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In chemistry, the descriptor geminal () refers to the relationship between two
''Mechanism and synthesis''
Book 10 of ''Molecular world''. Open University, Royal Society of Chemistry; . 368 pages The related term vicinal refers to the relationship between two functional groups that are attached to adjacent atoms. The relative arrangement of two functional groups can also be described by the descriptors α and β.
atom
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons.
Every solid, liquid, gas, ...
s or functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
s that are attached to the same atom. A geminal diol
In chemistry, the descriptor geminal () refers to the relationship between two atoms or functional groups that are attached to the same atom. A geminal diol, for example, is a diol (a molecule that has two alcohol functional groups) attached t ...
, for example, is a diol (a molecule that has two alcohol functional groups) attached to the same carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
atom, as in methanediol
Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound with chemical formula . It is the simplest geminal diol, and the simplest sugar alcohol. In aqueous solutions it coexists with oligomers (short polym ...
. Also the shortened prefix ''gem'' may be applied to a chemical name to denote this relationship, as in a ''gem''-dibromide for "geminal dibromide".
The concept is important in many branches of chemistry, including synthesis and spectroscopy, because functional groups attached to the same atom often behave differently from when they are separated. Geminal diols, for example, are easily converted to ketones or aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s with loss of water.Peter Taylor (2002)''Mechanism and synthesis''
Book 10 of ''Molecular world''. Open University, Royal Society of Chemistry; . 368 pages The related term vicinal refers to the relationship between two functional groups that are attached to adjacent atoms. The relative arrangement of two functional groups can also be described by the descriptors α and β.
1H NMR spectroscopy
In 1H NMR spectroscopy, the coupling of two hydrogen atoms on the same carbon atom is called a geminal coupling. It occurs only when two hydrogen atoms on amethylene group
In organic chemistry, a methylene group is any part of a molecule that consists of two hydrogen atoms chemical bond, bound to a carbon atom, which is connected to the remainder of the molecule by two single bond, single bonds. The group may be re ...
differ stereochemically from each other. The geminal coupling constant is referred to as 2''J'' since the hydrogen atoms couple through two bonds. Depending on the other substituents, the geminal coupling constant takes values between −23 and +42 Hz.D. H. Williams, I. Fleming: ''Strukturaufklärung in der organischen Chemie; Eine Einführung in die spektroskopischen Methoden.'' 6. überarbeitete Auflage, Georg Thieme Verlag, Stuttgart 1991, S. 109.
Synthesis
The following example shows the conversion of a cyclohexyl methyl ketone to a ''gem''-dichloride through a reaction with phosphorus pentachloride. This ''gem''-dichloride can then be used to synthesize an alkyne.
References
Chemical nomenclature {{Navbox stereochemistry