kilomentor

A Real-life Real-time Problem

kilomentor | 17 June, 2007 12:23

 

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com

 

A Practical Scheme for Working Up a Reaction Mixture based upon real Liquid-Liquid Extraction Possibilities and Logical Solubility Testing (An updated entry)

kilomentor | 09 June, 2007 11:42

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com. Use the internal search engine to find pertinent articles.

 

pKas of Common Organic Substances.

kilomentor | 07 June, 2007 19:40

 

 

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com

What does pKa mean? The pKa of a neutral molecule or ion is the negative logarithm of dissociation constant of a particular hydrogen atom under defined solvent conditions. Thus for a molecule A with a particular attached hydrogen H that we designate as A-H then Ka=[H

+] [A- ] / [H-A] this is the hydrogen ion concentration in the solution multiplied by the concentration of the anion of A all divided by the concentration of the undeprotonated H-A. pKa is –log Ka = -log [H+] –log [A- ] + log [H-A].

 

 

pKa = pH –log [A- ] + log [H-A]

pKa is best understood as the pH at which equal amounts of H-A and A- exist in the solution. Put another way the pK is the pH at which H-A is one-half deprotonated in the reference solution.

The concept is not just applied to neutral substances or substances that are commonly recognized as Bronsted acids. Basic molecules are also characterized by pKa values but these are the pKas of the corresponding fully protonated base.

then Ka=[H+] [B] / [H-B+; ] pKa is –log Ka = -log [H+] –log [B] + log [H-B+]

Thus pKa= pH –log [B] + log [H-B+]

The solvent systems commonly used are aqueous sulphuric acid for measuring the pKa of strong acids. The pKas of strong acids are usually negative numbers. The more negative the number, the stronger the acid. The strongest commonly known acid is hydrogen iodide.

Water is the solvent used for measuring the pKa of moderate acids and DMSO is common for measuring the pKas of weak acids such as the important class of carbon acids (hydrogens bonded directly to a carbon).

As an example of proper interpretation of the pKa acidity of a particular proton, we can note the two pKas important for the common solvent methanol, The pKa of Me-OH2 is -2.5 and the pKa of the hydrogen bonded to oxygen in neutral methanol, Me-OH, is 15.5. What the first number says is that at a pH of -2.5 (something like molar sulphuric acid), methanol in the solution is one-half protonated. The second number tells me that even in the strongest aqueous base (pH 14) methanol is not yet half deprotonated.

Traditional Bronsted Acids

pKa

HI

-10

HBr

-9

HCl

-8

CF3SO3H

-5.1 (-5.9)

HClO4

-5.0

FSO3H

-4.8 (-6.4)

PhSO3H

-2.8

H2SO4

-2.8

HNO3

-1.3

CF3CO2H

-0.6

CCl3CO2H

-0.5

H3PO4

2.1

HF

3.2

H2CO3

3.7

In the same vein one can look at the pKas of common protic solvents.

Acids with Deprotonated Form as Common Bases

Approx pKa

Isopropanol (isopropoxide)

17.1

t-butanol (t-butoxide)

18.0

cyclopentadiene (cyclopentandienyl anion)

18.1

acetylene (acetylide)

25.0

triphenylmethane (triphenylmethide)

30.6

diisopropylamine (diisopropylamide)

39.0

ammonia (amide)

41.0

benzene (phenyl lithium)

43.0

ethane (like butyl lithium)

50.0

methane (methyl lithium)

58 ± 5

The table below shows the pKas of different common carboxylic acids.

Acidity of Carboxylic Acids

pKa

Reference Solvent

CF3CO2H

-0.6

water

CCl3CO2H

-0.5

water

HO2CCO2H

1.25

water

Cl2HCO2H

1.35

water

FCH2CO2H

2.60

water

ClCH2CO2H

2.86

water

O2N-Ph-CO2H

3.44

water

HCO2H

3.75

water

HO-CO-OH

3.70

water

PhCO2H

4.20

water

PhCO2H

11.0

DMSO

CH3CO2H

4.76

water

CH3CO2H

12.3

DMSO

H2N-Ph-CO2H

4.92

water

H2N-CO2H

9.8

water

The pka of the conjugate acids of solvents are a guide to how active a Bronsted acid will be in different solvents, The more negative the pKa the more reactive the proton will be.

Conjugate Acids of Possible Solvents

pKa

CH3SCH3

-6.99

CH3O-Ph

-6.5

Ph-COOEt

-6.2

CH3OCH3

-3.8

(CH3)2CO

-2.85

CH3OH

-2.5

H2O

-1.7

CH3SOCH3

-1.5

NH2(CS)NH2

-1.26

H ( CO )NH2

-0.48

CH3(CO)NH2

0.1

NH2(CO)NH2

0.5

NH2CH2CO2H

2.35

Probably the most valuable table is the one showing carbon acids alongside other reference acids.

This helps when deciding how strong a base is required for a particular deprotonation.

Organic acids

pKa

PhSO2H

1.2

PhCH=NHOH (a protonated oxime)

2.0

MeSO2H

Improved Extractive Separations with Organic/Organic Biphasic Solvent Systems: Catalyzed Total Silylation to Improve Partition Coefficients

kilomentor | 14 May, 2007 09:42

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com . This particular article appears at http://www.kilomentor.com/2007/05/improved-extractive-separations-with-organicorganic-biphasic-solvent-systems-catalyzed-total-silylation-to-improve-partition-coefficients

 

 

Imminium Perchlorates & Fluoborates: Solid Crystalline Reversible Derivatives of Carbonyls

kilomentor | 06 May, 2007 11:37

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com. This particular article appears at http://www.kilomentor.com/2007/05/imminium-perchlorates--fluoborates-solid-crystalline-reversible-derivatives-of-carbonyls-a-publishable-undergrad-project

 

 

 

 

Solvent Exchanges for Special High Boiling Solvents

kilomentor | 29 April, 2007 15:14

 

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com . This particular article appears at http://www.kilomentor.com/2007/04/solvent-exchanges-for-special-high-boiling-solvents

 

 

 

 

 

 

 

 

Urea Complexes for the Separation of Straight Chain Solvents

kilomentor | 29 April, 2007 09:19

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com. This particular article can be found at http://www.kilomentor.com/2007/04/urea-complexes-for-the-separation-of-straight-chain-solvents 

 

 

Purification of Chemical Products by Treatment with Mixtures of Solid Adsorbants like Charcoal: Identifying Useful Absorbants by a Combinatorial Method

kilomentor | 14 April, 2007 10:11

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com. This particular article appears at http://www.kilomentor.com/2007/04/purification-of-chemical-products-by-treatment-with-mixtures-of-solid-adsorbants-like-charcoal-identifying-useful-absorbants-by-a-combinatorial-method

 

 

Solvent Replacement: The need to change solvent either from a reaction solvent to a crystallizing solvent or during reaction telescoping in a process

kilomentor | 09 April, 2007 19:29

 

 

 

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com This particular article is available at http://www.kilomentor.com/2007/04/solvent-replacement-the-need-to-change-solvent-either-from-a-reaction-solvent-to-a-crystallizing-solvent-or-during-reaction-telescoping-in-a-process

Otherwise, insert your search terms into the search engine on this site to see which of the more than 130 articles are pertinent.

 

 

 

 

 

ALCOHOLS: Organic Chemistry Isolations with Reversible Derivatives particularly Phthalate Esters

kilomentor | 01 April, 2007 12:16

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com. This particular article appears at http://www.kilomentor.com/2007/04/alcohols-organic-chemistry-isolations-with-reversible-derivatives-particularly-phthalate-esters

 

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Stoichiometry & the Rate of Addition of Reactants: An Important Consideration for Mentoring / Training in Chemical Process Development

kilomentor | 09 March, 2007 12:09

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com This particular article can be found at http://www.kilomentor.com/2007/03/stoichiometry--the-rate-of-addition-of-reactants-an-important-consideration-for-mentoring--training-in-chemical-process-development

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Getting a Job as a Process Development Chemist

kilomentor | 05 March, 2007 07:29

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com This particular article can be found at http://www.kilomentor.com/page/25

 

 

Balancing Chemical Equations and Calculating Heats of Reaction: Two Often Overlooked Helps for Chemical Process Developers.

kilomentor | 28 February, 2007 12:11

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com This particular article can be found at http://www.kilomentor.com/2007/02/balancing-chemical-equations-and-calculating-heats-of-reaction-two-often-overlooked-helps-for-chemical-process-developers

 

Inorganic Non-Stoichiometric Metal Salt Complexes with Organic Molecules as a particularly Useful Method for Purifying Neutral Substances.

kilomentor | 16 February, 2007 07:22

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com This particular article can be found at http://www.kilomentor.com/2007/02/inorganic-nonstoichiometric-metal-salt-complexes-with-organic-molecules-as-a-particularly-useful-method-for-purifying-neutral-substances

Swishing and Swish TLC: The Most Important Analytical Paper Ever for Chemical Process Development

kilomentor | 12 February, 2007 18:06

 

The Kilomentor Blog is now available in a more complete form at www.kilomentor.com This particular article can be found at http://www.kilomentor.com/2007/02/swishing-and-swish-tlc-the-most-important-analytical-paper-ever-for-chemical-process-development

 

 

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