Why lactose is reducing sugar

In other words, those sugars are also reducing sugars. So why does fructose give a positive test? Great question. Although fructose is a keto sugar, and ketones generally give a negative test with the Benedict, there is an exception. Likewise, some disaccharides such as maltose and lactose contain a hemiacetal.

They are also reducing sugars that give a positive Fehlings, Benedict, or Tollens test picture of lactose positive test is further below. We saw at the top of the post that hemiacetals are in equilibrium with an aldehyde or ketone. In contrast, acetals ketals are locked in place and can only be converted back to the aldehyde or ketone with aqueous acid. The poster child for a non-reducing sugar is sucrose , a. Sucrose is a disaccharide of glucose and fructose.

See if you can find a hemiacetal in its structure, below:. This is obtained by heating glucose in acidic methanol. Sugars are able to form long chains with each other in arrangements known as polysaccharides. Common examples of polysaccharides are starch, cellulose, and glycogen.

Hemiacetals are present, but only at the termini of the polymer. Starch, for example, generally has about individual units of glucose, but only one unit the terminus has a hemiacetal. Therefore these polysaccharides are not considered reducing sugars.

For example, starch gives a negative test see below. Make sense? Quiz yourself on whether the following sugars are reducing sugars or non-reducing sugars. But if you want to go further down the rabbit hole, I invite you to read further to learn about…. One thing about all three tests is that the active reagent is not particularly bench stable and has to be freshly prepared.

The purpose behind using the tartrate is that it coordinates to the copper II and helps prevent it from crashing out of solution. Once prepared, the substance to be analyzed is added, and the mixture is heated for a brief period. The ingredients are copper II sulphate, sodium carbonate note: hydroxide is also needed! Note: in the quantitative test, potassium thiocyanate is added, which results in a colourless white precipitate. The first three lines below describe the procedure. Silver nitrate is converted to silver hydroxide, which forms silver I oxide, Ag 2 O.

Then, addition of aqueous ammonia NH 3 results in formation of the silver-ammonia complex which is the active oxidant. The sample to be tested is then added to the freshly prepared active oxidant in a basic solution.

A positive test results in a beautiful mirror of silver metal being precipitated out on the reaction vessel. A variant of this procedure is used for the preparation of mirrors. Bottom line here is that adding base has the effect of increasing the concentration of the starting aldehyde. If I am wrong, please tell me leave a comment. One of the access points for the initiation of a single-electron transfer reaction is a carbon-metal bond, which can be achieved through base-promoted formation of an enolate.

That requires that the aldehyde have a proton on the alpha carbon i. Thus it would appear that the reaction needs to proceed through an enol. Hover here for a pop-up image or [ click for image of a hypothetical mechanism ]. Image sources: Benedicts solution. Tollens test. Note 1. The oxidation and reduction reactions also called redox reactions are the chemical reactions in which the oxidation number of the chemical species that are taking part in the reaction changes.

The redox processes are the wide range of reactions that include the majority of the chemical and biological processes taking part around us. Rusting and dissolution of the metals, browning of the fruits, fire reactions, respiration and the process of photosynthesis are all oxidation-reduction processes. The redox reactions involve the transfer of hydrogen, oxygen, or electrons where two very important characteristics are common in all three reactions.

Secondly , they always involve a net chemical change where new substituents are formed by the reaction of reactants. The examples of all three forms of chemical reaction have been elaborated on below. Two very important tests are often performed to identify the presence of reducing sugar. These tests are the Benedict test and the Fehling test. After around ten minutes the solution starts to change its color.

If the color changes to blue it means that there is no reducing sugar present. But if the color changes to green, yellow, orange, red, and then finally to dark red or brown color confirms the presence of reducing sugar in the food.

The chemical composition of the Benedict solution states that it is made of an anhydrous solution of sodium citrate, sodium carbonate, and copper II sulfate pentahydrate.

During its reaction with the reducing sugar, the blue copper sulfate in the solution is converted into red-brown copper sulfide. It is worth mentioning here that these tests only show the qualitative analysis of reducing sugar. In the Fehling test, the solution is warmed until the sample where the availability of reducing sugar has to be tested is homogeneously mixed in water after which the Fehling solution is added. If the reducing sugar is present the color of the solution will be changed to a red precipitate color resembling rust.

This test is specifically used for the identification of monosaccharides, especially ketoses and aldoses. These tests can be used in the laboratory for the determination of reducing sugar present in the urine which can be used to diagnose diabetes mellitus. It must be noted here that the reduction of aldehydes results in the formation of primary alcohols while the reduction of ketones gives secondary alcohols. The most common example of reducing sugar and monosaccharides is glucose.

In the human body, glucose is also referred to as blood sugar. It is essential for the proper functioning of brains and as a source of energy in various physical activities. Another reducing sugar is fructose, which is the sweetest of all monosaccharides. Galactose is another example of reducing sugar. It is a component of lactose available in many dairy products.

Moreover, the list of reducing sugars also includes maltose, arabinose, and glyceraldehyde. Carbohydrates, especially reducing sugar are the most abundant organic molecules that can be found in nature. They have a wide range of functions in biology. They provide a significant fraction of daily used dietary calories in most of the living organisms living on the earth. Also, their major role is to act as the storage of energy in living bodies.

Read: Glycolysis , Fermentation , and Aerobic respiration. Carbohydrates also serve as one of the cell membrane components and function primarily in mediating various intermolecular communications in the bodies of living organisms. Lastly, via Maillard reactions, carbohydrates are responsible for determining the crust color and the taste of the food such as coffee, bread, and roasted food items.

There are many uses of reducing sugar in our daily life activities. In medicines, the Fehling solution has been used as a test to detect diabetes in human blood. The relative measurement of the number of oxidizing agents reduced by the available glucose makes it easy to calculate the concentration of glucose present in the human blood or urine. Consequently, the blood galactose level is markedly elevated, and galactose is found in the urine.

An infant with galactosemia experiences a lack of appetite, weight loss, diarrhea, and jaundice. The disease may result in impaired liver function, cataracts, mental retardation, and even death.

If galactosemia is recognized in early infancy, its effects can be prevented by the exclusion of milk and all other sources of galactose from the diet. As a child with galactosemia grows older, he or she usually develops an alternate pathway for metabolizing galactose, so the need to restrict milk is not permanent. The incidence of galactosemia in the United States is 1 in every 65, newborn babies. Sucrose, probably the largest-selling pure organic compound in the world, is known as beet sugar , cane sugar , table sugar , or simply sugar.

The dark brown liquid that remains after the recrystallization of sugar is sold as molasses. This linkage gives sucrose certain properties that are quite different from those of maltose and lactose. Thus, sucrose is incapable of mutarotation and exists in only one form both in the solid state and in solution.

In addition, sucrose does not undergo reactions that are typical of aldehydes and ketones. Therefore, sucrose is a nonreducing sugar. The hydrolysis of sucrose in dilute acid or through the action of the enzyme sucrase also known as invertase gives an equimolar mixture of glucose and fructose. This mixture is referred to as invert sugar because it rotates plane-polarized light in the opposite direction than sucrose.

The hydrolysis reaction has several practical applications. Sucrose readily recrystallizes from a solution, but invert sugar has a much greater tendency to remain in solution. In the manufacture of jelly and candy and in the canning of fruit, the recrystallization of sugar is undesirable.

Therefore, conditions leading to the hydrolysis of sucrose are employed in these processes. Moreover, because fructose is sweeter than sucrose, the hydrolysis adds to the sweetening effect. Bees carry out this reaction when they make honey. The average American consumes more than lb of sucrose every year. About two-thirds of this amount is ingested in soft drinks, presweetened cereals, and other highly processed foods. The widespread use of sucrose is a contributing factor to obesity and tooth decay.

It is a reducing sugar that is found in sprouting grain. It is a reducing sugar that is found in milk. It is a nonreducing sugar that is found in sugar cane and sugar beets. Identify each disaccharide in Exercise 3 as a reducing or nonreducing sugar.