Volume 4
4.6. ORGANIC COMPOUNDS OF OXYGEN: A BRIEF OVERVIEW
What is a Functional Group? • Alcohols, Polyols, and Phenols • Aldehydes and Ketones • Carboxylic Acids. How Soap Works • Exercises
What is a Functional Group? • Alcohols, Polyols, and Phenols • Aldehydes and Ketones • Carboxylic Acids. How Soap Works • Exercises
4.6.1. What is a Functional Group? We have familiarized ourselves with different types of hydrocarbons: alkanes, alkenes (olefins), alkynes, and arenes. Compounds within each of these classes exhibit distinct chemical properties that are not characteristic of compounds of another class.
- Alkanes undergo radical substitution reactions.
- Alkenes and alkynes add various molecules across their multiple bonds.
- Aromatic hydrocarbons (arenes) engage in non-radical substitution reactions.
Imagine that a particular atom or a group of atoms, such as OH, is introduced into the molecule of a hydrocarbon of each class (Figure 4-81). Then, in addition to the intrinsic and distinctive chemical properties characteristic of each particular hydrocarbon type, all of these molecules will share the new reactivity conferred on them by virtue of the presence of the OH group.
- Alkanes undergo radical substitution reactions.
- Alkenes and alkynes add various molecules across their multiple bonds.
- Aromatic hydrocarbons (arenes) engage in non-radical substitution reactions.
Imagine that a particular atom or a group of atoms, such as OH, is introduced into the molecule of a hydrocarbon of each class (Figure 4-81). Then, in addition to the intrinsic and distinctive chemical properties characteristic of each particular hydrocarbon type, all of these molecules will share the new reactivity conferred on them by virtue of the presence of the OH group.
Figure 4-81. Examples of an alkane, akene, alkyne, and arene bearing an OH group.
Specific atoms or groups of atoms within organic molecules, which are responsible for characteristic chemical reactions of these molecules, are called functional groups. A functional group exhibits its distinctive chemical properties regardless of the rest of the molecule it is attached to.
The OH group used in the example above (Figure 4-81) is a functional group. All organic compounds containing an OH group are called alcohols. Although aromatic compounds bearing an OH group on the benzene ring are also alcohols, they are traditionally and customarily called phenols. Other important functional groups include NH2, CHO, C=O, COOH, NO2, and C≡N (Table 6).
Table 6. Selected important functional groups.
Specific atoms or groups of atoms within organic molecules, which are responsible for characteristic chemical reactions of these molecules, are called functional groups. A functional group exhibits its distinctive chemical properties regardless of the rest of the molecule it is attached to.
The OH group used in the example above (Figure 4-81) is a functional group. All organic compounds containing an OH group are called alcohols. Although aromatic compounds bearing an OH group on the benzene ring are also alcohols, they are traditionally and customarily called phenols. Other important functional groups include NH2, CHO, C=O, COOH, NO2, and C≡N (Table 6).
Table 6. Selected important functional groups.
4.6.2. Alcohols, Polyols, and Phenols. An organic compound containing one OH group is an alcohol (or a phenol if the OH is bonded to an aromatic ring). Methanol, also known as "wood alcohol" (CH3OH), ethanol, conventionally referred to as just "alcohol" (CH3CH2OH), and propanol (CH3CH2CH2OH) are examples of simplest alcohols. Rubbing alcohol is isopropanol, (CH3)2CHOH, an isomer of propanol.
In our course, we will deal only with a limited number of simple small alcohols. To name such an alcohol, we first find in the molecule the longest straight chain of carbon atoms containing the one attached to the OH group. Next, we number the carbon atoms in that chain such that the one bearing the OH group gets the lowest number. This number with a dash is placed before the name of the alcohol, which is derived by changing the ending e in the name of the parent alkane to ol. If there is one or more substituents on the main chain, their position numbers and names, separated by dashes, are included at the beginning of the name, the way it is done for alkanes, alkenes, and alkynes.
Alternatively, an alcohol can be named by naming the alkyl group connected to the OH, followed by "alcohol". Examples are presented in Figure 4-82.
In our course, we will deal only with a limited number of simple small alcohols. To name such an alcohol, we first find in the molecule the longest straight chain of carbon atoms containing the one attached to the OH group. Next, we number the carbon atoms in that chain such that the one bearing the OH group gets the lowest number. This number with a dash is placed before the name of the alcohol, which is derived by changing the ending e in the name of the parent alkane to ol. If there is one or more substituents on the main chain, their position numbers and names, separated by dashes, are included at the beginning of the name, the way it is done for alkanes, alkenes, and alkynes.
Alternatively, an alcohol can be named by naming the alkyl group connected to the OH, followed by "alcohol". Examples are presented in Figure 4-82.
Figure 4-82. Examples of simple alcohols with names.
At room temperature and atmospheric pressure, all alcohols deriving from straight-chain alkanes with up to 10 carbon atoms are liquids. Methanol, ethanol, propanol, and isopropanol are miscible with water in any proportion. Butanol and other higher alcohols have only limited solubility in water.
Hydrogen bonding is as characteristic of alcohols (Figure 4-83) as it is of water (Volume 2). Just like water, methanol and ethanol would be gases at room temperature and atmospheric pressure if hydrogen bonds did not exist. Although hydrogen bonds are weak, their extended network in water, methanol, and other alcohols makes all of these liquid compounds much less volatile. It is instructive to compare the boiling point of the smallest alcohol, methanol (CH3H; MW = 32) with those of its heavier analogs CH3SH (MW = 48), CH3F (MW = 34), and CH3Cl (MW = 50.5). The latter three are all gases, whereas methanol is a liquid that boils at 65 oC due to the hydrogen bonding (Figure 4-83).
At room temperature and atmospheric pressure, all alcohols deriving from straight-chain alkanes with up to 10 carbon atoms are liquids. Methanol, ethanol, propanol, and isopropanol are miscible with water in any proportion. Butanol and other higher alcohols have only limited solubility in water.
Hydrogen bonding is as characteristic of alcohols (Figure 4-83) as it is of water (Volume 2). Just like water, methanol and ethanol would be gases at room temperature and atmospheric pressure if hydrogen bonds did not exist. Although hydrogen bonds are weak, their extended network in water, methanol, and other alcohols makes all of these liquid compounds much less volatile. It is instructive to compare the boiling point of the smallest alcohol, methanol (CH3H; MW = 32) with those of its heavier analogs CH3SH (MW = 48), CH3F (MW = 34), and CH3Cl (MW = 50.5). The latter three are all gases, whereas methanol is a liquid that boils at 65 oC due to the hydrogen bonding (Figure 4-83).
Figure 4-83. Hydrogen bonds between molecules of methanol.
Digression. Various solutions of fermentation-derived ethanol (beer, wine, hard liquor) have been widely used throughout the history of mankind, sometimes wisely and sometimes less so. It is not uncommon that alcoholism drives the addict to seek illegal sources of spirits, ranging from relatively harmless "moonshine" to adulterated booze, which may be lethal. Of special concern is methanol poisoning that causes permanent blindness, coma, and ultimately death. Just 10-15 mL of methanol consumed by an adult can make the person irreversibly blind. A dose of 30-50 mL can be lethal.
How can one tell if an alcohol sample is methanol or ethanol? The two have somewhat similar odors and often cannot be distinguished by smell alone. They have quite different boiling points, 78 oC for ethanol and 65 oC for methanol, but to measure the boiling point of a liquid, special laboratory equipment is needed. There is a simple way to distinguish between undiluted ethanol and methanol: ethanol is miscible with gasoline (alkanes), whereas methanol is not.
Methanol poisoning can be and has been treated with – guess what – ethanol in the form of alcoholic beverages, usually hard liquor. The treated remains "under the influence" or, in other words, heavily drunk throughout the treatment, which can take several days. The basis for this treatment is scientific. To understand how the treatment works, we first need to know the mechanism of methanol poisoning. The alcohol dehydrogenase enzyme in the liver oxidizes methanol to formaldehyde, which is then further oxidized to formic acid (Figure 4-84). Ethanol undergoes similar transformations in the liver, the intermediate and final product of the metabolism being acetaldehyde and acetic acid, respectively.
How can one tell if an alcohol sample is methanol or ethanol? The two have somewhat similar odors and often cannot be distinguished by smell alone. They have quite different boiling points, 78 oC for ethanol and 65 oC for methanol, but to measure the boiling point of a liquid, special laboratory equipment is needed. There is a simple way to distinguish between undiluted ethanol and methanol: ethanol is miscible with gasoline (alkanes), whereas methanol is not.
Methanol poisoning can be and has been treated with – guess what – ethanol in the form of alcoholic beverages, usually hard liquor. The treated remains "under the influence" or, in other words, heavily drunk throughout the treatment, which can take several days. The basis for this treatment is scientific. To understand how the treatment works, we first need to know the mechanism of methanol poisoning. The alcohol dehydrogenase enzyme in the liver oxidizes methanol to formaldehyde, which is then further oxidized to formic acid (Figure 4-84). Ethanol undergoes similar transformations in the liver, the intermediate and final product of the metabolism being acetaldehyde and acetic acid, respectively.
Figure 4-84. Metabolism of methanol and ethanol in the liver.
Acetic acid, the metabolite of ethanol in the liver, is nontoxic — vinegar, which can be found in every kitchen, is a ~5% solution of acetic acid. In contrast, the oxidation of methanol in the liver leads to formic acid, a toxic compound. The formic acid produced destroys the cells of the optic nerve causing blindness and deactivates certain enzymes, without which living cells in the body die from oxygen starvation and overproduction of acidic species.
So, what does the ethanol do in the body of the treated from methanol poisoning? When present in large enough numbers, the molecules of ethanol successfully compete with those of methanol for the active sites of the alcohol dehydrogenase in the liver. In this way, the binding of the methanol molecules to the enzyme is effectively blocked. As the liver works on the oxidation of the ethanol, the methanol is slowly excreted through the kidneys unchanged.
So, what does the ethanol do in the body of the treated from methanol poisoning? When present in large enough numbers, the molecules of ethanol successfully compete with those of methanol for the active sites of the alcohol dehydrogenase in the liver. In this way, the binding of the methanol molecules to the enzyme is effectively blocked. As the liver works on the oxidation of the ethanol, the methanol is slowly excreted through the kidneys unchanged.
To name a substituted phenol, the position of the substituent is specified, followed by its name before the parent name "phenol" (Figure 4-85). The position of the substituent can be indicated by either a number, or ortho, meta, para prefixes. Common names are also used, such as cresols for phenols bearing one methyl group on the ring.
Figure 4-85. Examples of phenols and their names.
Phenol, also known as carbolic acid, is a white crystalline solid that melts at 40 oC when pure. Very often, however, samples of phenol are pink or red from the colored products of its slow oxidation in air. Also, many samples of phenol are not solid but rather syrupy liquids because even a slight contamination depresses the already low melting point of phenol to below room temperature. Although the solubility of phenol in water is moderate, just about 10 g in 100 mL at room temperature, phenol is hygroscopic (absorbs moisture from the air). Phenol has a distinct, easily recognizable odor that has been described as "sickeningly sweet and tarry". Knowing the smell of phenol well, I agree with this description.
Phenol, also known as carbolic acid, is a white crystalline solid that melts at 40 oC when pure. Very often, however, samples of phenol are pink or red from the colored products of its slow oxidation in air. Also, many samples of phenol are not solid but rather syrupy liquids because even a slight contamination depresses the already low melting point of phenol to below room temperature. Although the solubility of phenol in water is moderate, just about 10 g in 100 mL at room temperature, phenol is hygroscopic (absorbs moisture from the air). Phenol has a distinct, easily recognizable odor that has been described as "sickeningly sweet and tarry". Knowing the smell of phenol well, I agree with this description.
Digression. Phenol causes painful chemical burns that heal slowly and leave scars on the skin. In our 3rd year organic chemistry lab, there was an assistant, a highly skillful chemistry tehnician in her 40s. A beautiful woman, she had horrible, blood-curdling scars on her hands and wrists. Although she was not easily approachable to the students, I somehow made friends with her. Once I asked her about the scars. She said those were from phenol, from many years back when she just started her job and did not know enough about the hazards of some of the chemicals she was working with.
Hundreds of thousands of innocent incurably sick and mentally disabled adults and children were killed by the Nazis in Germany in the implementation of the criminally inhumane Aktion T4 euthanasia program (1939-1945). Deadly and cheap, phenol injections were conventionally used for that purpose. Maximilian Kolbe, a Polish priest, was killed with a phenol injection in Auschwitz, after he volunteered to die in place of another prisoner who was a total stranger to him. Kolbe, who was born to a German father and a Polish mother, was sent to Auschwitz for hiding Jews from the Nazis during World War II.
"What a horrible thing that phenol of yours is!", one might exclaim. But wait. Phenol was the first antiseptic ever used. Before it was pioneered by surgeon Dr. Joseph Lister (1827-1912) in Scotland in the mid-1860s, even a small wound could easily get infected and cause death. With his groundbreaking phenol method, Dr. Lister paved the way to the modern practice of wound treatment with antiseptics and sterile surgery, which since then have saved millions of lives. Although phenol-based antiseptics are no longer nearly as important as they were, phenol still finds key applications in certain areas of medicine and as a disinfectant.
Hundreds of thousands of innocent incurably sick and mentally disabled adults and children were killed by the Nazis in Germany in the implementation of the criminally inhumane Aktion T4 euthanasia program (1939-1945). Deadly and cheap, phenol injections were conventionally used for that purpose. Maximilian Kolbe, a Polish priest, was killed with a phenol injection in Auschwitz, after he volunteered to die in place of another prisoner who was a total stranger to him. Kolbe, who was born to a German father and a Polish mother, was sent to Auschwitz for hiding Jews from the Nazis during World War II.
"What a horrible thing that phenol of yours is!", one might exclaim. But wait. Phenol was the first antiseptic ever used. Before it was pioneered by surgeon Dr. Joseph Lister (1827-1912) in Scotland in the mid-1860s, even a small wound could easily get infected and cause death. With his groundbreaking phenol method, Dr. Lister paved the way to the modern practice of wound treatment with antiseptics and sterile surgery, which since then have saved millions of lives. Although phenol-based antiseptics are no longer nearly as important as they were, phenol still finds key applications in certain areas of medicine and as a disinfectant.
An organic compound bearing more than one OH group on its molecule is referred to as a polyol. Those with two and three OH groups on the molecule are diols and triols, respectively. In our course, we will touch on one diol, ethylene glycol, and one triol, glycerol (other names: glycerin and glycerine), whose structures are shown in Figure 4-86.
Figure 4-86. Structural formulas of ethylene glycol and glycerol.
Both ethylene glycol and glycerol are colorless and odorless viscous liquids with high boiling points, 197 and 290 oC, respectively. Both are miscible with water in any proportion and have a sweet taste. One important difference between the two is that while glycerol is nontoxic and widely used in food and cosmetics industries, ethylene glycol is a poison. The sweet taste of toxic ethylene glycol makes it attractive and therefore particularly dangerous to children and animals.
Both ethylene glycol and glycerol are colorless and odorless viscous liquids with high boiling points, 197 and 290 oC, respectively. Both are miscible with water in any proportion and have a sweet taste. One important difference between the two is that while glycerol is nontoxic and widely used in food and cosmetics industries, ethylene glycol is a poison. The sweet taste of toxic ethylene glycol makes it attractive and therefore particularly dangerous to children and animals.
Digression. There are other chemical compounds that, like ethylene glycol, have a sweet taste while being poisonous. Examples include some salts of beryllium (Be) and lead (Pb). Beryllium (Be) is one of the lightest and most toxic metals, whose original name was glucinium (Gl or G), from the Ancient Greek γλυκύς, meaning "sweet". Lead (II) acetate, Pb(OOCCH3)2, also known as lead sugar or sugar of lead due to its sweet taste, is toxic (like all lead compounds). It is quite astonishing that, being unaware of lead toxicity, the ancient Romans used lead (II) acetate as a sweetener. It has been suggested that lead poisoning might have been the cause of death for Pope Clement II and Ludwig van Beethoven due to consumption of wines sweetened with sugar of lead.
4.6.3. Aldehydes and Ketones. Aldehydes are organic compounds containing the functional group -C(H)O. We have briefly encountered aldehydes in our course already. As described earlier in section 4.4, acetaldehyde, CH3C(H)O, is the product of the catalytic addition of water to acetylene. The biooxidation of methanol and ethanol in the liver to formaldehyde and acetaldehyde was considered in a digression in the previous subsection. The name aldehyde appeared in the mid-19th century as a contraction of the Latin alcohol dehydrogenatum, meaning "alcohol dispossessed of hydrogen". Indeed, the product of an abstraction of a molecule of H2 from an alcohol is an aldehyde (Figure 4-87).
Figure 4-87. Formation of an aldehyde (acetaldehyde) via dehydrogenation of an alcohol (ethanol).
The aldehyde functional group, also known as formyl (Table 6), is planar, with the H-C-O, C-C-O, and H-C-C bond angles being approximately 120o. This is explained by sp2 hybridization of the C atom of the aldehyde group.
To name an aldehyde, find the longest straight carbon chain with the aldehyde group at an end. Number all carbon atoms in the chain, starting at the one of the aldehyde group. Change the e in the name of the corresponding alkane to al. Place the numbering locations and names of the substituents on the main chain before the parent name. The number 1 position (aldehyde carbon) is not included in the name. A handful of illustrative examples are given in Figure 4-88. Note that the simplest aromatic aldehyde is called benzaldehyde, not benzenal.
The aldehyde functional group, also known as formyl (Table 6), is planar, with the H-C-O, C-C-O, and H-C-C bond angles being approximately 120o. This is explained by sp2 hybridization of the C atom of the aldehyde group.
To name an aldehyde, find the longest straight carbon chain with the aldehyde group at an end. Number all carbon atoms in the chain, starting at the one of the aldehyde group. Change the e in the name of the corresponding alkane to al. Place the numbering locations and names of the substituents on the main chain before the parent name. The number 1 position (aldehyde carbon) is not included in the name. A handful of illustrative examples are given in Figure 4-88. Note that the simplest aromatic aldehyde is called benzaldehyde, not benzenal.
Figure 4-88. Examples of aldehydes with names.
Many aldehydes have common names, including formaldehyde (methanal), acetaldehyde (ethanal), and benzaldehyde. Benzaldehyde is the simplest aldehyde derivative of benzene. Naming substituted benzaldehydes is self-explanatory, so long as you remember how to name benzene derivatives bearing substituents on the ring (Figure 4-89).
Many aldehydes have common names, including formaldehyde (methanal), acetaldehyde (ethanal), and benzaldehyde. Benzaldehyde is the simplest aldehyde derivative of benzene. Naming substituted benzaldehydes is self-explanatory, so long as you remember how to name benzene derivatives bearing substituents on the ring (Figure 4-89).
Figure 4-89. Examples of aromatic aldehydes with names.
Formaldehyde is a water-soluble, toxic, pungent-smelling gas with a boiling point of -19 oC at atmospheric pressure. The conventional commercial form of formaldehyde is its 37% solution in water, called formalin. Aqueous solutions of formaldehyde are a well-known and broadly used disinfectant that kills most bacteria and fungi. Acetaldehyde boils at 20 oC and is an irritant with a characteristic "green apple" smell. Benzaldehyde is a colorless liquid at room temperature, boils at 178 oC, and has an almond-like scent. Formaldehyde, acetaldehyde, and benzaldehyde are precursors to many important industrial chemicals and materials.
Ketones are structurally similar to aldehydes. While in aldehydes the carbonyl group (CO) is bonded to one hydrogen atom and one carbon atom, both substituents at the CO of ketones are carbon groups (Figure 4-90). One way to name ketones is by naming each of the two substituents at the CO group, followed by 'ketone'. If both substituents are identical, then the ketone is named by naming the substituent with the prefix 'di', followed by 'ketone'.
Formaldehyde is a water-soluble, toxic, pungent-smelling gas with a boiling point of -19 oC at atmospheric pressure. The conventional commercial form of formaldehyde is its 37% solution in water, called formalin. Aqueous solutions of formaldehyde are a well-known and broadly used disinfectant that kills most bacteria and fungi. Acetaldehyde boils at 20 oC and is an irritant with a characteristic "green apple" smell. Benzaldehyde is a colorless liquid at room temperature, boils at 178 oC, and has an almond-like scent. Formaldehyde, acetaldehyde, and benzaldehyde are precursors to many important industrial chemicals and materials.
Ketones are structurally similar to aldehydes. While in aldehydes the carbonyl group (CO) is bonded to one hydrogen atom and one carbon atom, both substituents at the CO of ketones are carbon groups (Figure 4-90). One way to name ketones is by naming each of the two substituents at the CO group, followed by 'ketone'. If both substituents are identical, then the ketone is named by naming the substituent with the prefix 'di', followed by 'ketone'.
Figure 4-90. Selected ketones with names.
The simplest ketone is acetone, a colorless liquid with a fruity smell. Acetone boils at 56 oC and is broadly used as a solvent owing to its remarkable ability to dissolve various organic compounds while being non-carcinogenic and almost non-toxic. Acetone finds many household applications and is sold in some hardware stores.
4.6.4. Carboxylic Acids. How Soap Works. The functional group of carboxylic acids is –COOH (Table 6). Carboxylic acids are all around us. A few well-known carboxylic acids that we encounter on a daily basis are shown in Figure 4-91. Vinegar, as we know already, is a weak solution of acetic acid in water. We also know that benzoic acid is used to make food preservatives. Citric acid, the strongest edible acid, is used as a flavoring agent in beverages and foods as well as a key component of many kinds of hard candy and some seasonings, such as lemon pepper.
The simplest ketone is acetone, a colorless liquid with a fruity smell. Acetone boils at 56 oC and is broadly used as a solvent owing to its remarkable ability to dissolve various organic compounds while being non-carcinogenic and almost non-toxic. Acetone finds many household applications and is sold in some hardware stores.
4.6.4. Carboxylic Acids. How Soap Works. The functional group of carboxylic acids is –COOH (Table 6). Carboxylic acids are all around us. A few well-known carboxylic acids that we encounter on a daily basis are shown in Figure 4-91. Vinegar, as we know already, is a weak solution of acetic acid in water. We also know that benzoic acid is used to make food preservatives. Citric acid, the strongest edible acid, is used as a flavoring agent in beverages and foods as well as a key component of many kinds of hard candy and some seasonings, such as lemon pepper.
Figure 4-91. Examples of carboxylic acids widely used in foods and beverages.
Each time we wash our hands with soap, we use a carboxylic acid salt. Soaps are salts of long chain carboxylic acids, mostly stearic acid, C17H35COOH (Figure 4-92). Solid bar soap is a mixture of sodium salts of such carboxylic acids. Liquid soap is potassium salts of the same acids.
Each time we wash our hands with soap, we use a carboxylic acid salt. Soaps are salts of long chain carboxylic acids, mostly stearic acid, C17H35COOH (Figure 4-92). Solid bar soap is a mixture of sodium salts of such carboxylic acids. Liquid soap is potassium salts of the same acids.
Figure 4-92. Stearic acid, C17H35COOH, and its sodium salt, C17H35COONa (soap).
Why does soap clean and how does it work? First off, what is it that soap removes? Soap removes oils, fats, and grease, which are all nonpolar organic compounds that are insoluble in water. If they were, there would have been no need to use soap or any other detergent for cleaning. Soapy water has the magic ability to solubilize (not to dissolve, but to solubilize) fats and oils.
If we take a closer look at the molecule of sodium stearate (Figure 4-92), we will see that the molecule is made up of two different moieties. One end of the molecule is the long hydrocarbon "tail". This tail is like a long alkane and is nonpolar. The other end of the molecule is the highly polar, negatively charged "head", the ionized (dissociated) carboxy group. Since "like is attracted to like", the nonpolar tail of sodium stearate is attracted to nonpolar oils and fats, whereas the polar, negatively charged carboxy head is attracted to polar water dipoles. As a result, an assembly called a micelle is formed, wherein a droplet of oil is surrounded by the tails of the soap, with the polar heads sticking out into the water medium (Figure 4-93). Micelles are tiny particles, even though they are much larger in size than individual molecules or ions. Soap micelles form aqueous colloidal solutions that are washed away easily when we rinse our hands or kitchen dishes after cleaning them with soapy water.
Why does soap clean and how does it work? First off, what is it that soap removes? Soap removes oils, fats, and grease, which are all nonpolar organic compounds that are insoluble in water. If they were, there would have been no need to use soap or any other detergent for cleaning. Soapy water has the magic ability to solubilize (not to dissolve, but to solubilize) fats and oils.
If we take a closer look at the molecule of sodium stearate (Figure 4-92), we will see that the molecule is made up of two different moieties. One end of the molecule is the long hydrocarbon "tail". This tail is like a long alkane and is nonpolar. The other end of the molecule is the highly polar, negatively charged "head", the ionized (dissociated) carboxy group. Since "like is attracted to like", the nonpolar tail of sodium stearate is attracted to nonpolar oils and fats, whereas the polar, negatively charged carboxy head is attracted to polar water dipoles. As a result, an assembly called a micelle is formed, wherein a droplet of oil is surrounded by the tails of the soap, with the polar heads sticking out into the water medium (Figure 4-93). Micelles are tiny particles, even though they are much larger in size than individual molecules or ions. Soap micelles form aqueous colloidal solutions that are washed away easily when we rinse our hands or kitchen dishes after cleaning them with soapy water.
Figure 4-93. A micelle formed from a droplet of oil (or a fat particle) and soap (sodium stearate) in water.
Why do we use as soap sodium or potassium salts of the long chain acids, not the acids themselves? Stearic and similar long-chain acids are water-insoluble, weak acids (= weak electrolytes), which means that the carboxy group of the acid is not efficiently ionized (dissociated). In contrast, the sodium and potassium salts are soluble in water and, like most other salts, are strong electrolytes (Figure 4-94). Compared to -COOH, the -COO- group is much more polar as it bears a full negative charge and, consequently, has a much higher affinity for water.
Why do we use as soap sodium or potassium salts of the long chain acids, not the acids themselves? Stearic and similar long-chain acids are water-insoluble, weak acids (= weak electrolytes), which means that the carboxy group of the acid is not efficiently ionized (dissociated). In contrast, the sodium and potassium salts are soluble in water and, like most other salts, are strong electrolytes (Figure 4-94). Compared to -COOH, the -COO- group is much more polar as it bears a full negative charge and, consequently, has a much higher affinity for water.
Figure 4-94. Sodium stearate is active as soap, whereas stearic acid is not.
Why is soap much less efficient in hard water? As discussed earlier (Volume 3), water hardness is caused by the presence of calcium and magnesium ions. While potassium and sodium stearates are water-soluble, calcium and magnesium stearates are not. The stearate anions of soap react with the Ca2+ and Mg2+ in hard water to give insoluble calcium and magnesium stearates, which form soap scum that precipitates out (Figure 4-95).
Why is soap much less efficient in hard water? As discussed earlier (Volume 3), water hardness is caused by the presence of calcium and magnesium ions. While potassium and sodium stearates are water-soluble, calcium and magnesium stearates are not. The stearate anions of soap react with the Ca2+ and Mg2+ in hard water to give insoluble calcium and magnesium stearates, which form soap scum that precipitates out (Figure 4-95).
Figure 4-95. Soap deactivation with calcium and magnesium ions of hard water.
As mentioned above, carboxylic acids are weak acids. As an example, the degree of dissociation of acetic acid and benzoic acid in water (1.0 M) at room temperature is only about 0.5% and 0.8%, respectively. Carboxylic acids exist in the form of dimers due to hydrogen bonding, as shown in Figure 4-96.
As mentioned above, carboxylic acids are weak acids. As an example, the degree of dissociation of acetic acid and benzoic acid in water (1.0 M) at room temperature is only about 0.5% and 0.8%, respectively. Carboxylic acids exist in the form of dimers due to hydrogen bonding, as shown in Figure 4-96.
Figure 4-96. Hydrogen bond dimer of acetic acid.
Many carboxylic acids are malodorous. Everyone is familiar with the smell of vinegar, strongly diluted acetic acid. Pivalic acid, (CH3)3CCOOH, has one of the most revolting odors I have ever encountered. Yet, pivalic acid is a rather harmless, very low toxicity compound that is used to make beautiful highly reflective lacquers (which are devoid of the odor of pivalic acid). Again, all that glitters is not necessarily gold and all that stinks is not necessarily garbage.
A large number of carboxylic acids have common names, such as formic acid, acetic acid, benzoic acid, and citric acid. Alternatively, carboxylic acids can be named by changing the ending -e in the parent name of the corresponding alkane to -oic acid. For branched carboxylic acid, the numbering is done in the same manner as for aldehydes (Figure 4-97). Note that the simplest aromatic carboxylic acid is called benzoic acid, not benzenoic acid.
Many carboxylic acids are malodorous. Everyone is familiar with the smell of vinegar, strongly diluted acetic acid. Pivalic acid, (CH3)3CCOOH, has one of the most revolting odors I have ever encountered. Yet, pivalic acid is a rather harmless, very low toxicity compound that is used to make beautiful highly reflective lacquers (which are devoid of the odor of pivalic acid). Again, all that glitters is not necessarily gold and all that stinks is not necessarily garbage.
A large number of carboxylic acids have common names, such as formic acid, acetic acid, benzoic acid, and citric acid. Alternatively, carboxylic acids can be named by changing the ending -e in the parent name of the corresponding alkane to -oic acid. For branched carboxylic acid, the numbering is done in the same manner as for aldehydes (Figure 4-97). Note that the simplest aromatic carboxylic acid is called benzoic acid, not benzenoic acid.
Figure 4-97. Selected carboxylic acids with names.
The most widely used carboxylic acid, acetic acid, is manufactured on a very large scale of approximately 5 million tons annually. Pure acetic acid containing less than 1% water is often called glacial acetic acid. This name comes from the fact that pure acetic acid freezes out into an ice-like crystalline mass on cooling slightly below room temperature (at ~16 oC).
4.6.5. Exercises.
1. What is a functional group in organic chemistry? Give examples of functional groups with their names. [Answer: See 4.6.1]
2. Why is methanol a liquid at room temperature, whereas methane, formaldehyde, and even chloromethane are gases? [Answer: See 4.6.2]
3. Why is methanol toxic? What is the reason for the blindness as one of the first symptoms of methanol poisoning? How does the ethanol treatment of methanol poisoning work? [Answer: See 4.6.2]
4. There are four bottles, each containing a different liquid: methanol, ethanol, acetic acid, and gasoline (a mixture of alkanes). Find out which liquid is in which bottle. You have water, ice, and plenty of empty bottles and test tubes for mixing the liquids, if necessary. You do not have anything else. Smelling the liquids is prohibited to avoid the risk of irritation of the respiratory organs, especially by vapors of acetic acid. Answer
5. Draw chemical structures for the following compounds: (a) isopropanol (rubbing alcohol); (b) methanol (wood alcohol); (c) phenol; (d) para-nitrophenol; (e) formaldehyde; (f) 3-chlorobenzaldehyde; (g) 3-methylpentanal; (h) formic acid; (i) benzoic acid; (j) acetic acid; (k) terephthalic acid; (l) butanoic acid; (m) 2,2-dimethylpropanoic acid; (n) potassium stearate; (m) glycerol; (n) methyl isopropyl ketone; (o) acetone.
6. Draw the structure of ethylene glycol. What is the danger of ethylene glycol, especially for animals and children? [Answer: See 4.6.2]
7. What is soap? How does soap work? Why does soap lose activity in hard water? Do you think adding diluted HCl to soapy water would enhance or diminish its cleaning ability? Answer
8. Draw the structure of benzoic acid dimer. Hint
9. What is the hybridization of the carbon atom in (a) methane; (b) methanol; (c) formaldehyde; (d) formic acid? Answer
10. What is the alternative name for the aldehyde group? Answer
11. May formic acid be viewed as an aldehyde? Answer
12. Old samples of phenol are pink or red in color, often appearing as crystals in a syrupy liquid. Why? Pure phenol is a white crystalline solid. [Answer: See 4.6.2]
13. What is (a) wood alcohol; (b) formalin; (c) carbolic acid; (d) sugar of lead? Answer
14. Formulations containing large quantities of phenol are widely used for permanent treatment of ingrown toenails, certainly not life-threatening yet painful and annoying condition. What do you think of the role of phenol in this treatment, called phenol cauterization or phenol matricectomy? Answer
The most widely used carboxylic acid, acetic acid, is manufactured on a very large scale of approximately 5 million tons annually. Pure acetic acid containing less than 1% water is often called glacial acetic acid. This name comes from the fact that pure acetic acid freezes out into an ice-like crystalline mass on cooling slightly below room temperature (at ~16 oC).
4.6.5. Exercises.
1. What is a functional group in organic chemistry? Give examples of functional groups with their names. [Answer: See 4.6.1]
2. Why is methanol a liquid at room temperature, whereas methane, formaldehyde, and even chloromethane are gases? [Answer: See 4.6.2]
3. Why is methanol toxic? What is the reason for the blindness as one of the first symptoms of methanol poisoning? How does the ethanol treatment of methanol poisoning work? [Answer: See 4.6.2]
4. There are four bottles, each containing a different liquid: methanol, ethanol, acetic acid, and gasoline (a mixture of alkanes). Find out which liquid is in which bottle. You have water, ice, and plenty of empty bottles and test tubes for mixing the liquids, if necessary. You do not have anything else. Smelling the liquids is prohibited to avoid the risk of irritation of the respiratory organs, especially by vapors of acetic acid. Answer
5. Draw chemical structures for the following compounds: (a) isopropanol (rubbing alcohol); (b) methanol (wood alcohol); (c) phenol; (d) para-nitrophenol; (e) formaldehyde; (f) 3-chlorobenzaldehyde; (g) 3-methylpentanal; (h) formic acid; (i) benzoic acid; (j) acetic acid; (k) terephthalic acid; (l) butanoic acid; (m) 2,2-dimethylpropanoic acid; (n) potassium stearate; (m) glycerol; (n) methyl isopropyl ketone; (o) acetone.
6. Draw the structure of ethylene glycol. What is the danger of ethylene glycol, especially for animals and children? [Answer: See 4.6.2]
7. What is soap? How does soap work? Why does soap lose activity in hard water? Do you think adding diluted HCl to soapy water would enhance or diminish its cleaning ability? Answer
8. Draw the structure of benzoic acid dimer. Hint
9. What is the hybridization of the carbon atom in (a) methane; (b) methanol; (c) formaldehyde; (d) formic acid? Answer
10. What is the alternative name for the aldehyde group? Answer
11. May formic acid be viewed as an aldehyde? Answer
12. Old samples of phenol are pink or red in color, often appearing as crystals in a syrupy liquid. Why? Pure phenol is a white crystalline solid. [Answer: See 4.6.2]
13. What is (a) wood alcohol; (b) formalin; (c) carbolic acid; (d) sugar of lead? Answer
14. Formulations containing large quantities of phenol are widely used for permanent treatment of ingrown toenails, certainly not life-threatening yet painful and annoying condition. What do you think of the role of phenol in this treatment, called phenol cauterization or phenol matricectomy? Answer