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L-5 Bond Length

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L-5 Bond Length National University – Sudan Faculty of Clinical and Industrial Pharmacy First year (Batch 15) second Semester Organic Chemistry in Pharmacy (PA-ORG-127) Course Coordinator: Us. Maria M. Elamin Hamid, M.Sc. Pharmaceutical Analysis and Quality Control Email. [email protected] Mobile Number (00249-913714903 ) Specific Objectives • By the end of this Lecture the student, should be able to: L-5 • Define the various common types of physical bonding • Explain the various structure of Bond length physical bonding. and bond • Distinguish between various types strength of physical bonding. • Explain the concept of bond length and strength. All matter is held together by forces. The force between atoms within a molecule is (a chemical or intramolecular) force, strong force. The force between molecules is (a physical The Intermolecular or intermolecular) force , Forces are weaker than a weak force. Interamolecular Forces. : Strong interamolecular forces result in room temperature solids with high melting and boiling points. Examples: Ionic bonding, metallic bonding, and covalent bonding. : Hydrogen bonding, vander valls forces or London dispersion forces. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom. "Noble gas" valence electron configurations The idea that the noble-gas configuration is a particularly favorable, one which can be achieved through formation of electron-pair bonds with other atoms is known as the octet rule . Noble gas configuration (in this case, that of neon, s2p6) is achieved when two fluorine atoms (s2p5) are able to share an electron pair, which becomes the covalent bond Involve the sharing of a pair of valence electrons by two atoms There are two types of covalent bonding: : •with an equal sharing of electrons. A nonpolar covalent bond has a uniform distribution of electron charge between the bonded atoms. The simplest nonpolar covalent bonds exist in "homonuclear diatomic" molecules like H2 and Cl2. Both atoms attract the shared electrons equally. : Iodine forms a diatomic non-polar covalent molecule. The graphic on the top left shows that iodine has 7 electrons in the outer shell. Since 8 electrons are needed for an octet, two iodine atoms EQUALLY share 2 electrons. : Molecules of oxygen, also a covalent molecules. There are 6 electrons in the outer shell, therefore, 2 electrons are needed to complete the octet. The two oxygen atoms share a total of four electrons in two separate bonds, called double bonds. : The number of shared electrons depends on the number of electrons needed to complete the octet. unsymmetrical electron cloud distribution. The electrons in the bond are not shared equally. An example is HCl, the electrons in the bond spend more time around the chlorine nucleus. Formula Description Example Molecules with an A OH C H OH x OH at one end 2 5 Polar Molecules with an O A H O x y O at one end 2 Molecules with an N at one end NxAy NH3 Diatomic A2 molecules of the O2 Non-polar same element Most carbon C A CO x y compounds 2 The greater the electronegativity difference between atoms in a bond, the more polar the bond. Partial negative charges are found on the most electronegative atoms, the others are partially positive. The combination of carbons and hydrogens as in hydrocarbons or in the hydrocarbon portion of a molecule with a functional group is always NON-POLAR. An abbreviated list to know well is: Amide > Acid > Alcohol > Amine > Ether > Alkane Ionic Bonds Bond in which one or more electrons from one atom are removed and attached to another atom, resulting in positive and negative ions which attract each other. Ionic bonding is directly proportional to ionic charge and inversely proportional to ionic size. Ionic compounds Those molecules that consist of charged ions with opposite charges are called IONIC. These ionic compounds are generally solids with high melting points and conduct electrical current. Ionic compounds are generally formed from metal and a non-metal elements. Ionic Compounds Covalent Compounds 1. Crystalline solids (made of 1. Gases, liquids, or solids ions) (made of molecules) 2. High melting and boiling 2. Low melting and boiling points points 3. Conduct electricity when 3. Poor electrical conductors melted in all phases 4. Many soluble in water but 4. Many soluble in nonpolar not in nonpolar liquid liquids but not in water Metallic bonding The electromagnetic interaction between delocalized electrons, called conduction electrons and the metallic nuclei within other metals. Thus, the force of attraction between the mobile electrons and the positive kernels that binds the metal atoms together. Comparison of Ionic bond Covalent bond and Metallic bond Ionic Bond Covalent Bond Metallic Bond The bond is formed by This bond is formed due transfer of electrons This bond is formed by to the attraction between between two atoms the sharing of electrons kernels and the mobile having different electro between same or electrons in a metal negativities. different elements . lattice. This is a weak bond due This is a strong bond due This is also a fairly strong to the simultaneous to electrostatic force of bond because the attraction of the electrons attraction. electron pair is strongly by a large number of attracted by two nuclei. kernels This is a non-directional This is a non-directional bond. This is a directional bond. bond. This bond makes This bond makes This bond make substances hard and substances hard and substances malleable and brittle. incompressible. ductile. Summary of Types of Intermolecular Forces Weak Intermolecular Forces •Hydrogen bonding The unusually strong dipole- dipole interaction that occurs when a highly electronegative atom (N, O, or F) is bonded to a hydrogen atom. Hydrogen bonding is stronger than the dipole-dipole interactions which are in turn stronger than London dispersion forces. Hydrogen bonding exists only in molecules with an N-H, O-H, or F-H bond. The molecules which have this extra bonding are water •Dipole-Dipole The attraction between a partially negative portion of one molecule and a partially positive portion of a nearby molecule. Dipole-dipole interaction occurs in any polar molecule . Dipole-Dipole Interaction Hydrogen Bonding (Unusually strong Dipole-Dipole) London Dispersion• These forces are most important in systems that have no other types of molecular, like the rare gases, it is dispersion forces that hold the atoms together (no electrostatic or inductive forces exits). London Force It also goes by two other names, Van der Waals forces (VDW), dipole- induced dipole Effects of Intermolecular Forces: The strength of intermolecular forces present in a substance is related to the boiling point and melting point of the substance. Stronger intermolecular forces cause higher melting and boiling points. Methane: has only very weak London dispersion forces (lowest b.p. & m.p.) Chloroform: has dipole-dipole interaction (moderate b.p. & m.p.) Ammonia: has hydrogen bonding and dipole-dipole interaction (high b.p. & m.p.) Hydrophobic and hydrophilic molecule. 1- Hydrophobic = "water fearing" Hydrophobic molecules are non-polar and so do not mix with water; instead, they form their own separate "phase". Think about pouring oil into water: the oil, being hydrophobic, does not mix with water. Examples of hydrophobic molecules include the alkanes, oils, fats, and greasy substances in general. 2- Hydrophilic = "water loving" Hydrophilic molecules are polar or ionic, so do mix with water. If you pour salt into water it will dissolve and then the individual Na+ and Cl- ions will remain in solution. hydrophilic (water-loving) substances tend to dissolve in water and other hydrophilic substances. Lipophobic and Lipophilic molecule •Lipophobic: "fat fearing" chemical compound, not soluble in fats, oils or on other non-polar solvents. From the other point of view, they do not absorb fats. •Lipophilic: "fat loving" the ability of a chemical compound to dissolve in fats, oils, lipids, and on other non-polar solvents such as hexane or toluene, these non-polar solvents are themselves lipophilic . Thus lipophilic substances tend to dissolve in other lipophilic substances..
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