Brad Gietman's List: 2nd Week of 28th

• In a double bond, two electron pairs are shared between a pair of atomic nuclei
• three electron pairs in a triple bond behave as an “extra-fat” bond.

• 
  The ionic compounds are almost all solids with melting temperatures above 600°C. By contrast, most substances which contain simple molecules are either gases or liquids at room temperature.

• When two polar molecules approach each other, they can arrange themselves in such a way that the negative side of one molecule is close to the positive end of the other:
• Forces between polar molecules which arise in this way are called dipole forces.The existence of dipole forces explains why polar molecules have higher boiling points and melting points than do nonpolar molecules

• The tetrahedral orientation of H atoms around O atoms which results from hydrogen-bond formation has a profound effect on the properties of ice
• most of the electron density of each H and O atom is enclosed by a boundary surface. As you can see, hydrogen bonding causes the H₂O molecules to adopt a rather open structure with hexagonal channels running through it. These channels contain an almost perfect vacuum-in them there is a little electron density from the surrounding atoms, but nothing else.
• By the time 4°C has been reached, increased molecular velocities allow each H₂O molecule to push its neighbors farther away. This counteracts the effect of breaking hydrogen bonds, and the volume of a given amount of H₂O begins to increase with temperature.
• Since water has maximum density at 4°C, water at that temperature sinks to the bottom of a deep lake, providing a relatively uniform environment all year around. If ice sank to the bottom, as most freezing liquids would, the surface of a lake would not be insulated from cold winter air
• Hydrogen bonding also contributes to the abnormally large quantities of heat that are required to melt, boil, or raise the temperature of a given quantity of water. Heat energy is required to break hydrogen bonds as well as to make water molecules move faster, and so a given quantity of heat raises the temperature of a gram of water less than for almost any other liquid. Even at 100°C there are still a great many unbroken hydrogen bonds, and almost 4 times as much heat is required to vaporize a mole of water than would be expected if there were no hydrogen bonding.

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• s. The strength of the carbon-carbon bond permits long chains to form:
• This behavior is referred to as catenation. Such a chain contains numerous sites to which other atoms (or more carbon atoms) can bond, leading to a great variety of carbon  compounds, or organic compounds.The hydrocarbons contain only hydrogen and carbon.

• These hydrocarbons can be in straight chains of varying length, or they can branch out, with one carbon bonded to three or four other carbons. This allows for isomers, such as iso-butane, a branched hydrocarbon:

  Isobutane

• Hydrocarbons can also form ring structures, which are referred to as cycloalkanes. An example is cyclohexane:

  Cyclohexane

• Carbons are capable of forming double and triple bonds with other carbons. This leads to molecules called alkenes, which contain a double bond, and alkynes, which contain a triple bond. An example of an alkene is ethene (ethylene), and an example of a alkyne is ethyne (acetylene):

  Ethene
• Together, they are referred to as unsaturated hydrocarbons, since there are fewer hydrogen atoms in the molecule due to multiple bonds as compared to alkanes
• The gas fraction contains hydrocarbons containing 1 to 4 carbon atoms in each molecule. These can be used for fuels. Another use is to derive materials such as plastics and synthetic fibers from such hydrocarbons, accomplished by polymerization techniques.
• Gasoline consists of hydrocarbons with 5 to 12 carbon atoms in each molecule. It is difficult to overstate the importance of gasoline to modern society, given the central role of automobile travel in our society. Gasoline also serves as an industrial solvent.
• An example of a hydrocarbon found in gasoline is toluene:
• Kerosene consists of hydrocarbons containing between 12 and 16 carbon atoms per molecule. The foremost uses of kerosene are as lamp oil, diesel fuel, and for catalytic cracking, a processes discussed in the section on unsaturated hydrocarbons.
• Fuel Oils consist of hydrocarbons ranging between 15 and 18 carbon atoms per molecule
• Lubricating oils consist of 16 to 20 carbon atoms per hydrocarbon molecule. Referred to sometimes as mineral oil, lubricating oils are used to decrease friction between moving parts. Perhaps the most familiar application is motor oil.
• Hydrocarbons which are not boiled away remain after the distillation as hydrocarbons with more than 20 carbon atoms per molecule. These hydrocarbons can be used as asphalt

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• Other things being equal, the more carbon  atoms, the higher the boiling point of an alkane
• The alkanes are typical examples of nonpolar compounds. The electronegativities of carbon and hydrogen are 2.5 and 2.1, respectively, and so the C—H bond has almost no polarity. Consequently, even the most unsymmetrical hydrocarbon molecules have very small dipole moments.

• When two or more carbon atoms are present, however, isomeric structures in which oxygen is bonded to two different carbons become possible. Such compounds are called ethers. For example, dimethyl ether is isomeric with ethanol, and methyl ethyl ether is isomeric with propanol:
• In an ether there are no hydrogen atoms connected to a highly electronegative neighbor, and so, unlike alcohols, ether molecules cannot hydrogen bond among themselves. Each C—O bond is polar, but the bonds are at approximately the tetrahedral angle. The polarity of one partially cancels the polarity of the other.
• The chemical reactivity of ethers is also closer to that of the alkanes than that of the alcohols. Ethers undergo few characteristic reactions other than combustion, and so they are commonly used as solvents

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• Drinking methanol is harmful, not because of the CH₃OH molecules themselves, but rather because the human body converts these molecules into methanal (formaldehyde) molecules by combination with oxygen:
• Formaldehyde, H₂CO, is very reactive—in the pure state it can combine explosively with itself, forming much larger molecules.
• The functional group found in formaldehyde is called a carbonyl group.
• In ketones the carbonyl group is attached to two carbon  atoms
• Note that every ketone is isomeric with at least one aldehyde. Acetone, for example, has the same molecular formula (C₃H₆O) as propanal.
• Aldehyde and ketone molecules cannot hydrogen bond among themselves for the same reason that ethers cannot—they do not contain hydrogens attached to highly electronegative atoms.
• The carbonyl group is rather polar, however, since the difference between the electronegativities of carbon (2.5) and oxygen (3.5) is rather large, and there are usually no other dipoles in an aldehyde or ketone molecule to cancel the effect of C==O.

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• A chemical fuel is any substance which will react exothermically with atmospheric oxygen, is available at reasonable cost and quantity, and produces environmentally acceptable reaction products

• Certain bacterial enzymes are capable of converting ethanol to ethanoic acid (acetic acid) when oxygen is present:
• Acetic acid, CH₃COOH, is an example of the class of compounds called carboxylic acids, each of which contains one or more carboxyl groups
• Since the carboxyl group contains a highly polar as well as an OH group, hydrogen bonding is extensive among molecules of the carboxylic acids.
• Boiling Points of Some Organic Compounds Whose Molecules Contain 32 or 34 Electrons.
• Acetic acid is synthesized commercially according to the reaction shown above, but silver is used as a catalyst instead of bacterial enzymes. It is also prepared by reading air with propane separated from natural gas. The liquid acetaldehyde obtained in this reaction is then combined with oxygen in the presence of manganese(II) acetate to make acetic acid. About half the acetic acid produced in the United States goes into cellulose acetate from which acetate fibers are made.

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• Cellulose acetate is a complicated example of another group of organic compounds, esters, which can be made by combining alcohols with acids.