Organic chemistry

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. Current trends in organic chemistry are chiral synthesis, green chemistry, microwave chemistry and fullerene chemistry.

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 Wöhler synthesis. The name organic chemistry comes from the idea that carbon chains were only produced biologically. This has been proven false, but organic chemistry remains predominantly a study of the molecules of living organisms, also called biochemistry.

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, boil, sublimate, 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. Another unique property of carbon in organic compounds is the ease of formation of carbon carbon double bonds and triple bonds. When these bonds are arranged in a special way it gives rise to conjugated systems and aromaticity.

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 aliphatic compounds such as alkanes, aromatic compounds such as benzene and heterocyclic compounds such as Pyrrole and Indole.

Examples of functional group based categories are Alcohols, Aldehydes, Ketones, Amides, Amines, Carboxylic acids, Ethers and Esters.

Polymers

Polymers consist of long chains of repeating segments of small molecular units. 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 can be organic or inorganic. Commonly-encountered polymers are usually organic such as polyethylene, polypropylene, Nylon or Plexiglass. But inorganic polymers for instance silicone are also familiar within everyday items.

Biomolecules

Biomolecular 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 or biopolymers. The main classes are carbohydrates, Amino acids, polysaccharides, Lipids, Proteins and Nucleic acids.

Molecular structure of an organic compound

Organic 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. The knowledge of the chemical formula for an organic compound is not sufficient information because many isomers can exist. Organic compounds often exist as mixtures. Because many organic compound have relatively low boiling points and/or dissolve easily in organic solvents there exist many methods for separating mixtures into pure constituents that are specific to organic chemistry such as distillation, crystallization and chromatography techniques. 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

Additional methods are provided by Analytical chemistry.

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 Gibbs free energy, what heat is produced or needed in terms of Enthalpy and what unintended products are formed as well.