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Neutralization Reactions In Non-aqueous medium

• Non-aqueous titration: deals with the analysis in a medium completely free from water. -using solvents other than water.

-Uses: Determination of very weak acids and bases.

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II-Bronsted-Lowry Theory:

1-An acid is defined as proton donor.

2-A base is defined as proton acceptor.

3-Acid-base reaction is a proton transfer from acid to base.

HA + B → BH+ + A-

Acid base conj. Acid conj. Base

قاعدة , * او حمض كانت اذا المادة احدد مقدرش اناحسب على H2O بحددها المثال , بتتفاعلهيا و ايه مع

ال كدة على

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H2O + CH3COOH→ CH3COO- + H30+

H2O + NH3 → NH4+ + OH-

CH3COOH + NH3 → NH4+ + CH3COO-

From these equations it is clear that acidity and basicity of a substance and a solvent are relative to each other i.e. substance can react as a base in presence of more proton donor one and as an acid in presence of a more proton acceptor.

• H2O in presence of proton acceptor H2O is acidic• H2O in presence of proton donor H2O is basic

Acidity and basicity of a substance is relative to the medium.

Substance acts as an acid in the presence of proton acceptor.

Substance acts as an base in the presence of proton donor.

HCl not acidic to dry L.P.

-CH3COOH reacts as acid in pyridine

-CH3COOH reacts as base in HClO4, H2SO4

CH3COOH + HClO4 ClO4- + CH3COOH2

+

*Strongest acid in non aqueous medium

Ex.1

Ex.2

Why?Due to the absence of water which is the H+

acceptor. * V.imp note: الوسط حسب تحديد يتم تانيفيه الموجود

base or acidic المادة

N NCH3COOH + + CH3COO-

H+

A1 B2 Conj. acid (A2) Conj. base (B1)

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Classification of acids and bases according to Lewis Theory:

A-Defined acids as that accept a lone pair of electrons (unshared) to form co-ordinate covalent bond.

Examples: Cl

1-Uncharged molecules B- Cl , AlCl3 , ASCl3

Cl , BF3 , AlBr3

2-Cations Ag+, Cu2+, Zn2+, Co2+

coordinate bond :رابطة لتكوين االلكترونات من زوج تقبل التي المادة هي الحمض

ال

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B-defined a base as electron-pair donor to form co-ordinate covalent bond.

Examples: Me

1-Uncharged molecules: NH3 and :N Me

Me

2-Ions: Cl-, Br-, OH-, SO42-

Coordinate : ال رابطة لتكوين االلكترونات من زوج تمنح التي المادة هي القاعدة

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Roles of solvent in non-aqueous titrations:

1-relative acidity and basicity:According to Bronsted theory acidity and basicity of

substance is relative to the solvent.

If substance is more acidic than the solvent, it will behave as an acid.

If more basic than solvent, behave as a base

e.g. potassium acid phthalate dissolved in water acts as primary acid, while in glacial acetic acid acts as a base and used as primary basic substance.

•Levelling effect & differentiating ability

المستوى :of non-aqueous solvents بعليHCl , CH3COOH in NH4OH

• NH4OH attack the same no. of H+ so both acids have the same acidity

HCl , CH3COOH in H2O

• HCl donate H+ more than CH3COOH

NH4OH: leveling & non-differentiating

H2O: non-leveling &

differentiating

NH4OسحبتH*

+H من Acetic acid

and HClالقوة بنفس

بالتالي وجعلت

Acetic acid and HClبنفس المستوى

H2O اما*H+سحبت

HCl مناقوى بشكل

ال منAcetic acid

بالتالي وH2O

بين فرقتالحمضين

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2-Levelling effect of solvents:

• It is the ability of solvent to increase the strength of weak acids or weak bases to reach that of strong ones, respectively.

• Basic solvents have a levelling effect on weak acids, generally strong acids gives it’s H+ easily, while the basic solvent attracts H+ easily from the weak acid so that both, the strong & weak acids, will have the same strength.

القوي الحمض انو عارفين احنالكن , و بسهولة بروتن بيفقد

ميقدرش الضعيف فقد يالحمضلما بالتالي و و بسهولة البروتنهتقدر , قوية قاعدة استخدمالحمض من البروتن تسحبالحمض هتخلي و الضعيف

القوي الحمض زي يبان الضعيف

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• Also acidic solvents have levelling effect on weak bases, normally strong base attracts H+ from the solvent, while in weak bases the acidic solvent donates or gives its proton easily to it , i.e. weak base and strong base are of equal strength.

Examples:1-Glacial acetic acid has levelling effect on bases such as amines i.e. they are strong bases in acetic acid.

2-Liquid ammonia and ethylene diamine which are more basic than H2O have levelling effects on both

mineral and carboxylic acids (acetic), they all nearly have the same strength in such solvents.

المذيب يكون لما الفكرة نفسبسهولة , بروتن هيمنح قوي حمضقاعدة من خليط عندي يكون فلمافطبيعي , ضعيفة اخرى و قويةبروتون هتستقبل القوية القاعدةالحمض , الضعيفة القاعدة و بسهولةاستقبال على هيجبرها القويالقاعدة كفاءة بنفس البروتون القوية

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3-Differentiating effect of solvents:• i.e. the ability of the solvent to differentiate between

the strength of acids and bases:

Examples:

1-Water is differentiating solvent for amines (weak bases) they are much weaker than mineral alkalies

( NaOH & KOH).

2-When HClO4, HCl and HNO3 are dissolved in

glacial acetic acid it is only HClO4 is the strong

acid i.e. acetic differentiates between strength of mineral acids.

Levelling ال فكرة نفسان هنا االختالف بسفي المستخدم المذيبالDifferentiating

ضعيفة قاعدة يكون اماحمضضعيف او

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Solvent properties that affect

levelling & differentiating effect of

solvent and completeness of neutralization reactions.

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• 1) Acid-base properties of solvent relative to the solute.Solvent such as formic, sulphuric and glacial acetic acid is proton donor more than being proton acceptor i.e. acidic in nature having levelling effect on bases

e.g. aniline weak base in water (Ka = 10-10) can not be directly titrated, whoever it is strong base in glacial acetic acid and can be directly titrated.

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• Solvents such as ethylene diamine, n-butylamine and liquid ammonia are more proton acceptor than donor i.e. basic in nature have levelling effect on weak acids as phenol which has Ka = 10-10 in H2O,

but strong enough in such solvents to be directly titrated.

H2O , ال في جدا ضعيفة بصورة بتبان االحماض القواعد بعض في( titration ) اعملها مقدرش بالتالي و

Strong basic solvent , الحمض بأضع المشكلة احل عشان وفي الضعيف

Acidity ال أزود علشانStrong acidic base الضعيف في .للحمض الضعيفة القاعدة أضع و

أعمل أقدر بالتالي و الضعيفة ال Bascicity للقاعدة أزود علشانTitration

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2) Autoprotolysis constant (Kap) of the solvent

Autoprotolysis is self-dissociation of solvent

2HA → [A-.HAH+] → A- + H2A+

Kap = [A-] [H2A+]

2 NH3 → NH4+ + NH2

-

2 CH3OH → CH3OH2+ + CH3O

-

H2O + H2O → H3O+ + OH-

2 CH3COOH → CH3COOH2+ + CH3COO-

Conj. Acid Conj. Base- i.e. two solvent moles interact one acts as a proton

donor and the other acts as proton acceptor.

عندها مواد فينفس في انها خاصيةتبقى ممكن الوسطتبقى ممكن و حمض

نفس في قاعدة. الوقت

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• The smaller the autoprotolysis constant Kap of

solvent the greater will be its ability to be differentiating.

- The smaller the Kap the more complete the reaction because solvents with high Kap will produce a considerable amount of ionic species which will compete with substance to be determined for the protons. (decrease competition of conjugate base of the solvent with the basic sample for protons and also to decrease competition between conjugate acid of the solvent and acidic sample).

كان ما كل

Kapده , عاليعلى دليلالمادة انقدرة ليهاانها عالية تعطي

Conjugate base and conjugate

acid

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3-The dielectric constant (DE):• It is the ability to separate positively charged ions

from negatively charged ions. Solvent with high DE minimum work is required for such separation.

• The higher DE of solvent the greater the separations by decreasing force of attraction between +ve and –ve ions.

- H2O is unique solvent it has low Kap but highest DE of

all solvent which allows completeness of reactions.

Kdiss DE Kap differentiating

على المذيب قدرة هيال من افصل السالبة يوتات

الموجبة االيوتات• زادت كلما ,DE كلما

المذيب قدرة زادتعن االيونات فصل على

العكسصحيح و بعدها

Classifications of solvents used in non-aqueous titrations

1) Amphiprotic• Acts as H+ donor & acceptor• Acts as acid &base (amphoteric).

• e.g. H2O (neutral), CH3COOH (acidic, but has no definite acidic characters) , NH3 (basic, but has no definite basic characters).

3) Protophilic• definite basic characters without

any acidic characters.• No autoprotolysis• Exert levelling effect on weak

acids.• e.g. ethylene diamine , pyridine ,

DMF, DMSO, butylamine.

2) Aprotic• Can’t accept or donate H+ (inert solvents).•No autoprotolysis, low DE.• Used to prevent solvolysis of the reaction products near e.p so more sharp e.p.• e.g. Hydrocarbon (HC), halogenated HC

4) Protogenic• definite acidic characters without

any basic characters.• No autoprotolysis• Exert levelling effect on weak

bases.• e.g. H2SO4,HClO4.

انها قدرة ليهاحمض تكون

قاعدة اوالوسط حسب

فيه الموجود

على قدرة ليهااستقبال

بس بروتوناتمحتاجة مش

تاني

- Aprotic:- يمنح أن على قدرة لها ليس

بروتونات يستقبل او- المذيب تأين لمنع بيستخدم

قراءة ليعطيEnd point دقيقة

- Acetic acid لما أنا فمثالان , عارفين احنا كمذيب أستخدمعلى القدرة ليه االسيتك حمض

مرافقين حمض و قاعدة , انتاجTitration أعمل لما بالتالي و

Endpoint قبل titrantلل مع هتتفاعل

Conjugated base and conjugated acid

قراءة بالتالي وهتبقى مش ال Titrationدقيقة

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Choice of solvents:Best solvent has the following characters:

1-Lowest possible Kap↓

2-Highest possible DE↑

3-Acidic solvents are used to dissolve basic substances to increase their dissociation and strength, therefore allow complete reaction, similarly basic solvents are used for acidic substances

4- Substance must be soluble in the chosen solvent.

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Advantages of non aqueous titrations :1.Very weak acids or bases directly titrated in non-aqueous medium using leveling solvent.2.Sharper e.p obtained using aprotic solvent which hinder solvolysis of the product of the reaction.3. Differentiating solvents enables differential titration of mixture of acids or bases as they increase the difference between their dissociation constant to be 104 at least.4. If the sample insoluble in H2O could be dissolved in non-aqueous solvent.5. Low surface tension small droplet size< aqueous so more accurate volume of titration.

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Disadvantages of non aqueous titrations :1. Non-aqueous solvents have high thermal

coefficient, therefore titration must be carried out at the same temp. of preparation of standard or the volume of titrant should be multiplied by C.F.

C.F = 1 ± 0.001 t˚ C t˚ C = difference in temp. at which standard prepared & used

2. Non-aqueous titration is non specific (any acidic or basic impurities can interfere.

Titration نعمل لما الزمNon- aqueous نستخدم لما و

Solventالحرارة درجة نفس في يبقى

ده المذيب فيها اتجهز .اللي

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Application of Non-aqueous titration

I-Determination of basic substances:1-Solvents used: glacial acetic acid, less common

formic acid.

free from H2O

Acetic acid must be free from water. Therefore water content of acetic acid must be

carefully determined (using Karl-Fischer reagent), then the solvent is refluxed with a suitable amount of acetic anhydride for removal of water.

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3-Detection of the end point

(a) Instrumental methods (potentiometrically):

measurement of potential of ion selective (glass electrode) which responds to the concentration of solvated protons.

(b) Visual method:using acid-base indicators e.g.

Acetus crystal violet: violet to bluish green.

2-Titrant: acetous perchloric acid, 0.1N HClO4 in

glacial acetic acid standardize against primary standard base KHphthalate.

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Applications:

• Weak bases e.g. amines, purine bases such as coffiene, theophylline …, alkaloids, urea, amides… i.e. all can be titrated in glacial acetic using acetous perchloric and crystal violet indicator.

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A) Primary, secondary, tertiary Amines Mixtures:

1. Total amines directly titrated with acetous perchloric in glacial acetic acid using crystal violet , according to the following equations:

RNH2 + HClO4 → RNH3+ + ClO4

-

R2NH + HClO4 → R2NH2+ + ClO4

-

R3N + HClO4 → R3NH+ + ClO4-

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2. Determination of tertiary amine only: Equal amount of the mixture (used in (1) is refluxed with acetic anhydride where primary and secondary amines are acetylated only. The remaining tertiary amine is determined by titration against hydrochloric acid in isopropanol.

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+ RNH2 CH3CO.NHR + CH3COOH

+ R NH CH3CO.NR2 + CH3COOH

R

O

CH3C

OC

O

H3C

O

CH3C

OC

O

H3C

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3-Determination of secondary and tertiary amines together: Equal volume of sample is heated with salicylaldehyde which forms schiff’s base with primary amine only. The product is titrated with hydrochloric acid in isopropanol, mls = tertiary + secondary amines

OH

CHO

+ RNH2 + H2OOH

CH NR

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• Calculation:• Volume of standard perchloric in (2) = tertiary

amine• Volume of standard in (3) – (2) = secondary

amine• Volume of standard (1) – (3) = primary amine

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B. Amino acids

• Amino acids such as alanine, glycine…in glacial acetic, dissociation of -COOH group is suppressed while –NH2 group is levelled up and can be titrated with acetous perchloric using crystal violet.

+ HClO4 + ClO4

-H3C

H

C COOH

NH2NH3+

H3C

H

C COOH

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II-Determination of Acidic substances:

1. Solvents: Benzene-Methanol (or Ethanol) for moderate

strong acids• Methyl isobutyl ketone ,acetone and acetonitrile

have differentiating effect on acids and can be used for step wise titration of mixed acids

• leveling solvents e.g. ethylene diamine, DMF, pyridine, butylamine

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2. Titrant: Sodium Methoxide (NaOCH3), NaOC2H5 (ethoxide) in

methanol or ethanol-benzene mixture, used for carboxylic and acidic substance of similar strength.

Prepared by dissolving Na° in the corresponding alcohol. Standardize against benzoic acid

3. Indicators:• Thymol blue changes its colour from yellow in acid side

to blue in alkali one. • Azo violet changes its colour from red to blue.

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Determination of sulphonamides:

Sulfonamides contain – NH2 which is a basic group and–SO2-NH- which is weak acidic group. i.e., sulfonamides are amphoteric therefore we have toadd strong basic solvent to increase its acidic character.-Solvent: DMF, butylamine-Standard: Na methoxide.-Indicator: thymol blue or azo-violet.

Or for NH2 (basic) sulfonamides add strong acidic solvent to increase its basic character -Solvent: glacial acetic acid. -Standard: perchloric acid.

H2N CH3ONa

CH3OH H2N

Na

SO2 NH R +

+ SO2 N R

OR

H2N HClO4SO2 NH R +

ClO4-

SO2 NH R++H3N

Weak acids Weak bases

Solvent ethylene diamine, DMF, pyridine, butylamine

Glacial acetic acid

Titrant CH3ONa, C2H5ONa,tetrabutyl ammonium hydroxide

Acetus perchloric acid

Standardization of titrant

Against benzoic acid Against primary standard base KHphthalate

Detection of end point

Potentiometrically or by using acid-base indicators, e.g., thymol blue (yellow to blue) or azo violet (red to blue)

Potentiometrically or by using acid-base indicators, e.g., acetous crystal violet (violet to bluish green)

Examples Phenol, sulphonamides, amino acids

Amines, sulphonamides, amino acids, alkalie metal salts and ammonium salts of carboxylic acids eg. acetate, oxalate, benzoate, salicylate, citrate.