Intro to Carbohydrates: Videos & Practice Problems

Carbohydrates, also known as saccharides, are organic compounds made up of carbon, hydrogen, and oxygen, typically characterized by the presence of hydroxyl groups (–OH). They serve as the primary source of energy for living organisms. The simplest form of carbohydrates is monosaccharides, which have the general formula of C_nH_2nO, where n is a whole number equal to or greater than 3. This formula highlights the relationship between carbohydrates and water, as the term "hydrate" implies the addition of water to carbon.

Monosaccharides can exist in either straight-chain or ring forms. Two important types of monosaccharides are identified based on their functional groups: aldoses and ketoses. An aldose contains an aldehyde group, which is a carbonyl group (C=O) bonded to a hydrogen atom (H), while a ketose contains a ketone group, characterized by a carbonyl group bonded to two other carbon atoms. Thus, the naming convention reflects their structure: "aldose" for sugars with aldehyde groups and "ketose" for those with ketone groups.

Simple carbohydrates, also known as monosaccharides, are organic compounds that follow the general formula of CH₂O, where n represents the number of carbon atoms. This means that for every carbon atom, there are typically two hydrogen atoms and one oxygen atom, establishing a ratio where the number of hydrogens is double that of the carbons and oxygens.

When evaluating different chemical formulas to identify simple carbohydrates, it is essential to check this ratio. For instance, the formula C₆H₁₂O₆ exemplifies a simple carbohydrate, as it maintains the required ratio of hydrogens to carbons and oxygens. In this case, there are six carbon atoms, twelve hydrogen atoms, and six oxygen atoms, confirming that the number of hydrogens is indeed double that of the carbons and oxygens.

In contrast, other options may not satisfy this ratio. For example, if a formula has fewer or more hydrogens than twice the number of carbons or oxygens, it does not represent a simple carbohydrate. Therefore, the correct identification of a simple carbohydrate among the options provided is crucial, and in this case, the answer is b: C₆H₁₂O₆.

Monosaccharides, the simplest form of carbohydrates, can be named using both generic and specific nomenclature. To derive the generic name of a monosaccharide, follow a systematic approach based on its structure. The first step is to identify the type of carbonyl group present: if the sugar contains an aldehyde group, it is classified as an aldo sugar, while a ketone group designates it as a keto sugar. For instance, if a monosaccharide has a carbonyl group with a carbon on each side, it is categorized as a ketone sugar.

Next, count the number of carbon atoms in the molecule. Use numerical prefixes to denote the number of carbons: "tri" for three, "tetra" for four, "pent" for five, "hex" for six, and "hept" for seven. When numbering the carbon atoms, start from the end closest to the carbonyl carbon. For example, if there are five carbon atoms, the prefix "pent" is used.

Finally, since monosaccharides are carbohydrates, the name is completed by adding the suffix "ose." Therefore, if the monosaccharide is a ketone sugar with five carbons, its generic name would be "ketopentose." This name indicates that it is a keto sugar (due to the ketone group), contains five carbons (pent), and is a type of sugar (ose).