Experiment 7 – Aldehydes, Ketones, and Carboxylic Acids, Slides of Organic Chemistry

Download Experiment 7 – Aldehydes, Ketones, and Carboxylic Acids and more Slides Organic Chemistry in PDF only on Docsity! 35 Experiment 7 – Aldehydes, Ketones, and Carboxylic Acids Aldehydes and ketones are molecules that contain a carbonyl group, which is an oxygen atom with a double bond to a carbon atom. In an aldehyde, the carbonyl group is on the end of the molecule. In a ketone, the carbonyl group is somewhere in the middle of the molecule. Example structures of aldehydes and ketones are given below. Carboxylic acids contain the carboxyl functional group, shown below. Physical Properties Aldehydes and ketones are polar because they contain a carbonyl group (oxygen is very electronegative compared to carbon). Aldehydes and ketones can form hydrogen bonds with water, so small aldehydes and ketones are water-soluble. However, they cannot form hydrogen bonds to each other, so their boiling points are not very high. (Their boiling points are higher than the corresponding alkanes, but lower than the corresponding alcohols.) Carboxylic acids are polar and can form hydrogen bonds with water. Small carboxylic acids are therefore water-soluble. They can also form hydrogen bonds with each other, so they have relatively high boiling points. As a comparison, the following molecules have very similar molar masses, but different boiling points. The differences in the boiling points are thus due to differences in polarity and hydrogen-bonding ability. C H2 CH3C H3C O H3C H2 C C H2 C H O CH3 C H3C O H C H3C O An aldehyde An aldehyde A ketone A ketone C OH O H3C C OH O HO C C H2 O CH3 A carboxyl group A carboxylic acid A carboxylic acid CH3CH2CH2CH3 Butane (MM = 58) bp = 0°C CH3 C H3C O Acetone (MM = 60) bp = 56°C H3C C H2 C H O Propanal (MM = 60) bp = 50°C 36 Many aldehydes and ketones have pleasant odors. Examples of good-smelling aldehydes are vanillin, cinnamaldehyde, and benzaldehyde. Because aldehydes and ketones are somewhat polar but not extremely polar, they are good solvents for organic reactions. Oxidation Aldehydes can be oxidized to carboxylic acids by almost any oxidizing agent. Some common oxidizing agents are chromic acid, Benedict’s reagent, and Fehling’s reagent. Chromic acid is an orange solution and it contains chromium in the +6 oxidation state. It can be reduced to a green solution of chromium (III) ion (in the +3 oxidation state). This reaction is useful, because the reduction of chromium is accompanied by a color change from orange to green. If the test solution turns green when chromic acid is added, it means that the test solution is being oxidized. Fehling’s reagent and Benedict’s reagent are blue solutions that contain copper ions under basic conditions. The copper ions are in the +2 oxidation state, and can be reduced to the +1 oxidation state, where they form a red precipitate of Cu2O in a basic solution. Therefore, the formation of a red precipitate in the blue solution is a visual indication that the compound being tested has been oxidized. Ketones and carboxylic acids cannot be oxidized and therefore will not react with chromic acid or Benedict’s reagent. Iodoform Test Compounds that contain a methyl group attached to a carbonyl carbon will react with I2 in a basic solution to produce a yellow precipitate of iodoform (CHI3). Many (but not all) ketones will undergo this reaction. Acidity Carboxylic acids are weak acids, and will ionize to a slight extent in water. A solution of a carboxylic acid in water will be acidic. The most common carboxylic acid is acetic acid, which contains only two carbons. (This is the acid present in vinegar.) C H3 H2 C H3C OH C H3C O Acetic acid (MM = 60) bp = 118°C OH Propanol (MM = 60) bp = 97°C R C O CH3 R C O CI3 R C O O- I2 NaOH I2 NaOH + CHI3 (s) 39 thoroughly. Record the relative solubility of hexanoic acid in each of the solutions. Part 8: Sodium Bicarbonate Test 9. In this part, you will test propanoic acid, acetic acid, acetone, and your unknown. Label four test tubes and add 3 mL of 10% NaHCO3 (aq) to each tube. Add 10 drops of each substance to be tested to the appropriate tube. While you are adding the substances to the sodium bicarbonate solution, watch for the formation of bubbles, which indicate a positive reaction. Record your observations. Part 9: Identification of the Unknown Substance 10. Based on the results from each of the experiments, discuss what you know about the structure of your unknown. Identify it as an aldehyde, ketone, or carboxylic acid, and include any additional conclusions based on your results. Questions 1. Draw the condensed structural formulas for each of the following compounds. a. benzaldehyde b. 3-chlorobutanoic acid c. 1,4-cyclohexanedione d. 3-ethyl-2,2-dimethylpentanal 2. Name each of the following compounds. a. b. c. d. 3. An unknown has a chemical formula of C3H6O. When chromic acid was added to the unknown, the solution remained orange. The unknown was soluble in water. When NaOH and KI/iodine were added to the unknown, a yellow precipitate formed. What is the structure of the unknown compound? O C OH O H3C C C H2 C C H2 CH3 O CH3 H3C C H O CH3CH2CF2 40 4. Write the structure of the product(s) of each of the following reactions. Also include a description of the changes you would expect to see (if any) in each solution. C O HO CH2CH2CH3 NaHCO3 (aq)a. b. H C O CH2CHBrCH2CH3 NaHCO3 (aq) H3C C O CH CH3 CH3c. I2 NaOH CH3CH2 C O CH2CH3 I2 NaOH d. H C O CH2CH3 [O] e. f. CH3CH2 C O OH [O] 35 Experiment 7 – Aldehydes, Ketones, and Carboxylic Acids Aldehydes and ketones are molecules that contain a carbonyl group, which is an oxygen atom with a double bond to a carbon atom. In an aldehyde, the carbonyl group is on the end of the molecule. In a ketone, the carbonyl group is somewhere in the middle of the molecule. Example structures of aldehydes and ketones are given below. Carboxylic acids contain the carboxyl functional group, shown below. Physical Properties Aldehydes and ketones are polar because they contain a carbonyl group (oxygen is very electronegative compared to carbon). Aldehydes and ketones can form hydrogen bonds with water, so small aldehydes and ketones are water-soluble. However, they cannot form hydrogen bonds to each other, so their boiling points are not very high. (Their boiling points are higher than the corresponding alkanes, but lower than the corresponding alcohols.) Carboxylic acids are polar and can form hydrogen bonds with water. Small carboxylic acids are therefore water-soluble. They can also form hydrogen bonds with each other, so they have relatively high boiling points. As a comparison, the following molecules have very similar molar masses, but different boiling points. The differences in the boiling points are thus due to differences in polarity and hydrogen-bonding ability. C H2 CH3C H3C O H3C H2 C C H2 C H O CH3 C H3C O H C H3C O An aldehyde An aldehyde A ketone A ketone C OH O H3C C OH O HO C C H2 O CH3 A carboxyl group A carboxylic acid A carboxylic acid CH3CH2CH2CH3 Butane (MM = 58) bp = 0°C CH3 C H3C O Acetone (MM = 60) bp = 56°C H3C C H2 C H O Propanal (MM = 60) bp = 50°C 36 Many aldehydes and ketones have pleasant odors. Examples of good-smelling aldehydes are vanillin, cinnamaldehyde, and benzaldehyde. Because aldehydes and ketones are somewhat polar but not extremely polar, they are good solvents for organic reactions. Oxidation Aldehydes can be oxidized to carboxylic acids by almost any oxidizing agent. Some common oxidizing agents are chromic acid, Benedict’s reagent, and Fehling’s reagent. Chromic acid is an orange solution and it contains chromium in the +6 oxidation state. It can be reduced to a green solution of chromium (III) ion (in the +3 oxidation state). This reaction is useful, because the reduction of chromium is accompanied by a color change from orange to green. If the test solution turns green when chromic acid is added, it means that the test solution is being oxidized. Fehling’s reagent and Benedict’s reagent are blue solutions that contain copper ions under basic conditions. The copper ions are in the +2 oxidation state, and can be reduced to the +1 oxidation state, where they form a red precipitate of Cu2O in a basic solution. Therefore, the formation of a red precipitate in the blue solution is a visual indication that the compound being tested has been oxidized. Ketones and carboxylic acids cannot be oxidized and therefore will not react with chromic acid or Benedict’s reagent. Iodoform Test Compounds that contain a methyl group attached to a carbonyl carbon will react with I2 in a basic solution to produce a yellow precipitate of iodoform (CHI3). Many (but not all) ketones will undergo this reaction. Acidity Carboxylic acids are weak acids, and will ionize to a slight extent in water. A solution of a carboxylic acid in water will be acidic. The most common carboxylic acid is acetic acid, which contains only two carbons. (This is the acid present in vinegar.) C H3 H2 C H3C OH C H3C O Acetic acid (MM = 60) bp = 118°C OH Propanol (MM = 60) bp = 97°C R C O CH3 R C O CI3 R C O O- I2 NaOH I2 NaOH + CHI3 (s) 39 thoroughly. Record the relative solubility of hexanoic acid in each of the solutions. Part 8: Sodium Bicarbonate Test 9. In this part, you will test propanoic acid, acetic acid, acetone, and your unknown. Label four test tubes and add 3 mL of 10% NaHCO3 (aq) to each tube. Add 10 drops of each substance to be tested to the appropriate tube. While you are adding the substances to the sodium bicarbonate solution, watch for the formation of bubbles, which indicate a positive reaction. Record your observations. Part 9: Identification of the Unknown Substance 10. Based on the results from each of the experiments, discuss what you know about the structure of your unknown. Identify it as an aldehyde, ketone, or carboxylic acid, and include any additional conclusions based on your results. Questions 1. Draw the condensed structural formulas for each of the following compounds. a. benzaldehyde b. 3-chlorobutanoic acid c. 1,4-cyclohexanedione d. 3-ethyl-2,2-dimethylpentanal 2. Name each of the following compounds. a. b. c. d. 3. An unknown has a chemical formula of C3H6O. When chromic acid was added to the unknown, the solution remained orange. The unknown was soluble in water. When NaOH and KI/iodine were added to the unknown, a yellow precipitate formed. What is the structure of the unknown compound? O C OH O H3C C C H2 C C H2 CH3 O CH3 H3C C H O CH3CH2CF2 40 4. Write the structure of the product(s) of each of the following reactions. Also include a description of the changes you would expect to see (if any) in each solution. C O HO CH2CH2CH3 NaHCO3 (aq)a. b. H C O CH2CHBrCH2CH3 NaHCO3 (aq) H3C C O CH CH3 CH3c. I2 NaOH CH3CH2 C O CH2CH3 I2 NaOH d. H C O CH2CH3 [O] e. f. CH3CH2 C O OH [O] 41 5. Would you expect 2-octanone to be soluble in water? Would you expect butanal to be soluble in water? Would either of them be more soluble in hexane than in water? Explain. 6. Would you expect pentanal to give a positive iodoform test? Would you expect acetaldeyde to give a positive iodoform test? Explain.