[Vighour]

I'm given the structures of molecules that are potentially liquid crystals for use in LCDs and asked to choose the most appropriate molecules.



I know that in order for the molecule to function in LCDs, they need to be polar.
From the image above, I already know that molecule 1 can be used as a liquid crystal. From the options I'm given in the question, I know that the answer is molecule 1 with molecule 2 OR 3. However, it seems like a toss-up between molecules 2 and 3 to me.

I think that the high electronegativity of the Cl atom on molecule 2 makes the molecule polar. However, just like molecule 1, molecule 3 has the carbon-nitrogen triple bond which gives molecule 1 it's polarity. Would molecule 2 be chosen over molecule 3 because it has a similar structure/size as molecule 1?

[nj_bartel]

Why is molecule one definitely a liquid crystal?

[Vighour]

Molecule 1 was presented in a lesson when we looked at liquid crystals. We were told that the carbon-nitrogen triple bond contributed to it's polarity by giving that end a slight negative charge (I think) and then when an electric field is turned on, the molecule realigns so that opposite charges attract.

I'm still not sure if the lone Cl atom will cause a dipole moment (or if it does make a dipole moment, I'm not sure if it'll be strong enough) and also whether the shape make one molecule more suitable than the other.

[nj_bartel]

The Cl absolutely makes a dipole moment, and I'd say due to bond rotation the direction of that dipole would be in rougly the same direction as the nitrile.  So I'd have to go with B (only knowing about LCD's that information you've given me).

[Vighour]

Alright. Thanks!