Woher weiß man die polarität bei langen verbindungen wie Ethanal, Essigsäure oder Chlorethan?

Hi.

the polarity in total can be determined from the binding polarities and the influence on the molecule. For you can first only set a binding on whether it is polar or not.

Let’s take Ethanal first. This molecule has the following bonds: C-H, C-C, C=O. EN(C)=2.5; EN(H)=2.1; EN(O)=3.5. You can check these values in the Pauling scale. The difference of the electronegativities of a bond yields a value:

• ΔEN(C-H) = 2.5 – 2.1 = 0.4
• ΔEN(C-C) = 2.5 – 2.5 = 0
• ΔEN(C=O) = 3.5 – 2.5 = 1.0

These values correspond to a polarity. For a value of 0.4 or less, one speaks of nonpolar binding. The bonds C-H and C-C are therefore nonpolar. However, we have the polar C=O bond in the ethanal molecule. The molecule is small and the C=O bond takes up a relatively large part, since the nonpolar part (namely the –CH3) is very small. The polar part, i.e. the carbonyl group, takes up a large proportion of the molecule, so that ethanal as a whole polar Mole is.

In acetic acid we proceed as well. There, O-H is added as a bond: ΔEN(O-H)=3.5 – 2.1=1.4=> polar bond. So now we have a polar acid group (carboxy group) and a nonpolar methyl radical (-CH3). Again, the polar part of the molecule takes up a lot of space and therefore acetic acid is also a polar Connection.

In chloroethane, you have the bonds C-C, C-H and C-Cl. The electronegativity difference is ΔEN(C-Cl)=3.5-3.2=0.3=> nonpolar. The compound chloroethane is thus non-polar.

You always look at what bonds you have and determine their polarity if you don’t already know them by experience (sometimes you have a few values in your head). Then look at the molecule itself and see how large the polar and nonpolar components are in the molecule. The larger the nonpolar component, the more nonpolar is the molecule and vice versa.

LG