Azide-alkyne Huisgen cycloaddition
The azide-alkyne Huisgen cycloaddition is a 1,3-dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3-triazole. Rolf Huisgen was the first to understand the scope of this organic reaction. American chemist Karl Barry Sharpless has referred to this cycloaddition as "the cream of the crop" of click chemistry and "the premier example of a click reaction." In the reaction above azide 2 reacts neatly with alkyne 1 to afford the triazole 3 as a mixture of 1,4-adduct and 1,5-adduct at 98 °C in 18 hours. The standard 1,3-cycloaddition between an azide 1,3-dipole and an alkene as dipolarophile has been ignored due to lack of reactivity as a result of electron-poor olefins and elimination side reactions; some success has been found with non-metal-catalyzed cycloadditions, such as the reactions using dipolarophiles that are electron-poor olefins or alkynes. Although azides are not the most reactive 1,3-dipole available for reaction, they are preferred for their relative lack of side reactions and stability in typical synthetic conditions.
A notable variant of the Huisgen 1,3-dipolar cycloaddition is the copper catalyzed variant, no longer a true concerted cycloaddition, in which organic azides and terminal alkynes are united to afford 1,4-regioisomers of 1,2,3-triazoles as sole products. The copper-catalyzed variant was first reported in 2002 in independent publications by Morten Meldal at the Carlsberg Laboratory in Denmark and Valery Fokin and K. Barry Sharpless at the Scripps Research Institute. While the copper-catalyzed variant gives rise to a triazole from a terminal alkyne and an azide, formally it is not a 1,3-dipolar cycloaddition and thus should not be termed a Huisgen cycloaddition; this reaction is better termed the Copper-catalyzed Azide-Alkyne Cycloaddition. While the reaction can be performed using commercial sources of copper such as cuprous bromide or iodide, the reaction works much better using a mixture of copper and a reducing agent to produce Cu in situ; as Cu is unstable in aqueous solvents, stabilizing ligands are effective for improving the reaction outcome if tris-amine is used.
The reaction can be run in a variety of solvents, mixtures of water and a variety of miscible organic solvents including alcohols, DMSO, DMF, tBuOH and acetone. Owing to the powerful coordinating ability of nitriles towards Cu, it is best to avoid acetonitrile as the solvent; the starting reagents need not be soluble for the reaction to be successful. In many cases, the product can be filtered from the solution as the only purification step required. NH-1,2,3-triazoles are prepared from alkynes in a sequence called the Banert cascade; the utility of the Cu-catalyzed click reaction has been demonstrated in the polymerization reaction of a bis-azide and a bis-alkyne with copper and TBTA to a conjugated fluorene based polymer. The degree of polymerization exceeds 50. With a stopper molecule such as phenyl azide, well-defined phenyl end-groups are obtained; the copper-mediated azide-alkyne cycloaddition is receiving widespread use in material and surface sciences. Most variations in coupling polymers with other polymers or small molecules have been explored.
Current shortcomings are that the terminal alkyne appears to participate in free-radical polymerizations. This requires protection of the terminal alkyne with a trimethyl silyl protecting group and subsequent deprotection after the radical reaction are completed; the use of organic solvents and inert atmospheres to do the cycloaddition with many polymers makes the "click" label inappropriate for such reactions. An aqueous protocol for performing the cycloaddition with free-radical polymers is desirable; the CuAAC click reaction effectively couples polystyrene and bovine serum albumin. The result is an amphiphilic biohybrid. BSA contains a thiol group at Cys-34, functionalized with an alkyne group. In water the biohybrid micelles with a diameter of 30 to 70 nanometer form aggregates; the use of a Cu catalyst in water was an improvement over the same reaction first popularized by Rolf Huisgen in the 1970s, which he ran at elevated temperatures. The traditional reaction thus requires high temperatures.
However, the azides and alkynes are both kinetically stable. As mentioned above, copper-catalysed click reactions work on terminal alkynes; the Cu species undergo metal insertion reaction into the terminal alkynes. The Cu species may either be introduced as preformed complexes, or are otherwise generated in the reaction pot itself by one of the following ways: A Cu2+ compound is added to the reaction in presence of a reducing agent which reduces the Cu from the to the oxidation state; the advantage of generating the Cu species in this manner is it eliminates the need of a base in the reaction. The presence of reducing agent makes up for any oxygen which may have gotten into the system. Oxygen oxidises the Cu to Cu. One of the more used Cu compounds is CuSO4. Oxidation of Cu metal Halides of copper may be used. However, the iodide and bromide Cu salts require either the presence of amines or higher temperatures. Used solvents are polar aprotic solvents such as THF, DMSO, acetonitrile, DMF as well as in non-polar aprotic solvents such as toluene.
Neat solvents or a mixture of solvents may be used. DIPEA and Et3N are used bases. A mechanism for the reaction has been suggested based on density functional
Lead azide is an inorganic compound. More so than other azides, Pb2 is explosive, it is used in detonators to initiate secondary explosives. In a commercially usable form, it is a white to buff powder. Lead azide is prepared by metathesis between lead nitrate. Dextrin can be added to the solution to stabilize the precipitated product; the solid is not hygroscopic, water does not reduce its impact sensitivity. It is shipped in a dextrinated solution that lowers its sensitivity; when protected from humidity, it is stable in storage. An alternative method involves dissolving lead acetate in a sodium azide solution. Lead azide in its pure form was first prepared by Theodor Curtius in 1891. Due to sensitivity and stability concerns, the dextrinated form of lead azide was developed in the 1920s and 1930s with large scale production by DuPont Co beginning in 1932. Detonator development during World War II resulted in the need for a form of lead azide with a more brisant output. RD-1333 lead azide, a version of lead azide with sodium carboxymethylcellulose as a precipitating agent, was developed to meet that need.
The Vietnam War saw an accelerated need for lead azide and it was during this time that Special Purpose Lead Azide was developed. After the Vietnam War, the use of lead azide decreased. Due to the size of the US stockpile, the manufacture of lead azide in the US ceased by the early 1990s. In the 2000s, concerns about the age and stability of stockpiled lead azide led the US government to investigate methods to dispose of its stockpiled lead azide and obtain new manufacturers. Lead azide is sensitive and handled and stored under water in insulated rubber containers, it will explode after a fall of around 150 mm or in the presence of a static discharge of 7 millijoules. Its detonation velocity is around 5,180 m/s. Ammonium acetate and sodium dichromate are used to destroy small quantities of lead azide. Lead azide has immediate deflagration to detonation transition this means that small amounts undergo full detonation. Lead azide reacts with copper, cadmium, or alloys containing these metals to form other azides.
For example, copper azide is more explosive and too sensitive to be used commercially. Lead azide was a component of the six.22 caliber Devastator rounds fired from a Röhm RG-14 revolver by John Hinckley, Jr. in his assassination attempt on U. S. President Ronald Reagan on March 30, 1981; the rounds consisted of lead azide centers with lacquer-sealed aluminum tips designed to explode upon impact. Lead styphnate National Pollutant Inventory – Lead and Lead Compounds Fact Sheet
Sodium nitrite is an inorganic compound with the chemical formula NaNO2. It is a white to yellowish crystalline powder, soluble in water and is hygroscopic, it is a useful precursor to a variety of organic compounds, such as pharmaceuticals and pesticides, but it is best known as a food additive used in processed meats and in fish products. Nitrate or nitrite under conditions that result in endogenous nitrosation has been classified as "probably carcinogenic to humans" by International Agency for Research on Cancer. Nitrite is an easy way to give a pink shade to processed meats. Nitrite reacts with the meat myoglobin to cause color changes, first converting to nitrosomyoglobin on heating, to nitrosohemochrome. According to the meat industry, nitrite is used to prevent botulism, yet several large meat processors produce safe processed meats without relying on nitrite or nitrate. The main use of sodium nitrite is for the industrial production of organonitrogen compounds, it is a reagent for conversion of amines into diazo compounds, which are key precursors to many dyes, such as diazo dyes.
Nitroso compounds are produced from nitrites. These are used in the rubber industry, it is used for phosphatizing and detinning. Sodium nitrite is an effective corrosion inhibitor and is used as an additive in industrial greases, as an aqueous solution in closed loop cooling systems, in a molten state as a heat transfer medium. Sodium nitrite is on the World Health Organization's List of Essential Medicines as it is an efficient drug in case of cyanide poisoning, it is used together with sodium thiosulfate. Salt has been used for the preservation of meat; the salt-preserved meatproduct was brownish-gray in color. When sodium nitrite is added with the salt, the meat develops a red pink color, associated with cured meats such as ham, hot dogs, bologna. In the early 1900s, irregular curing was commonplace; this led to further research surrounding the use of sodium nitrite as an additive in food, standardizing the amount present in foods to minimize the amount needed while maximizing its food additive role.
Through this research, sodium nitrite has been found to give color to the meat. The ability of sodium nitrite to address the above-mentioned issues has led to production of meat with extended storage life and has improved desirable color/taste. According to scientists working for the meat industry, nitrite has improved food safety. Nitrite has the E number E250. Potassium nitrite is used in the same way, it is approved for usage in the New Zealand. The appearance and taste of meat is an important component of consumer acceptance. Sodium nitrite is responsible for the desirable red color of meat. Little nitrite is needed to induce this change, it has been reported that as little as 2 to 14 parts per million is needed to induce this desirable color change. However, to extend the lifespan of this color change higher levels are needed; the mechanism responsible for this color change is the formation of nitrosylating agents by nitrite, which has the ability to transfer nitric oxide that subsequently reacts with myoglobin to produce the cured meat color.
The unique taste associated with cured meat is affected by the addition of sodium nitrite. However, the mechanism underlying this change in taste is still not understood. Sodium nitrite is known for its role in inhibiting the growth of Clostridium botulinum spores in refrigerated meats; the mechanism for this activity results from the inhibition of iron-sulfur clusters essential to energy metabolism of Clostridium botulinum. However, sodium nitrite has had varying degrees of effectiveness for controlling growth of other spoilage or disease causing microorganisms. Though the inhibitory mechanisms for sodium nitrite are not well known, its effectiveness depends on several factors including residual nitrite level, pH, salt concentration, reductants present and iron content. Furthermore, the type of bacteria affects sodium nitrites effectiveness, it is agreed upon that sodium nitrite is not considered effective for controlling gram-negative enteric pathogens such as Salmonella and Escherichia coli.
Other food additives provide similar protection against bacteria, but do not provide the desired pink color. Sodium nitrite is able to delay the development of oxidative rancidity. Lipid oxidation is considered to be a major reason for the deterioration of quality of meat products. Sodium nitrite acts as an antioxidant in a mechanism similar to the one responsible for the coloring affect. Nitrite reacts with heme proteins and metal ions. Neutralization of these free radicals terminates the cycle of lipid oxidation that leads to rancidity. While this chemical will prevent the growth of bacteria, it can be toxic in high amounts for animals and humans. Sodium nitrite's LD50 in rats is 180 mg/kg and its human LDLo is 71 mg/kg, meaning a 65 kg person would have to consume at least 4.6 g to result in death. To prevent toxicity, sodium nitrite sold as a food additive is dyed bright pink to avoid mistaking it for plain salt or sugar. Nitrites are not occurring in vegetables in significant quantities. However, nitrates are found in commercially available vegetables and a study in an intensive agricultural area in northern Portugal found residual nitrate levels in 34 vegetable samp
A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed by the reception of a proton by a base—in other words, it is a base with a hydrogen ion added to it. On the other hand, a conjugate base is what is left over after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a species formed by the removal of a proton from an acid; because some acids are capable of releasing multiple protons, the conjugate base of an acid may itself be acidic. In summary, this can be represented as the following chemical reaction: Acid + Base ⇌ Conjugate Base + Conjugate Acid Johannes Nicolaus Brønsted and Martin Lowry introduced the Brønsted–Lowry theory, which proposed that any compound that can transfer a proton to any other compound is an acid, the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge. A cation can be a conjugate acid, an anion can be a conjugate base, depending on which substance is involved and which acid–base theory is the viewpoint.
The simplest anion which can be a conjugate base is the solvated electron whose conjugate acid is the atomic hydrogen. In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid: Conjugates are formed when an acid loses a hydrogen proton or a base gains a hydrogen proton. Refer to the following figure: We say that the water molecule is the conjugate acid of the hydroxide ion after the latter received the hydrogen proton donated by ammonium. On the other hand, ammonia is the conjugate base for the acid ammonium after ammonium has donated a hydrogen ion towards the production of the water molecule. We can refer to OH- as a conjugate base of H2O, since the water molecule donates a proton towards the production of NH+4 in the reverse reaction, the predominating process in nature due to the strength of the base NH3 over the hydroxide ion. Based on this information, it is clear that the terms "Acid", "Base", "conjugate acid", "conjugate base" are not fixed for a certain chemical species.
The strength of a conjugate acid is directly proportional to its dissociation constant. If a conjugate acid is strong, its dissociation will have a higher equilibrium constant and the products of the reaction will be favored; the strength of a conjugate base can be seen as the tendency of the species to "pull" hydrogen protons towards itself. If a conjugate base is classified as strong, it will "hold on" to the hydrogen proton when in solution and its acid will not dissociate. On the other hand, if a species is classified as a strong acid, its conjugate base will be weak in nature. An example of this case would be the dissociation of Hydrochloric acid HCl in water. Since HCl is a strong acid, its conjugate base will be a weak conjugate base. Therefore, in this system, most H+ will be in the form of a Hydronium ion H3O+ instead of attached to a Cl anion and the conjugate base will be weaker than a water molecule. If an acid is weak, its conjugate base will be strong; when considering the fact that the Kw is equal to the product of the concentrations of H+ and OH.
A weak acid will have a low concentration of H+. The Kw divided by a low H+ concentration will result in a low OH- concentration as well. Therefore, weak acids will have weak conjugate bases, unlike the misconception that they have strong conjugate bases; the acid and conjugate base as well as the base and conjugate acid are known as conjugate pairs. When finding a conjugate acid or base, it is important to look at the reactants of the chemical equation. In this case, the reactants are the acids and bases, the acid corresponds to the conjugate base on the product side of the chemical equation. To identify the conjugate acid, look for the pair of compounds that are related; the acid–base reaction can be viewed in a before and after sense. The before is the reactant side of the after is the product side of the equation; the conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the compound that has one less hydrogen ion of the conjugate acid is the base.
The conjugate base in the after side of the equation lost a hydrogen ion, so in the before side of the equation, the compound that has one more hydrogen ion of the conjugate base is the acid. Consider the following acid–base reaction: HNO3 + H2O → H3O+ + NO−3Nitric acid is an acid because it donates a proton to the water molecule and its conjugate base is nitrate; the water molecule acts as a base because it receives the Hydrogen Proton and its conjugate acid is the hydronium ion. One use of conjugate acids and bases lies in buffering systems. In a buffer, a weak acid and its conjugate base, or a weak base and its conjugate acid, are used in order to limit the pH change during a titration process. Buffers have both non-organic chemical applications. For example, besides buffers being used in lab processes, our blood acts as a buffer to maintain pH; the most important buffer in our bloodstream is the carbonic acid-bicarbonate buffer, which prevents drastic pH changes when CO2 is introduced. This functions as such: CO 2 + H 2 O ↽ − − ⇀ H 2 CO 3 ↽
Nitrous oxide known as laughing gas or nitrous, is a chemical compound, an oxide of nitrogen with the formula N2O. At room temperature, it is a colourless non-flammable gas, with taste. At elevated temperatures, nitrous oxide is a powerful oxidiser similar to molecular oxygen, it is soluble in water. Nitrous oxide has significant medical uses in surgery and dentistry, for its anaesthetic and pain reducing effects, its name "laughing gas", coined by Humphry Davy, is due to the euphoric effects upon inhaling it, a property that has led to its recreational use as a dissociative anaesthetic. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system, it is used as an oxidiser in rocket propellants, in motor racing to increase the power output of engines. Nitrous oxide occurs in small amounts in the atmosphere, but has been found to be a major scavenger of stratospheric ozone, with an impact comparable to that of CFCs, it is estimated that 30% of the N2O in the atmosphere is the result of human activity, chiefly agriculture.
Nitrous oxide may be used as an oxidiser in a rocket motor. This is advantageous over other oxidisers in that it is much less toxic, due to its stability at room temperature is easier to store and safe to carry on a flight; as a secondary benefit, it may be decomposed to form breathing air. Its high density and low storage pressure enable it to be competitive with stored high-pressure gas systems. In a 1914 patent, American rocket pioneer Robert Goddard suggested nitrous oxide and gasoline as possible propellants for a liquid-fuelled rocket. Nitrous oxide has been the oxidiser of choice in several hybrid rocket designs; the combination of nitrous oxide with hydroxyl-terminated polybutadiene fuel has been used by SpaceShipOne and others. It is notably used in amateur and high power rocketry with various plastics as the fuel. Nitrous oxide may be used in a monopropellant rocket. In the presence of a heated catalyst, N2O will decompose exothermically into nitrogen and oxygen, at a temperature of 1,070 °F.
Because of the large heat release, the catalytic action becomes secondary, as thermal autodecomposition becomes dominant. In a vacuum thruster, this may provide a monopropellant specific impulse of as much as 180 s. While noticeably less than the Isp available from hydrazine thrusters, the decreased toxicity makes nitrous oxide an option worth investigating. Nitrous oxide is said to deflagrate at 600 °C at a pressure of 309 psi. At 600 psi, for example, the required ignition energy is only 6 joules, whereas N2O at 130 psi a 2,500-joule ignition energy input is insufficient. In vehicle racing, nitrous oxide allows the engine to burn more fuel by providing more oxygen than air alone, resulting in a more powerful combustion; the gas is not flammable at a low pressure/temperature, but it delivers more oxygen than atmospheric air by breaking down at elevated temperatures. Therefore, it is mixed with another fuel, easier to deflagrate. Nitrous oxide is a strong oxidant equivalent to hydrogen peroxide, much stronger than oxygen gas.
Nitrous oxide is stored as a compressed liquid. Sometimes nitrous oxide is injected into the intake manifold, whereas other systems directly inject, right before the cylinder to increase power; the technique was used during World War II by Luftwaffe aircraft with the GM-1 system to boost the power output of aircraft engines. Meant to provide the Luftwaffe standard aircraft with superior high-altitude performance, technological considerations limited its use to high altitudes. Accordingly, it was only used by specialised planes such as high-altitude reconnaissance aircraft, high-speed bombers and high-altitude interceptor aircraft, it sometimes could be found on Luftwaffe aircraft fitted with another engine-boost system, MW 50, a form of water injection for aviation engines that used methanol for its boost capabilities. One of the major problems of using nitrous oxide in a reciprocating engine is that it can produce enough power to damage or destroy the engine. Large power increases are possible, if the mechanical structure of the engine is not properly reinforced, the engine may be damaged, or destroyed, during this kind of operation.
It is important with nitrous oxide augmentation of petrol engines to maintain proper operating temperatures and fuel levels to prevent "pre-ignition", or "detonation". Most problems that are associated with nitrous oxide do not come from mechanical failure due to the power increases. Since nitrous oxide allows a much denser charge into the cylinder, it increases cylinder pressures; the increased pressure and temperature can cause problems such as melting valves. It may crack or warp the piston or head and cause pre-ignition due to uneven heating. Automotive-grade liquid nitrous oxide differs from medical-grade nitrous oxide. A small amount of sulfur dioxide is added to prevent substance abuse. Multiple washes through a base can remove this, decreasing the corrosive properties observed when SO2 is further oxidised during combustion into sulfuric
Hydrolysis is a term used for both an electro-chemical process and a biological one. The hydrolysis of water is the separation of water molecules into hydrogen and oxygen atoms using electricity. Biological hydrolysis is the cleavage of biomolecules where a water molecule is consumed to effect the separation of a larger molecule into component parts; when a carbohydrate is broken into its component sugar molecules by hydrolysis, this is termed saccharification. Hydrolysis or saccharification is a step in the degradation of a substance. Hydrolysis can be the reverse of a condensation reaction in which two molecules join together into a larger one and eject a water molecule, thus hydrolysis adds water to break down, whereas condensation builds up by removing water and any other solvents. Some hydration reactions are hydrolysis. Hydrolysis is a chemical process in which a molecule of water is added to a substance. Sometimes this addition causes both water molecule to split into two parts. In such reactions, one fragment of the target molecule gains a hydrogen ion.
It breaks a chemical bond in the compound. A common kind of hydrolysis occurs when a salt of weak base is dissolved in water. Water spontaneously ionizes into hydroxide anions and hydronium cations; the salt dissociates into its constituent anions and cations. For example, sodium acetate dissociates in water into acetate ions. Sodium ions react little with the hydroxide ions whereas the acetate ions combine with hydronium ions to produce acetic acid. In this case the net result is a relative excess of hydroxide ions. Strong acids undergo hydrolysis. For example, dissolving sulfuric acid in water is accompanied by hydrolysis to give hydronium and bisulfate, the sulfuric acid's conjugate base. For a more technical discussion of what occurs during such a hydrolysis, see Brønsted–Lowry acid–base theory. Acid–base-catalysed hydrolyses are common, their hydrolysis occurs when the nucleophile attacks the carbon of the carbonyl group of the ester or amide. In an aqueous base, hydroxyl ions are better nucleophiles than polar molecules such as water.
In acids, the carbonyl group becomes protonated, this leads to a much easier nucleophilic attack. The products for both hydrolyses are compounds with carboxylic acid groups; the oldest commercially practiced example of ester hydrolysis is saponification. It is the hydrolysis of a triglyceride with an aqueous base such as sodium hydroxide. During the process, glycerol is formed, the fatty acids react with the base, converting them to salts; these salts are called soaps used in households. In addition, in living systems, most biochemical reactions take place during the catalysis of enzymes; the catalytic action of enzymes allows the hydrolysis of proteins, fats and carbohydrates. As an example, one may consider proteases, they catalyse the hydrolysis of interior peptide bonds in peptide chains, as opposed to exopeptidases. However, proteases do not catalyse the hydrolysis of all kinds of proteins, their action is stereo-selective: Only proteins with a certain tertiary structure are targeted as some kind of orienting force is needed to place the amide group in the proper position for catalysis.
The necessary contacts between an enzyme and its substrates are created because the enzyme folds in such a way as to form a crevice into which the substrate fits. Therefore, proteins that do not fit into the crevice will not undergo hydrolysis; this specificity preserves the integrity of other proteins such as hormones, therefore the biological system continues to function normally. Upon hydrolysis, an amide converts into an amine or ammonia. One of the two oxygen groups on the carboxylic acid are derived from a water molecule and the amine gains the hydrogen ion; the hydrolysis of peptides gives amino acids. Many polyamide polymers such as nylon 6,6 hydrolyse in the presence of strong acids; the process leads to depolymerization. For this reason nylon products fail by fracturing. Polyesters are susceptible to similar polymer degradation reactions; the problem is known as environmental stress cracking. Hydrolysis is related to energy storage. All living cells require a continual supply of energy for two main purposes: the biosynthesis of micro and macromolecules, the active transport of ions and molecules across cell membranes.
The energy derived from the oxidation of nutrients is not used directly but, by means of a complex and long sequence of reactions, it is channelled into a special energy-storage molecule, adenosine triphosphate. The ATP molecule contains pyrophosphate linkages. ATP can undergo hydrolysis in two ways: the removal of terminal phosphate to form adenosine diphosphate and inorganic phosphate, or the removal of a terminal diphosphate to yield adenosine monophosphate and pyrophosphate; the latter undergoes further cleavage in
An airbag is a vehicle occupant restraint system using a bag designed to inflate rapidly quickly deflate during a collision. It consists of a flexible fabric bag, inflation module and impact sensor; the purpose of the airbag is to provide the occupants a soft cushioning and restraint during a crash event. It can reduce injuries between the interior of the vehicle; the airbag provides an energy absorbing surface between the vehicle's occupants and a steering wheel, instrument panel, as well as the body pillars and windshield. Modern vehicles may contain multiple airbag modules in various configurations including, passenger, side curtain, seat-mounted side impact, knee bolster, inflatable seat-belt, front right and left side sensors and pedestrian airbag modules. During a crash, the vehicle's crash sensors provide crucial information to the airbag electronic controller unit, including collision type and severity of impact. Using this information, the airbag electronic controller unit's crash algorithm determines if the crash event meets the criteria for deployment and triggers various firing circuits to deploy one or more airbag modules within the vehicle.
Working as a supplemental restraint system to the vehicle's seat-belt systems, airbag module deployments are triggered through a pyrotechnic process, designed to be used once. Newer side-impact airbag modules consist of compressed air cylinders that are triggered in the event of a side on vehicle impact; the first commercial designs were introduced in passenger automobiles during the 1970s with limited success and caused some fatalities. Broad commercial adoption of airbags occurred in many markets during the late 1980s and early 1990s with a driver airbag, a front passenger airbag as well on some cars. Airbags are considered a "passive" restraint and act as a supplement to "active" restraints, i.e. seat belts. Because no action by a vehicle occupant is required to activate or use the airbag, it is considered a "passive" device; this is in contrast to seat belts, which are considered "active" devices because the vehicle occupant must act to enable them. This terminology is not related to active and passive safety, which are systems designed to prevent accidents in the first place, systems designed to minimize the effects of accidents once they occur.
In this usage, a car Anti-lock Braking System will qualify as an active-safety device, while both its seatbelts and airbags will qualify as passive-safety devices. Further terminological confusion can arise from the fact that passive devices and systems—those requiring no input or action by the vehicle occupant—can operate independently in an active manner. Vehicle safety professionals are careful in their use of language to avoid this sort of confusion, though advertising principles sometimes prevent such semantic caution in the consumer marketing of safety features. Further confusing the terminology, the aviation safety community uses the terms "active" and "passive" in the opposite sense from the automotive industry; the airbag "for the covering of aeroplane and other vehicle parts" traces its origins to a United States patent submitted in 1919 by two dentists, Harold Round & Arthur Parrott of Birmingham and approved in 1920. Air-filled bladders were in use as early as 1951; the airbag for automobile use is credited independently to the American John W. Hetrick who filed for an airbag patent on 5 August 1952, granted #2,649,311 by the United States Patent Office on 18 August 1953.
German engineer Walter Linderer, who filed German patent #896,312 on 6 October 1951, was issued on 12 November 1953 three months after American John Hetrick. Hetrick and Linderer's airbags were both based on a compressed air system, either released by spring, bumper contact or by the driver. Research during the 1960s showed that compressed air could not inflate the mechanically based airbags fast enough for maximum safety, leading to the current chemical and electrically based airbags. In patent applications, manufacturers sometimes use the term "inflatable occupant restraint systems". Hetrick was an industrial member of the United States Navy, his airbag was designed based on his experiences with compressed air from torpedoes during his service in the Navy, combined with a desire to provide protection for his family in their automobile during accidents. Hetrick worked with the major American automobile corporations at the time, but they chose not to invest in it. Although airbags are now required in every automobile sold in the United States, Hetrick's 1951 patent filing serves as an example of a "valuable" invention with little economic value to its inventor because its first commercial use did not occur until after the patent expired when in 1971, it was installed as an experiment in a few Ford cars.
In 1964, a Japanese automobile engineer, Yasuzaburou Kobori, started developing an airbag "safety net" system that harnesses an explosive to inflate an airbag, for which he was awarded patents in 14 countries. He died in 1975 without seeing widespread adoption of airbag systems. In 1967, a breakthrough occurred in the development of airbag crash sensors, when Allen K. Breed invented a mechanically-based ball-in-tube component for crash detection, an electromechanical sensor with a steel ball attached to a tube by a magnet that would inflate an airbag in under 30 milliseconds. A small explosion of sodium azide instead of compressed air was used for the first time during inflation. Breed Corporation marketed this innovation first to Chrysler. A similar "Auto-Ceptor" crash-restraint, developed by the Eato