The Theory and Use of Melting Point and Refractive Index to Verify or Identify Organic Compounds

Transcription

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 Pavia Tech 2; 3.9, 24 Tech #9 ( ; ) Dr. Schornick Web Site 1/17/2015 1

2 Melting Point Theory & Background Melting Point Uses 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 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 1/17/2015 2

3 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 curve (the 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 1/17/2015 3

4 Melting Point Depression Melting Point 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 The mixture displays both a melting point depression and a melting point range The size of the Melting Point Depression depends on the relative concentrations of the compounds in the mixture 1/17/2015 4

5 Melting Point Melting Point Depression and Degree of Purity The Melting Point depression of a mixture decreases as the concentration of one component increases relative to the other compound, e.g., antifreeze in coolant system Generally, a 1% impurity results in a 0.5 o C depression As the melting point of a mixture decreases with increasing contamination of one of the components, the melting point range initially increases then decreases until the range is minimal 1/17/2015 5

6 Melting Point Temperature The temperature at which the melting point of the mixture is the lowest and the range is at a minimum is called The Eutectic Point The Eutectic Point represents the solubility limit of the contaminating compound relative to the other compound mp A mpb > mpa mp B Liquid A + B Range Clear Point Onset Point MP Range Solid A + B Eutectic Point 0% B 0% A 1/17/2015 6

7 Melting Point & Refractive Index Elements of the Melting Point Experiment Pre-lab report Melting Point Melting point of: 2 known compounds with similar melting points A mixture of the two known compounds An unknown compound A mixture of the unknown compound and a known compound from the list of known compounds Final Report This process will be repeated until the known/unknown mixture produces a melting point similar to the pure unknown 1/17/2015 7

8 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) 1/17/2015 8

9 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 1/17/2015 9

10 Melting Point MelTemp Apparatus 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 o C per minute) Determine a 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 20 o C below rough MP Reset temperature knob so that temperature rises no more than 0.5 o C/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 Repeat for next compound or mixture 1/17/

11 Melting Point SRS DigiMelt Press Start/Temp button and then use the /2 and /3 buttons to set the starting temperature about 10 degrees below the lowest expected melting point of the two known compounds Note: For an unknown compound set the Start/Temp at 100 o C (applies to this experiment only) Press the Ramp/Rate button and use the /2 and /3 buttons to set the ramp rate to 0.5 o C/min for compounds of known melting point Note: For an unknown compound set the initial ramp rate to 5 o C/min When the approximate melting point of the unknown compound is determined, reset the ramp rate to 0.5 o C and retest the sample for the actual melting point 1/17/

12 Melting Point SRS DigiMelt Press the Stop/Temp button and use the /2 and /3 buttons to set the Stop Temperature to at least 5 o C above the expected melting point Note: For an unknown set the Stop/Temp to 175 o C 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) Temporarily remove the capillary tube from the instrument 1/17/

13 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: Let the sample to equilibrate for at least 2 minutes before proceeding with the measurement Note: Each slot is associated with a keypad button: Left slot (keypad 1) Middle slot (keypad 2) Right slot (keypad 3) 1/17/

14 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 melting point of the sample. When the sample reaches the Onset Point (the particles will begin to glisten) press the 1 st keypad (1) 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 2 nd keypad button (2) again to record the 2 nd data point When the sample becomes completely liquid at the Clear (or Liquefaction) Point, press the 3 rd keypad button (3) to record the 3 rd data point 1/17/

15 Melting Point SRS DigiMelt Sample Sequence Determine the melting point values for the first known sample: Onset Point Meniscus Point Clear Point Determine the melting point values for the second known compound Set the Start Temp button to about 20 o C below the lowest melting point of the two known compounds Prepare a 50/50 mixture of the two known compounds Determine the melting point values of this mixture 1/17/

16 Melting Point SRS DigiMelt Determine the melting point values for an unknown compound obtained from the instructor s desk From the table on page 27 of the Slayden lab manual select a compound the closely matches the melting point of your unknown Prepare a 50/50 mixture of the unknown and selected known Determine the melting point values for this mixture If the melting point range 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 1/17/

17 Melting Point 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., C a H b X c 1/17/

18 Refractive Index The Determination Of The Refractive Index Of Organic Compounds Study Materials Slayden pp Pavia Tech #24 pp Dr. Schornick Web Site 1/17/

19 Refractive Index 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 1/17/

20 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 1/17/

21 Refractive Index The Refractive Index for a given medium depends on two (2) variables: Refractive Index (n D ) 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 (n D ) 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 1/17/

22 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). c v V V air medium = n (Refractive Index) 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. V sin sin V air 1 Constant n medium 2 (Refractive Index) 1 - Angle of Incidence (air) 2 - Angle of Refraction (sample) 1/17/

23 Refractive Index Consider two (2) media: air (or vacuum) & organic liquid Frequency of light in both media remains constant f2 = f2 = f v (velocity) v f (Frequency * Wavelength) 1 f 1 v2 = f λ2 Divide 1 by 2 v v f f /17/

24 Refractive Index Since: Then: v c n = & v = Substitute in original refractive index equation sinφ1 v1 λ1 n2 = = = = n = Refractive Index sinφ2 v2 λ2 n1 c n c n Note: n 1 for air (or vacuum) = 1.0 v v n 2 n n n 2 1 c n 1/17/

25 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 just enough sample to cover the rectangular prism Close the hinged prisms together - Gently Turn on the light by pushing up on the switch on the left side of instrument. Note: The 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 1/17/

26 Refractive Index Abbe Refractometer (Con t) Rotate coarse adjustment knob 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 demarcation line dividing light & dark areas appears as a colored band (chromatic aberration), adjust with the knurled drum knob on the front of the instrument until demarcation line is as sharp as possible Press switch on left side of instrument down completely to make the scale visible Read refractive index value to 4 decimal placese, e.g., /17/

27 Refractive Index View through eyepiece of ABBE Refractometer Light Half Dark Half 1/17/

28 Refractive Index Reading the Refractive ValueI The Index of Refraction (N D ) decreases with increasing temperature, i.e., velocity of light in medium increases as density decreases Measured values of (N D ) must be adjusted to 20 o C Temp Correction Factor = t * = (Room Temp 20) * For temp > 20 o C (t is positive) Correction Factor is added to Raw Value, i.e., the Refractive index value at 20 o C is greater than the value determined at a higher temperature For temp < 20 o C (t is negative) Correction Factor is subtracted from Raw Value, i.e., the value at 20 o C is less than the value at a lower temperature 1/17/

29 Refractive Index The equation for adjusting the raw Refractive Index value is: N D 20 = N D Rm Temp + (Rm Temp 20) * Note the equation automatically adjusts the correction factor up or down from 20 o C N D 20 = (16 20) * (-4) * = Note: The scale must be read to 4 decimal places Typical Range of Values for Organic Liquids: /17/

30 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 1/17/

31 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 just 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 1/17/

32 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 1/17/

33 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 1/17/

34 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 1/17/

35 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 1/17/

36 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 it could be) Create a 1:1 mixture of your unknown and the selected known compound Determine melting point range of known/unknown mixture 1/17/