Why is galactose a reducing sugar

Once you have a way of quickly and easily measuring the concentration of sugar, then you can determine how much insulin is needed to counteract it. Next question. What would be an easy, visual way to detect the presence of glucose?

This brings us via aldehydes to the topic of reducing sugars , since they are the basis of a historically important color-based test for blood glucose. During this process, the aldehyde is oxidized and the oxidizing agent is reduced.

Another way of framing this is to say that the aldehyde is the reducing agent in this process. The list of reagents that can be used to oxidize aldehydes to carboxylic acids is loooong.

Of these, a few methods stand out in providing a particularly clear visual indication that the reaction has proceeded to completion. The above tests were also a useful way of distinguishing aldehydes from ketones in the dark days before IR and NMR spectroscopy made this routine. But what if we want to determine the exact concentration of glucose in a solution of, say, urine or blood?

In this case, a slightly different formulation of Benedicts solution is used [ Note 2 ] which results in a colourless precipitate rather than a red color. A solution of the sample to be analyzed is added, via buret, to a flask containing a known amount of Benedicts solution until the blue color of the Cu II disappears. Benedicts assay was the method of choice for quantifying glucose for over 50 years.

One researcher recalls that all inductees into the U. So it will also give a positive test for other reducing sugars. 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.

In sucrose, there are glycosidic bonds between their anomeric carbons to retain the cyclic form of sucrose, avoiding its conversion into the form of an open chain with an aldehyde group. Contrarily, maltose and lactose, which are the reducing sugar, have a free anomeric carbon that can get converted into an open-chain form by forming a bond with the aldehyde group. Is starch a reducing sugar? It should be remembered here that starch is a non-reducing sugar as it does not have any reducing group present.

Any carbohydrate that is capable of causing the reduction of some other substances without being hydrolyzed first is the reducing sugar whereas sugars that do not possess a free ketone or an aldehyde group are called the non-reducing sugar. It is worth mentioning here that the non-reducing sugars never get oxidized. The most common example of non-reducing sugar is sucrose. The very important question that needs to be addressed here is this: why sucrose is the non-reducing sugar?

The reason is that in sucrose the two units of monosaccharides units are held together very tightly by the glycosidic linkages between the C-2 carbon of the fructose and the C-1 of glucose. Since the reducing groups of fructose and glucose are involved in the glycosidic bond formation, sucrose, therefore, is a non-reducing sugar.

The reducing sugar is also mentioned as the compounds such as sugar or an element, for instance, calcium that lose an electron to another chemical or biological species in the reactions stated as the oxidation-reduction often abbreviated as the redox reactions. Some of the most significant characteristics of reducing sugar have been summarized in the points below.

What is reduction? The loss of electrons during a reaction of a molecule is called oxidation while the gain of single or multiple electrons is called reduction. 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. The chair form of galactose follows the same pattern as that for glucose.

The anomeric carbon is the center of a hemiacetal functional group. A carbon that has both an ether oxygen and an alcohol group is a hemiacetal. In the chair structure this results in a horizontal projection Haworth - an upwards projection.

The Alpha position is defined as the -OH being on the opposite side of the ring as the C 6. In the chair and Haworth structure this results in a downward projection. The position of the -OH group on the carbon 4 is the only distinction between glucose and galactose.