 Organic chemistry is the scientific study of the structure, properties, composition, reactions, and synthesis of organic compounds that by definition contain carbon. It is a specific discipline within the subject of chemistry. Organic compounds are molecules composed of carbon and hydrogen, and may contain any number of other elements. Many organic compounds contain nitrogen, oxygen, halogens, and more rarely phosphorus or sulphur.
History
Organic chemistry as a science is generally agreed to have started in 1828 with Friedrich Woehler's synthesis of the organic, biologically significant compound urea by accidentally evaporating an aqueous solution of ammonium cyanate NH4OCN now called the Wohler synthesis. The name organic chemistry comes from the idea that carbon chains were only produced by living things or organisms. This has been proven false, but organic chemistry remains predominantly a study of the molecules of living organisms.
Characteristics of organic substances
Organic compounds are generally covalently bonded. This allows for unique structures such as long carbon chains and rings. The reason carbon is excellent at forming unique structures and that there are so many carbon compounds is that carbon atoms form very stable covalent bonds with one another (catenation). In contrast to inorganic materials, organic compounds typically melt, sublime, or decompose below 300°C. Neutral organic compounds tend to be less soluble in water compared to many inorganic salts, with the exception of certain compounds such as ionic organic compounds and low molecular weight alcohols and carboxylic acids where hydrogen bonding occurs. Organic compounds tend to be much more soluble in organic solvents such as ether or alcohol, but the solubility in each solute depends upon the functional groups present and on the overall structure. Like inorganic salts, organic compounds form crystals.
Categories of organic substances
Because so very many compounds exist, a clear, unambiguous naming system is necessary. Organic nomenclature is the system established for naming and grouping organic compounds. Organic subtances are classified by their molecular structural arrangement, and by what other atoms are present: hydrogen is impicitly assumed. Other atoms such as O, N, or Cl almost always bond in certain relative ways, forming functional groups. In chemistry, structure is quite synonymous with function, and so the structural categories double as categories of property or activity. The main organizational categories are: (HC=pure hydrocarbons, FG=containing functional groups):
Aliphatic compounds
Aliphatic compounds are organic molecules that do not contain aromatic parts. Typically, they contain hydrocarbon chains.
- HC: Alkanes - Alkenes - Dienes or Alkadienes - Alkynes - Alicyclic compounds
- FG: Haloalkanes
Aromatic compounds
Aromatic compounds are organic molecules that contain one or more aromatic rings,a carbon ring structure (usually from 5-8 ring members) with conjugated double bonds. Some may also contain nitrogen, oxygen, or sulphur within the ring.
- HC: Benzene - Toluene - Styrene - Xylene - Naphthalene - Azulene - Biphenyl - Anthracene - Phenanthrene - Benzopyrene - Coronene
- FG: Phenol - Aniline - Haloarenes - Acetophenone - Benzonitrile
Heterocyclic compounds
Heterocyclic compounds are cyclic organic molecules whose ring(s) contain at least one heteroatom. These heteroatoms can include oxygen, nitrogen, phosphorus, and sulfur.
- single ring: Pyrrole - Pyridine - Imidazole - Pyrimidine - Thiophene - Furan
- multi ring: Indole - Purine
Functional group based
- Alcohols - Aldehydes - Ketones - Amides - Amines - Carboxylic acids - Ethers - Esters - Thiols - Nitriles
Polymers
Polymers are a special kind of molecule. Generally considered "large" molecules, polymers get their reputation regarding size because they are molecules that consist of chains of multiple smaller segments. The segments could be chemically identical, which would make such a molecule a homopolymer - or, the segments could vary in chemical structure, which would make that molecule a heteropolymer. Polymers are a subset of "macromolecules" which is basically a classification for all molecules that are considered large.
Polymers can be organic or inorganic. Commonly-encountered polymers are usually organic (e.g., polyethylene, polypropylene, Nylon, Plexiglass, etc.). But inorganic polymers (e.g., silicone) are also familiar within everyday items.
Bio-molecules
Bio-molecular chemistry is a major category within organic chemistry. Many complex multi-functional group molecules are important in living organisms. Some of them are considered to be polymers (biopolymers).
- Carbohydrates - Amino acids
- Polysaccharides - Lipids - Proteins - Nucleic acids
Molecular structure of an organic compound
Compounds are generally made from the building blocks of carbon atoms, hydrogen atoms, and functional groups. The valence of carbon is 4, and hydrogen is 1, functional groups are generally 1. Many, but not all structures can be envisioned by the simple valence rule that there will be one bond for each valence number. Currently, there exist several methods for deducing the structure an organic compound. In general usage are (in alphabetical order):
- Crystallography: This is the most precise method; however, it is very difficult to grow crystals of sufficient size and high quality to get a clear picture, so it remains a secondary form of analysis.
- Elemental Analysis: A destructive method used to determine the elemental composition of a molecule.
- Infrared spectroscopy: Chiefly used to determine the presence (or absence) of certain functional groups.
- Mass spectrometry: Used to determine the molecular weight of a compound and the fragmentation pattern.
- Nuclear magnetic resonance (NMR) spectrometry
- UV/VIS spectroscopy: Used to determine degree of conjugation in the system
Organic reactions
Organic reactions are chemical reactions involving organic compounds. While pure hydrocarbons undergo certain classes of reactions, many more reactions which organic compounds undergo is largely determined by functional groups. The general theory of these reactions involves careful analysis of such properties as the electron affinity of key atoms, and bond strengths. These issues can determine the relative stability of short-lived reactive intermediates, which usually directly determine the path of the reaction.
A common reaction is generically written here as an example:
R-F + X-Y > R-Y + X-F
where F is some functional group such as the hydroxyl or -OH group . It is presumed that functional group F is bonded to one of the carbon atoms in R. R is often one of the hydrocarbon categories mentioned previously. The example above is a substitution reaction, since Y is substituted for F.
Important concerns of a reaction include whether it will occur spontaneously determined by the Gibb's free energy, what heat is produced or needed in terms of Enthalpy and what unintended products are formed as well.
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