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Home >> Chemistry Dictionary >>Alkynes (Acetylenes) - Allotropy(Allotrope)
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Alkanols.
Alcohols. Compounds ROH Where R is-an alkyl group.
Alkenes.
The straight chain and branched chain hydrocarbons in which one or more of the carbon—carbon bonds is a double bond. There are unsaturated hydrocarbons and the simplest alkene in ethene. They possess the genral formula CnH2n forming a homologous series. Higher members of the series show isomerism depending on position of the double bond. Alkenes typically undergo addition reactions to the double bond.
Alklxides, Alcoholates.
Compounds formed by reaction of alcohols with sodium or potassium metal. Alkoxides are like compounds containing the ion R-O.
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Alkyd resins.
Products of the condensation of polyhdric alcohols and polybasic (or sometimes monobasic) acids, e.g., phthalic acid. Used widely in coatings, particularly in paints.
Alkyl.
Refers to any organic group resulting from the removal of a hydrogen atom from an aliphatic compound, e.g., the methyl group (CH3-); the ethyl group (C2H5-); the acetylenic group(HC≡C-). In an abbreviated form an alkyl group may be represented generally in chemical formulae by R—.
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Alkylbenzenes.
Organic compounds that are having an alkyl group bond to a benzene ring. The simplest example is methylbenzene CH3C6H5. Alkyl benzenes can be made by the Friedal Crafts reaction.
Alknes (acetylenes).
Unsaturated hydro-carbons that contain one or more triple carbon-carbon bonds in their molecules. In systematic chemical nomenclature alkyne names end in the suffix –yne. Alkynes that have only one triple bond form a homologous series; ethyne (acetylene), CH≡CH, propyne, CH3CH≡CH, etc. They are made by the action of potassium hydroxide in alcohol solution on haloalkanes containing halogen atoms on adjacent carbon atoms; for example:
RCHCICH2CI + 2KOH – 2KCI + 2H2O + RCH≡CH
Like alkenes, alkynes undergo addition reactions.
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Allo.
The prefix allo-before the name of an organic compound indicates that two compounds have the same empirical formula. With sterols and related substances the prefix allo should indicate that rings A and B are in the trans position to each other, but allocholesterol is an exception.
Allomone.
A compound or mixture acting between different species in the environment which adoptively favours the emitting species.
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Allotropy. Allotrope.
The ability of certain elements to exist in more than one physical form. Allotrophy is very common in groups IV V and VI of the periodic table than in other groups. The graphic and diamonds are the two important allotropic forms of carbon. The sulphur also exist in various allotropic forms like rhombic monoclune, plastic and colloidal sulphur etc.
Alloy.
A material consisting of two or more metals (e.g. brass is an alloy of copper and zinc) or a metal and a non-metal (e.g. steel is an alloy of iron and carbon, sometimes with other metals included). Alloying is one of the common methods of increasing useful properties in metals e.g. strength and hardness.
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Alloy elements.
Elements added to give special properties to alloys, such as resistance to corrosion, heat, creep, etc.
Allyl.
The trivial name given to the propenyl group CH2 : CHCH2-.
Allyl alcohol.
(CH2CHCH2OH) A colourless pigment liquid b.p. 96.50C. It is used in the manufacture of synthetic resin and pharmaceuticals.
Allyl derivatives of metals.
Allyl complexes are important catalysts and important as intermediates in many reactions involving metals. Allyl derivatives are the simplest members of a series of complex of higher homologues.
Alnico alloys.
Alloys containing Al, Ni, Co and Fe. Used for permanent magnets
Alpha particle.
A helium nucleus emitted by a larger nucleus during the course of the type of radioactive decays known as alpha decay. As a helium nucleus consists of two protons and two neutrons bound together as a stable entity the loss of an alpha particle involves a decrease in nucleon number of 4 and decrease of 2 in the atomic number, e.g. the decay of a uranium—238 nucleus into a thorium—234 nucleus. A stem of alpha particles is known as an alpha ray or alpha radiation.
Alternating axis of symmetry.
An axis of symmetry which combines rotation with reflection thought a plane at right angles to the axis.
Alum.
KAl(SO4)2, 12H2O. A substance which crystallizes in large colurless octahedral. Crystals become white on exposure to the air due to the production of a basic salt. Dehydrated at a dull red heat, a porous friable mass is formed, which is known as burnt alum. A mineral approximating in composition to potash alum occurs in nature. An allied mineral alunite may be used a source of alum. Other common starting materials for the manufacture of alum are aluminium schists and shales, which contain aluminium silicate and iron pyrites. The latter furnishes the sulphuric acid required, potassium sulphate or chloride being derived from an independent source. Alum is used in the dyeing industries for the production of mordants and pigments, in dressing leather, sizing paper, in water-proofing fabrics, in fireproofing, in gelatin hardening and medicinally as a styptic and astringent, and as a coagulant for water purification.
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