Deoxyribonucleic acid, DNA, is the genic material which is
a major constituent of the chromosomes within the nucleus of cells. It plays a central role in the determination of
hereditary characteristics by controlling protein synthesis
in cells.
A Dewar flask is a vessel for storing hot or cold liquids so that
they maintain their temperature independently of the surroundings.
Heat transfer to or from the flask is reduced to a minimum. The
walls of the vessel consists of two thin layers separated by
a vacuum to reduce conduction and convection. The inside wall is
coated with silver to reduce radiation. It was first introduced
by a British physicist Sir James Dewar about 1872 and it
was known under the trade name Thermos flask.
Diamagnetism is magnetic characteristic of those transition metals
where all the electrons in the d sub-orbitals of the atoms are
paired, and results in these elements not attracted by a magnetic field.
A diatomic molecule consists of two atoms. The atoms in
diatomic molecules may be similar (e.g. oxygen, nitrogen,
hydrogen, etc.) or they may be dissimilar (e.g. carbon monoxide,
hydrogen chloride, etc.).
A dibasic acid has two acidic hydrogen atoms in its molecules
which can be ionised. Sulphuric acid is a
dibasic acid, because it
contains two hydrogens atoms which ionise in aqueous solution
to become hydrogen ions.
H2SO4 ==> 2 H(+) + SO4(2 -)
A Diels-Alder reaction is an addition reaction in which a
conjugated diene, (i.e. a compound containing two double bonds separated by a single bond), becomes attached to the double
bond of a dienophile (i.e. an unsaturated compound). The result
of the reaction is invariably the formation of a compound
containing a six-membered ring. The reaction is named after the
German chemists, Otto Diels and Kurt Alder, who discovered it
in 1928AD.
Dienes
Dienes are Alkenes which have two
Carbon-Carbon Double Bonds
per molecule. Conjugated dienes, (i.e. dienes which have
alternate double and single carbon to carbon bonds along the
chain), are more stable than alkenes and undergo addition reactions
at either the 1,2-position or the 1,4-position.
The simplest diene compounds is Buta-1,3-Diene, CH2=CH-CH=CH2.
Diffusion is the gradual mixing of two or more different
gases, or liquids, so that a uniform mixture results. This mixing
is caused by molecules of the different substances colliding and
intermingling, so that all substances are eventually distributed
uniformly in the space. The diffusion of gases is described
by Dihydrate
A dihydrate is a compound which contains two molecules of
water per molecule of compound. Usually, a hydrate is the
water of crystallisation present in crystals of an ionic solid.
Dihydric alcohols contain two hydroxyl groups in the molecules,
and are called diols or
glycols.
Dihydric alcohols are prepared by the oxidation of an
Alkene with potassium permanganate, KMnO4.
A dilute solution contains a relatively low
concentration of solute.
There are\two distinct meanings associated with the term, dilution.
- Dilution involves the addition of more solvent to a solution.
- Dilution is the volume of solvent in which a given
amount of solute is dissolved.
A diol is a dihydric alcohol.
Dioxan, C4H8O2, is a colourless liquid which is freely miscible with
water and is used as a solvent.
- Its relative density is 1.03,
- Its melting point is 11 degC.
- Its boiling point is 101.5 degC.
The molecule consists of a six membered ring
containing four methylene groups, CH2, and two oxygen atoms.
The dissociation of water involves the loss of a hydroxyl ion, H(+),
from the water molecule, H2O, with the formation of a corresponding
hydroxide ion, HO(-). The reverse reaction occurs simultaneously.
Thus, water is in a dynamic equilibrium.
H2O <==> H(+) + HO(-)
Dissolved oxygen refers to the concentration of oxygen, which is
dissolved in environmental waters. The concentration of
dissolved oxygen is expressed as the number of mg per liter or as the percentage of the saturation concentration.
Because fish require dissolved oxygen for respiration, the
concentration of dissolved oxygen is a measure of the
quality of the water to support aquatic organisms.
Since both methanol and ethanol are widely used it may be
necessary to distinguish between them, using either their
physical properties or their chemical properties as
appropriate.
- Using Physical Properties
- Their Boiling Points are different,
- Their Specific Gravities are different,
- Their Refractive Indices. are different,
- Using Chemical Properties
- The Iodoform Test works for Ethanol only.
Distillation is the process whereby a liquid is heated in a
distillation flask to evaporate the liquid into the gaseous
state, and the vapour is then cooled by passing them over
a condenser to condense the vapour back into a liquid.
Distillation is used to separate liquids which have greatly differing
boiling points and to obtain a pure solvent from a solution
containing a dissolved solid.
Distilled water is water which has been purified by removing
dissolved salts and other compounds by distillation.
A donor is an ion or molecule that provides a pair of
electrons in the formation of a dative bond.
The double bond between carbon atoms in Alkenes
involves a total of four electrons (i.e. two shared pairs of electrons). The
two bonds in the double bonds (i.e. two separate chemical bonds) are
formed in different ways and behave differently in chemical reactions
and the first and second bonds have different chemical properties, which
indicates a difference in the nature of these bonds. The
chemical bonding in alkenes can be illustrated by reference to
the simplest alkene, ethene. This compound has the following
structural
H H
C = C
H H
The double bond represents a four electron bond (i.e. two shared
pairs of electrons). However, the two bond between the carbon
atoms are significantly different, and are formed in different
ways. The first bond between the carbon atoms in ethene is
a sigma bond and is similar to the carbon to carbon
bond found in the alkane series. However, the second bond between the
carbon atoms in the ethene molecule is a Pi
Bonds,
which is much more reactive than the Sigma
Bond
and behaves differently in a variety of experimental conditions.
In order to form bonds with three other atoms, carbon uses three equivalent hybrid orbitals,
which are known as sp2 orbitals. They are formed by
hybridising one s and two p orbitals.
These orbitals lie in the plane of the carbon nucleus and point towards the corners
of an equilateral triangle. By so doing, the orbitals lie in one plane and are as far apart bas
possible. The angle between any pair of orbitals is 120 deg.
In ethane, sp2 orbitals achieve the greatest possible overlap when they are cylindrically
symmetrical about a line joining the two carbon nuclei. The strong bond which results is a
sigma bond. The maximum overlapping of sp2 orbitals and the s orbitals of the four
hydrogen atoms can be achieved by an arrangement in which each carbon atom is at the
center of a triangle with two hydrogen atoms and the other carbon at the corners of the
triangle:
H H
C C
H H
Note that the remaining p orbital, which is occupied by one electron, consists of two equal
lobes, one lying above and the other below the plane of the sp2 orbitals. When the p orbital
of carbon atom 1 overlaps the p orbital of carbon atom 2, a second bond is formed between
the carbons. As this bond is formed by a slight overlap above and below the plane of the
carbon atoms, it is weaker than a sigma bond and is called a pi bond. Such overlap
can take place only if all the constituent atoms lie in the same plane.
This means that ethene is a flat molecule, the distance between the carbon atoms being less
than that in ethene.
A doublet is a pair of electrons in a covalent chemical bond.
A Dry Cell is a primary or secondary cell in which the electrolytes are in the form of a paste.
Dry ice is a solid carbon dioxide used as a refrigerant. It
is a convenient refrigerant because it sublimes at -78 degC.
The dual nature of the electron is the observed behaviour of
the electron in experiments, where sometimes it shows particle
behaviour and other times shows the properties of a wave.
Dynamic equilibrium is the apparent steady state of a
chemical system, where the rate of a forward reaction is
just equal to the rate of the reverse reaction.
Start of Hypertext ....
Elements ....
Compounds ....
Index
Hypertext Copyright (c) 2000 Donal O'Leary. All Rights Reserved.