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Trans vs Cis Fats

Unsaturated fats that occur in nature overwhelmingly contain unsaturated fatty acids with a cis arrangement of H atoms around the double bonds. During manufacturing processes that convert unsaturated fats into saturated fats, some unsaturated fats get converted into the trans arrangement. These trans fats are believed to pose serious health risks. All foods containing more than 1 g per serving are required by U.S. law to indicate the amount of trans fats on the nutrition label.

Cis and Trans Unsaturated Double Bonds

The hydrogen atoms can be arranged differently around the double bond.
  • Inspect the two molecules below. Look at the arrangement of the hydrogen atoms on the carbon atoms with the double bonds.
  • What is the same in these two molecules?
  • What is different about the placement of the H atoms?

A trans-unsaturated fatty acid

Look at the structure of the most common trans-unsaturated fatty acid, elaidic acid. Compare its structure to that of its cis counterpart, oleic acid and the saturated counterpart, stearic acid.

Click on the button below to highlight the hydrogen atoms around the double bond.

Click on the buttons below to view the fatty acid molecules in different ways.


Instructions for manipulating images.

To rotate the molecule, move the cursor inside the Jmol frame while holding down the left mouse button. To enlarge the molecule, move the cursor inside the Jmol frame, simultaneously shift and left click the mouse then drag the mouse downward; to reduce molecule size, shift-click and drag the mouse upward... To access a pop-up menu use the right button on a PC or do a command-click on a Mac while the cursor is inside the java frame. To measure a bond distance, double-click on two atoms. To measure a bond angle, do double-click, single-click, double-click on three atoms.