ديسمبر 2nd, 2013

 

anabolic steroids

Steroid hor­mones related to the male sex hor­mone testosterone. They promote the development of masculine char­acteristics and increase muscle growth. Anabolic steroids have been used medically for various conditions but are also used illegally by sports­men and women and by body­builders. They have a number of deleterious side effects and are a con­trolled drug in the UK and many other countries. Their use is banned by nearly all…

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amperometric titration

A method of determining the chemical composition of a solution by measur­ing the current passing through a cell containing the solution; the po­tential is held constant during the titration for both the indicator and reference electrodes, with changes in the current being measured. The cur­rent flowing through the cell is meas­ured as a function of the amount of substance being titrated.

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ammonia clock

ammonia clock A form of atomic clock in which the frequency of a quartz oscillator is controlled by the vibrations of excited ammonia molecules. The ammonia molecule (NH3) consists of a pyramid with a ni­trogen atom at the apex and one hy­drogen atom at each corner of the triangular base. When the molecule is excited, once every 20.9 microsec­onds the nitrogen atom passes through the base and forms…

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alternant

Describing a conjugated molecule in which the atoms can be divided into two sets of alternate atoms such that no atom has a direct link to another atom in the same set. Naphthalene, for example, has an al­ternant conjugated system.

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alpha helix

The most common form of secondary structure in pro­teins, in which the polypeptide chain is coiled into a helix. The helical structure is held in place by weak hy­drogen bonds between the N-H and C=O groups in successive turns of the helix.

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barbiturate

Any one of a group of drugs derived from barbituric acid that have a depressant effect on the central nervous system. Barbiturates were originally used as sedatives and sleeping pills but their clinical use is now limited due to their toxic side ­effects; prolonged use can lead to ad­diction. Specific barbiturates in clinical use include phenobarbitone, for treating epilepsy, and methohexi­tane sodium, used as an anaesthetic.

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Barbier-Wieland degradation

The stepwise degradation of a car­boxylic acid to the next lower homo­logue. First the ester is converted into a tertiary alcohol using a Grig­nard reagent (PhMgX) and acid (HX):

RCH2COOCH3 → RCH2C(OH)Ph2

The secondary alcohol is then dehy­drated using ethanoic anhydride (CH3COOCOCH3) to give an alkene:

RCH2C(OH)Ph2 → RCH=CPh2

The alkene is oxidized with chromic acid:

RCH=CPh2 → RCOOH + Ph2CO

The result is conversion…

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baking powder

baking powder A mixture of pow­dered compounds added to dough or cake mixture to make it rise in cook­ing. It is used as a substitute for yeast in bread-making. Baking powders consist of a source of carbon dioxide, such as sodium hydrogencarbonate or ammonium hydrogencarbonate, and an acidic substance such as cal­cium hydrogenphosphate, potassium hydrogentartrate (cream of tartar), or sodium hydrogenphosphate. In the hot wet mixture, the acid…

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Baeyer-Villiger reaction

A re­arrangement reaction, sometimes known as the Dakin reaction, com­monly used in organic synthesis in which a ketone reacts with a peroxy acid to form an ester. For example,

R-CO-R → R-CO-O-R

The reaction is equivalent to the in­sertion of an oxygen atom next to the ketone’s carbonyl (>C=O) group. Meta-chloroperbenzoic acid (m-CPBA; ClC6H4.CO.O.OH) and trifluoro­perethanoic acid (CF3.CO.O.OH) are typical peroxy acids employed in the reaction….

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Baeyer test

A test for unsaturated compounds in which potassium per­manganate is used. Alkenes, for ex­ample, are oxidised to glycols, and the permanganate loses its colour:

3R2C=CR2 + 2KMnO4 + 4H2O → 2MnO2 + 2KOH + 3R2COHR2COH

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