Hydrolysis of Carbohydrates

Carbohydrates like disaccharides and polysaccharides can be broken down into monosaccharides.
In the lab we use acid hydrolysis, but in your body you use enzymes to do this.
AUS-e-TUTE has just added a new tutorial, game, test and exam to help you understand these chemical reactions. Members should log-in to access these new resources (under Biochemical reactions).
If you are not an AUS-e-TUTE member you can access a "free-to-view" tutorial at https://www.ausetute.com.au/hydrolysiscarbs.html

Sucralose and Acesulfame potassium

Artificial sweeteners,  especially aspartame, have hit the news headlines once again with PepsiCo's decision to start selling Diet Pepsi without aspartame from August 2015 in the USA. The aspartame is to be replaced by a blend of sucralose and acesulfame potassium.

Sucralose was discovered in 1976 when Shashikant Phadnis at Queen Elizabeth College was asked to "test" a chlorinated sugar compound but he thought he'd been asked to "taste" it, so he did, and found it be very sweet! Sucralose, shown on the right, is synthesised from sucrose in a number of steps in which 3 of sucrose's hydroxyl groups are substituted for chlorine atoms.
Although sucralose is about 300 times sweeter than sucrose, it is not broken down during digestion and therefore does not contribute to ingested calories (kilojoules).
Splenda is a brand name for a common sucralose-based sweetener.


Acesulfame potassium was accidentally discovered in 1967 by German chemist Karl Clauss, who found it to be sweeter than sucrose. It is not as sweet as sucralose, however, and is often used in combination with other artificial sweeteners such as aspartame or sucralose. It has the structural formula shown below:

Acesulfame potassium is currently sold under the brand names Sunette, Sweet One and Sweet 'n Safe and is found in many "sugar-free" foods such as chewing gum, jelly (Jell-O), even in alcoholic drinks.

Reference:
http://www.smh.com.au/business/retail/diet-pepsi-dumps-aspartame-as-consumer-backlash-hurts-sales-20150425-1msz1b.html

Further Reading:

• http://www.ausetute.com.au/sugars.html 
• http://www.ausetute.com.au/proteins.html
• http://www.ausetute.com.au/fungroup.html
• http://www.ausetute.com.au/intermof.html
• http://www.ausetute.com.au/skeletal.html
• http://www.ausetute.com.au/structural2D.html
• http://www.ausetute.com.au/condensedsf.html
• http://www.ausetute.com.au/molecularformula.html

Suggested Study Questions:

1. What is the molecular formula for:
   • sucralose
   • sucrose
   • acesulfame potassium
2. Draw the structural formula for sucrose and circle the hydroxyl functional groups.
3. Draw the structural formula for sucralose and
   • circle the hydroxyl functional groups in red
   • circle the halogen functional groups in green
   • circle any other functional groups blue and name them 
4. What class of compounds does sucrose belong to?
5. Draw the structural formula for acesulfame potassium and identify the functional groups.
6. Is it appropriate to call acesulfame potassium a potassium salt? Explain your answer.
7. Aspartame is the methyl ester of a didpeptide. What functional groups do you expect the aspartame molecule to have? Explain your answer. 
8. Do you expect sucralose and acesulfame potassium to be soluble in water? Explain your answer.

Sulfuric Acid and Sugar

November 28, 2014, The Canberra Times reported that, "Students were looking on as a teacher conducted an experiment involving sugar and sulfuric acid inside a cabinet when the glass container holding the acid exploded."
The experiment referred to is probably similar to the one shown in the YouTube video below in which concentrated sulfuric acid from a container is poured over sugar in a beaker.

Safety Notes

Sulfuric acid will cause permanent damage if it comes into contact with the eyes or skin. Concentrated solutions of sulfuric acid are extremely corrosive. When sulfuric acid is dissolved in water enough heat is released to make water boil! Carbon monoxide and carbon dioxide are both toxic gases. Sulfur dioxide gas is toxic in high concentration and is a severe respiratory irritant at lower concentration.Some people, especially those prone to asthma, may be especially sensitive to sulfur dioxide. In the presence of moisture, sulfur dioxide forms an acidic, corrosive solution, which in contact with the skin or eyes may lead to burns.

You will notice that the reaction seems to proceed slowly at first. The reaction mixture turns yellow as the reaction begins. This reaction releases heat, it is said to be an exothermic reaction. The heat produced by the reaction then speeds up the rate of further reactions, and, in the video, this is also accelerated by stirring the mixture.

Table sugar is made up sucrose, molecular formula C₁₂H₂₂O₁₁ and structural formula as shown below:

The reaction between sucrose and sulfuric acid in which solid carbon, water vapour and heat are produced is known as a dehydration reaction  :
C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(g)     ΔH = -918.9 kJ mol^-1 Since the enthalpy change for this reaction is negative, the reaction is exothermic, the reaction gives off heat.
Sulfuric acid molecules have a great affinity for water, that is, sulfuric acid will readily and spontaneously dissolve in water. The water produced by the dehydration of sucrose will then be used to dilute the sulfuric acid that is present. This reaction is also exothermic.
H₂SO₄(l) → H₂SO₄(aq)     ΔH = - kJ mol^-1 The heat produced by this reaction also speeds up the rate of the dehydration reaction and subsequent dilution reactions.
Solid carbon is black, so the "black snake" is just carbon.
But what causes the carbon to "rise up" out of the beaker? This must be the result of evolving gases forcing their way through the mixture as the reaction proceeds. The gases that have been identified as products of this reaction  are:

• carbon monoxide (CO) 66% of the dry gas volume (ie, water has been condensed out)
• carbon dioxide (CO₂) 17% of the dry gas volume
• sulfur dioxide (SO₂) 17% of the dry gas volume

Since sulfuric acid does not oxidize carbon, it is most unlikely that the carbon monoxide and carbon dioxide gases are the result of a reaction between the black snake carbon and sulfuric acid. It is much more reasonable to assume that some of the sucrose undergoes dehydration by the sulfuric acid while some this sucrose (and/or some of the resulting intermediate organic products) is oxidized by the sulfuric acid to produce carbon monoxide gas and carbon dioxide gas. During this process, the sulfuric acid will itself be reduced, resulting in the formation of sulfur dioxide gas.

Reference:
http://www.canberratimes.com.au/act-news/students-treated-after-spill-in-burgmann-anglican-school-sciencbe-laboratory-20141128-11vzad.html

Further Reading:
http://ausetute.com.au/safety.html
http://www.ausetute.com.au/mmcalcul.html  
http://www.ausetute.com.au/moledefs.html
http://www.ausetute.com.au/massmole.html
http://www.ausetute.com.au/concsols.html 
http://www.ausetute.com.au/molarvol.html
http://www.ausetute.com.au/molreact.html

Suggested Study Questions:

1.  Describe two hazards in the YouTube Video.
2.  Describe the safety precautions you would take to minimize the risk of the hazards identified in question 1 above.  
3.  Why do you think the concentrated sulfuric acid is added to the sugar rather than adding the sugar to the container of sulfuric acid?
4.  In a typical experiment, 25 mL of 18 mol L^-1 sulfuric acid is added to 50 g of granulated sugar (sucrose). Calculate the amount in moles of :
   • sucrose used
   • sulfuric acid used
5. Calculate the mass of carbon that could be produced in the typical experiment given in question 4.
6. What assumptions have you made in order to calculate the mass of carbon in question 5?
7. Assume that all 50 g of the sucrose is now oxidized at 25^oC to produce carbon dioxide gas and liquid water. What is the maximum volume of carbon dioxide gas, in litres, that could be produced?
8.  Predict what you think might happen if a 50 g of granulated sugar were quickly added to 25 mL of concentrated sulfuric acid in a 100 mL conical flask that was being swirled continuously.
9. Sucrose is a disaccharide, made up of the monosaccharide glucose and the monosaccharide fructose. Starch and cellulose are both polysaccharides, that is, they are made of repeating glucose monomer units. Predict what would happen if you spilled concentrated sulfuric acid on:
   • a paper cup (cellulose)
   • a piece of raw potato (starch)
   • a cotton shirt (cellulose)
10. Design an experiment that could safely be performed in the laboratory to test your predictions in question 9.
11. Do you think that concentrated sulfuric acid will react with the monosaccharide glucose? Explain your answer using a chemical equation.
12.  Design an experiment that could safely be performed in the laboratory to test your prediction in question 11.

Cyclodextrin

Researchers at Temple University's Water and Environmental Technology (WET) Center have been investigating the use of cyclodextrins in the treatment of waste water.
Cyclodextrins are made up of sugar molecules, bonded together to form a ring. An example of the structure of a cyclodextrin, alpha-cyclodextrin which is a 6-membered sugar ring, is shown below:

Cyclodextrins can be used in water treatment because they can bond to substances such as toxic organic compounds or heavy metals and hold them inside the ring structure. Owing to the many polar OH functional groups, cyclodextrins are water soluble, but the researchers have produced insoluble materials by coating a thin layer of cyclodextrin on sand, glass, silica and filter paper. Doing this allows the absorbent material to be collected, washed and re-used.

Reference:
Temple University (2013, September 24). New adsorbent is more effective and environmentally friendly for treating wastewater. ScienceDaily. Retrieved September 30, 2013, from http://www.sciencedaily.com­ /releases/2013/09/130924174152.htm

Further Reading
Carbohydrates: http://ausetute.com.au/members/sugars.html 
Functional Groups: http://ausetute.com.au/fungroup.html
Solubility: http://ausetute.com.au/intermof.html

Zeolites: http://ausetute.com.au/members/zeolites.html
Polymers: http://ausetute.com.au/members/polymers.html 

Suggested Study Questions:

1. What does the "cyclo" part of the name in cyclodextrin refer to?
2. Use the structure above to determine the molecular formula for alpha-cyclodextrin.
3. Use the molecular formula to determine the empirical formula for alpha-cyclodextrin.
4. Calculate the percentage composition of alpha-cyclodextrin.
5. Draw the structure of alpha-cyclodextrin and locate and identify the functional groups.
6. Use a diagram to explain how alpha-cyclodextrin dissolves in water.
7. Use a diagram to explain how alpha-cyclodextrin could be used to remove lead ions from water.
8. Draw the structure of alpha-cyclodextrin an clearly identify all 6 of the sugar molecules making up the structure. 
9. Activated carbon is commonly used to remove contaminants from water. What benefits do you think there might be in using cyclodextrin-derived absorbant materials instead of activated carbon? 

Sweet Chemistry

Table sugar, or sucrose, C₁₂H₂₂O₁₁, is a disaccharide of glucose and fructose.
When a pure substance melts, it melts at a consistent, repeatable temperature, and retains its chemical identity when moving from the solid state to the liquid state.
The melting point of sucrose is usually given as a range, such as 160^oC to 180^oC, not as a well-defined melting point. This is because sucrose decomposes as it is heated. University of Illinois scientists have shown that glucose and fructose also decompose on heating.

If no oxygen were present, then sucrose would decompose into carbon and water. Caramelized sugar is due to the process of sucrose decomposing, and the brown colour is due to the presence of carbon in the mixture.

Carbon and water are also the products of the exothermic reaction between sucrose and sulfuric acid. This is an example of a dehydration reaction in which water is eliminated from the organic molecule during the reaction. The sulfuric acid then oxidizes the carbon produced as a result of the dehydration reaction, producing both carbon dioxide and toxic sulfur dioxide gases.

If potassium nitrate is used to oxidize sucrose, the products are potassium carbonate, water, nitrogen gas and carbon dioxide gas.

48 KNO₃ + 5 C₁₂H₂₂O₁₁ → 24 K₂CO₃ + 24 N₂ + 55 H₂O + 36 CO₂

This reaction can be used as the basis for a propellant for model rockets, such as in the Sugar Shot to Space program.

Reference
Joo Won Lee, Leonard C. Thomas, Shelly J. Schmidt. Can the Thermodynamic Melting Temperature of Sucrose, Glucose, and Fructose Be Measured Using Rapid-Scanning Differential Scanning Calorimetry (DSC)? Journal of Agricultural and Food Chemistry, 2011; 59 (7): 3306 DOI: 10.1021/jf104852u

Further Reading
Carbohydrates
Pure Substances and Mixtures

Study Questions

1. Draw a structural formula for both glucose and fructose.
2. Give the molecular formula for both glucose and fructose.
   
3. Explain why glucose and fructose are considered to be monosaccharides.
4. Explain what is meant by the term disaccharide.
5. Draw the structural formula for a disaccharide composed of:
   • glucose
   • fructose
   • glucose and fructose
6. Draw a sketch of the temperature vs time graph you expect to see if a sample of sucrose was:
   • a pure sample
   • a mixture of different sugar
7. Write a balanced chemical equation to represent the decomposition of sucrose into carbon and water in the absence of oxygen.
   
8. Write a balanced chemical equation for the dehydration of sucrose using sulfuric acid.
9. What is the difference between the decomposition of sucrose reaction and the dehydration of sucrose sucrose? Why is the reaction with sulfuric acid not considered to be a decomposition reaction?
   
   
   

Tasty Chemistry

Taste refers to the ability to detect the flavour of substances. We receive tastes through sensory organs called taste buds which are concentrated on the upper surface of the tongue.
Among the 50 or so cells in each taste bud there are cells responding to each of the five tastes:

• sweetness
• bitterness
• sourness
• saltiness
• umami-ness or savoriness
  

Sweetness is often associated with foods rich in simple carbohydrates such as glucose and sucrose, but many compounds taste sweet. Examples include the amino acids alanine, glycine and serine as well as the glycosides glycyrrhizin (found in licorice root) and stevioside (from the Stevia rebaudiana shrub). Even some inorganic compounds, such as beryllium chloride and lead acetate, taste sweet.

Bitterness is perceived by many people to be unpleasant. It helps prevent us ingesting toxic substances. A bitterant is the chemical that makes a substance taste bitter. Examples of bitterants are sucrose octaacetate which is used as an inert ingredient in pesticides and herbicides, and, brucine which is a bitter alkaloid closely related to strychnine that is found naturally in a number of plant species.

Sourness is the sensation evoked by substances that are acidic such as lemons and pickles. The acids we ingest release protons which enter the cell and cause a direct, detectable, electronic response.

Saltiness is the taste produced by the presence of alkali metal cations such as Na⁺ and K⁺. The less sodium-like the ion is, the less salty the sensation will be, eg, Rb⁺ and Cs⁺ ions are larger than Na⁺ ions so they do not taste as salty.

The umami taste is due to the detection of the carboxylate anion of glutamic acid, a naturally occurring amino acid found in meat, cheese, and other protein-rich foods. Glutamates, the salts of glutamic acid, easily ionize resulting in the same carboxylate anions and therefore producing the same umami taste. As a consequence, glutamates are often used as flavour enhancers, the most common of which is monosodium glutamate (MSG).

Further Reading
Carbohydrates (sugars)
Amino Acids
Properties of Acids and Bases

Study Questions

1. Name the 3 elements common to all carbohydrates.
2. What is the structural difference between molecules classified as monosaccharides and those that are classed as disaccharides or polysaccharides?
   
3. Is glucose an example of a monosaccharide, a disaccharide or a polysaccharide?
4. Is sucrose an example of a monosaccharide, a disaccharide or a polysaccharide?
5. What elements are common to all amino acids?
6. What functional group or groups must a molecule contain in order for it be classified as an amino acid?
7. Draw the structures for glycine, alanine and serine. Identify the functional groups present in each molecule.
8. A number of amino acids are said to taste sweet, but acids generally are said to taste sour. Can you explain these apparently contradictory statements?