spectroscopy experiment (nmr) determination of the identity of an unknown organic compound using...

Spectroscopy Experiment (NMR)

Determination of the Identity of an unknown organic compound using selected physical properties, Infrared Spectroscopy, and NMR Spectroscopy

Unknown List: pp. 126 – 127 in Slayden Lab Manual

ReferencesSlayden, et al. – pp. 59 – 60Pavia, et al. – pp. 909 – 964Schornick – http://classweb.gmu.edu/jschorni/chem318

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Overview

Physical Properties

Boiling Point & Purification (Simple Distillation)

Physical Characteristics of Unknown

Refractive Index with Temperature Correction

Solubility (Relative to Water)

Density (Relative to Water)

Infrared Spectra Analysis

NMR Spectra Analysis

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The Laboratory Report:

Determination of an Unknown

Procedures

Title – Use Short, Concise terms

Simple Distillation, Refractive Index, IR Spectrum, etc.

Materials & Equipment (2 Columns in list (bullet) form)

Note: include all reagents & principal equipment used

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The lab Report (Cont’d) Procedure Description:

Descriptions must be detailed, but concise Use list (bullet) form Use your own words (don’t copy book)

Results – Neat, logically designed template to present results

Summary of Results Paragraph summarizing experimental results,

computed results, and principal absorptions from the IR & NMR spectra.

Analysis & Conclusions A logical step by step set of arguments,

utilizing selected results, to support the proposed identity of the Unknown compound

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Organic Lab – Unknowns, Purification, Boiling Point

Several experiments in Chem 315/318 (Org Lab I & II) involve the identification of an unknown compound

Liquid samples that students receive in Lab may contain some impurities in addition to the unknown compound that could produce ambiguous results when determining the chemical or physical properties of the compound

Simple Distillation is used to purify the sample by separating the pure compound that comes over in a narrow temperature range – corresponding to its boiling point – from impurities that have boiling points either lower than or higher than the compound

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Vapor Pressure / Boiling Point Boiling Point

The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the temperature at which the vapor pressure of the liquid is equal to 1 atmosphere (atm), the atmospheric pressure at sea levelAt that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid.The standard boiling point is now (as of 1982) defined by IUPAC as the temperature at which boiling occurs under a pressure of 1 bar1 bar = 105 Pascals = 0.98692 atmospheres = 14.5038 psi (pounds per square inch) = 29.53 in Hg (inches of mercury) = 750.06 mm

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Distillation / Boiling Point Measurement

Note: The temperature range you obtain for your boiling point may be inaccurate for three (3) reasons

1. The atmospheric pressure in the lab may not be:

1 bar (0.98692 atm)

2. The thermometers used in the lab may not

reflect the actual temperature

3. The thermal inefficiency of the glassware used for the boiling point determination may result in a

lower than expected measured value by as much as 2 – 5oC

You should take this potential temperature differential into account when you compare your measured results with the list of possible unknowns in lab manual tables

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Spectroscopy Experiment (NMR) Simple Distillation Setup

Note: Equipment used in distillation is expensive. Use care to avoid breakage.

ASK BEFORE YOU ACT!!

Note: The distillation procedure description includes the equipment setup process and the steps taken to collect the dataEquipment

Heating Plate (from cabinet to right of entrance door)

Sand Bath to hold distillation flask 25 mL Distillation Flask Distillation Head Thermometer & Thermometer Adapter

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Simple Distillation Setup (Con’t)

Condenser (with rubber tubing for cooling water)

Receiving containers

50 mL beaker for low boiling point and high boiling point impurities.

50 mL Erlenmeyer flask for purified sample

Procedure

Use Ring Stands to support apparatus

Note: Adjust height of apparatus so that sample receiving containers (small beaker & small Erlenmeyer flask) can sit on bench about an inch below where the condensate emerges from the condenser

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Simple Distillation Setup (Con’t)

Procedure (Con’t)

Use tablets, books, etc., to adjust height of Heating Plate

Use blue clamp to attach distillation flask to distillation head

Use blue plastic clamp to secure condenser tube to distillation head

Tighten tongs of clamp supporting the condenser tube just enough to ensure condenser tube water-input/output ports point outward or slightly up

Insert thermometer through adapter so that bulb is positioned just below elbow opening to condenser

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Typical Distillation Setup (Simple Distillation)

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Spectroscopy Experiment (NMR) Simple Distillation - Data Collection

Set Hot Plate Heat control unit to 4-5

Use 50 mL beaker to collect low boiling point impurities

Collect drops of condensate in waste beaker until temperature rise slows down and becomes constant

Note: It may be necessary to increase the temperature setting on the controller

Collect purified sample condensate in 50 mL Erlenmeyer flask throughout the period when the temperature is relatively constant

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Simple Distillation - Data Collection

If the temperature begins to increase again, turn the heat off, remove the Erlenmeyer flask, and collect any remaining condensate in the waste beaker

Put the purified sample back into the original vial for further processing

Dispose of any waste organic liquid in the jar in the hood

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Solubility/Density (Relative to Water) Add 2-3 mL Distilled water to a small to

medium test tube Add 4-5 drops of the purified distillate or 3 mg

of solid sample to the test tube Shake the test tube vigorously Note the following:

Did the sample dissolve, i.e., single clear solution?

Determine relative density Does the sample float on top of the liquid? Does the sample float in the middle of the

liquid or disperse throughout the liquid? Does the solid settle to the bottom?

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Refractive Index

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 sufficient drops of sample to cover

pridm surface Close hinged prisms together - Gently Use switch on left to turn on the light Move hinged lamp up into position

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Refractive Index (Con’t)

Rotate coarse and fine adjustment knob on the right side of instrument until horizontal dividing line (may not be sharp at first) between the light (Top) and dark (Bottom) halves 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 switch on left side of instrument down to make the scale visible

Read Refractive Index value to 4 decimal places04/19/23 16


Refractive Index - Reading the Instrument Index of Refraction (ND) is the ratio of the speed of light

in a vacuum (air) vs. the speed of light in a medium The speed of light in a medium increases with decreasing

density and decreases with increasing density Thus, Nd increases with increasing temperature (lower

density) and decreases with decreasing temperature (higher density)

Measured values of “nD” are adjusted to a standard 20oC.

Read the room temperature using the thermometer attached to the instrument or use the one in your set.

Correction Factor = t * 0.00045 = (Rm Temp – 20) * 0.00045

04/19/23 17(Con’t on next slide)


If temp > 20oC then t is positive - correction factor is added

If temp < 20oC then t is negative - correction factor subtracted

Ex: For an observed value of 1.5523 at 25oC, the correction is:

Instrument is read to “4” decimal places

The value corrected for temperature is:

nD20 = nD

rm temp + t (0.00045)

t = Room Temp - 20.0oC

nD20 = 1.5523 + (25 – 20)*(0.00045) = 1.5523 + 0.00225 = 1.5546

Note: nD at 25oC, as expected, is lower than nD at 20oC

Typical Range of Values for Organic Liquids: 1.3400 - 1.560004/19/23 18

1.5500 1.5523 1.56001.5550 1.5580


IR Spectrum

Salt Plate Preparation

Liquid Samples

1 to 2 drops of liquid sample are placed between two single crystals of sodium chloride (Salt Plates)

Note: NaCL plates are water soluble – do not use water or methanol to clean plates

Solid Samples soluble in Acetone

Dissolve sample in acetone

Evaporate on Salt Plate

Solid Samples not soluble in acetone

Make Potassium Bromide (KBR) pellet04/19/23 19


IR Spectrum

Salt Plate Preparation (Con’t)

Plate Holder

Place Salt Plate sandwich into base of Plate Holder

Place Plate Holder top over base and press down

Obtain Spectrum

Instructor - At the start of session enter new “background”

Student - Insert Plate Holder into beam slot

Select Memory location (X, Y, or Z)04/19/23 20


IR Spectrum

Obtain Spectrum (Con’t)

Push “SCAN” button

Verify “No. of Scans” box shows “4”

Press “Execute”

Note: If spectrum bottoms out - remove Cell Holder and reload Salt Plate with less sample; rerun Scan again

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Spectroscopy Experiment (NMR) When the “Scan” box in lower right corner

indicates “Ready”, press “Plot” to produce chart

Remove Cell Holder Clean & dry Salt Plates with Acetone Place in desiccator, replace desiccator cap

An NMR Spectrum of your unknown will be handed out during the laboratory session.

Summarize the principal NMR peaks in the : Results section of the report opposite the

procedure description The Results Summary Section

Analyze the NMR spectrum as part of the “Analysis & Conclusion” section

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spectroscopy experiment (nmr) determination of the identity of an unknown organic compound using...

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