معلومات مقرر دراسي

EXPERIMENT 25
AMINES

Organic amines are compounds in which one or more hydrogens of an ammonia molecule, NH₃ have been replaced by R- or Ar- groups. Amines may be classified as:

(1)Primary R-NH₂ or ArNH₂

(2)Secondary R₂NH or ArRNH₂ or Ar₂NH

(3)Tertiary R₃N, ArR₂N, Ar₂RN

Amines are quite basic. Ammonia itself has a Kb value of 1.8 x 10^-5. Aliphatic amines have Kb values of 10^-4, while aromatic amines have Kb values as 10^-10. These differences are explained by the relative stability of the cation produced in the reaction: G-NH₂ + H⁺ àG-NH₃⁺ When G = R, base strength is increased because the R- group is electron donating group and therefore stabilizes the cation produced. When G = Ar, the base strength is decreased because the net effect of the aryl group is to withdraw electrons and destabilize the cation formed.

The relative base strength of amines can be tested in several ways. Aqueous solutions of each base can be tested with indicators. Several drops of the amine to be tested can be added to FeC1₃ solution. If the amine produces sufficient OH^- to exceed the Ksp for Fe(OH)₃, a brown gelatinous precipitate forms. Because iron(III) hydroxide has a very small Ksp value, this test is positive for very weak bases.

In addition, amines are readily protonated by strong acids such as HCl and H₂SO₄.

The water solubility of amines is similar to that of other organic compounds that contain H-bonding portions. Small aliphatic amines are water soluble. As the carbon number increases, solubility in water decreases. The usual limit occurs at about four or five carbons. Thus, methylamine is soluble in water while aniline is not soluble. Although most amines are NOT water soluble, amine salts, being ionic, are generally water soluble.

Most pure amines are clear or very pale in color. Often, stock samples are found to be of various colors, from dark brown to almost black. Amines are quite easily oxidized to form these highly colored products. However, most amines can be readily purified by distillation or by recrystallization. This oxidizability of amines is shown by simple reaction with bleaching powder.

Whether an amine is primary, secondary, or tertiary can be determined by the Hinsberg test. The amine in question is treated with benzenesulfonyl chloride in KOH and then acidified with HCl. The results are interpreted as follows:

Often, this test is particularly difficult to interpret because of impurities present in the amine.

Many amines form solid derivatives with the appropriate reagents. Among the most frequently prepared derivatives are amides and carbanilides. When a primary or secondary amine is reacted with acetic anhydride or acetyl chloride, an acetamide is produced.

If benzoyl chloride is used, the resulting product is a benzamide.

These reactions are examples of nucleophilic acyl substitution. Many times, reactions with acid chlorides are carried out in a basic solution to avoid the formation of HCl. This is called the Schotten-Baumann technique of amide formation.

Carbanilides and thiocarbanilides are readily formed by the reaction between primary and secondary amines with phenylisocyanate and phenylisothiocyanate respectively. Aniline and substituted anilines react well with either of these reagents.

ph-N=C=O       +           HNH-ph    à     ph-NH-CO-NH-ph

phenylisocyanate           aniline          diphenylurea

This reaction works very well. However, because the phenylisocyanate is a lachrymator, this reaction is not performed often. Similar in reactivity but less dangerous to use is phenylisothiocyanate.

ph-N=C=S       +             HNH-ph      à         ph-NH-CS-NH-ph

phenylisothiocyanate       aniline             diphenylthiourea

Amide formation and substituted urea formation both require the transfer of a proton after nucleophilic attack has occurred. Thus, tertiary amines are incapable of making derivatives of this kind. Tertiary amines are derivatized by the formation of specific salts.

In summary, amines represent a major functional group among organic compounds. They are characterized by their basicity, their ease of oxidation, and by their ability to form derivatives by nucleophilic acyl substitution (amides) and nucleophilic acyl addition (substituted areas).

Procedure

 I. Solubility of Amines

1. Place 3-mLs of distilled water in a reaction tube. Add ethylamine in 5-drop increments until a second layer is observed. Record on the data sheet.

2.Repeat the procedure using aniline. Record the results.

3.Repeat the procedure using solid aniline hydrochloride small pea-sized increments.

II. Basicity of Amines

1. pH paper

a) Place 3-mLs of distilled water in each of three medium sized test tubes. Number each tube as 1 - 3.

b) Add 5 drops of concentrated NH₃ to Tube 1; 5 drops of ethylamine to Tube 2;  and 5 drops of aniline to Tube 3.

c) Test each solution with pH paper. Record the results on the data sheet.

2. Ferric Chloride Test

Add about 2-mLs of 5% ferric chloride solution to each of the test tubes from above step 1. Observe and record the results.

3. Reaction with Strong Acids

a) Place one drop of concentrated sulfuric acid on a clean dry watchglass. Add 1-2 drops of aniline. Note and record the results.

b) Repeat the procedure using a drop of concentrated HC1. Note and record the results.

III. Ease of Oxidation

Place about 2.5-mLs of water and one drop of aniline in a reaction tube. Add a small amount of bleaching powder [Ca(OC1)₂] about the size of a small pea. Stopper and shake well. Observe and record.

IV. Derivative Preparation

A. Acetyl (To be prepared in the HOOD!)

(1)Place about 1-mL of acetic anhydride into a medium sized test tube. Add about 20-25 drops of aniline. Warm gently in the sand bath or in a water bath on the hot plate for three to four minutes. This should complete the reaction.

(2)Add about 0.5 mL of water, continue to heat. This should hydrolyze any remaining acetic anhydride. During this time white crystals of the acetamide  should appear.

(3)Add another 0.5-mL of water, heat to boiling. If all product crystals do not dissolve, add more water as needed. Once the crystals completely dissolve, remove from the heat. (This is recrystallization.)

(4)Cool thoroughly. Suction filter. Press dry. Obtain and record a melting point for the product.

B. Benzoyl (To be prepared in the HOOD!)

(1)Place about 20 drops of aniline in a medium-size test tube. Add 5-mLs of 10% NaOH. Stopper the tube well.

(2)Add about 5 drops of benzoyl chloride. Stopper and shake gently. Benzoyl chloride is a lachrymator. It should be handled carefully in the hood. It should NEVER be washed down an open sink. Rapid hydrolysis increases eye irritation.

(3)Add more benzoyl chloride in 5-drop increments. After each addition, stopper the tube and gently shake it. CAUTIOUSLY smell the tube after shaking. If no irritating odor is present, add another 5 drops of benzoyl chloride. This technique of addition prevents a large excess of the benzoyl chloride being present.

(4)Once the addition seems complete and product crystals of the benzanilide appear, cool the reaction mixture and suction filter using the Hirsch funnel. Wash the crystals with cold water. (NOTE: if excess benzoyl chloride is present, the filtration process will be accompanied by irritating fumes.)

(5)Recrystallize the crystals from hot ethyl alcohol.

(6)Cool, suction filter, press dry, and obtain a melting point.

C. Diphenylthiourea

(1) Place about 20 drops of aniline in a medium test tube. Add aboutt. two mLs of  ethyl alcohol.

(2) Add about one-mL of phenylisothiocyanate to the test tube. Stopper and shake.

(3) Crystals of diphenylthiourea should appear within a few minutes. If no crystallization is observed, follow the steps outlined below.

(a)Cool thoroughly in an ice-water bath.

(b)Scratch the inside of the test tube with a stirring rod.

(c)Add a little ice-water.

(d)Wait.

(4) Once crystals form, cool thoroughly. Suction filter using the Hirsch funnel.

(5) Recrystallize from a little hot ethyl alcohol.

EXPERIMENT 25: Report and Worksheet
AMINES

Student Name:  __________________         Day: ___________

Student Number: ________________________Date: _____________

DATA

I. solubility of Amines

II. Basicity of Amines

Aniline + Concentrated Sulfuric Acid _________________________

Aniline + Concentrated HCl ________________________

III. Ease of Oxidation

Aniline + Bleaching Powder _______________________

IV. Derivative Preparation

QUESTIONS

1. Which one is most soluble and which one is least soluble for (a) ethylamine, (b) aniline, and.(c) aniline hydrochloride.? And why?

2. Which one is the strongest base and which is the weakest base for (a) ammonia, (b) ethylamine, and (c) aniline. And Why? EXPLAIN in terms of electronic effects.

3. Write a BALANCED EQUATION for the reaction of (a) ammonia, (b) ethylamine, and (c) aniline with ferric chloride in water.

4. Write a BALANCED EQUATION for the reaction of aniline with H₂SO₄ and With HCl .

5. Explain, using appropriate equations how Primary, Secondary, and Tertiary amines react in the Hinsberg Test.

6. What is the equation for the reaction of aniline with acetic anhydride?

7. What is the equation for the reaction of aniline with benzoyl chloride? Why is the base necessary ?

8. What is the equation for the reaction of aniline with phenyl-isothiocyanate?

9. What characteristic IR absorbances are given by (a) primary amines, and (b) secondary amines ?EXPERIMENT 25

AMINES

Organic amines are compounds in which one or more hydrogens of an ammonia molecule, NH₃ have been replaced by R- or Ar- groups. Amines may be classified as:

(1)Primary R-NH₂ or ArNH₂

(2)Secondary R₂NH or ArRNH₂ or Ar₂NH

(3)Tertiary R₃N, ArR₂N, Ar₂RN

Amines are quite basic. Ammonia itself has a Kb value of 1.8 x 10^-5. Aliphatic amines have Kb values of 10^-4, while aromatic amines have Kb values as 10^-10. These differences are explained by the relative stability of the cation produced in the reaction: G-NH₂ + H⁺ àG-NH₃⁺ When G = R, base strength is increased because the R- group is electron donating group and therefore stabilizes the cation produced. When G = Ar, the base strength is decreased because the net effect of the aryl group is to withdraw electrons and destabilize the cation formed.

The relative base strength of amines can be tested in several ways. Aqueous solutions of each base can be tested with indicators. Several drops of the amine to be tested can be added to FeC1₃ solution. If the amine produces sufficient OH^- to exceed the Ksp for Fe(OH)₃, a brown gelatinous precipitate forms. Because iron(III) hydroxide has a very small Ksp value, this test is positive for very weak bases.

In addition, amines are readily protonated by strong acids such as HCl and H₂SO₄.

The water solubility of amines is similar to that of other organic compounds that contain H-bonding portions. Small aliphatic amines are water soluble. As the carbon number increases, solubility in water decreases. The usual limit occurs at about four or five carbons. Thus, methylamine is soluble in water while aniline is not soluble. Although most amines are NOT water soluble, amine salts, being ionic, are generally water soluble.

Most pure amines are clear or very pale in color. Often, stock samples are found to be of various colors, from dark brown to almost black. Amines are quite easily oxidized to form these highly colored products. However, most amines can be readily purified by distillation or by recrystallization. This oxidizability of amines is shown by simple reaction with bleaching powder.

Whether an amine is primary, secondary, or tertiary can be determined by the Hinsberg test. The amine in question is treated with benzenesulfonyl chloride in KOH and then acidified with HCl. The results are interpreted as follows:

Often, this test is particularly difficult to interpret because of impurities present in the amine.

Many amines form solid derivatives with the appropriate reagents. Among the most frequently prepared derivatives are amides and carbanilides. When a primary or secondary amine is reacted with acetic anhydride or acetyl chloride, an acetamide is produced.

If benzoyl chloride is used, the resulting product is a benzamide.

These reactions are examples of nucleophilic acyl substitution. Many times, reactions with acid chlorides are carried out in a basic solution to avoid the formation of HCl. This is called the Schotten-Baumann technique of amide formation.

Carbanilides and thiocarbanilides are readily formed by the reaction between primary and secondary amines with phenylisocyanate and phenylisothiocyanate respectively. Aniline and substituted anilines react well with either of these reagents.

ph-N=C=O       +           HNH-ph    à     ph-NH-CO-NH-ph

phenylisocyanate           aniline          diphenylurea

This reaction works very well. However, because the phenylisocyanate is a lachrymator, this reaction is not performed often. Similar in reactivity but less dangerous to use is phenylisothiocyanate.

ph-N=C=S       +             HNH-ph      à         ph-NH-CS-NH-ph

phenylisothiocyanate       aniline             diphenylthiourea

Amide formation and substituted urea formation both require the transfer of a proton after nucleophilic attack has occurred. Thus, tertiary amines are incapable of making derivatives of this kind. Tertiary amines are derivatized by the formation of specific salts.

In summary, amines represent a major functional group among organic compounds. They are characterized by their basicity, their ease of oxidation, and by their ability to form derivatives by nucleophilic acyl substitution (amides) and nucleophilic acyl addition (substituted areas).

Procedure

 I. Solubility of Amines

1. Place 3-mLs of distilled water in a reaction tube. Add ethylamine in 5-drop increments until a second layer is observed. Record on the data sheet.

2.Repeat the procedure using aniline. Record the results.

3.Repeat the procedure using solid aniline hydrochloride small pea-sized increments.

II. Basicity of Amines

1. pH paper

a) Place 3-mLs of distilled water in each of three medium sized test tubes. Number each tube as 1 - 3.

b) Add 5 drops of concentrated NH₃ to Tube 1; 5 drops of ethylamine to Tube 2;  and 5 drops of aniline to Tube 3.

c) Test each solution with pH paper. Record the results on the data sheet.

2. Ferric Chloride Test

Add about 2-mLs of 5% ferric chloride solution to each of the test tubes from above step 1. Observe and record the results.

3. Reaction with Strong Acids

a) Place one drop of concentrated sulfuric acid on a clean dry watchglass. Add 1-2 drops of aniline. Note and record the results.

b) Repeat the procedure using a drop of concentrated HC1. Note and record the results.

III. Ease of Oxidation

Place about 2.5-mLs of water and one drop of aniline in a reaction tube. Add a small amount of bleaching powder [Ca(OC1)₂] about the size of a small pea. Stopper and shake well. Observe and record.

IV. Derivative Preparation

A. Acetyl (To be prepared in the HOOD!)

(1)Place about 1-mL of acetic anhydride into a medium sized test tube. Add about 20-25 drops of aniline. Warm gently in the sand bath or in a water bath on the hot plate for three to four minutes. This should complete the reaction.

(2)Add about 0.5 mL of water, continue to heat. This should hydrolyze any remaining acetic anhydride. During this time white crystals of the acetamide  should appear.

(3)Add another 0.5-mL of water, heat to boiling. If all product crystals do not dissolve, add more water as needed. Once the crystals completely dissolve, remove from the heat. (This is recrystallization.)

(4)Cool thoroughly. Suction filter. Press dry. Obtain and record a melting point for the product.

B. Benzoyl (To be prepared in the HOOD!)

(1)Place about 20 drops of aniline in a medium-size test tube. Add 5-mLs of 10% NaOH. Stopper the tube well.

(2)Add about 5 drops of benzoyl chloride. Stopper and shake gently. Benzoyl chloride is a lachrymator. It should be handled carefully in the hood. It should NEVER be washed down an open sink. Rapid hydrolysis increases eye irritation.

(3)Add more benzoyl chloride in 5-drop increments. After each addition, stopper the tube and gently shake it. CAUTIOUSLY smell the tube after shaking. If no irritating odor is present, add another 5 drops of benzoyl chloride. This technique of addition prevents a large excess of the benzoyl chloride being present.

(4)Once the addition seems complete and product crystals of the benzanilide appear, cool the reaction mixture and suction filter using the Hirsch funnel. Wash the crystals with cold water. (NOTE: if excess benzoyl chloride is present, the filtration process will be accompanied by irritating fumes.)

(5)Recrystallize the crystals from hot ethyl alcohol.

(6)Cool, suction filter, press dry, and obtain a melting point.

C. Diphenylthiourea

(1) Place about 20 drops of aniline in a medium test tube. Add aboutt. two mLs of  ethyl alcohol.

(2) Add about one-mL of phenylisothiocyanate to the test tube. Stopper and shake.

(3) Crystals of diphenylthiourea should appear within a few minutes. If no crystallization is observed, follow the steps outlined below.

(a)Cool thoroughly in an ice-water bath.

(b)Scratch the inside of the test tube with a stirring rod.

(c)Add a little ice-water.

(d)Wait.

(4) Once crystals form, cool thoroughly. Suction filter using the Hirsch funnel.

(5) Recrystallize from a little hot ethyl alcohol.

EXPERIMENT 25: Report and Worksheet
AMINES

Student Name:  __________________         Day: ___________

Student Number: ________________________Date: _____________

DATA

I. solubility of Amines

II. Basicity of Amines

Aniline + Concentrated Sulfuric Acid _________________________

Aniline + Concentrated HCl ________________________

III. Ease of Oxidation

Aniline + Bleaching Powder _______________________

IV. Derivative Preparation

QUESTIONS

1. Which one is most soluble and which one is least soluble for (a) ethylamine, (b) aniline, and.(c) aniline hydrochloride.? And why?

2. Which one is the strongest base and which is the weakest base for (a) ammonia, (b) ethylamine, and (c) aniline. And Why? EXPLAIN in terms of electronic effects.

3. Write a BALANCED EQUATION for the reaction of (a) ammonia, (b) ethylamine, and (c) aniline with ferric chloride in water.

4. Write a BALANCED EQUATION for the reaction of aniline with H₂SO₄ and With HCl .

5. Explain, using appropriate equations how Primary, Secondary, and Tertiary amines react in the Hinsberg Test.

6. What is the equation for the reaction of aniline with acetic anhydride?

7. What is the equation for the reaction of aniline with benzoyl chloride? Why is the base necessary ?

8. What is the equation for the reaction of aniline with phenyl-isothiocyanate?

9. What characteristic IR absorbances are given by (a) primary amines, and (b) secondary amines ?