experiment 8b

Experiment 8BQualitative Analysis II

Introduction

• Qualitative analysis of an unknown organic

compound is an indispensible tool in experimental

organic chemistry. This method is done in order to

determine the possible structures of an

unknown sample. The different functional groups

present in an organic compound are identified by

reacting the sample with different reagents that react

that gives a visible result. It is an indispensable tool

in experimental organic chemistry as well as other

fields such as the study of natural products and

biochemistry.

Sodium Metal

• Results

Tests for Alcohols

Sample Visible ResultStructure/Formula of compound

responsible for visible results

n-butyl

alcoholEvolution of gas H2

Sec-butyl

Tert-butyl

Sodium Metal

• Type of Reaction: Acid-Base Reaction

• General Formula:

Tests for Alcohols

Sodium Metal

• Mechanism

Tests for Alcohols

sodium

alkoxidebubbles

Sodium Metal

• Complications

– Dealing with sodium metal can be exciting.

Make sure that all samples are dry before

proceeding with test.

Tests for Alcohols

Sodium Metal

1. What property of alcohol is demonstrated in the reaction with Na metal? What is the formula of the gas liberated?

– The alcohol exhibits acidic property in the reaction with Na metal. The hydrogen atom is replaced by Na producing H2 shown in the reaction:

– 2 R–O–H + 2 Na 2 R–O(–)Na(+) + H2

(http://www2.uni-siegen.de/~pci/versuche/english/v44-1-1.html)

Tests for Alcohols

Sodium Metal

2. Dry test tube should be used in the

reaction between the alcohols and Na

metal. Why?

– A dry test tube should be used because Na

metal is highly reactive with water and may

ignite violently due to the H2 gas produced

(http://www2.uni-siegen.de/~pci/versuche/english/v44-1-1.html)

Tests for Alcohols

Lucas Reagent (HCl/ZnCl2)

• Results

Tests for Alcohols

Sample Visible Result

Structure/Formula of

compound responsible

for visible results

n-butyl No layer None

Sec-butyl

alcohol

Formation of

layers(CH3)2CHCl + H2O

Tert-butyl

alcohol

Formation of

layers(CH3)3CCl + H2O

• Type of Reaction: Nucleophilic Substitution

• Substrate:

• Attacking Agent: Cl-

Tests for Alcohols

• Mechanism (SN1 for 2º and 3º ROH)

Tests for Alcohols

insoluble layer

• Mechanism (SN2 for 1º ROH)

Tests for Alcohols

insoluble layer

• ZnCl2 is a Lewis acid that complexes strongly

with the lone-pairs oxygen. This weakens the C-

O bond and creates a better leaving group.

• ZnCl2 also enhances the reactivity of the HCI by

polar coordination.

Tests for Alcohols

• Complications

– The test applies only to those alcohols soluble

in the reagent (monofunctional alcohols lower

than hexyl and some polyfunctional alcohols).

Tests for Alcohols

3. Why is the Lucas test not used for alcohols

containing more than eight carbon atoms?

– The Lucas test applies only to alcohols soluble in

Lucas reagent (monofunctional alcohols with less

than 6 carbons and some polyfunctional alcohols).

Long chains of carbon makes the compound nonpolar

and the –OH group less functional. This results to the

insolubility of the alcohol in the reagent, rendering the

test ineffective.

Tests for Alcohols

4. Explain why the order of reactivity of the

alcohols toward Lucas reagent is 3°>2°>1°?

– The reaction of alcohols with Lucas reagent is a

nucleophilic substitution reaction. In an Sn1 reaction,

the rate determining step is the carbocation. The

carbocation is most stable in a tertiary alcohol and is

therefore formed faster than a secondary and a

primary carbocation. (Solomons and Fryhle)

Tests for Alcohols

Potassium Dichromate

• Results

Tests for Alcohols

compound responsible for

visible results

n-butyl Blue-green soln Cr3+

Sec-butyl Blue-green Cr3+

Tert-butyl Yellow-orange Cr2O72-

• Type of Reaction: Reduction-Oxidation

Tests for Alcohols

• Mechanism

Tests for Alcohols

• Mechanism

Tests for Alcohols

• Mechanism

Tests for Alcohols

chromate ester

elimination

(due to chromium ion, Cr3+)

• For 1º ROH, RCHO products are further

oxidized to form RCOOH

Tests for Alcohols

• Tertiary alcohols do not react because there will

not be any b-hydrogens to eliminate.

Tests for Alcohols

• Complications

– Aldehydes, which also give a positive test, are

better characterized in other ways. The color

usually develops in 5 - 15 seconds.

– Enols may give a positive test.

– Phenols give a dark colored solution which is

not blue-green like a positive test.

Tests for Alcohols

5. Illustrate with equations the reactions of the following alcohols with Potassium Dichromate solution:

Proof that dichromate is a strong oxidizing reagent in an acidic sol’n (ε0= 1.33 V) (ε0 = −0.13 V)

• n-butylalcohol

– CH3CH2CH2CH2OH + Cr2O72- CH3CH2CH2CHO + [O]

CH3CH2CH2COOH

• sec-butylalcohol

– CH3CH2CH(OH)CH3 + Cr2O72- CH3CH2COCH3 + H2O

• tert-butylalcohol– (CH3)3COH + Cr2O7

2- no reaction

Tests for Alcohols

Ferric Chloride

• Results

Tests for Phenols

visible results

Phenol Brown-black Fe(OAr)3

Catechol Brownish Fe(OAr)3

Resorcinol Brownish green Fe(OAr)3

a-naphthol Purple solution Fe(OAr)3

Ferric Chloride

• Type of Reaction: Complexation

Tests for Phenols

Ferric Chloride

• Mechanism

Tests for Phenols

blue to violet color

Ferric Chloride

• Complications

– Not all phenols or enols give positive results.

– Most oximes, hydroxamic acids, and sulfinic acids give a positive test.

– Bulky groups on the benzene ring, especially in the ortho position, may cause the experiment to show a negative result.

– Activating groups attached to the ring decreases the wavelength caused by less electron excitation, thus causing a different color of positive result.

Tests for Phenols

Bromine Water

• Mechanism

Tests for Phenols

for visible results

Phenol Light brown 2,4,6-tribromophenol

Catechol Dark solutionBromination of benzene

Resorcinol Yellow with precipitate Tribromoresorcinol

a-naphthol Dark cloudy solution 2,4-dibromo-1-naphthol

Bromine Water

• Type of Reaction: Electrophilic Substitution

• Substrate: ArOH

• Attacking Agent: Br+

Tests for Phenols

brown to black solution

yellow Br color is removed

Bromine Water

• Mechanism

Tests for Phenols

Bromine Water

• The aromatic ring is activated by the hydroxy

group, therefore, it can react by electrophilic

addition

Tests for Phenols

Bromine Water

• Complications

– Mercaptans react readily.

– HBr is generated, but will not be observed

since the reagent is aqueous.

Tests for Phenols

Millon’s (Hg(NO3)2 in H2O)

• Results

Tests for Phenols

visible results

PhenolDark upper layer, light

bottomHg complex with phenols

Catechol Dark solution Hg complex with phenols

Resorcinol Dark purple Hg complex with phenols

a-naphthol Greenish yellow Hg complex with phenols

• Type of Reaction: Electrophilic Addition

and Complexation

• Substrate: Ar-OH and tyrosine

• Attacking Agent: Hg(NO3)2

Tests for Phenols

• Mechanism

1. Nitration of aromatic ring

2. Complexation with mercuric ion

• Produces: red complexes and ppt

Tests for Phenols

6. What functional group is responsible for

the observed result in Millon’s test?

– Phenol interacts with nitric acid, nitrated

phenol reacts with mercury and mercury salts

are formed. This reaction accounts for the

observed positive result in Millon’s test.

Tests for Phenols

2,4-dinitrophenylhydrazine

• Results

Tests for Aldehydes and Ketones

visible results

Formaladehyde Yellow ppt 2,4-dinitrophenylhydrazone

AcetaldehydeOrange,

brown(heated)2,4-dinitrophenylhydrazone

Acetone Orange ppt 2,4-dinitrophenylhydrazone

Benzaldehyde Orange ppt 2,4-dinitrophenylhydrazone

• Type of Reaction: Nucleophilic Addition

• Substrate:

• Attacking Agent:

• Mechanism

yellow to red ppt

• Complications

– Some ketones give oils which will not solidify.

– Some allylic alcohols are oxidized by the

reagent to aldehydes and give a positive test.

7. Why is it disadvantageous to use a strong acid catalyst in the reaction of aldehyde or ketone with 2,4-DNPH?– The use of a strong acid reverses the sequence of the

reaction. Instead of the nucleophilic attacking the substrate followed by the electrophile, in the presence of a strong acid, the weaker ucleophile attacks the carbon to stabilize the forming hemiacetal. Water abstracts the H+ of the hemiacetal. Hemiacetals are less stable and will form acetals which do not show the visible changes expected of the test.

8. Show the mechanism for the reaction of

acetaldehyde with the following reagents:

a. 2,4 – DNPH

Bisulfite

• Mechanism

visible results

Formaldehyde Ppt formation H2CH(OH)SO3- Na+

Acetaldehyde Ppt formation H3C(OH) SO3- Na+

Acetone No Ppt H3CC(OH)SO3- Na+

Benzaldehyde Ppt formation (C6H6)CH(OH)SO3- Na+

Bisulfite

• Type of Reaction: Nucleophilic Addition

• Substrate:

Bisulfite

• Mechanism

insoluble

Bisulfite

• Complications

– Aryl methyl ketones form the precipitate

slowly or not at all.

– Addition complex stable only in neutral

solution.

Bisulfite

b. NaHSO3

Schiff’s Test (Fuschine in NaHSO3 solution)

• Results

Formaldehyde Purple

Acetaldehyde Red-purple

Acetone No change

Benzaldehyde purple

• Type of Reaction: Nucleophilic Addition and

Elimination

• Substrate:

• Attacking Agent: Schiff’s Reagent

• Schiff’s Reagent (fuchsia/pink)

– (bis-N-sulfinic acid of p-rosaline hydrochloride) aka

fuchsin-aldehyde reagent or Leucosulfonic acid

• Mechanism

• Product: Quinoid Dye (purple/magenta)

• Complications

– In this test the reagent should not be heated, and the

solution tested should not be alkaline.

– When the test is used on an unknown, a

simultaneous test on a known aldehyde and a known

ketone should be performed for comparison.

9. Why is the Schiff’s test considered a general

test for aldehydes?

– Differentiation of aldehydes from ketones becomes

the main purpose of using Schiff’s test since it cannot

distinguish one kind of aldehyde from another.

• Results

for visible results

Formaladehyde Silver mirror Ag

Acetaldehyde Silver mirror Ag

Acetone Clear None

Benzaldehyde Silver mirror Ag

Tollen’s Test (Ag(NH3)2OH)

• Reducing Agent:

• Oxidizing Agent: Ag(NH3)2+

• Mechanism

silver mirror

• Complications

– The test tube must be clean and free of oil if a silver

mirror is to be observed.

– Easily oxidized compounds give a positive test. For

example: aromatic amine, some phenols, a-alkoxy

and a-dialkylaminoketones.

10.What test will you use to differentiate

each of the following pairs? Give also the

visible result.

– Acetaldehyde and acetone

• Schiff’s reagent: acetaldehyde – purple solution;

acetone – no reaction

• Tollen’s test: acetaldehyde – silver mirror; acetone

– no reaction

10.What test will you use to differentiate each of the following pairs? Give also the visible result.– Acetaldehyde and bezaldehyde

• Acetaldehyde and benzaldehyde can be distinguished from each other using the Bisulfite test since it is also an indication whether the sample is an aliphatic aldehyde or an aromatic aldehyde. Aldehyde will react faster than benzaldehyde because an aromatic ring makes the compound more electron-rich, making the compound less susceptible to nucleophilic attack. Both will form a red precipitate due to cuprous oxide.

Iodoform Test (NaOH/I2KI)

• Results

for visible results

Formaladehyde None None

Acetaldehyde Yellow Ppt CHI3

Acetone Yellow Ppt CHI3

Benzaldehyde None None

• Type of Reaction: Nucleophilic Addition + Substitution

• Substrate: I+

• Attacking Agent: OH-

addition substitution

• Mechanism: Secondary alcohols/ethanol with an

adjacent methyl group are oxidized to methyl

ketones/ethanal by iodine bleach.

• Mechanism (Nucleophilic Addition Part)

• Mechanism (Nucleophilic Substitution Part)

yellow

• Complications

– Test will not be positive if the R group is a di-

ortho substituted aryl group.

11.What structural feature in a compound is

required for a positive iodoform test? Will

ethanol give a positive iodoform test? Why or

why not?

– Ketones and alcohols with a methyl group directly

adjacent to a carbonyl group or a carbon atom

bearing a hydroxyl group react with an alkaline

solution of iodine to produce a yellow solid, iodoform.

Ethanol will give a positive result because it can be

oxidized to an aldehyde with a methyl group attached

to a carbonyl group.

12.Show the mechanisms for the iodoform using

acetaldehyde as the test sample:

• Results

visible results

Formaladehyde Brick Red Precipitate Cu2O

Acetaldehyde Brick Red Precipitate Cu2O

Acetone None No rxn

Benzaldehyde Brick Red Precipitate Cu2O

Fehling’s Test [Cu(C4H4O6)2]4-

• Reducing Agent: (reducing sugar)

• Oxidizing Agent: [Cu(C4H4O6)2]4-

• Fehling’s Reagent: Sodium tartrate, NaOH and

CuSO4 forming a copper-tartrate complex

• Mechanism

brick red ppt

• Complications

– None.

• Results

Test Samples Visible Result

Structure Formula of

Compound Responsible

for the Visible Result

Glucose Violet Ring

1% Sucrose Violet Ring

Maltose Violet Ring

1% Boiled Starch Violet Ring

Molisch (a-naphthol in ethanol)

• Substrate: furfural or hydroxymethylfurfural

• Attacking Agent: a-naphthol

• Mechanism

– Dehydration of Hexose Sugar by H2SO4 (elimination)

• Mechanism

– Dehydration of Pentose Sugar by H2SO4 (elimination)

• Mechanism

violet

Benedict’s Test [Cu3(citrate)2]

• Results

visible results

Glucose Brick Red Precipitate Cu2O

Maltose Brick Red Precipitate Cu2O

Sucrose Brick Red Precipitate Cu2O

Boiled starch No change No rxn

• Reducing Agent: (reducing sugar)

• Oxidizing Agent: Cu3(citrate)2

• Benedict’s Reagent: Sodium citrate, CuSO4 and

NaHCO3 forming copper-citrate

• Mechanism

brick red ppt

• Complications

– Not general for simple aldehydes and ketones.

– Hydrazine derivatives give a positive test.

Hinsberg (benzenesulfonyl chloride)

• Results

Tests for Amines

visible results

Methylamine ppt in acid C6H5SO2NRH

Dimethylamine ppt in base/acid C6H5SO2NR2

Trimethylamine ppt in base R3NHCl

Aniline ppt in baseacid C6H5SO2NRH

n-methylamine ppt in acid C6H5SO2NRH

• Substrate:

Tests for Amines

• Mechanism (1º amines)

Tests for Amines

water soluble water insoluble

Tests for Amines

water insoluble

Tests for Amines

water insoluble

Tests for Amines

water soluble

• Complications

– Amphoteric compounds give erroneous results.

– Some sodium salts of benzenesulfonamides of

primary amines are insoluble in the Hinsberg solution

and may appear to be secondary amines.

– Some tertiary amine hydrochloride salts are insoluble

in dilute HCl and water and may also appear to be

secondary amines.

Tests for Amines

Esterification

• Results

Tests for Carboxylic Acid + Derivatives

for visible results

Salicylic acidColorless with

wintergreen odor

methyl 2-

hydroxybenzoate

Esterification

• Substrate:

Esterification

• Mechanism

ester with odor

Esterification

• Complications

– None.

Hydrolysis of Benzamide

• Results

for visible results

BenzamideRed to blue litmus

Pungent odorammonia

• Substrate:

• Attacking Agent: OH-

• Mechanism

basic and pungent

• Complications

– none

Hydrolysis of Ester

• Results

for visible results

EthylacetateLoss of Sweet

OdorAcetic acid and Ethanol

Hydrolysis of Ester

• Substrate:

Hydrolysis of Ester

• Mechanism

Hydrolysis of Ester

• Complications

– none

Hydrolysis of Anhydride

• Results

for visible results

Acetic anhydride Acidic Acetic acid

• Substrate:

• Attacking Agent: HOH

• Mechanism

• Complications

– Higher aliphatic anhydrides and aromatic anhydrides

are not readily hydrolyzed with water and thus may

not give a positive test.

Hydroxamic Acid Test(hydroxylamine hydrochloride)

• Results

EthylacetateBurgundy or magenta

color Hydroxamate complexes

Benzamide Red to violet color Hydroxamate complexes

Acetic anhydrideBurgundy or magenta

BenzoylchlorideBurgundy or magenta

• Substrate:

Tests for Carboxylic Acid + Derivativeshydroxamic acid

• Formation of Hydroxylamine

• KOH is added to consume the HCl, so that the reaction will not proceed backward

• Mechanism for Anhydrides

• Mechanism for Acyl Halides

• Mechanism for Esters

• Mechanism for Amides

• Ferric Hydroxamate Complex Formation

• The solution is acidified by HCl to prevent the

excess base from reacting with FeCl3 (a Lewis Acid)

red/violet/burgundy/magenta color

• Complications

– Some acids will give a positive test.

– Primary and secondary nitro compounds, imides,

some amides, most nitriles, and aldehydes (with no a-

hydrogens give a positive test.

– Some sterically hindered amides fail to react.

Last Guide Question!

13. A colorless liquid has a bp 199-201°C and burns with a smoky flame. The sodium fusion test proved negative for the presence of halogens, nitrogen and sulfur. It was not soluble in water, 5% aqueous sodium hydroxide, or 5% hydrochloric acid. However, it dissolved in sulfuric acid with evolution of heat. It did not give a precipitate with 2,4-DNPH solution and did not decolorize bromine-methylene chloride solution. The unknown liquid did give a positive hydroxamate test and was found to have a saponification equivalent of 136. Identify the unknown liquid.

– Smoky flame indicates that the compound is aromatic. It tested negative for 2,4-DNPH, telling us that it is neither an aldehyde nor a ketone. Negative tests for bromine in methylene chloride solution also indicates that it is neither an alcohol nor a phenol. A positive test for hydroxamate test tells us that it is an acid derivative, which can undergo saponification—therefore it is an ester. Finally, an aromatic ester with a boiling point range of 199-201oC is methyl benzoate.

References

• http://chemed.chem.purdue.edu/genchem/topicreview/bp

/ch9/active.php

• http://www.biologie.uni-hamburg.de/b-

online/library/newton/Chy251_253/Lectures/Oxidation_of

_Alcohols/Oxidation.html

• http://homepages.ius.edu/DSPURLOc/c122/ket.htm

• http://www.curvedarrowpress.com/partd/tollens.html

• http://web.pdx.edu/~wamserc/C335W99/Fans.htm

• http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/

amine1.htm

• http://www.demochem.de/p30_Z_mol-e.htm

References

• Hornback, J.M. (2006). Organic Chemistry. 2nd Ed.

Belmont, CA: Thomas Brooks/Cole.

• Shriner, R.L. (2004). Systematic Identification of Organic

Compounds. NJ: John Wiley and Sons Inc.

• Vogel, A.I. (1989). Vogel’s Textbook of Practical Organic

Chemistry 5th Ed. UK: Longman Group.

experiment 8b

nucleophilic substitution substrate

brick red precipitate

nucleophilic addition substrate

oxidation reducing agent

experimental organic chemistry

brick red ppt

resultssample visible result

methylene chloride solution

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