What are Carbohydrates?
Carbohydrates are macronutrients and one of the three basic sources of energy for our body. Carbohydrates are named for the chemical elements carbon, hydrogen, and oxygen that they contain. Carbohydrates, which comprise sugars, fibres, and starches, are necessary nutrients. They can be found in cereals, vegetables, fruits, and dairy products such as milk and cheese.
They are the basic dietary types that are essential for living a healthy lifestyle. The term is most commonly used in biochemistry as a synonym for saccharide, which refers to sugars, starch, and cellulose. Monosaccharides, disaccharides, oligosaccharides, and polysaccharides are the four chemical groups of saccharides. The smallest (lowest molecular weight) carbohydrates, monosaccharides and disaccharides, are generally referred to as sugars.
This page will study carbohydrate chemical reactions and glucose reactions in detail.
The Chemical Reactions of Carbohydrates
Given below are the reactions of carbohydrates
1. Alkylation
Through a simple \[SN^{2}\] reaction, the -OH groups in carbohydrates react quickly with alkylating agents to produce ethers.
Methylation can be accomplished with silver oxide \[Ag_{2}O\] and methyl iodide, CH3I.
Benzylation can be accomplished with the help of benzyl halides (Cl, Br, I), \[C_{6}H_{5}CH_{2}X\] , and a base like NaH or \[Ag_{2}O\].
Because the products act as protective groups for the reactive -OH functional group, these reactions are frequently utilised in carbohydrate chemistry.
With aqueous acid, the -OR group at the acetal core can be changed back to a -OH.
2. Acylation
Esters are formed when the -OH groups in carbohydrates combine with acylating substances such as acid halides or acid anhydrides.
The most prevalent esters utilised are ethanoate esters (also commonly known as acetate and represented as "Ac")
Because the products act as protective groups for the reactive -OH functional group, these reactions are frequently utilised in carbohydrate chemistry.
3. Reduction
Sodium borohydride, \[NaBH_{4}\], or catalytic hydrogenation (\[H_{2}\], Ni, EtOH/\[H_{2}O\]) can convert the C=O groups in open-chain carbohydrates to alcohols.
"Alditols" are the goods' names.
Because of its difficulty with polar solvents, LiAlH4 is rarely used.
Aldehydes, also known as aldoses, are converted into primary alcohols.
Ketones, sometimes known as ketones, are converted to secondary alcohols.
4. Oxidation
Carbohydrate C=O can be converted to carboxylic acids via oxidation.
Based on Benedicts or Fehling’s tests, these carbohydrates are classed as lowering sugars.
5. Hydrolysis
Aqueous acid is a common reagent.
The -OR bond outside the ring at the anomeric core is broken and replaced with a -OH.
A mixture of a-and b-isomers is usually formed.
It's important to note that this is only acetal or ketal hydrolysis.
This reaction is the inverse of the one used to make glycosides.
Polysaccharides can be broken down into their constituent monosaccharides via this process.
6. Glycoside Formation
Excess ROH, acid catalysts are common reagents.
The "ROH" can also come from another saccharide, resulting in the union of two saccharide units.
Glycosides are simply acetals or ketals.
Simple acetals and ketals are used in this chemistry.
Monosaccharide - Glucose
In this section, we will study the chemical reaction of carbohydrates (glucose).
With six carbon atoms and one aldehyde group, glucose is a simple sugar. The molecular formula for this monosaccharide is \[C_{6}H_{12}O_{6}\].
Dextrose is another name for it. It's called aldohexose because it has six carbon atoms and an aldehyde group. It can open in two different ways: as an open chain or as a ring structure. Animals produce it in their livers and kidneys. It can be present in plants' fruits as well as other sections of the plant. D-glucose is the most common kind of glucose found in nature. It can take the form of a solid or a liquid. It is water-soluble as well as acetic acid-soluble. It has no odour and is delicious to the taste. Andreas Marggraf, a German chemist, extracted glucose from raisins in 1747. The term glucose was invented by Jean Baptiste Dumas in 1838.
Properties of Glucose
Glucose Formation Reaction
Glucose is primarily obtained from two sources: starch and sucrose. Let's have a look at how to make glucose from these sources.
Glucose is made from starch hydrolysis on a large and commercial scale by boiling it with dilute \[H_{2}SO_{4}\]. The following is the chemical reaction:
\[(C_{6} H_{10} O_{5}) + n (H_{2}O) \rightarrow n (C_{6}H_{12}O_{6})\]
Also, another way of preparing glucose, with fructose as a joint or by-product, is to boil sucrose in dilute HCl or even \[H_{2}SO_{4}\] in an alcoholic solution. This chemical reaction is as follows
\[C_{12} H_{22} + H_{2} + O_{11} \rightarrow C_{6}H_{12}O_{6} + C_{6} H_{12} O_{6}\]
Chemical Reaction of Glucose
Aldohexose and dextrose are two names for glucose. Many bigger molecules, such as carbohydrates, starch, and cellulose, are monomers of it. This is the most common organic compound on the planet.
The structure seen above was assigned based on the following glucose reactions:
\[C_{6}H_{12}O_{6}\] is its molecular formula.
n-hexane is created when HI is heated for a long duration, indicating that all six carbon atoms are connected in a straight chain.
When hydrogen cyanide is added to glucose, it interacts with hydroxylamine and cyanohydrins to generate oxime. The existence of the carbonyl group in glucose can be confirmed using this reaction.
When glucose reacts with a mild oxidising agent such as bromine water, the glucose is converted to a carboxylic acid with six carbon atoms. The carbonyl group is present as an aldehyde group in this case. After acetylation of glucose with acetic acid, which produces glucose pentaacetate, the existence of the -OH group is established.
When glucose and gluconic acid are oxidised by nitric acid, they produce dicarboxylic acid and saccharic acid. This indicates the existence of primary alcohol.
Glucose to Fructose Chemical Reaction
The glycolysis cycle, which transforms glucose into pyruvate, includes the isomerization of glucose to fructose. This is accomplished by isomerizing aldehyde (hemiacetal) glucose to ketone (hemiacetal) fructose and forming a new phosphate ester.
Applications of Carbohydrates
Carbohydrates' primary job is to deliver energy and food to the body and nervous system.
Carbohydrates, which include sugars, starch, and fibre, are abundant in grains, fruits, and dairy products, and are known as one of the basic components of the diet.
Carbs are also known by other names such as starch, simple sugars, and complex carbohydrates.
It also plays a role in fat metabolism and helps to keep you out of ketosis.
Proteins are the primary source of energy, hence it prevents them from being broken down for energy.
Amylase is an enzyme that aids in the breakdown of starch into glucose, which is then converted into energy for metabolism.
Applications of Glucose
It's a drug that's used to treat hypoglycemia (low blood sugar).
It is given to patients who are unable to eat because it contains carbohydrate calories.
It's used to treat blood potassium levels that are too high (hyperkalemia)
It is utilised in the synthesis of substances as a precursor.
Health Effects of Carbohydrates
Low-carbohydrate diets may overlook the health benefits of high-quality carbs found in legumes and pulses, whole grains, fruits, and vegetables, such as increased dietary fibre consumption. Halitosis, headaches, and constipation are common side effects of the diet, and the potential negative consequences of carbohydrate-restricted diets remain under-researched in general, notably for probable hazards of osteoporosis and cancer incidence.
When overall calorie intake is lowered, carbohydrate-restricted diets can be just as successful as low-fat diets in assisting weight loss in the short run. "When calorie intake is held constant, the body-fat formation does not appear to be altered by even very noticeable variations in the quantity of fat vs carbohydrate in the diet," according to an Endocrine Society research statement. Effective weight loss or maintenance, in the long run, is dependent on calorie restriction, not the macronutrient ratio in a diet.
Did You Know?
In chemistry, racemization is the transformation of an optically active compound into a racemic (optically inactive) form using heat or a chemical process. Racemic mixtures (which contain equal amounts of (+) and (-) forms) are formed when half of an optically active material transforms into its mirror counterpart (enantiomer). The resulting sample is termed as a racemic mixture or a racemate if the racemization produces a mixture in which the D and L enantiomers are present in equal amounts. Racemization can occur via a variety of methods, and it's especially important in pharmacology because various enantiomers might have distinct medicinal effects.
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FAQs on Chemical Reactions of Carbohydrates
1. What is the basic chemical definition of a carbohydrate?
Chemically, carbohydrates are defined as polyhydroxy aldehydes or ketones or substances that produce these on hydrolysis. This means they are molecules containing multiple hydroxyl (-OH) groups and either an aldehyde (-CHO) or a ketone (C=O) functional group. The general formula, Cn(H₂O)n, is a common but not universal rule.
2. What are the main types of chemical reactions that carbohydrates undergo?
Carbohydrates like glucose participate in several key chemical reactions based on their functional groups. The most common reactions include:
- Oxidation: Forming acids when reacting with oxidising agents.
- Reduction: Forming polyhydric alcohols (like sorbitol) when reacting with reducing agents.
- Glycoside Formation: Reacting with an alcohol, which is how monosaccharides link together to form disaccharides and polysaccharides.
- Esterification: Reacting with acids to form esters.
- Fermentation: The biochemical breakdown by enzymes, often from yeast, into ethanol and carbon dioxide.
3. Why are some sugars called 'reducing' and others 'non-reducing'?
The difference is based on their chemical structure. A reducing sugar has a free aldehyde or ketone group, or a hemiacetal group that can open up to form one. This allows it to reduce other compounds, like in Fehling's or Tollens' test. Glucose and fructose are examples. A non-reducing sugar, like sucrose, has its reactive groups locked in a glycosidic bond, making it unable to act as a reducing agent.
4. How does the chemical reaction of digestion break down carbohydrates?
Carbohydrate digestion is primarily a process of hydrolysis, where water and enzymes break down large molecules into smaller ones. In the mouth, the enzyme amylase begins breaking down starch into smaller sugars. In the small intestine, other enzymes like maltase, sucrase, and lactase complete the process, breaking disaccharides into monosaccharides (like glucose) that can be absorbed by the body.
5. How can you perform a chemical test to confirm the presence of a reducing sugar?
A simple and common method is Fehling's test. To perform it, you add Fehling's solution (which is blue due to copper(II) ions) to the food sample and gently heat it. If a reducing sugar is present, it will reduce the copper(II) ions to copper(I) oxide, causing a visible colour change from blue to a red or brown precipitate. This positive result confirms the presence of a reducing sugar.
6. What is the chemical difference between the oxidation of glucose with a mild agent versus a strong one?
The product of oxidation depends on the strength of the oxidising agent used.
- Using a mild oxidising agent like bromine water only oxidises the aldehyde group of glucose, resulting in gluconic acid.
- Using a strong oxidising agent like nitric acid oxidises both the aldehyde group and the terminal primary alcohol group, resulting in saccharic acid.
7. Why does starch not taste sweet, even though it's made of sugar units?
Sweetness is perceived when a molecule fits into specific taste receptors on the tongue. Simple sugars like glucose have a size and shape that fit these receptors perfectly. Starch, however, is a polysaccharide—a very large polymer made of thousands of glucose units linked together. This massive, complex structure is too large to fit into the sweetness receptors, so it doesn't trigger a sweet taste.
8. How do the chemical reactions of monosaccharides, like glucose, differ from those of polysaccharides, like starch?
The main difference is reactivity. Monosaccharides like glucose are small and have accessible functional groups, so they readily undergo reactions like oxidation (as in Fehling's test). Polysaccharides like starch are much less reactive in this way because most of their reactive aldehyde groups are tied up in glycosidic bonds linking the many glucose units together. The most significant reaction for starch is hydrolysis, where it breaks back down into its constituent monosaccharides.
