prostaglandins

Prostaglandin 

in mammals, a hormone that has a broad spectrum of physiological action. Prostaglandins were discovered in human semen by the Swedish scientist U. Euler in 1936. Initially, they were thought to be secretions of the prostate gland (hence the name). They were obtained in a pure form in 1956–65 by Swedish and American scientists.
About 20 natural prostaglandins are known, including thick liquids and low-melting crystalline substances. All prostaglandins are unsaturated hydroxy fatty acids that have a skeleton of 20 carbon atoms. According to their chemical structure, prostaglandins are divided into four groups—A, B, E, and F—E and F prostaglandins being the most important biologically. The subscripts in the formula below indicate the number of double bonds in the lateral chains of the molecule.

Prostaglandins are found in low concentrations (about 1 μg/g) in almost all organs, tissues, and biological fluids of higher animals. The most important physiological effect that is stimulated by prostaglandins is the ability to contract smooth muscles, especially the muscles of the uterus and fallopian tubes; at childbirth and during menstruation, the concentration of prostaglandins in uterine tissues increases substantially. For this reason, they are used in obstetrics and gynecology to facilitate normal labor and to artificially terminate pregnancy in its early stage.
Prostaglandins are also cardiotonics and bronchodilators. Arterial pressure is lowered by A and E prostaglandins and raised by F prostaglandin. A, E, and F prostaglandins intensify coronary and renal blood flow, inhibit gastric secretion, and affect the endocrine glands, including the thyroid gland; they also affect water-salt metabolism by altering the ratio Na⁺: K⁺ and blood coagulation by inhibiting the aggregation of thrombocytes.
The biosynthesis of prostaglandins occurs in the cells of different tissues. The precursors of prostaglandins are phospholipids; polyunsaturated fatty acids with a linear chain of 20 carbon atoms are released from phospholipids by the enzyme phospholipase. The oxidative cyclization of the carbon atoms, which occurs with the participation of prostaglandin synthetases (a special system of enzymes), results in the synthesis of E and F prostaglandins.
The classification of prostaglandins as local, or cellular, hormones is justified by their varied functions and the absence of a special organ for their biosynthesis. Their mechanism of action is still unclear. It has been established that prostaglandins affect the activity of the enzyme adenyl cyclase, which regulates the concentration of cyclic adenosine 3’: 5’-monophosphate (cyclic AMP) in the cell. Since prostaglandins influence the biosynthesis of cyclic AMP and since cyclic AMP participates in hormonal regulation, a possible mechanism of action of prostaglandins could consist in correcting (intensifying or weakening) the action of other hormones.

Clinical tests have shown prostaglandins to be promising in the treatment of such conditions as gastric ulcer, asthma, hypertonia, thromboses, arthritides, and inflammations of the nasopharynx. For medical and research purposes, prostaglandins are produced: (1) by enzymatic synthesis based on polyunsaturated fatty acids that are produced in the food-processing industry, (2) by complete chemical synthesis in nine to 13 stages chiefly based on cyclopentadiene, and (3) by partial synthesis in three to five stages based on prostaglandin A₂ and E₂ derivatives that are present in high concentrations (reaching 1.4 percent of the raw mass) in several varieties of the soft marine coral Plexaura homomalla.

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SUBSTITUTION, ADDITION, AND ELIMINATION REACTIONS

Introduction:

Substitution, addition, and elimination reactions are of great importance in a major branch of chemistry known as Organic Chemistry, which covers the chemistry of compounds of carbon. These reactions, which generally involve covalently bonded molecules, are also found, to a much more limited extent with other compounds.

Substitution reactions:

A substitution reaction is a reaction in which an atom (or group of atoms) in a molecule is replaced by another atom or group of atoms:

Example 1:

The gas ethane, CH₃CH₃ reacts with bromine vapour in the presence of light to form bromoethane, CH₃CH₂Br and hydrogen bromide, HBr. In the process, a hydrogen atom in ethane has been substituted for a bromine atom:

                          

Example 2:

Ethanol, CH₃CH₂OH, reacts with hydrogen iodide, HI, to form iodoethane and water. Here, a group of atoms, OH, has been replaced by an iodine atom:

Example 3:

Benzene, C₆H₆, reacts with bromine in the (presence of iron bromide as catalyst) to form bromobenzene, C₆H₅Br. This results in a hydrogen atom being replaced by a bromine atom:

Addition reactions:

An addition reaction is a reaction whereby a molecule reacts with another molecule having one or more multiple covalent bonds so as to form a molecule whose molecular mass is the sum of the molecular masses of the reacting molecules:

Example 3:

Ethene, CH₂=CH₂ has a double bond joining the two carbon atoms. This substance can add a hydrogen molecule (in the presence of platinum as catalyst) to form ethane, CH₃CH₃:

Example 5:

Ethyne, C₂H₂ has a triple bond joining the two carbon atoms. Hydrogen bromide adds onto this triple bond to form 1,1-dibromoethane, CH₃CHBr₂:

Elimination reactions:

An elimination reaction is a reaction whereby a multiple covalent bond is formed in a molecule by the removal of another, usually smaller molecule:

Example 6:

Ethanol, CH₃CH₂OH, when treated with concentrated sulphuric acid, H₂SO₄, loses 2 hydrogen atoms and one oxygen atom, forming ethene, CH₂=CH₂ and water (the atoms that have been eliminated are shown in red):

When an elimination reaction removes the elements of water from a compound, as in the reaction above, the reaction is called a DEHYDRATION REACTION.

Example 7:

Bromoethene, CH₂=CHBr, when treated with potassium hydroxide dissolved in ethanol, loses one hydrogen atom and one bromine atom, forming ethyne, CH≡CH (the atoms that have been eliminated are shown in red):

When an elimination reaction removes the elements of a halogen acid (HCl, HBr, HI) from a compound, as in the reaction above, the reaction is called a DEHYDROHALOGENATION REACTION

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Hyperconjuction

Definition of hyperconjugation

In the formalism that separates bonds into  and  types, hyperconjugation is the interaction of -bonds (e.g. C-H, C-C, etc.) with a  network. This interaction is customarily illustrated by contributing structures, e.g. for toluene (below), sometimes said to be an example of "heterovalent" or "sacrificial hyperconjugation", so named because the contributing structure contains one two-electron bond less than the normal Lewis formula for toluene


At present, there is no evidence for sacrificial hyperconjugation in neutral hydrocarbons.
The concept of hyperconjugation is also applied to carbenium ions and radicals, where the interaction is now between -bonds and an unfilled or partially filled  or p-orbital. A contributing structure illustrating this for the tert-butyl cation is:


This latter example is sometimes called an example of "isovalent hyperconjugation" (the contributing structure containing the same number of two-electron bonds as the normal Lewis formula).
Both structures shown on the right hand side are also examples of "double bond- no-bond resonance".
The interaction between filled  or p orbitals and adjacent antibonding * orbitals is referred to as "negative hyperconjugation", as for example in the fluoroethyl anion:


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