Three representations of an organic compound,5α-Dihydroprogesterone (5α-DHP), asteroid hormone. For molecules showing color, the carbon atoms are in black, hydrogens in gray, and oxygens in red. In the line angle representation, carbon atoms are implied at every terminus of a line and vertex of multiple lines, and hydrogen atoms are implied to fill the remaining needed valences (up to 4).
Organic chemistry is typically taught at the college or university level.[4] It is considered a very challenging course but has also been made accessible to students.[5]
Before the 18th century,chemists generally believed thatcompounds obtained from living organisms were endowed with a vital force that distinguished them frominorganic compounds. According to the concept ofvitalism (vital force theory), organic matter was endowed with a "vital force".[6] During the first half of the nineteenth century, some of the first systematic studies of organic compounds were reported. Around 1816Michel Chevreul started a study ofsoaps made from variousfats andalkalis. He separated the acids that, in combination with the alkali, produced the soap. Since these were all individual compounds, he demonstrated that it was possible to make a chemical change in various fats (which traditionally come from organic sources), producing new compounds, without "vital force". In 1828Friedrich Wöhler produced theorganic chemicalurea (carbamide), a constituent ofurine, frominorganic starting materials (the saltspotassium cyanate andammonium sulfate), in what is now called theWöhler synthesis. Although Wöhler himself was cautious about claiming he had disproved vitalism, this was the first time a substance thought to be organic was synthesized in the laboratory without biological (organic) starting materials. The event is now generally accepted as indeed disproving the doctrine of vitalism.[7]
After Wöhler,Justus von Liebig worked on the organization of organic chemistry, being considered one of its principal founders.[8]
In 1856,William Henry Perkin, while trying to manufacturequinine, accidentally produced the organicdye now known asPerkin's mauve. His discovery, made widely known through its financial success, greatly increased interest in organic chemistry.[9]
A crucial breakthrough for organic chemistry was the concept of chemical structure, developed independently in 1858 by bothFriedrich August Kekulé andArchibald Scott Couper.[10] Both researchers suggested thattetravalent carbon atoms could link to each other to form a carbon lattice, and that the detailed patterns of atomic bonding could be discerned by skillful interpretations of appropriate chemical reactions.[11]
The era of thepharmaceutical industry began in the last decade of the 19th century when the German company,Bayer, first manufactured acetylsalicylic acid—more commonly known asaspirin.[12] By 1910Paul Ehrlich and his laboratory group began developing arsenic-basedarsphenamine (Salvarsan) as the first effective medicinal treatment ofsyphilis, and thereby initiated the medical practice ofchemotherapy. Ehrlich popularized the concepts of "magic bullet" drugs and of systematically improving drug therapies.[13][14] His laboratory made decisive contributions to developing antiserum fordiphtheria and standardizing therapeutic serums.[15]
An example of an organometallic molecule, a catalyst calledGrubbs' catalyst. Its formula is often given as RuCl2(PCy3)2(=CHPh), where the ball-and-stick model is based onX-ray crystallography.[16] The single metal atom ruthenium (Ru) (in turquoise) is at the very center of the structure. Two chlorines (green) are bonded to the ruthenium atom. Carbon atoms are black, hydrogens gray-white, and phosphorus orange. A phosphorus-ligand bond, tricyclohexylphosphine, PCy, is below center. Another PCy ligand appears at the top of the image where its rings are obscuring one another. The ring group projecting to the right, analkylidene, contains a metal-carbon double bond to ruthenium.
Early examples of organic reactions and applications were often found because of a combination of luck and preparation for unexpected observations. The latter half of the 19th century however witnessed systematic studies of organic compounds. The development ofsynthetic indigo is illustrative. The production of indigo from plant sources dropped from 19,000 tons in 1897 to 1,000 tons by 1914 thanks to the synthetic methods developed byAdolf von Baeyer. In 2002, 17,000 tons of synthetic indigo were produced frompetrochemicals.[17]
In the early part of the 20th century,polymers andenzymes were shown to be large organic molecules, and petroleum was shown to be of biological origin.
The multiple-step synthesis of complex organic compounds is called total synthesis.Total synthesis of complex natural compounds increased in complexity toglucose andterpineol. For example,cholesterol-related compounds have opened ways to synthesize complexhuman hormones and their modified derivatives. Since the start of the 20th century, complexity of total syntheses has been increased to include molecules of high complexity such aslysergic acid andvitamin B12.[18]
Thetotal synthesis of vitamin B12 marked a major achievement in organic chemistry.
The discovery ofpetroleum and the development of thepetrochemical industry spurred the development of organic chemistry. Converting individual petroleum compounds intotypes of compounds by various chemical processes led toorganic reactions enabling a broad range of industrial and commercial products including, among (many) others:plastics,synthetic rubber, organicadhesives, and various property-modifying petroleum additives andcatalysts.
The majority of chemical compounds occurring in biological organisms are carbon compounds, so the association between organic chemistry andbiochemistry is so close that biochemistry might be regarded as in essence a branch of organic chemistry. Although thehistory of biochemistry might be taken to span some four centuries, fundamental understanding of the field only began to develop in the late 19th century and the actual termbiochemistry was coined around the start of 20th century. Research in the field increased throughout the twentieth century, without any indication of slackening in the rate of increase, as may be verified by inspection of abstraction and indexing services such asBIOSIS Previews andBiological Abstracts, which began in the 1920s as a single annual volume, but has grown so drastically that by the end of the 20th century it was only available to the everyday user as an online electronicdatabase.[19]
Organic compounds were traditionally characterized by a variety of chemical tests, called "wet methods", but such tests have been largely displaced by spectroscopic or other computer-intensive methods of analysis.[20] Listed in approximate order of utility, the chief analytical methods are:
Nuclear magnetic resonance (NMR) spectroscopy is the most commonly used technique, often permitting the complete assignment of atom connectivity and even stereochemistry usingcorrelation spectroscopy. The principal constituent atoms of organic chemistry – hydrogen and carbon – exist naturally with NMR-responsive isotopes, respectively1H and13C.
Elemental analysis: A destructive method used to determine the elemental composition of a molecule. See also mass spectrometry, below.
Mass spectrometry indicates themolecular weight of a compound and, from thefragmentation patterns, its structure. High-resolution mass spectrometry can usually identify the exact formula of a compound and is used in place of elemental analysis. In former times, mass spectrometry was restricted to neutral molecules exhibiting some volatility, but advanced ionization techniques allow one to obtain the "mass spec" of virtually any organic compound.
Crystallography can be useful for determiningmolecular geometry when a single crystal of the material is available. Highly efficient hardware and software allows a structure to be determined within hours of obtaining a suitable crystal.
Traditional spectroscopic methods such asinfrared spectroscopy,optical rotation, andUV/VIS spectroscopy provide relatively nonspecific structural information but remain in use for specific applications. Refractive index and density can also be important for substance identification.
The physical properties of organic compounds typically of interest include both quantitative and qualitative features. Quantitative information includes a melting point, boiling point, solubility, and index of refraction. Qualitative properties include odor, consistency, and color.
Organic compounds typically melt and many boil. In contrast, while inorganic materials generally can be melted, many do not boil, and instead tend to degrade. In earlier times, the melting point (m.p.) and boiling point (b.p.) provided crucial information on the purity and identity of organic compounds. The melting and boiling points correlate with the polarity of the molecules and their molecular weight. Some organic compounds, especially symmetrical ones,sublime. A well-known example of a sublimable organic compound ispara-dichlorobenzene, the odiferous constituent of modern mothballs. Organic compounds are usually not very stable at temperatures above 300 °C, although some exceptions exist.
Neutral organic compounds tend to behydrophobic; that is, they are lesssoluble in water than in organic solvents. Exceptions include organic compounds that containionizable groups as well as lowmolecular weightalcohols,amines, andcarboxylic acids wherehydrogen bonding occurs. Otherwise, organic compounds tend to dissolve in organicsolvents. Solubility varies widely with the organic solute and with the organic solvent.
Various names and depictions for one organic compound
The names of organic compounds are either systematic, following logically from a set of rules, or nonsystematic, following various traditions. Systematic nomenclature is stipulated by specifications fromIUPAC (International Union of Pure and Applied Chemistry). Systematic nomenclature starts with the name for aparent structure within the molecule of interest. This parent name is then modified by prefixes, suffixes, and numbers to unambiguously convey the structure. Given that millions of organic compounds are known, rigorous use of systematic names can be cumbersome. Thus, IUPAC recommendations are more closely followed for simple compounds, but not complex molecules. To use the systematic naming, one must know the structures and names of the parent structures. Parent structures include unsubstituted hydrocarbons, heterocycles, and mono functionalized derivatives thereof.
Nonsystematic nomenclature is simpler and unambiguous, at least to organic chemists. Nonsystematic names do not indicate the structure of the compound. They are common for complex molecules, which include most natural products. Thus, the informally namedlysergic acid diethylamide is systematically named(6aR,9R)-N,N-diethyl-7-methyl-4,6,6a,7,8,9-hexahydroindolo-[4,3-fg] quinoline-9-carboxamide.
With the increased use of computing, other naming methods have evolved that are intended to be interpreted by machines. Two popular formats areSMILES andInChI.
Organic molecules are described more commonly by drawings orstructural formulas, combinations of drawings and chemical symbols. Theline-angle formula is simple and unambiguous. In this system, the endpoints and intersections of each line represent one carbon, and hydrogen atoms can either be notated explicitly or assumed to be present as implied bytetravalent carbon.
This diagram shows 5 distinct structural representations of the organic compound butane. The left-most structure is a bond-line drawing where the hydrogen atoms are removed. The 2nd structure has the hydrogens added depicted—the dark wedged bonds indicate the hydrogen atoms are coming toward the reader, the hashed bonds indicate the atoms are oriented away from the reader, and the solid (plain) bonds indicate the bonds are in the plane of the screen/paper. The middle structure shows the four carbon atoms. The 4th structure is a representation just showing the atoms and bonds without 3-dimensions. The right-most structure is a condensed structure representation of butane.
By 1880 an explosion in the number of chemical compounds being discovered occurred assisted by new synthetic and analytical techniques. Grignard described the situation as "chaos le plus complet" (complete chaos) due to the lack of convention it was possible to have multiple names for the same compound. This led to the creation of theGeneva rules in 1892.[21]
The concept of functional groups is central in organic chemistry, both as a means to classify structures and for predicting properties. A functional group is a molecular module, and the reactivity of that functional group is assumed, within limits, to be the same in a variety of molecules. Functional groups can have a decisive influence on the chemical and physical properties of organic compounds. Molecules are classified based on their functional groups. Alcohols, for example, all have the subunit C-O-H. All alcohols tend to be somewhathydrophilic, usually formesters, and usually can be converted to the correspondinghalides. Most functional groups feature heteroatoms (atoms other than C and H). Organic compounds are classified according to functional groups, alcohols, carboxylic acids, amines, etc.[22] Functional groups make the molecule more acidic or basic due to their electronic influence on surrounding parts of the molecule.
As thepKa (akabasicity) of the molecular addition/functional group increases, there is a correspondingdipole, when measured, increases in strength. A dipole directed towards the functional group (higher pKa therefore basic nature of group) points towards it and decreases in strength with increasing distance. Dipole distance (measured inAngstroms) andsteric hindrance towards the functional group have an intermolecular and intramolecular effect on the surrounding environment andpH level.
Different functional groups have different pKa values and bond strengths (single, double, triple) leading to increased electrophilicity with lower pKa and increased nucleophile strength with higher pKa. More basic/nucleophilic functional groups desire to attack an electrophilic functional group with a lower pKa on another molecule (intermolecular) or within the same molecule (intramolecular). Any group with a net acidic pKa that gets within range, such as an acyl or carbonyl group is fair game. Since the likelihood of being attacked decreases with an increase in pKa,acyl chloride components with the lowest measuredpKa values are most likely to be attacked, followed by carboxylic acids (pKa = 4), thiols (13), malonates (13), alcohols (17), aldehydes (20), nitriles (25), esters (25), then amines (35).[23] Amines are very basic, and are great nucleophiles/attackers.
The aliphatic hydrocarbons are subdivided into three groups ofhomologous series according to their state ofsaturation:
alkanes (paraffins): aliphatic hydrocarbons without anydouble ortriple bonds, i.e. just C-C, C-H single bonds
alkenes (olefins): aliphatic hydrocarbons that contain one or more double bonds, i.e. di-olefins (dienes) or poly-olefins.
alkynes (acetylenes): aliphatic hydrocarbons which have one or more triple bonds.
The rest of the group is classified according to the functional groups present. Such compounds can be "straight-chain", branched-chain or cyclic. The degree of branching affects characteristics, such as theoctane number orcetane number in petroleum chemistry.
Both saturated (alicyclic) compounds and unsaturated compounds exist as cyclic derivatives. The most stable rings contain five or six carbon atoms, but large rings (macrocycles) and smaller rings are common. The smallest cycloalkane family is the three-memberedcyclopropane ((CH2)3). Saturated cyclic compounds contain single bonds only, whereas aromatic rings have an alternating (or conjugated) double bond.Cycloalkanes do not contain multiple bonds, whereas thecycloalkenes and thecycloalkynes do.
Benzene is one of the best-known aromatic compounds as it is one of the simplest and most stable aromatics.
Aromatic hydrocarbons containconjugated double bonds. This means that every carbon atom in the ring is sp2 hybridized, allowing for added stability. The most important example isbenzene, the structure of which was formulated byKekulé who first proposed thedelocalization orresonance principle for explaining its structure. For "conventional" cyclic compounds, aromaticity is conferred by the presence of 4n + 2 delocalized pi electrons, where n is an integer. Particular instability (antiaromaticity) is conferred by the presence of 4n conjugated pi electrons.
The characteristics of the cyclic hydrocarbons are again altered if heteroatoms are present, which can exist as either substituents attached externally to the ring (exocyclic) or as a member of the ring itself (endocyclic). In the case of the latter, the ring is termed aheterocycle.Pyridine andfuran are examples of aromatic heterocycles whilepiperidine andtetrahydrofuran are the correspondingalicyclic heterocycles. The heteroatom of heterocyclic molecules is generally oxygen, sulfur, or nitrogen, with the latter being particularly common in biochemical systems.
Heterocycles are commonly found in a wide range of products including aniline dyes and medicines. Additionally, they are prevalent in a wide range of biochemical compounds such asalkaloids, vitamins, steroids, and nucleic acids (e.g. DNA, RNA).
Rings can fuse with other rings on an edge to givepolycyclic compounds. Thepurine nucleoside bases are notable polycyclic aromatic heterocycles. Rings can also fuse on a "corner" such that one atom (almost always carbon) has two bonds going to one ring and two to another. Such compounds are termedspiro and are important in severalnatural products.
This swimming board is made ofpolystyrene; it is an example of a polymer.
One important property of carbon is that it readily forms chains, or networks, that are linked by carbon-carbon (carbon-to-carbon) bonds. The linking process is calledpolymerization, while the chains, or networks, are calledpolymers. The source compound is called amonomer.
Two main groups of polymers exist:synthetic polymers andbiopolymers. Synthetic polymers are artificially manufactured, and are commonly referred to asindustrial polymers.[24] Biopolymers occur within a respectfully natural environment, or without human intervention.
Biomolecular chemistry is a major category within organic chemistry which is frequently studied bybiochemists. Many complex multi-functional group molecules are important in living organisms. Some are long-chainbiopolymers, and these includepeptides,DNA,RNA and thepolysaccharides such asstarches in animals andcelluloses in plants. The other main classes areamino acids (monomer building blocks of peptides and proteins),carbohydrates (which includes the polysaccharides), thenucleic acids (which include DNA and RNA as polymers), and thelipids. Besides, animal biochemistry contains many small molecule intermediates which assist in energy production through theKrebs cycle, and producesisoprene, the most common hydrocarbon in animals. Isoprenes in animals form the importantsteroid structural (cholesterol) and steroid hormone compounds; and in plants formterpenes,terpenoids, somealkaloids, and a class of hydrocarbons called biopolymer polyisoprenoids present in thelatex of various species of plants, which is the basis for makingrubber. Biologists usually classify the above-mentioned biomolecules into four main groups, i.e., proteins, lipids, carbohydrates, and nucleic acids. Petroleum and its derivatives are considered organic molecules, which is consistent with the fact that this oil comes from the fossilization of living beings, i.e., biomolecules.[25]See also:peptide synthesis,oligonucleotide synthesis andcarbohydrate synthesis.
In pharmacology, an important group of organic compounds issmall molecules, also referred to as "small organic compounds". In this context, a small molecule is a small organic compound that is biologically active but is not apolymer. In practice, small molecules have amolar mass less than approximately 1000 g/mol.
Fullerenes andcarbon nanotubes, carbon compounds with spheroidal and tubular structures, have stimulated much research into the related field ofmaterials science. The first fullerene was discovered in 1985 by Sir Harold W. Kroto of the United Kingdom and by Richard E. Smalley and Robert F. Curl Jr., of the United States. Using a laser to vaporize graphite rods in an atmosphere of helium gas, these chemists and their assistants obtained cagelike molecules composed of 60 carbon atoms (C60) joined by single and double bonds to form a hollow sphere with 12 pentagonal and 20 hexagonal faces—a design that resembles a football, or soccer ball. In 1996 the trio was awarded the Nobel Prize for their pioneering efforts. The C60 molecule was namedbuckminsterfullerene (or, more simply, the buckyball) after the American architect R. Buckminster Fuller, whose geodesic dome is constructed on the same structural principles.
The number of possible organic reactions is infinite. However, certain general patterns are observed that can be used to describe many common or useful reactions. Each reaction has a stepwise reaction mechanism that explains how it happens in sequence—although the detailed description of steps is not always clear from a list of reactants alone.
The stepwise course of any given reaction mechanism can be represented usingarrow pushing techniques in which curved arrows are used to track the movement of electrons as starting materials transition through intermediates to final products.
Synthetic organic chemistry is anapplied science as it bordersengineering, the "design, analysis, and/or construction of works for practical purposes".[26] Organic synthesis of a novel compound is a problem-solving task, where a synthesis is designed for a target molecule by selecting optimal reactions from optimal starting materials. Complex compounds can have tens of reaction steps that sequentially build the desired molecule. The synthesis proceeds by utilizing the reactivity of the functional groups in the molecule. For example, acarbonyl compound can be used as anucleophile by converting it into anenolate, or as anelectrophile; the combination of the two is called thealdol reaction. Designing practically useful syntheses always requires conducting the actual synthesis in the laboratory. The scientific practice of creating novel synthetic routes for complex molecules is calledtotal synthesis.[22]
Strategies to design a synthesis includeretrosynthesis, popularized byE.J. Corey, which starts with the target molecule and splices it to pieces according to known reactions. The pieces, or the proposed precursors, receive the same treatment, until available and ideally inexpensive starting materials are reached. Then, the retrosynthesis is written in the opposite direction to give the synthesis. A "synthetic tree" can be constructed because each compound and also each precursor has multiple syntheses.
^Streitwieser, Andrew; Heathcock, Clayton H.; Kosower, Edward M. (2017).Introduction to Organic Chemistry. New Delhi: Medtech (Scientific International, reprint of revised 4th edition, Macmillan, 1998). pp. 3–4.ISBN978-93-85998-89-8.
^"Paul Ehrlich".Science History Institute. June 2016. Retrieved20 March 2018.
^Torker, Sebastian; Müller, Andre; Sigrist, Raphael; Chen, Peter (2010). "Tuning the Steric Properties of a Metathesis Catalyst for Copolymerization of Norbornene and Cyclooctene toward Complete Alternation".Organometallics.29 (12):2735–2751.doi:10.1021/om100185g.
^Steingruber, Elmar (2004) "Indigo and Indigo Colorants" inUllmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim.doi:10.1002/14356007.a14_149.pub2
^Nicolaou, K.C.; Sorensen, E.J. (1996).Classics in Total Synthesis: Targets, Strategies, Methods.Wiley.ISBN978-3-527-29231-8.
^Allan, Barbara. Livesey, Brian (1994).How to Use Biological Abstracts, Chemical Abstracts and Index Chemicus. Gower.ISBN978-0-566-07556-8
^Shriner, R.L.; Hermann, C.K.F.; Morrill, T.C.; Curtin, D.Y. and Fuson, R.C. (1997)The Systematic Identification of Organic Compounds. John Wiley & Sons,ISBN0-471-59748-1
