Download Chemistry for pharmacy students : general, organic and by Satyajit D Sarker; Lutfun Nahar, Ph. D PDF

By Satyajit D Sarker; Lutfun Nahar, Ph. D

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98 D The arrow indicates the electrons are towards the more electronegative atom fluorine. The dþ and dÀ symbols indicate partial positive and negative charges. 2 van der Waals forces Relatively weak forces of attraction that exist between nonpolar molecules are called van der Waals forces or London dispersion forces. Dispersion forces between molecules are much weaker than the covalent bonds within molecules. Electrons move continuously within bonds and molecules, so at any time one side of the molecule can have more electron density than the other side, which gives rise to a temporary dipole.

The number of valence electrons present in each of the elements is to be considered first. The number of valence electrons determines the number of electrons needed to complete the octet of eight electrons. Simple ions are atoms that have gained or lost electrons to satisfy the octet rule. However, not all compounds follow the octet rule. Elements in organic compounds are joined by covalent bonds, a sharing of electrons, and each element contributes one electron to the bond. The number of electrons necessary to complete the octet determines the number of electrons that must be contributed and shared by a different element in a bond.

Any compounds that rotate plane-polarized light are called optically active. If the rotation is in a clockwise direction, the enantiomer is said to be dextrorotatory and is given the (þ) sign in front of its name. Anticlockwise rotation gives an enantiomer which is known as levorotatory and is given the sign (À) in front of its name. or Before passing through a solution of an enantiomer Rotates clock-wise (right) Dextrorotatory (+) Rotates anti-clock-wise (left) Levorotatory (−) After passing through a solution of an enantiomer The amount of rotation can be measured with an instrument called a polarimeter.

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