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Melting Point & Refractive Index

Melting Point & Refractive Index. The Theory and use of Melting Point and Refractive Index to Verify or Identify Organic Compounds Study Materials Slayden – pp. 25 - 27 Pavia – Tech 2; 3.9, 24 – Tech #9 (9.1 – 9.5; 9.7 – 9.9) Dr. Schornick Web Site

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Melting Point & Refractive Index

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  1. Melting Point & Refractive Index The Theory and use of Melting Point and Refractive Index to Verify or Identify Organic Compounds • Study Materials Slayden – pp. 25 - 27 Pavia – Tech 2; 3.9, 24 – Tech #9 (9.1 – 9.5; 9.7 – 9.9) Dr. Schornick Web Site http:/classweb.gmu.edu/jschorni/meltpoint

  2. Melting Point & Refractive Index • Elements of the Experiment • Pre-lab report • Melting Point • Theory and Background • Uses • Measurement Techniques & Equipment • Melting Point Range • Melting Point Ranges of Known Compounds, Mixtures, Unknown • Refractive Index • Theory and Background • Temperature Correction • Measurement Techniques & Equipment • Refractive Index, with temperature correction for a known and unknown compounds

  3. Melting Point • Theory & Background • Melting Point • Temperature at which a transition occurs between solid and liquid phases • Temperature at which an equilibrium exists between the well-ordered crystalline state and the more random liquid state • Melting Point Range • The Onset point (lower temperature) is the temperature at which the liquid phase first appears in coexistence with the crystals • The Meniscus point is when a solid phase is at the bottom and a liquid phase on top with a well defined meniscus – Used as “Pelting Point” in Europe • The Clear point is when the substance becomes completely liquid – Used as “Melting Point” in USA

  4. Melting Point • Uses • Identify Compounds • Establish Purity of Compounds • Melting Point Depression • Pure compounds display little, if any, “melting point” range, i.e., they have “sharp” melting points • Mixtures of substances, i.e., the contamination of one compound by another, whose components are insoluble in each other in the liquid phase, display both a melting point depression and, instead of a sharp melting point, a melting point range • The size of the melting point depression depends on the composition of the mixture • Generally, a 1% impurity results in a 0.5oC depression

  5. Melting Point • Melting Point Indicates Purity in Two Ways • The Purer the Compound, the Higher the Melting Point • The Purer the Compound, the Narrower the Melting Point Range • Melting point of A decreases as impurity B is added • Eutectic Point is the Solubility Limit of B in A; Thus, it is the Lowest Melting Point of an A/B mixture(Note: Sharp melting point, i.e., no range, at eutectic point) mp B mpB > mpA Liquid A + B mp A Solid + Liquid Temperature Clear Point Onset Point Solid + Liquid  Range MP Range Solid A + B Eutectic Point 0% B 0% A

  6. Melting Point • The Experiment • Determine the melting point range of: • Two Known Compounds • A Mixture of the Two Known Compounds • An Unknown Compound • Mixture of Unknown Compound and a Known compound.Note: The Unknown might have to be mixed with additional known compounds until the melting point of the known and the known/unknown mixture match. • Identify the unknown compound. • Equipment • Capillary Tubes • Mel-Temp Melting Point Apparatus (Obtain from Prep Room)

  7. Melting Point • Procedure • Obtain: • Mel-Temp Melting Point apparatus from Prep Room • Two known samples in sequence from table on page 20of the Slayden manual • Unknown sample from Prep room(Note: Record unknown No. in your report) • Loading the Capillary Tube • Crush sample using spatula or open end of Capillary tube • Tap open end of tube into sample (1-2 mm of sample) • Drop tube (closed end down) down a length of glass tubing letting it bounce on table – sample is transferred to closed end of capillary tube. Repeat, if necessary

  8. Melting Point • Obtaining the “Melting Point Range” • Place capillary tube with sample at the bottom of the tube in a Mel-Temp apparatus • Adjust temperature knob until temperature rises about (2-3 oC per minute) • Determine rough melting point • Allow capillary tube to cool until liquid solidifies • Reset temperature knob for a slower rate of temperature increase • Allow temperature to rise to 10oC below “rough MP” • Reset temperature knob so that temperature rises no more than 0.5oC/Min • Record “Melting Point Temperature Range, i.e., the temperature when the “initial drop of liquid forms” and the temperature when the entire mass turns to clear liquid

  9. Melting Point • Prepare capillary tubes for the following: • Two of the known compounds in sequence from theTable 1, p 20, in Slayden manual. • Sample of a 1:1 mixture of the two known compounds. • Sample of your unknown compound. • Determine melting point range of each sample. • Select from Table 1 a compound with a melting point close to the melting point of your unknown. • Create a 1:1 mixture of your unknown and the known compound • Determine melting point range of known/unknown mixture.

  10. Melting Point • If the melting point range of the unknown/known mixture and your unknown differ by several degrees or more, create a new known/unknown mixture and determine its MP range. • Repeat process with a new known for the mixture until the difference in the two ranges is minimal • Compare your results against literature values • Give IUPAC (formal chemical name) and synonyms for the unknown • Provide Molecular Structure of unknown, e.g., CaHbXc

  11. Refractive Index • Refractive Index • Uses – Purity and identification of unknowns • Background • Measurement & Equipment • Temperature Correction • Experiment – Refractive Index of Known Compounds & Unknown Compound • Study Materials • Slayden – pp. 28 - 30 • Pavia – Tech #24 pp. 845 – 850 • Dr. Schornick Web Site http:/classweb.gmu.edu/jschorni/meltpoint.ppt

  12. Refractive Index • Uses • Identification • Measure of Purity • Background • Refractive Index is a physical property of liquids & solids related to the velocity and wavelength of light in a medium • Refractive Index is the ratio of the velocity of light in a vacuum (air) to the velocity of light in a medium • The Velocity and Wavelength of light in a medium are functions of temperature, thus refractive index is a function of temperature • The velocity of light in a medium increases as the density decreases and decreases as the density increases

  13. Refractive Index • The Refractive Index for a given medium depends on two (2) variables: • Refractive Index (nD) is wavelength () dependent • Beams of light with different wavelengths are refracted to different extents in the same medium, thus, produce different refractive indices • Refractive Index (nD) is temperature dependent • As the temperature changes, the density of the medium changes, thus, the velocity () changes • As temperature increases, the medium density decreases • As the medium density decreases, the velocity of light increases • As the velocity of light increases, the ratio of the speed of light in vacuum vs. speed of light in medium decreases • Thus, the Refractive Indexdecreases as temperature rises

  14. Refractive Index • For a given liquid and temperature, the ratio of the speed of light in a vacuum (c) and speed of light in the medium () is a constant (n). • The speed of light ratio is also proportional to the ratio of the sin of the angle of incidence and the sin of the angle of refraction. (Index of Refraction) (Refractive index) 1 - Angle of Incidence (air) 2 - Angle of Refraction (sample)

  15. Refractive Index Consider two (2) media: air (or vacuum) & organic liquid Frequency of light in both media remains constant Divide 1 by 2

  16. Refractive Index • Since: • Then: • Substitute in original refractive index equation • Note: n1 for air (or vacuum) = 1.0

  17. Refractive Index • The Instrument – Abbe Refractometer (Bausch & Lomb) • Clean prisms with tissues & Methyl Alcohol – BE GENTLE!! • Do not touch prism with fingers or other hard objects, use tissues • Use 3 – 4 drops of sample • Close hinged prisms together - Gently • Turn on the light - Preferred light source is a sodium discharge lamp producing yellow light at 589 nm – also called Sodium “D” light. • Move hinged lamp up into position

  18. Refractive Index • AbbeRefractometer (Con’t) • Rotate coarse and fine adjustment knobs on the right side of instrument until the horizontal dividing line (may not be sharp at first) between the light upper half and dark lower halve of the visual field coincide with the center of the cross-hairs. • Use eyepiece to focus cross-hairs • If horizontal line dividing light & dark areas appears as a colored band (chromatic aberration), adjust with the knurled drum knob on the front of the instrument • Press small button on left side of instrument to make the scale visible. Read refractive index value to 4 decimal places

  19. Refractive Index • The Measurement • Place 3-4 drops of sample on Prism • Close Prism and raise lamp in front of Prism Portal • Flip switch on left side to turn on light • Use large dial on right to bring light/dark image into view • If image cannot be found, flip switch on left down and use large dial on right to bring the Scale into view around 1.4000 • Release switch on left and use large dial on right to bring light/dark image into view • Sharpen line of demarcation using Drum dial on front of instrument • Use Eyepiece to sharpen Cross-Hairs • Align the line of demarcation with the Cross-Hairs • Flip switch on left down and read value to 4 decimal places, e.g., 1.3875 Light Half Dark Half

  20. Refractive Index • Reading the Instrument • Index of Refraction (ND) decreases with increasing temperature, i.e., velocity of light in medium increases as density decreases • Measured values of (ND) are adjusted to 20oC • Temp Correction Factor = t * 0.00045 = (Room Temp – 20) * 0.00045 • For temp > 20oC (t is positive), i.e., add correction factor • For temp < 20oC (t is negative), i.e., subtract correction factor • The following equation automatically accounts for temperature correction: ND20 = NDRm Temp + (Rm Temp – 20) * 0.00045 Ex: For an observed value of 1.5523 at 16oC, the correction is: ND20 = 1.5523 + (16 – 20) * 0.00045 = 1.5523 + (-4) * 0.00045 = 1.5505 Note: Instrument can be read to “4” decimal places • Typical Range of Values for Organic Liquids: 1.3400 - 1.5600 1.5500 1.5523 1.5550 1.5580 1.5600

  21. Refractive Index • Procedure • Use the ABBE refractometer to measure the Refractive Index of a compound with a known refractive index • Note the temperature using the thermometer on the right side of the refractometer • Record the refractive index value to 4 decimal places • Repeat the measurement • Obtain an unknown sample from Instructor’s desk • Determine Refractive Index, noting temperature • Repeat the measurement

  22. Refractive Index • Procedure (con’t) • In your lab report correct the Refractive Index value for Temperature • Identify your unknown from the list of unknowns given in Table 2 on page 30 of the Slayden lab manual Note: The values for the unknown possibilities in the table are shown to 2 decimal places Use Google, CAS nos, and literature resources to find the refractive index values (to 4 decimal places) for the compounds in the table that closely match your measured value Match values and deteermine your unknown

  23. Melting Point & Refractive Index • The Laboratory Report (Review Points) • The report must reflect the appropriate number of procedures • A new procedure is defined when the experimental process changes to a logically different series of steps • Remember that each unique computation is considered a new procedure • When the procedure involves a computation, the equation must be set up in the procedure description and must include the definition of each variable

  24. Melting Point & Refractive Index • The laboratory Report (Review Points) (Con’t) • When the results for a computation are reported in the “Results” section, the calculation of each result must by shown along with the applicable units and appropriate precision, i.e., decimal places & significant figures. • When multiple samples or sub-samples are processed with the same procedure, it is not necessary to set up a separate procedure for each sample. Setup a suitable template in “Results” to report all of the results obtained.

  25. Melting Point & Refractive Index • The laboratory Report (Review Points) (Con’t) • Literature references for specific compounds are usually cited in the “References” section of the lab report and must include the page number and the item no., if available. Note: The Slayden manual and the Pavia text are not citable references for compounds. • Use the following sources for compound citations: • CRC handbook of Chemistry & Physics • The Merck Index • The CRC Handbook of Data on Organic Compounds

  26. Melting Point & Refractive I • The laboratory Report (Review Points) (Con’t) • Summarize in paragraph form, all of the results obtained in the experiment. • Use a logical organization and order of the results. • The “Conclusion” for the Melting Point & Refractive Index experiment must present arguments, using applicable results, that support the identification of the melting point and refractive index unknowns.

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