Question

Is more than one shape possible for the caproic acid molecule? If you built a model of stearic acid by extending the carbon chain of caproic acid to18 carbons, would it have more, less, or the same number of possible shapes as the number of shapes for caproic acid? Can you write a general rule that relates the length of a fatty acid chain (carbon skeleton, backbone) to the number of shapes possible?

Unrelated but... Do the open chain and closed ring forms of glucose have the same molecular weight?

Answer 1

Caproic acid: CH3CH2CH2CH2CH2COOH. This is a small (short chain) member of the fatty acid family. Aside from the hydrophilic carboxyl group at one end, the rest of a fatty acid molecule (the bulk of it) is made of methyl groups linked together (-CH2-) and those are hydrophobic. As discussed in class, members of this family having more than about five carbons are water-insoluble.

Build a second model of caproic acid and put a double bond between C3 and C4. This molecule is "unsaturated", by definition, because of this C=C double bond. The first one you made is "saturated", by definition, because there are no C=C double bonds in the carbon chain. Note that "saturation" here has nothing to do with water. The presence of the double bond means that there are two geometric isomer forms of this unsaturated fatty acid molecule: the cis isomer and the trans isomer. Recall the two different arrangements of the atoms around the C=C double bond in the 2-butenes that you constructed. In the cis isomer the double bond interrupts the regularity of the zigzag in the chain; i.e. it causes a "bend" in the chain.

Saturated fatty acids have no C=C double bonds. They have the same formula CH3(CH2)nCOOH, with variations in "n". An important saturated fatty acid is stearic acid (n = 16), which when neutralized with lye is the most common form of soap.

In most naturally occurring unsaturated fatty acids, each double bond has three (n-3), six (n-6), or nine (n-9) carbon atoms after it, and all double bonds have a cis configuration.

Most fatty acids are even-chained, e.g. stearic (C16) and oleic (C18), meaning that an even number of carbon atoms comprise them. Some fatty acids have odd numbers of carbon; they are referred to as odd-chained fatty acids (OCFA). The most common OCFA are the saturated C15 and C17 derivatives, respectively pentadecanoic acid and heptadecanoic acid, which are found in dairy products.

In these cyclic forms, the ring usually has 5 or 6 atoms. These forms are called furanoses and pyranoses, respectively — by analogy with furan and pyran, the simplest compounds with the same carbon-oxygen ring (although they lack the double bonds of these two molecules). For example, the aldohexose glucose may form a hemiacetal linkage between the hydroxyl on carbon 1 and the oxygen on carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose. The same reaction can take place between carbons 1 and 5 to form a molecule with a 6-membered ring, called glucopyranose. Cyclic forms with a 7-atom ring (the same of oxepane), rarely encountered, are called heptoses. For many monosaccharides (including glucose), the cyclic forms predominate, in the solid state and in solutions, and therefore the same name commonly is used for the open- and closed-chain isomers. Thus, for example, the term "glucose" may signify glucofuranose, glucopyranose, the open-chain form, or a mixture of the three.So we can conclude that the molecular weight remains same.