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Lecture: 5 Properties, structures, existence and synthesis/perparation
Dr. A.K.M. Shafiqul Islam
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Carbon-Halogen Bond Lengths and Bond Strengths
The shorter the bond length, the stronger the bond strength
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Structure and electrostatic potential map of methanol
The oxygen atom of an alcohol is sp3 hybridized
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Structure and electrostatic potential map of dimethyl ether
The oxygen in an ether is sp3 hybridized
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Structure of methylamine, dimethylamine, and trimethylamine
The nitrogen is sp3 hybridized
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The physical properties of Alkanes, Alkyl Halides, Alcohol, Esters, and Amines
The boiling point of a compound is the temperature at which the liquid form become gas In order for a compound to vaporize, the force that held the individual molecules close to each other in the liquid must be overcome This means that the boiling point of a compound depends on the strength of the attractive forces between the individual molecules If the molecules are held together by strong force, a lot of energy will be needed to pull the molecules away from each other and the compound will have a high boiling point
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Van der Waals forces are induced-dipole–induced-dipole interactions
The molecules of an alkane are held together by van der Waals forces
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Boiling points of alkanes
Differences in the boiling points of several alkanes Branching decreases the boiling point
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Boiling Point The boiling points of the compounds in any homologous series increase as their molecular weights increase Because of the increase in van der Waals forces
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The boiling points are also affected by the polar character of the C-Z bond
The C-Z bond is polar because nitrogen, oxygen, and halogens are more electronegative than the carbon to which they are attached
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Dipole moment The magnitude of the charge differential between the two bonded atoms is indicated by the bond dipole moment. The dipole moment of a bond is equal to the magnitude of the charge on one of the bonded atoms times the distance between the bonded atoms
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Dipole-dipole interactions
Molecules with dipole moments are attracted to one another Dipole-dipole interactions are stronger than London forces in small molecules
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Boiling points of cyclopentane and tetrahydrofuran
Ethers generally have higher boiling points than alkanes of comparable molecular weight because both van der Waals forces and dipole-dipole interactions must be overcome for an ether to boil
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Hydrogen bonding in water, ammonia, and hydrogen fluoride
Hydrogen bonds occur between molecules that contain O, N, or F with a hydrogen directly attached to the O, N, or F
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Hydrogen bonding in water
The hydrogen bond forms between the hydrogen of one water molecule and a nonbonding pair of electrons on the oxygen of the other water molecule
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For amines with the same molecular weight, a primary amine has a higher boiling point than a secondary amine and a secondary amine has a higher boiling point than a tertiary amine
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Melting Point The melting point (mp) of a compound is the temperature at which its solid form is converted to liquid They increase with homogenous series as the molecular weight increases
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Melting Point
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Melting points of straight-chain alkanes
Alkanes with even numbers of carbon atoms fall on a melting-point curve that is higher than the melting-point curve for alkanes with odd numbers of carbon atoms Because alkanes with odd no. of carbon atoms pack tightly than alkanes with even no. of carbon atoms
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Alkanes with odd numbers of carbon atoms have weaker intermolecular attractions and corresponding lower melting points. Alkanes with an odd number of carbons pack less tightly than alkanes with an even number of carbon atoms.
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Solubility The general rule that govern solubility is “like dissolves like” Polar compounds dissolve in polar solvent, and nonpolar compound dissolve in nonpolar solvent.
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Solvation of a polar compound by water
The negative poles of the solvent molecules surround the positive poles of the polar solute and the positive poles of the solvent molecules surround the negative poles of the polar solute
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Solubility of Ether in Water
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Rotation about a carbon-carbon single bond
Orbital depiction and structure of a carbon-carbon single bond, showing allowed rotational motion
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Conformations of ethane
Since there is free rotation about the carbon-carbon bond there are two types of conformations that exist: staggered and eclipsed
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