Melting Point & Refractive Index The Theory and use of Melting Point and 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:/mason.gmu.edu/~jschorni/meltpoint 4/16/2017

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 Uses Establish Purity of Compounds The Purer the Compound, the Higher the Melting Point The Purer the Compound, the Narrower the Melting Point Range Identify Compounds 4/16/2017

Melting Point Melting Point Range The Onset point (lower temperature) is the temperature at which the liquid phase first appears in coexistence with the crystals (crystals will begin to glisten) The Meniscus point is when a solid phase is at the bottom and a liquid phase is on top separated by a well defined downward pointing meniscus The Meniscus Point is used as the “Melting Point” in Europe The Clear Point is when the substance becomes completely liquid The Clear Point is used as the “Melting Point” in USA 4/16/2017

Melting Point 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 4/16/2017

Melting Point Melting Point Depression and Degree of Purity The Melting Point of a mixture decreases as the concentration of one component increases relative to the other compound, e.g., antifreeze in coolant system As the melting point of a mixture decreases with increasing contamination of one of the components, the melting point range initially increases then decrease until the range is minimal 4/16/2017

Melting Point The Lowest Melting Point of an A/B mixture – The Eutectic Point - is reached when the contaminating compound (B) reaches its solubility limit in component A Solid + Liquid Liquid A + B Solid A + B Range Clear Point Onset Point mp A mp B 0% B 0% A Temperature Eutectic Point mpB > mpA  MP Range 4/16/2017

Melting Point & Refractive Index Elements of the Melting Point Experiment Pre-lab report Melting Point Melting point of: 2 known compounds A mixture of the two known compounds An unknown compound A mixture of the unknown compound and a known compound from the list of knowns (repeated with other known compounds until one is found that matches the unknown Final Report 4/16/2017

Melting Point Procedure Equipment Capillary Tubes Mel-Temp Melting Point Apparatus or SRS Digimelt Apparatrus Obtain: Two known samples in sequence as listed in the table on page 27 of the Slayden manual; i.e., two samples with similar melting points Unknown sample from Prep room or Instructor’s desk (Note: Record unknown No. in your report) 4/16/2017

Melting Point Procedure Loading the Capillary Tube If necessary, crush the sample using a spatula, pestle, or open end of Capillary tube Tap the open end of the capillary tube into the sample until 1-2 mm of sample is obtained 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 Prepare capillary tubes for the following: Two of the known compounds in sequence from the Table 1, p 27, in Slayden manual Sample of a 1:1 mixture of the two known compounds Sample of your unknown compound 4/16/2017

Melting Point – Mel_Temp 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 4/16/2017

Melting Point – SRS DigiMelt Press Start/Temp button and then use the /2 and /3 buttons to set the starting temperature about 5 degrees below the lowest expected melting point of the two known compounds Note: For an unknown compound set the Start/Temp at 100oC (applies to this experiment only) Press the Ramp/Rate button and use the /2 and /3 buttons to set the ramp rate to 0.5oC/min for compounds of known melting point Note: For an unknown compound set the initial ramp rate to 5oC/min When the approximate melting point of the unknown compound is determined, reset the ramp rate to 0.5oC and retest the sample for the actual melting point 4/16/2017

Melting Point – SRS DigiMelt Press the Stop/Temp button and use the /2 and /3 buttons to set the Stop Temperature to at least 5oC above the expected melting point Note: For an unknown set the Stop/Temp to 175oC Press the Stop/Temp button again to return to the current temperature display Insert the capillary tube, closed end down, into one of the chassis holes near the Tube Tapper button (right side) Press the Tube Tapper button to transfer the sample to the bottom of the tube (repeat as necessary) 4/16/2017

Melting Point – SRS DigiMelt Press the Start/Stop button to preheat the block to the starting temperature. The Preheat LED will light When the Ready LED becomes lit, the oven is holding at the start temperature Insert capillary tube containing sample into heating block Note: The heating block can accommodate up to 3 capillary tubes Each slot is associated with a keypad button: Left slot (keypad 1) Middle slot (keypad 2) Right slot (keypad 3) 4/16/2017

Melting Point – SRS DigiMelt Press the Start/Stop button to begin ramping the temperature at the ramp rate – The Melt LED will light Observe the sample(s) When the sample reaches the “Onset Point” (the particles will begin to glisten) press the appropriate keypad button to record the first data point (repeat for each capillary tube if multiple samples are being tested) When the sample begins to exhibit a meniscus (liquid phase on top, solid phase on bottom with a well defined downward curved interface) press the applicable keypad button again to record the 2nd data point When the sample becomes completely liquid at the Clear (or Liquefaction) Point, press the keypad button again to record the 3rd data point 4/16/2017

Melting Point If the melting point ranges of the unknown/known mixture and your unknown differ by several degrees or more, select a new known compound from the table and create a new known/unknown mixture and determine its MP range Repeat this 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 4/16/2017

The Determination Of The Refractive Index Of Organic Compounds Study Materials Slayden – pp. 28 - 30 Pavia – Tech #24 pp. 845 – 850 Dr. Schornick Web Site http:/mason.gmu.edu/~jschorni/meltpoint.ppt 4/16/2017

Elements of the Refractive Index Experiment Pre-lab Report Uses – Purity and identification of unknowns Background Measurement & Equipment Temperature Correction Experiment – Refractive Index of a Known Compound & an Unknown Compound Final Report 4/16/2017

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 4/16/2017

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 Index decreases as temperature rises 4/16/2017

Refractive Index (Index of Refraction) (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) 4/16/2017

Refractive Index Consider two (2) media: air (or vacuum) & organic liquid Frequency of light in both media remains constant Divide 1 by 2 4/16/2017

Refractive Index Since: Then: Substitute in original refractive index equation Note: n1 for air (or vacuum) = 1.0 4/16/2017

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 4/16/2017

Refractive Index Abbe Refractometer (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 4/16/2017

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 4/16/2017

Refractive Index 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 Dark Half Light Half 4/16/2017

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) Correction Factor is added to Raw Value, i.e., the Refractive index value at 20oC is greater than the value determined at a higher temperature For temp < 20oC (t is negative) Correction Factor is subtracted from Raw Value, i.e., the value at 20oC is less than the value at a lower temperature 4/16/2017

ND20 = NDRm Temp + (Rm Temp – 20) * 0.00045 Refractive Index The equation from the previous slide correctly accounts for temperature correction factor: 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 1.5600 1.5550 1.5580 4/16/2017

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 4/16/2017

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 determine your unknown 4/16/2017

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\ 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 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 4/16/2017

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 4/16/2017

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 4/16/2017

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 4/16/2017

Melting Point 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 Note: Selection of this compound is probably not related to either of the original known compounds (but could be) Create a 1:1 mixture of your unknown and the selected known compound Determine melting point range of known/unknown mixture 4/16/2017