ديسمبر 2nd, 2013
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…
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.
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…
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.
كلمات دليلية: alternant
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.
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.
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…
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…
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….
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