Chemical nature of carbohydrates
The term "carbohydrates" arose because the empirical formulas of many of them look like a combination of carbon and water (for example, glucose — $6C \cdot 6H_2O$, sucrose — $12C \cdot 11H_2O$). In reality, the structure of these substances is different.
The basis of the molecule is an unbranched chain of carbon atoms. Hydroxyl groups (-OH) are attached to them. One of the carbon atoms forms a double bond with oxygen, forming a carbonyl group. It is the position of this group that largely determines the chemical properties of the compound.
Classification of monosaccharides
Simple carbohydrates are classified according to two main criteria:
- By type of carbonyl group:
- Aldoses contain an aldehyde group (-CHO) at the end of the carbon chain. They combine the properties of polyhydric alcohols and aldehydes. A typical representative is glucose.
- Ketoses have a ketone group (-CO-) inside the chain. They exhibit the properties of polyhydric alcohols and ketones. An example is fructose.
- By carbon chain length:
- Pentoses contain 5 carbon atoms.
- Hexoses have 6 carbon atoms (most common in nature, including glucose and fructose).
Thus, according to the full classification, glucose is an aldohexose.
Stereoisomerism of carbohydrates
The complexity of carbohydrate structure is largely due to the presence in their molecules of chiral centers — asymmetric carbon atoms bonded to four different substituents.
In aldohexoses, such centers are atoms C2, C3, C4, and C5, and in ketohexoses — C3, C4, and C5. This gives rise to the phenomenon of enantiomerism. The total number of stereoisomers depends on the number of asymmetric atoms ($n$) and is equal to $2^n$. For glucose ($n=4$), there are 16, yielding 8 pairs of enantiomers.
Isomers are divided into:
- Enantiomers: molecules that are complete mirror images of each other (differing in configuration at all chiral centers). Example: D-glucose and L-glucose.
- Diastereomers: isomers that differ in configuration at only some asymmetric atoms. These are completely different substances with their own names and properties (for example, galactose and mannose relative to glucose).
D- and L-series
The assignment of a carbohydrate to the D- or L-stereochemical series is determined by the configuration of the last asymmetric atom in the chain. For hexoses, this is the C5 atom.
In biological systems, the vast majority of carbohydrates, including glucose synthesized by plants, belong to the D-series. L-isomers are much rarer in nature.
Briefly: answers to topical questions
Which pentoses are part of nucleic acids?
Nucleic acids contain the most important aldopentoses — ribose and deoxyribose. In the polymer chains of DNA and RNA, they exist in cyclic $\beta$-forms.
- Ribose contains hydroxyl groups (-OH) at all carbon atoms of the ring.
- Deoxyribose differs in that it lacks a hydroxyl group (-OH) at the second carbon atom ($^2C$).
In what cyclic forms do monosaccharides exist in aqueous solution?
Sources indicate that fructose in solutions exists in the form of furanose rings — in the $\alpha$-D-fructose and $\beta$-D-fructose forms. For D-glucose, the transition from a linear form to a cyclic form with the formation of a six-membered pyranose ring is described.
What qualitative reactions are used to prove the presence of an aldehyde group in aldoses?
The presence of an aldehyde group is proven using qualitative oxidation reactions with weak oxidizing agents.
- Silver mirror test — interaction with an ammoniacal solution of silver oxide (diamminesilver(I) complex — $[Ag(NH_3)_2]OH$). As a result, a layer of metallic silver settles on the cleanly washed walls of the test tube.
- Reaction with copper(II) hydroxide — using $Cu(OH)_2$ as a weak oxidizing agent.
Which carbohydrates do not correspond to the general empirical formula Cn(H2O)m?
The general empirical formula of carbohydrates $C_n(H_2O)_m$ does not correspond to D-deoxyribose ($C_5H_{10}O_4$). The $^2C$ atom lacks a hydroxyl group (-OH). Despite the discrepancy in chemical composition with the general formula, deoxyribose is structurally a typical carbohydrate.
Class and exam questions
What is the difference between aldoses and ketoses?
In aldoses, the carbonyl group is located at the end of the carbon chain (aldehyde group), whereas in ketoses, it is located within the chain (ketone group).
Why are D-galactose and D-mannose not enantiomers of glucose?
They differ from glucose in configuration at only one asymmetric atom (galactose at C4, mannose at C2), which is why they are diastereomers (epimers). Enantiomers must differ in configuration at all asymmetric centers.
How to calculate the number of aldohexose isomers?
One must use the formula $2^n$, where $n$ is the number of asymmetric carbon atoms. Aldohexoses have 4 of them, so the number of isomers is 16.
From what simple substances can carbohydrates be synthesized?
Historically, A.M. Butlerov proved the possibility of synthesizing carbohydrates from formaldehyde in the presence of calcium hydroxide.
What else to study on this topic
- Biological role of sucrose and starch in the food chain
- Structural features of fructose as a ketohexose
- Detailed analysis of the nomenclature of glucose enantiomers
- Specific chemical properties of hydroxyl and carbonyl groups
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