Are organic compounds that contain a nitrogen atom bonded to one, two, or three alkyl groups.

In the hair protein, keratin, disulfide bonds are formed by oxidation of the sulfhydryl groups on cysteine. Different protein chains or loops within a single chain are held together by the strong covalent disulfide bonds.  In the permanent wave process, a basic reducing substance (usually ammonium thioglycolate) is first added to reduce and disrupt the disulfide cross-links.

Figure 10.4 First Step in Receiving a Permanent Hair Treatment.  The hair is first rolled tight onto curlers. The first reagent, a basic reducing agent is then added to the hair. The reducing agent disrupts the disulfide bonds in the keratin hair proteins, allowing the folding pattern of the keratin to shift and accommodate the curled hair structure.

To stabilize the hair in the new position an oxidizing agent, usually a dilute solution of hydrogen peroxide, (also called the neutralizer) is added to reform the disulfide bonds in their new positions. The permanent will hold these new disulfide bond positions until the hair grows out. The new hair growth has not been treated and will thus, adopt it’s natural disulfide bond structure.

Figure 10.5 Second Step in a Permanent Hair Treatment.  After treatment of the curled hair with the reducing agent. An oxidizing agent is added to the hair to reform the disulfide bridges while the hair is in the curled position. Upon unrolling the hair, the hair will now retain the curly conformation due to the formation of the new disulfide bridges.

Esters of pyrophosphoric acid and triphosphoric acid are also important in biochemistry.

Esters of these acids are present in every plant and animal cell. They are biochemical intermediates in the transformation of food into usable energy. The bonds between phosphate units in adenosine triphosphate (ATP) are called phosphoanhydride bonds. These are high-energy bonds that store energy from the metabolism of foods. Most cellular functions need energy in order to be carried out: synthesis of proteins, synthesis of membranes, movement of the cell, cellular division, transport of various solutes etc. The ATP is the molecule that carries energy to the place where the energy is needed. When ATP is hydrolyzed into ADP (Adenosine diphosphate) and Pi (phosphate), the breakdown of the last covalent link of phosphate (a simple -PO4) liberates energy that is used in reactions where it is needed. 

Phosphate esters are also important structural constituents of phospholipids and nucleic acids, DNA and RNA. In the next chapter, you will be introduced to the major macromolecules of the body and will revisit the phosphoester linkages found in DNA, RNA, and the phospholipids.

Inorganic acids such as H3PO4 form esters. The esters of phosphoric acid are especially important in biochemistry.

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An amine is any compound that has one or more organic groups bonded to a nitrogen atom. Recall that nitrogen atoms have 5 electrons in their valence shell.  Thus, they tend to form three covalent bonds and have one lone pair of electrons associated with them.

Amines are classified according to the number of carbon atoms bonded directly to the nitrogen atom. A primary (1°) amine has one carbon (alkyl) group attached to the nitrogen atom, a secondary (2°) amine has two, and a tertiary (3°) amine has three.

Figure 10.6 The Structure of Amines Compared to Water, an Alcohol, and an Ether

This differs from the way that we classified alcohols as primary, secondary and tertiary.  To classify alcohols, we look at the number of carbon atoms bonded to the carbon atom bearing the OH group, not the oxygen atom itself. Thus, although isopropylamine looks similar to isopropyl alcohol, the former is a primary amine, while the latter is a secondary alcohol. Note that both primary and secondary amines have hydrogen atoms attached to the nitrogen.  Thus, they can both participate in hydrogen bonding.  This causes primary and secondary amines to have higher boiling points than alkanes of similar size. However, because nitrogen has less electronegativity than oxygen, primary and secondary amines have lower boiling points than alcohols of similar size. All amines can form hydrogen bonds with water due to the lone pair of electrons on the nitrogen atom. Due to the polarity of the amine, amines with up to six carbons are soluble in water.

Interestingly, due to the lone pair of electrons that nitrogen has, they can also form a coordinate covalent bond.  Recall that a coordinate covalent bond is one where a single atom shares two electrons with another atom that doesn’t bring any electrons into the shared bond. When nitrogen forms a bond with a fourth group through this lone pair of electrons, the resulting molecule is called a quaternary ammonium ion. This ion has a positive charge and can for ionic bonds with common anions. 

Summary

An amine is a derivative of ammonia in which one, two, or all three hydrogen atoms are replaced by hydrocarbon groups. The amine functional group is as follows:

Amines are classified as primary, secondary, or tertiary by the number of hydrocarbon groups attached to the nitrogen atom. Amines are named by naming the alkyl groups attached to the nitrogen atom, followed by the suffix –amine.

Key Takeaways

• Primary and secondary amines have higher boiling points than those of alkanes or ethers of similar molar mass because they can engage in intermolecular hydrogen bonding. Their boiling points are lower than those of alcohols because alcohol molecules have hydrogen atoms bonded to an oxygen atom, which is more electronegative.
• The boiling points of tertiary amines, which cannot engage in hydrogen bonding because they have no hydrogen atom on the nitrogen atom, are comparable to those of alkanes and ethers of similar molar mass.
• Because all three classes of amines can engage in hydrogen bonding with water, amines of low molar mass are quite soluble in water.

Concept Review Exercises

1. Which compound has the higher boiling point, CH3CH2CH2CH2CH2NH2 or CH3CH2CH2CH2CH2CH3? Explain.

2. Which compound is more soluble in water, CH3CH2CH2CH2CH3 or CH3CH2NHCH2CH3? Explain.

Answers

1. CH3CH2CH2CH2CH2NH2 because the nitrogen-to-hydrogen (N–H) bonds can engage in hydrogen bonding; CH3CH2CH2CH2CH2CH3 cannot engage in hydrogen bonding

2. CH3CH2NHCH2CH3 because amines can engage in hydrogen bonding with water; alkanes cannot engage in hydrogen bonding

To Your Health: The Smell of Death

Amines have “interesting” odors. The simple ones smell very much like ammonia. Higher aliphatic amines smell like decaying fish. Or perhaps we should put it the other way around: Decaying fish give off odorous amines. The stench of rotting fish is due in part to two diamines: putrescine and cadaverine. They arise from the decarboxylation of ornithine and lysine, respectively, amino acids that are found in animal cells.

Example Problems:

Classify each compound as a primary, secondary, or tertiary amine.

1. CH3CH2CH2NH2
3. CH3CH2NHCH2CH3
4. CH3CH2CH2NHCH3

Solution

1. There is only one alkyl group attached to the nitrogen atom, so the amine is primary.
2. There are two methyl groups and one ethyl group on the nitrogen atom. The compound is a tertiary amine.
3. There are two ethyl groups attached to the nitrogen atom; the amine is secondary.
4. The nitrogen atom has a methyl group and a propyl group, so the compound is a secondary amine.

Classify each compound as a primary, secondary, or tertiary amine..

Solution

1. There is an isopropyl group (in red) and two methyl groups (in green) attached to the nitrogen atom; the amine is tertiary.

   3. There is one n-butyl group and two hydrogens attached to the nitrogen atom; the amine is primary.

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The ability of the nitrogen to form coordinate covalent bonds accounts for many of the chemical features of amines. For example, the lone pair electrons of the nitrogen in amines enables them to act as a weak base.  In this case, the nitrogen is acting as a Lewis base. A Lewis base, by definition, is any compound that can donate a pair of electrons to an acceptor.  Typically, this will be a H+ (proton) that is donated from a water molecule or an acid. In the case of the water molecule, you can see that amines effectively increase the basicity (alkalinity) of water by increasing the concentration of hydroxide (OH–) ions that are in solution. Note that amines are only weak bases, they do not fully ionize.

As a weak base, amines can also participate in acid-base neutralization reactions.

The primary amine in which the nitrogen atom is attached directly to a benzene ring has a special name—aniline. Aryl amines are named as derivatives of aniline.  Like most volatile amines, aniline possesses the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds.

Analine was first discovered by the destructive distillation of the dye, indigo.  Soon after, analine was used to produce the first synthetic dye, mauveine. Other aniline-baed dyes (also known as azo dyes) followed, such as fuchsine and safranine.

Figure 10.7 Aniline and the Creation of Synthetic Dyes. A. Aniline was originally isolated from the decomposition of indigo. Following it’s isolation, aniline has been used to create several synthetic dyes including B. Mauveine, C. Fuchsine, and D. Safranine.

Photo of Indigo provided by: David Stroe, Photo of Mauveine provided by: Science Museum Group Collection, Photo of Fuchsine and Safranine provided by: LHcheM

Following on the early work with aniline as the starting material for building synthetic dyes, it was discovered that aniline also had analgesic or pain relieving properties. While the toxic side effects of aniline limited its widespread use, it sparked research on the medicinal properties of synthetic dyes.  In 1932, Bayer Laboratories discovered that a red azo dye also had antibacterial properties.  The resulting compound, Prontosil (also known as a sulfa drug because it contains sulfur) became the first widely used antibiotic.

Prontosil

In addition to aromatic nitrogen compounds, nitrogen can also be incorporated directly into cyclic structures forming a heterocycle. Recall that a heterocycle is any cyclic organic molecule that contains a heteroatom (typically N, S, or O). Heterocycles that contain nitrogen are widely found in nature and contribute to the biological activity of many compounds.

Many heterocyclic amines occur naturally in plants. Like other amines, these compounds are basic. Such a compound is called an alkaloid, a name that means “like alkalis.” Many alkaloids are physiologically active, including the familiar drugs caffeine, nicotine, and cocaine.

Caffeine is a stimulant found in coffee, tea, and some soft drinks. Its mechanism of action is not well understood, but it is thought to block the activity of adenosine, a heterocyclic base that acts as a neurotransmitter, a substance that carries messages across a tiny gap (synapse) from one nerve cell (neuron) to another cell. The effective dose of caffeine is about 200 mg, corresponding to about two cups of strong coffee or tea.

Nicotine acts as a stimulant by a different mechanism; it probably mimics the action of the neurotransmitter acetylcholine. People ingest this drug by smoking or chewing tobacco. Its stimulant effect seems transient, as this initial response is followed by depression. Nicotine is highly toxic to animals. It is especially deadly when injected; the lethal dose for a human is estimated to be about 50 mg. Nicotine has also been used in agriculture as a contact insecticide.

Cocaine acts as a stimulant by preventing nerve cells from taking up dopamine, another neurotransmitter, from the synapse. High levels of dopamine are therefore available to stimulate the pleasure centers of the brain. The enhancement of dopamine action is thought to be responsible for cocaine’s “high” and its addictive properties. After the binge, dopamine is depleted in less than an hour. This leaves the user in a pleasureless state and (often) craving more cocaine.

Cocaine is used as the salt cocaine hydrochloride and in the form of broken lumps of the free (unneutralized) base, which is called crack cocaine.

Because it is soluble in water, cocaine hydrochloride is readily absorbed through the watery mucous membranes of the nose when it is snorted. Crack cocaine is more volatile than cocaine hydrochloride. It vaporizes at the temperature of a burning cigarette. When smoked, cocaine reaches the brain in 15 seconds.

Amines are bases; they react with acids to form salts. Salts of aniline are properly named as anilinium compounds, but an older system is used to name drugs: the salts of amine drugs and hydrochloric acid are called “hydrochlorides.” Heterocyclic amines are cyclic compounds with one or more nitrogen atoms in the ring.

1. Explain the basicity of amines.
2. Contrast the physical properties of amines with those of alcohols and alkanes.
3. What is a heterocyclic compound?

1. Amines have a lone pair of electrons on the nitrogen atom and can thus act as proton acceptors (bases).
2. The solubilities of amines are similar to those of alcohols; the boiling points of primary and secondary amines are similar to those of alcohols; the boiling points of tertiary amines, which cannot engage in hydrogen bonding because they do not have a hydrogen atom on the nitrogen atom, are comparable to those of alkanes.
3. Heterocyclic compounds are ring compounds with atoms other than carbon atoms in the ring.

Exercises

1. What salt is formed in each reaction? Write its condensed structural formula.

   1. CH3NH2(aq) + HBr(aq) →
   2. CH3NHCH3(aq) + HNO3(aq) →

2. What salt is formed in each reaction? Draw its structure.

Answer

1. 1. CH3NH3+Br−(aq)
   2. [CH3NH2CH3]+NO3−(aq)

Paramedics are highly trained experts at providing emergency medical treatment. Their critical duties often include rescue work and emergency medical procedures in a wide variety of settings, sometimes under extremely harsh and difficult conditions. Like other science-based professions, their work requires knowledge, ingenuity, and complex thinking, as well as a great deal of technical skill. The recommended courses for preparation in this field include anatomy, physiology, medical terminology, and—not surprisingly—chemistry. An understanding of basic principles of organic chemistry, for example, is useful when paramedics have to deal with such traumas as burns from fuel (hydrocarbons) or solvent (alcohols, ethers, esters, and so on) fires and alcohol and drug overdoses.

To become a paramedic requires 2–4 y of training and usually includes a stint as an emergency medical technician (EMT). An EMT provides basic care, can administer certain medications and treatments, such as oxygen for respiratory problems and epinephrine (adrenalin) for allergic reactions, and has some knowledge of common medical conditions. A paramedic, in contrast, must have extensive knowledge of common medical problems and be trained to administer a wide variety of emergency drugs.

Paramedics usually work under the direction of a medical doctor with a title such as “medical director.” Some paramedics are employed by fire departments and may work from a fire engine that carries medical equipment as well as fire-fighting gear. Some work from hospital-sponsored ambulances and contiAmidesnue to care for their patients after reaching the hospital emergency room. Still other paramedics work for a government department responsible for emergency health care in a specific geographical area. Finally, some work for private companies that contract to provide service for a government body.

An experienced paramedic has a broad range of employment options, including training for mountain or ocean rescue, working with police department special weapons and tactics (SWAT) teams, or working in isolated settings such as on oil rigs. With their expertise at treating and stabilizing patients before quickly moving them to a hospital, paramedics often provide the first critical steps in saving an endangered life. The following quotation, inscribed on the Arlington National Cemetery headstone of Army Lieutenant R. Adams Cowley, who is often called the “father” of shock trauma medicine, serves as the motto for many paramedic units: “Next to creating a life the finest thing a man can do is save one.” —Abraham Lincoln

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With the exception of formamide (HCONH2), which is a liquid, all simple amides are solids (Table 10.1) The lower members of the series are soluble in water, with borderline solubility occurring in those that have five or six carbon atoms. Like the esters, solutions of amides in water usually are neutral—neither acidic nor basic.

The amides generally have high boiling points and melting points. These characteristics and their solubility in water result from the polar nature of the amide group and hydrogen bonding (Figure 10.8) Similar hydrogen bonding plays a critical role in determining the structure and properties of proteins, deoxyribonucleic acid [DNA], ribonucleic acid [RNA], and other macromolecules so important to life processes. This will be discussed in further detail in Chapter 11.

Figure 10.8 Hydrogen Bonding in Amides. Amide molecules can engage in hydrogen bonding with water molecules (a). Those amides with a hydrogen atom on the nitrogen atom can also engage in hydrogen bonding (b). Both hydrogen bonding networks extend in all directions.

• Most amides are solids at room temperature; the boiling points of amides are much higher than those of alcohols of similar molar mass.
• Amides of five or fewer carbon atoms are soluble in water?.

Concept Review Exercises

1. Which compound has the higher boiling point—pentanamide (CH3CH2CH2CH2CONH2) or propyl acetate (CH3COOCH2CH2CH3)? Explain.
2. Which compound is more soluble in water—propanamide (CH3CH2CONH2) or 1-pentene (CH2=CHCH2CH2CH3)? Explain.

1. pentanamide because the nitrogen-to-hydrogen (N–H) and the carbon-to-oxygen double (C=O) bonds can engage in hydrogen bonding; propyl acetate cannot engage in hydrogen bonding
2. propanamide because the N–H and C=O bonds can engage in hydrogen bonding with water; 1-pentene cannot engage in hydrogen bonding with water

More Practice

1. Which compound has the higher boiling point—butyramide (CH3CH2CH2CONH2) or ethyl acetate (CH3COOCH2CH3)? Explain.

2. Which compound has the higher boiling point—butyramide or dimethylacetamide [CH3CON(CH3)2]? Explain.

3. Which compound is more soluble in water—acetamide (CH3CONH2) or 1-butene (CH2=CHCH2CH3)? Explain.

4. Which compound is more soluble in water—CH3CONHCH3 or 2-methylbutane [CH3CH(CH3)CH2CH3)]? Explain.

1. butyramide because the nitrogen-to-hydrogen (N–H) and the carbon-to-oxygen double (C=O) bonds can engage in hydrogen bonding; ethyl acetate cannot engage in hydrogen bonding

1. acetamide because the N–H and C=O bonds can engage in hydrogen bonding with water; 1-butene cannot engage in hydrogen bonding with water

Amides are formed by the addition of an amine to a carboxylic acid, and thus, resemble an ester in structure. Amide reactions occur very slowly in the laboratory at room temperature. Similar to ester formation, amide formation is a dehydration reaction where water molecules are split out, and a bond is formed between the nitrogen atom and the carbonyl carbon atom.

In living cells, amide formation is catalyzed by enzymes. Proteins are polyamides; they are formed by joining amino acids into long chains. In proteins, the amide functional group is called a peptide bond. The formation of proteins will be discussed in greater detail in Chapter 11.

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A Closer Look: Synthetic Polyamides

Many synthetic fabrics are polyamide structures, that is, they contain many amide bonds that link together smaller molecules that act as building blocks.  Two important ones are nylon and Kevlar. This section looks at the structures, formation, and hydrolysis of these polyamides.

Polyamides are polymers where the repeating units are held together by amide links. An amide group has the formula – CONH2. An amide link has this structure:

In nylon, the repeating units contain chains of carbon atoms. (That is different from Kevlar, where the repeating units contain benzene rings – see below.) There are various different types of nylon depending on the nature of the carbon chains. 

Nylon-6,6 is made from two monomers each of which contain 6 carbon atoms – hence its name. One of the monomers is a 6 carbon acid with a -COOH group at each end – it is called hexadioic acid. The other monomer is a 6 carbon chain with an amino group, -NH2, at each end. This is 1,6-hexadiamine.

10.9 Formation of Nylon-6,6.

When these two compounds polymerize, the amine and acid groups combine, each time with the loss of a molecule of water. This is known as condensation polymerization. Condensation polymerization is the formation of a polymer involving the loss of a small molecule. In this case, the molecule is water, but in other cases different small molecules might be lost. Note that this reaction does not readily occur at room temperature.  To get amide formation, the reaction must be subjected to high heat (~350oC) and high pressure conditions.

Figure 10.9 shows the loss of water between two of the monomers. As long as the reagents are readily available and conditions are favorable, condensation polymerization keeps on happening, and so you get a chain that alternates between the building block units and looks like this:

Kevlar is similar in structure to nylon-6,6 except that instead of the amide links joining chains of carbon atoms together, they join benzene rings. The two monomers are benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene.

10.10 The Formation of Kevlar

Similar to nylon, the reactions of kevlar will continue to react and undergo condensation polymerization building long fibers.

Uses of polyamides

• Nylon: Apart from obvious uses in textiles for clothing and carpets, a lot of nylon is used to make tire cords – the inner structure of a vehicle tire underneath the rubber. The fibers are also used in ropes, and nylon can be cast into solid shapes for cogs and bearings in machines, for example.
• 
• Figure 10.11 Examples of Nylon.  (A) Nylon 6,6 produced in a laboratory experiment. Nylon used to make (B) rope, (C) umbrellas, and (D) rain jackets. Photo of Nylon 6,6 is provided by: University of Ulm, Photo of nylon rope provided by: Angelsharum, Photo of nylon jackets provided by: Sam Kim.
  
• Kevlar: Kevlar is a very strong material – about five times as strong as steel, weight for weight. It is used in bulletproof vests, in composites for boat construction, in lightweight mountaineering ropes, and for lightweight skis and racquets – amongst many other things.
• 
• Figure 10.12 Examples of Kevlar. Kevlar used in (A) canoes, (B) bullet-proof vests, (C) knife-proof sleeves, and (D) workboots. Photo of canoe provided by: Franklin.vp, Photo of bullet-proof vest provided by: Cyril Thomas, Photo of kevlar sleeves provided by: Erin Carstens, and Photo of workboots provided by: Nextcrave

Key Takeaway

• Amides are prepared by the reaction of a carboxylic acid with an amine.
• Polyamides contain many amide bonds that link together smaller building blocks into a larger molecule.

Athletic training is an allied health-care profession recognized by the American Medical Association. The athletic trainer’s role is to recognize, evaluate, and provide immediate care for athletic injuries; prevent athletic injuries by taping, bandaging, and bracing vulnerable body parts; make referrals to medical doctors when necessary; and rehabilitate injured athletes. Athletic trainers work in high schools, colleges, and other organizations where athletics programs are found. Athletic trainers usually have a degree from an accredited athletic training program whose curriculum includes such basic science courses as biology, chemistry, and physics. These studies provide the necessary background for more applied courses, such as anatomy and physiology, exercise physiology, kinesiology, and nutrition. Knowledge of chemistry is necessary for understanding pharmacological and medical terminology. For example, athletic trainers must understand the action of numerous drugs, many of which are esters, amines, or amides like those mentioned in this chapter.

Athletic trainers may have administrative duties, such as the responsibility for ordering supplies. They also need to be able to evaluate nutritional supplements because providing the wrong one can get an athlete banned from competition and may bring sanctions against a school. In short, the athletic trainer is responsible for the overall health and well-being of the athletes in his or her charge.

• The hydrolysis of an amide produces a carboxylic acid and ammonia or an amine.

Exercises

1. Complete each equation.

2. Complete each equation.

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10.4 Chapter Summary

To ensure that you understand the material in this chapter, you should review the meanings of the following bold terms in the summary and ask yourself how they relate to the topics in the chapter. A carboxylic acid (R-COOH) contains the functional group COOH, called the carboxyl group, which has an OH group attached to a carbonyl carbon atom. An ester (R-COOR′) has an OR′ group attached to a carbonyl carbon atom. Sulfur can be incorporated into organic molecules as thiols (R-SH), disulfides (R-S-S-R’), thioethers (R-S-R’) or thioesters (R-COSR’). Phosphorus can be incorporated into organic molecules as phosphoesters (R-OPO32-) (An amine is derived from ammonia (NH3), with one, two, or all three of the hydrogen atoms of NH3 replaced by an alkyl (or an aryl) group. The amide (RCONR’) functional group has a carbonyl group joined to a nitrogen atom from NH3 or an amine.

Since sulfur is in the same family at oxygen, it has similar characteristics and reactivity when forming organic molecules. However, sulfur is not as electronegative as oxygen, thus, intermolecular forces between sulfur-containing molecules is similar but weaker than those containing oxygen. Thiols resemble the alcohol functional group and can undergo oxidation with other thiols to form disulfide bonds. Disulfide bonds are important in many biological molecules, such as proteins where they aid in maintaining the 3-dimensional structure of the molecules.

Amines are nitrogen-containing organic molecules derived from ammonia (NH3). A primary (1°) amine (RNH2) has one organic group bonded to the nitrogen atom, a secondary (2°) amine (R2NH) has two organic groups bonded to the nitrogen atom, and a tertiary (3°) amine (R3N) has three organic groups bonded to the nitrogen atom. Amines are basic compounds that react with strong acids to produce ammonium (NH4+) salts. A cyclic compound in which the ring contains one or more noncarbon atoms is called a heterocyclic compound. There are many heterocyclic amines, including many physiologically important ones. Alkaloids are heterocyclic amines found in many plants. Caffeine, nicotine, and cocaine are familiar alkaloids.

The formation of esters and the related compounds, thioesters, amides, and phosphoesters are formed by dehydration synthesis, involving the loss of water. Some of the most important esters in biochemistry are those formed from phosphoric acid. The structures of DNA and RNA contain phosphodiester linkages.

Most amides are colorless and odorless, and the lighter ones are soluble in water. Because they are polar molecules, amides have comparatively high boiling points and melting points. Amides are synthesized from carboxylic acids and NH3 or amines. Amides are neutral compounds. They resist hydrolysis in water, but acids, bases, and enzymes catalyze the breakdown reaction.

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1. Sulfur mustard. (2017, May 11). In Wikipedia, The Free Encyclopedia. Retrieved 00:30, May 16, 2017, from https://en.wikipedia.org/w/index.php?title=Sulfur_mustard&oldid=779831530
2. Nitrogen mustard. (2017, May 10). In Wikipedia, The Free Encyclopedia. Retrieved 00:31, May 16, 2017, from https://en.wikipedia.org/w/index.php?title=Nitrogen_mustard&oldid=77970721.
3. Sarin. (2017, May 9). In Wikipedia, The Free Encyclopedia. Retrieved 00:31, May 16, 2017, from https://en.wikipedia.org/w/index.php?title=Sarin&oldid=779604274
4. Ethanethiol. (2017, February 21). In Wikipedia, The Free Encyclopedia. Retrieved 01:44, May 16, 2017, from https://en.wikipedia.org/w/index.php?title=Ethanethiol&oldid=766730923
5. Disulfide. (2017, May 15). In Wikipedia, The Free Encyclopedia. Retrieved 02:06, May 16, 2017, from https://en.wikipedia.org/w/index.php?title=Disulfide&oldid=780480973
6. Ophardt, C. (2013) Biological Chemistry. Libretexts. Available at: https://chem.libretexts.org/Core/Biological_Chemistry/Proteins/Case_Studies%3A_Proteins/Permanent_Hair_Wave
7. Soderberg, T. (2016) Organic Chemistry with a Biological Emphasis. Libretexts. Available at: https://chem.libretexts.org/Textbook_Maps/Organic_Chemistry_Textbook_Maps/Map%3A_Organic_Chemistry_with_a_Biological_Emphasis_(Soderberg)
8. Ball, et al. (2016) MAP: The Basics of General, Organic, and Biological Chemistry. Libretexts. Available at: https://chem.libretexts.org/Textbook_Maps/Introductory_Chemistry_Textbook_Maps/Map%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)
9. Aniline. (2017, April 11). In Wikipedia, The Free Encyclopedia. Retrieved 14:01, May 17, 2017, from https://en.wikipedia.org/w/index.php?title=Aniline&oldid=774896013
10. Clark, J. (2017) Organic Chemistry. Libretexts. Available at: https://chem.libretexts.org/Core/Organic_Chemistry/Amides/Reactivity_of_Amides/Polyamides