1B: NutraSweet: Functional Groups

We can learn most of the important principles of organic chemistry through a study of the aspartame molecule. In doing so, we will see the scientific problems that chemists faced as they attempted to synthesize and evaluate this artificial sweetener.

We stated earlier that, by definition, organic compounds contain carbon. Compounds that contain only carbon and hydrogen are called hydrocarbons and those that contain only sp³ hybridized carbons and hydrogen are called saturated hydrocarbons. Many fuels, such as gasoline, are mixtures of hydrocarbons, but, other than combustion to carbon dioxide (CO₂) and water (H₂O), such compounds have limited chemical reactivity. When carbon forms double or triple bonds to other carbons or when it bonds to atoms or groups of atoms other than hydrogen, then the possibility of chemical reactions increases dramatically. There are several dozen atomic arrangements that occur regularly among the millions of organic compounds. These arrangements are called functional groups (functionalities). Such groups impart physical characteristics (e.g., boiling point, melting point, density, solubility properties, and physical state) to the compounds that possess the groups and also impart chemical properties related to the functional groups' atom arrangement. (Chapter 11 in Atkins and Jones discusses organic functional groups in more detail.)

We show a collection of several important functionalities.

One of the most important bonding arrangements in organic chemistry is C=O, the carbonyl group. The carbon of the carbonyl will have two other attachments. The actual functional group depends upon the identity of the other two groups. For simplicity, let’s assume that one of the two attachments is to a saturated hydrocarbon residue that does not itself have a functionality. Such an attachment could be a CH₃-, CH₃CH₂-, or (CH₃)₂CH- and will be designated by the symbol R, a standard designation. It is the other attachment that will define the functional group. Similarly, other atom combinations create other functional groups. We describe five types of carbonyl compounds below.

Organic Functional Groups

R1-	OH - alcohol
	OR1 - ether
R1R2C=O	R1R2=hydrocarbon - ketone
	R1=hydrocarbon,R2=H- aldehyde
	R1=hydrocarbon,R2=OH - acid
	R1=hydrocarbon,NH2 - amide
	R1=hydrocarbon,R2=ORx - ester
R1R2C=CR3R4	alkene
C6H5-	aryl,phenyl

An alcohol contains an OH group attached to a sp³ hybridized carbon, while an ether such as the anesthetic diethyl ether, CH₃CH₂OCH₂CH₃, has an R-O-R bonding arrangement. Another way to look at these two functional groups is to think of an alcohol as water (HOH) with one H replaced by an R and an ether as water with both hydrogens replaced by an R. The Rs could be the same or different. Similarly, ammonia (NH₃) is related to an amine by the replacement of one to three of the hydrogens with an R grouping. The chemistry of alcohols is defined by the reactions of the R-O (C-O) and O-H bonds, so an ether with an R-O-R structure has quite different chemistry and, hence, is a different functionality than an alcohol. Amine chemistry, however, is especially defined by the nitrogen’s unshared electron pair, so whether R or H is attached to nitrogen is less consequential to the chemistry. All three species (R₃N, R₂NH, and RNH₂) are amines.

Can you identify the five functional groups in NutraSweet? The answer appears later. It is the interaction of the functionalities with other chemicals or other functionalities that defines the "chemistry" of a compound. As we will see later, it is the interaction of functionalities that allows for the synthesis of NutraSweet from small chemical units.

Aspartame

In addition, the actual, energetically preferred three-dimensional shape of any molecule will depend upon the hybridization of atoms; the shapes associated with a given hybridization; the attraction/repulsion between and among functional groups, both within the molecule (intramolecular) and with other molecules (intermolecular); and the molecular movements (i.e., bond rotations) allowed for individual bonds. When molecules such as NutraSweet interact with a biological system, such as our taste buds, the resulting biochemistry depends greatly on the functionalities within the molecule and the molecule’s overall three-dimensionality. Many men and women have applied many years of effort to studying this kind of structure-activity relationships (SAR). This topic will be explained later.