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KWAME NKRUMAH UNIVERSITY OF SCIENCE ANDTECHNOLOGY

DEPARTMENT OF CHEMISTRY

YEAR TWO (CHEM 269)

PRACTICAL CHEMISTRY III

TITLE: INVERSION OF SUCROSE

NAME: OPOKU ERNEST

EMAIL: ernopoku@gmail.com

DATE: 14TH OCTOBER, 2013

TITLE: INVERSION OF SUCROSE

AIMS AND OBJECTIVES:

1. To know how to use the polarimeter to measure the angle of rotation of a given sample ofsolution.

2. To determine the reaction rate constant of cane sugar (sucrose) cleavage3. To determine the angle of rotation of light in the hydrolysis of sucrose4. Determine graphically the activation energy of hydrolysis from the determined

reaction rate constants.

INTRODUCTION

Sucrose is a disaccharide of glucose and fructose. By addition of an acid, the sucrose cleaves

into its components. The activation energy of the reaction can be determined from the reaction

rate constants of this cleavage reaction at different temperatures. To determine the reaction rate

constant, we make use of the differences in the optical rotation properties between sucrose and

its components (differing in terms of magnitude and, in some cases, in their signs). After

equilibration, the products of the splitting reaction show a different sign in the optical rotation

than the starting molecule, which led to the name "invert sugar".

Mechanism of sucrose splitting:

Sucrose consists of one molecule of D-glucose and one molecule of D-fructose. Both molecules

are acetal-like linked.

Saccharose (o-D-glucopyranosyl-β-D-fructofuranoside) or sucrose.

As a matter of fact, no cleavage occurs initially when sucrose is dissolved in water. However,

the cleavage can be catalyzed (the cleavage can be measure in a time scale of a few hours) by

adding an acid. By the addition of an acid, the bridging oxygen atom is protonated. This increases

the partial positive charge of the adjacent carbon atoms where the lone pair orbitals of oxygen in

the water molecules can attack. Consequently, the cleavage of the disaccharide and

subsequent deprotonation takes place:

While the protonation proceeds too fast to be measurable with our investigation method, the

splitting process is tracked by polarimetry.

Sucrose and invert sugar / stereochemistryGlucose and fructose are formed during the cleavage of sucrose (specific rotation [αD] = +66.5°).

In aqueous solution, glucose exists in three forms: the β-D-glucose, α-D-glucose and the open-

chain form.

When both pure α-D-glucose and pure β-D-glucose are dissolved in water, one can observe a

continuous change in optical rotation, until a constant final value is eventually reached. This so-

called "mutarotation" takes place with an immeasurably rapid rate upon addition of

small amounts of alkali. The conversion of α-and β-form into each other proceeds via the above-

mentioned open form, however, only about 0.024 mole-percent is present in solution at pH = 7 (see

structure below).

Saccharose, its cleavage products and the present forms in the equilibrium.For these reasons, the sucrose cleavage will, in addition to the direct splitting products α-D-

glucose and β-D-fructose, also result in β-D-glucose as well as the open form of glucose,

depending on the equilibrium of the mutarotation. In addition to the glucose molecules, there are

also two possible conformers (equatorial and axial form). The adjustment of all related

equilibria is faster than the cleavage reaction of sucrose itself.

The specific rotation values for D-glucose and D-fructose are [α] D = +52.5° and [α] D = -92°,

respectively.

Since the levorotatory fructose has a greater molar rotation than the dextrorotatory glucose, the

resulting mixture of glucose and fructose is slightly levorotatory.

The phenomenon of optical activity and its experimental measurement

Isomeric forms of molecules that only differ in their behaviour regarding linearly polarized light

(identical chemical and physical properties except for their ability to rotate the plane of linearly-

polarized light by equal amounts in opposite directions) are called enantiomers. The two

enantiomeric forms of molecules behave according to their symmetry as an object to its mirror

image. All kinds of isomerism of molecules, which do not fall within the definition of

enantiomers, are called diastereomer. If a type of molecules is an enantiomer to another one,

it can not simultaneously be diastereomeric to that type of molecules and vice versa.

Using the present experimental set-up, the optical rotation is only measured at a fixed wavelength,

namely that of the Na-D-line. In general, polarimetry is a spectroscopic method: the

optical rotation of optically active compounds changes over the wavelengths. Actually, to describe

the molecular conformation clearly, the chemist can use different nomenclature principles. Find

out about the D, L- and R-, S-notation! Keep in mind that neither the absolute nor relative

molecular conformation has anything to do about the occurring rotation. Therefore, it is essentially

necessary to denote the occurring rotation in the unified name. (By d, l-, or nowadays by +, -)

The measurement of optical activity is performed by means of a polarimeter. Such a device

basically consists of a light source with a monochromator, a polarizer, which produces

the linearly polarized light (how does the natural light differ from linearly polarized light?) and an

analyzer, which determines whether the plane of polarization is rotated, and if so, at what angle.

The inversion of sucrose

Let’s represent S: Sucrose, F: fructose, G:glucose.

The actual inversion reaction (1)

is catalyzed by protons. The actual cleavage reaction is in front of the equilibrium (2):

This equilibrium is relatively faster compared to the overall reaction (1).

The equilibrium constant of this equilibrium is given by:

Further reaction proceeds according to the scheme:

The water is, if working in (sufficiently) diluted solutions, available in large excess, the H2O

concentration may therefore be regarded as practically constant and is thus included in the

reaction rate constant k'. This results in c = cs:

The H+ ions are only catalytically active, hence, the concentration cH+ can also be considered as a

constant during the reaction. Thus we obtain: (k = k'·K·cH+)

(3)

Therefore, the cleavage proceeds in a pseudo-first reaction order.

Do you know any reactions with another reaction orders? Are there any zero-order reactions? The

order of a reaction does not coincide with the reaction stoichiometry. Try to understand the

differences between these values. Why is it so important for a chemist to get detailed information

about the kinetics of a reaction? What conclusions can be drawn from the reaction kinetics?

By integrating equation (3) (with c = c0 at time t = t0)

(4)

Since optical rotation is directly proportional to the concentration of optically active

molecular species (at least in the low concentration regime), a change in optical rotation can be

exploited to monitor the reaction. The variation of the rotation angle is proportional to the

change in concentration of sucrose. The rotation angle α of a chiral substance depends on the

temperature, the wavelength, the length d that the light passes through the substance, and the

concentration c of the chiral substance:

α = [α] d c

where [α] means the specific rotation.

During the inversion reaction, the concentration of sucrose (= c) decreases and the

concentration of invert sugar (= c ') increases. At a given time t, the angle of rotation is

obtained by:

α = [α] dc + [α '] d c' = [α] dc + [α '] d (C-c) (5)

(C: weight concentration of sucrose)

[α], [α']: specific rotation of sucrose or of invert sugar. Thus, with α0 being the angle of rotation

at time t0 :

(6)

From equation (5), the following equation is found for t →∞ (i.e., c → 0)

[α'] d C = α∞

Thus, we obtain from (4) and (6):

(7)Using this relationship, the reaction rate constant k can be calculated from the readings of the

rotation angle during the reaction. When transforming equation (4) into an exponential form, we

recognize that the concentration decreases exponentially with time. This means that the frequency

of measurements must be higher in the beginning than later in chronological order.

Are there other methods to determine reaction rate constants? Do you know any special procedures

for the determination of reaction rate constants for very rapid reactions? Does this optical method

have a special advantage ?

The reaction rate constants k determined by this method strongly depend on the temperature.

Considerations such as Arrhenius equation is an exponential dependence:

(8)

(A: prefactor, T: absolute temperature, ĒA molar activation energy)

The reaction rate constant is related to the energy of the reacting molecules according to the

Arrhenius law. The reaction can only occur for those molecules with an energy that is at least as

large as is the activation energy. The prefactor A essentially includes geometric characteristics of

molecule collision.

The qualitative form of the Arrhenius expression suggests a relation to the Boltzmann distribution.

What is this?

From equation (8) we obtain (with k0: rate constant at temperature T0):

So k is determined at different temperatures, one can calculate EA by lg(k/k0) plotting against l/T.

CHEMICALS AND EQUIPMENT

1. Cane sugar (Sucrose)2. Distilled water3. Hydrochloric acid of specific gravity 1.18g/cm3

4. Polarimeter ( Bellingham & Stanley limited, BSR96001, made in England)5. Electronic balance6. Beaker7. Conical flask8. 100ml measuring cylinder9. 50ml measuring cylinder10. 25ml pipette

PROCEDURE

1. 39 mL of the HCl stock solution was measured which was 10.17M concentration and wasdiluted into 1000mL of distilled water to get the required 0.4M HCl.

2. 20g of sugar was measured in a beaker using an electronic balance and added to 100ml ofdistilled water in a conical flask.

3. 25ml of 0.4M hydrochloric acid was added to 25ml of the sugar solution and swirled.4. A sample was poured into the polarimeter tube and put into the polarimeter.5. The angle of rotation of the light by the solution was taken at 10 minutes interval for 80

minutes and the results tabulated.6. Another 25ml of the sugar solution was taken and then 100ml of 0.4M hydrochloric acid

was added and the resulting solution heated for 30minutes.7. The solution was cooled and a sample poured into the polarimeter tube and the angle of

rotation taken.

TABLE OF RESULTS

Time(Minutes)

Angle of rotation(Degrees)

Actual angle of rotation (αt- α)(Degrees)

Log (αt- α)

10 93.85 15.90 1.20

20 86.55 8.60 0.93

30 89.45 11.50 1.60

40 84.55 6.60 0.82

50 85.70 7.75 0.89

60 86.50 8.55 0.93

70 87.00 9.05 0.96

80 85.40 7.45 0.87

The angle of rotation for the solution of sugar solution and 0.4M hydrochloric acid (α) = 77.95o

From slope= y2-y1 = 40.0-10.0= 30.0/-0.38= -78.94x2-x1 0.82-1.20

but slope= k/2.303; hence k = -78.94×2.303= -181.799

DISCUSSION

Since the levorotatory fructose has a greater molar rotation than the dextrorotatory glucose, theresulting mixture of the glucose is slightly levorotatory. As the sucrose is used up and the glucose-fructose mixture is formed, the angle of rotation to the right (as the observer looks in the directionopposite to that of the light propagation) becomes less and less, and finally the light is rotated to theleft. The observed rotation is dependent upon the path length of the light passing through thesample compartment and is also dependent upon the number of molecules of the isomer. Thenegative rate constant value shows the rate of decrease of the concentration of the sucrose.

PRECAUTIONS

1. Both ends of the polarimeter tube was thoroughly cleaned before readings were taken.

2. The sugar solution was swirled for complete dissolution.

3. The hydrochloric acid was added only when the polarimeter was ready for use.

4. The colour of light from the polarimeter turned yellow before the polarimeter was used

ERROR ANALYSIS

The changes in room temperature could have altered the results obtained. Some difficulties ascended in reading from the polarimerter. This might have affected the

readings. From the range of values obtained, it is fairly obvious that there were some errors in the

readings.

CHALLENGES

Numerous challenges were faced by students and the laboratory assistant (Graduate Assistant) duringthe lab session. Some of the challenges subsume the following:

There was only one polarimeter present for use by all the students in the laboratory. Thisresulted to some of the students snapping pictures in the lab since they had no much to do.

The student to laboratory assistant (Graduate Assistant) ratio was as large that it was difficultfor the instructor to have a full control of the entire class. The resulted to unusual noise makingin the lab.

RECOMMENDATIONS

The authorities in the department and the college as a whole should try their possible best tofind appreciable number of apparatus for use in the lab.

Students should be divided into smaller groups for lab instructors to handle us easily.

SUGGESTIONS FOR FURTHER WORK

Other than hydrochloric acid (HCl), sulphuric acid (H2SO4) and trichloroacetic acid, each 4N, may beused as catalysts. Trichloroacetic acid is about as highly dissociated as hydrochloric acid. In fact,modern studies indicate that enzymes can even be used as the catalyst.

CONCLUSION

Based on the experimental results, it may be concluded that the linearity of the plots show the kineticof first order for the hydrolysis of the sucrose. The constant of rate, also, increase with temperatureincreasing.

The magnitude of optical rotation is affected by the concentration of the solution, the length of thepath of the light in the solution, the wavelength of the light, the temperature, and the nature of thesolvent. The slope of the graph was easily determined from the rate constant K was deduced from it.

REFERENCES

General Chemistry, Raymond Chang, 5th edition, pages 630-640. Chemical Technician Ready Reference Handbook 2nd edition by Shugar and Bauman pages

592- 593. Health Chemistry Laboratory Experiment 2nd edition by Micheal .A. Dispezio pages 310-312. Textbook of Practical Chemistry 4th edition (2001) by Wesley. D. Smith pages 620-633. Practical Chemistry 4th edition (1990) by Francis Kinkier pages 745-748.

POST LAB

A

1. Inversion is the spatial rearrangement of atoms or groups of atoms in a dissymmetricmolecule, giving rise to a product with a molecular configuration that is a mirror image of thatof the original molecule.

2. Hydrochloric acid was added to speed up the rate of reaction since the reaction proceeds veryslowly in water.

3. The equation for the reaction isC12H22O11(sucrose) + H2O + H+ =>C6H12O6(fructose) + C6H12O6(glucose) + H+

B

1. Yes. The properties of matter as dielectric constant and refractive index of the sample canbe used.

2. Readings are to be taken at exactly the same interval of time because the wavelength isdependent of time and that the specific rotation of a solution depends on the changingwavelength travelling through it.

3. The reaction is a first order rate reaction. From the equation2.303log (αt-α)=kt, when log(αt-α) is plotted against t, a straight line graph is obtainedwhich gives the slope of k/2.303.

4. From data20g of sugar was dissolved in 100ml of waterMolar Mass of sucrose = 12(12) + 22(1) + 11 (16) = 342g/molmoles of sucrose = 20/342 = 0.0585 moldensity of water = 1g/cm3

from mass = density × volumemass = 1g/cm3 ×100cm3 = 100g =0.1kgmolality= number of moles of sucrose = 0.0585mol/0.1kg =0.585mol/kg

mass of the solvent in kg