonetheless it is much more viscous than it should be (jsut compare it to liquid ammonia or liquid methane). It is also unlikely to dimerize since it has 2 sites for making H-bonds. (two $$\delta+$$ charged hydrogen atoms and a $$\delta-$$ charged oxygen atom with 2 free e-pairs.) which lend themselves perfectly for a kind of chain-polimerisation rather and very poorly for dimerization.

10. Jul 15, 2005

### GCT

movies, trust me I know the difference, I specifically said

I wasn't really retaliating to your post, it's just my way of saying....I know the difference. I understand your point though, terminology is particularly significant in chemistry

11. Jul 15, 2005

### GCT

yeah, the conclusion seems too simplistic. There are probably more advanced methods, some of which I'll be learning in analytical and p. chem this fall.

12. Jul 15, 2005

### wangasu

It might seem to be simple, but it is not necessarily wrong. I totally agree "should be very careful when going from experimental results to the underlying theory". We do need evidences to verify it to be right or wrong. The structure of liquid alcohols is far more complicated than the dimer model. lots of studies have suggested various molecular configurations like chain-like and cyclic clusters in alcohol liquids.

I just got another information. Maybe, this topic can be considered on the basis of the basicity of the two liquids. we know that from amines to alcohols to acids, if they have similar alkane groups, their basicity decreases in the order. For the mixtues of amines and acids, ionic type binding forms which is stronger than H-bond. so I am thinking if there is the possibility that between amines and alcohols, a somewhat stronger binding forms than that in pure alcohols or amines as well.

13. Jul 15, 2005

### Gokul43201

Staff Emeritus
I too initially thought in terms of basicity (or acidity), but my thinking ended up with the opposite result. However, you seem to have some empirical knowledge of some kind of ionic bonding between molecules in a mixture of amines and acids, which I am not aware of. In fact, I wonder if this is not just a hydrogen-bond here too, after all. But still, I feel that knowledge (single data point) is insufficient to draw conclusions from.

Also, I can't see how, in general, one would experimentally determine the strength of the H-bond between a pair of molecules, if the nature of the macro-structure is unknown (dimers/chains/rings/networks) ? I guess it can't be done with a single test...