[Golden_4_Life]

I am trying to use UV-Vis spectroscopy to differentiate between a set of three phenolic compounds: chlorobenzyl phenol, amyl phenol, and ortho para phenol.  I have empirically determined that the first two compounds have as their wavelength of maximum absorbance = 280 nm; whereas the last one has w.m.a. of 283 nm.  How can I differentiate between cmpds 1 vs 2 by using my UV-Vis?
I don't have access to a Mass spectroscopy unit; all I have is a Uv Vis.
Any help?
Would I ionize one of them?

[MOTOBALL]

(1)  Since you have access to a UV spectrophotometer, you should determine the extinction coefficient of each of your two unknowns, then compare with literature values. see (2), below !!

(2)  Note that you need to check the values for the ortho- , meta- and para- forms of chlorobenzylphenol and amyphenol.

(3) Your reference to "ortho para phenol" is nonsensical; there must be two substituents on the phenyl ring to label a particular compound as ortho- , meta- or para-

(4) Let us know how this works out.

Good Luck,

Motoball

[Golden_4_Life]

Thanky Moto: I will endeavor to do just that--although I know what an "extinction coefficient" is by definition, I am not familiar wit how to empirically determine it with the resources I have available?

The proper names of the compounds are: "2-phenyl phenol" (OPP); 4-tert amyl phenol (PTAP); and 2-benzyl 4 clorophenol for (OBPCP).

What I have empirically determined so far is the following:
Compound                     Wavelength of Maximum Absorbance (W.M.A.) for a 1% solution (1 mg/mL)
OPP                                                282 nm
PTAP                                               274 nm
OBPCP                                             282 nm
equivolume mixture                            280 nm

*It is a mystery to me how I can differentiate betw. OPP and OBPCP since they have the same w.m.a.. I don't have a mass spec so my resources are limited--yet I don't what to just "throw up my hands" and plead I give up. ?

[marquis]

This is kind of a far shot, but...

Phenols are used in rubber accelerators.  One quick and dirty way used in the past to identify accelerators, was to extract them in a base (0.01 N NaOH typically). Run the UV from 400 nm to 200 nm.  Then acidify the extract with HCl and rerun the UV/VIS. 

Please note, you will have to replace the solution in the reference side of the UV/vis and probably rerun the blank after you acidify the extract.

Typically, you would see a shift of a few nm (2 to 5 nm) with this approach.  If it works, please let me know.

Good luck.

[Golden_4_Life]

Thanky Marquis:
I will attempt this experiment; however, when I do the extraction steps you cited---which of the analytes will be the one that is discernable from the 2 to 5 nm shift? Would it be the OBPCP or the OPP one?  I presume I would have to get primary STDs of the two analytes and treat them in the same fashion?

[marquis]

If you can, take standards of the three materials and run them in the 0.01 N NaOH. You may need to filter the solution.  You will probably see a slight reproducible wavelength difference of the phenol peaks in the three materials.  The run times for your scans may need to be long to detect the difference in the peaks.

The acidification step will also produce a different shift between the three materials.  Then it is comparing the knowns to the unknowns.

The acidification should be gentle.  Take the solution to pH 3 or 4. 

   

[Golden_4_Life]

OK, I will develop an experimental scheme based on that idea. Will post my results here for the forum's perusal.

[Golden_4_Life]

Procedure to differentiate iso-chromic phenols in an equivolume mixture.
1. prepare separate 1% STD solutions of each phenol from crystalline primary stock, diluent will be matrix blank. And, prepare an equivolume mixture of the STDs labelled as "combo STD".  This gives a grand total of 5 preps [1 blank matrix, 2 OPP analyte, 3 PTAP analyte, 4, OBPCP analyte, and 5 equivolume mixture).
2. Split samples in half and leave one-half as "untreated"; the other half will be "treate" by add 0.01 N NaOH, 1 ml of base per 1 ml of each of the five STD prep ,mix well, record pH, allow to set for 5 mins, then filter thru whatman 5 filter paper.
3. recover the filtrate and set the UV-Vis spectrophotometer to SCAN mode to determine the wavelength of maximum absorbance (WMA) for each STD prep.
 Hypotheses
1. In the "untreated" single analyte preps, the WMA for OPP and OBPCP should be the same (as I have seen as before) at 282nm, the WMA for PTAP should be 274nm.
2. In the "untreated" combo STD prep the WMA should be 280nm as I have shown before.
3. In the "treated single analyte preps there ought to be a discernable difference betw. WMA of OPP and OBPCP.
4. IN the ""treateed" combo prep STD there ought to be a discernable difference betwee all three analytes because of the hydrolysis step.

DOes this sound feasible?

[Golden_4_Life]

Marquis: Thanks, but pls clarify for me----is the reason to use base or acid in order to produce the phenolate anions of these parent compounds?  And, do these ionic forms of the parent phenols produce absorbance spectra than their parent compounds?  Pls clarify--thanky.

[Golden_4_Life]

We conducted the cited experiment to determine if a shift in WMA would occur as a result of conversion of the analytes to their respective phenolate anions.  We observed that only for the OPP analyte was there distinctive difference in WMA--"untreated" WMA = 279nm; "treated" WMA = 309nm.
For the other two analytes and their 'untreated' and 'treated' groups either a "soft peak" with no real peak per se but more so just like a plateau was observed.
pH before base treatment was 7.27
after base treatment pH was 10.98.
We conclude that: 1) no effect may have been due to the pH not being high enough above the pka of phenols (~10).
2) the samples could've been over-diluted;  3) whether a shift in WMA occurs is still open to question--perhaps it does but the shift was not measurable in this study; 4) to date we must state that it is inconclusive to claim that we can differentiate between these three analytes by conversion to their anions

[marquis]

You have the right idea.  The ionic forms sometimes produce different spectra.  Usually, you will see the difference bond lengths expressed as a different wavelength of the peak.  The shift can be major, but usually is rather small.

It was worth a shot, but it doesn't look like it will work for you.

Thank you for following up and posting the results.

[Golden_4_Life]

Was that effort worthy of a "mole snack"?  To date I have -1 mole snaks (not sure why).

I may actually re-run a part of the study just to confirm that  first set of results were valid--to satisfy this "itch".

[marquis]

Can't hurt to rerun it, except for the time it takes.

I didn't give a very good explanation.  Most chemists will tell you that phenols absorb at 280 nm in the UV VIS.  This is correct.  However, if you have a sensitive instrument, you will find that the exact wavelength is not always 280 nm.  It will often be 285nm or 279 nm, based on the exact form of phenol present.  I don't know what kind of uv/vis you are using.  I use a scanning UV/VIS spectrophotometer.  If you slow the scan down to 50 nm per minute or so, you can often identify the exact wavelength more easily.

Likewise, the form of phenol present will determine the nm of shift when you switch from a base to acid environment.

This only works if you are trying to identify a few closely related molecules.  In a mix of 100 similar molecules, it is worthless.  Hoped it would work for your phenols.  Sorry that it didn't.

[Golden_4_Life]

Well, that is typical of many proposed experiments--they often look quite elegant "on-paper"; however, when subjected to empirical rigor they often fall apart like a house of cards in a wind tunnel.