[ST_08_09]

I would like to arrange these in order of increasing viscosity at 50 'C   a)ThioPhenol  b)Phenol  c) Benzene .What I have completed is   Benzene(0.744) and Phenol(11.3) but unable to guess for ThioPhenol ?

2)I have to Identify the Intermolecular forces that might arise b.w molecule of the following substance.

 a)Cl2
 b)NH3
 c)C6H6
 D)C6H5Cl


Thanks in advance

[rahul2ghosh]

1.  We know that higher the Molecular Mass, more is the Van der Wall Forces, thus higher inter molecular forces and hence increased viscosity, in case of liquids. Now, with out getting into the experimental viscosity values of a, b and c, it can be seen that the Molecular Mass increases as:

Benzene < Phenol < Thiophenol

thus the viscosity should increase in the same order at Room Temperture. However,
its not the case. The increasing order viscosity at room temp. is:

Benzene < Thiophenol < Phenol

It is because we also have to consider the hydrogen bonding in case of Phenol.

As the temperature of observation is 50^oC, at this elevated temp. the viscosity of all three will decrease due to thermal agitation. But the H Bonding completely vanishes for phenol. Thus, the new viscosity order would be:

Benzene < Phenol < Thiophenol

2.  Again going by the Molecular Mass theory, the order should be:

NH₃ < Cl₂ < C₆H₆ < C₆H₅Cl

However, here also H bonding in NH₃ dominates Molecular Mass of Cl₂. Therefore, the actual order is:

Cl₂ < NH₃ < C₆H₆ < C₆H₅Cl

[macman104]

Just a curiosity, just wondering how you know H-bonding disappears at 50°C.

[rahul2ghosh]

Just a curiosity, just wondering how you know H-bonding disappears at 50°C.

Macman!! Its a kind of PROPHECY!!! 

Well H Bonding in normal conditions acts to a range of ~44 ^oC for Phenols and Benzoic acid derivatives. For, Salicylic acid the H Bonding almost becomes negligible by 48.7 ^oC.

[macman104]

Fair enough, I've just never heard of a hard value where H-Bonding stops or becomes negligible.  Thanks for the info.

[ST_08_09]

Thanks alot for such a great discussion ...