Carbohydrates

What Are Carbohydrates?

Carbohydrates form one of the largest groups of naturally occurring organic compounds, and plants are their primary source. Everyday examples include cane sugar, glucose and starch.

A great many carbohydrates fit a general formula $\text{C}_x(\text{H}_2\text{O})_y$ — historically this made them look like "hydrates of carbon," which is where the name comes from. Glucose, $\text{C}_6\text{H}_{12}\text{O}_6$, is a clean example: it can be rewritten as $\text{C}_6(\text{H}_2\text{O})_6$, matching the pattern exactly.

That formula, however, is only a coincidence of counting atoms — it is not a reliable definition, and the book is careful to show both kinds of failure:

• A compound can fit the formula and still not be a carbohydrate. Acetic acid, $\text{CH}_3\text{COOH}$, can be written as $\text{C}_2(\text{H}_2\text{O})_2$, yet it is a simple carboxylic acid, not a carbohydrate.
• A genuine carbohydrate can fail to fit the formula. Rhamnose, $\text{C}_6\text{H}_{12}\text{O}_5$, is a real carbohydrate but does not reduce to the $\text{C}_x(\text{H}_2\text{O})_y$ pattern.

Because the formula alone breaks down both ways, carbohydrates have to be defined by what they actually are chemically, based on the functional groups repeatedly found in them:

Carbohydrates that taste sweet are also called sugars, and the whole family carries the name saccharides, from the Greek sakcharon, meaning sugar. The sugar used at home is sucrose (cane sugar); the sugar found in milk is lactose. …