Question: In fatty acids why does the position of the double bond make such a great difference, e.g trans fats.

Ruhina Miller answered on 19 Jun 2015:

Trans fats have a linear structure that is similar to an alkane. This is because across the trans double bond, the R groups (R-C=C-R) are located in opposite planes/directions, so this causes the molecule to be a “straight” zig-zag. If that bond was cis-, the R groups would be on the same side of the molecule and it would cause a bend in the molecule.

Now, imagine trying to pack trans fat molecules – this is going to be relatively easy because the chains are straight and there won’t be many gaps so you will fit more molecules in this space – it will be quite dense. If you pack molecules with cis bonds in, these bends will mean that they don’t pack nicely, they occupy more space and therefore it will be less dense compared to the same volume of trans-fat.

So why does this matter? Imagine if you ate a lot of trans-fats, the body will “pack” these in nicely and they will be difficult to remove again. Ideally, when we consume fats we would want to consume fats that contain lots of double bonds (polyunsaturated fats), especially ones with cis-bonds in because this will cause lots of bends in the molecule to distrupt packing.

Also, because of this packing, trans fats have a higher melting point.