Brad Gietman's List: Sept 16

• The number of covalent substances is far larger than the number of ionic compounds, largely because of the ability of one element, carbon, to form strong bonds with itself. Hydrogen, oxygen, nitrogen, and a number of other elements also bond strongly to carbon, and a tremendous variety of compounds can result
• The number of covalent substances is far larger than the number of ionic compounds, largely because of the ability of one element, carbon, to form strong bonds with itself. Hydrogen, oxygen, nitrogen, and a number of other elements also bond strongly to carbon, and a tremendous variety of compounds can result
• The number of covalent substances is far larger than the number of ionic compounds, largely because of the ability of one element, carbon, to form strong bonds with itself. Hydrogen, oxygen, nitrogen, and a number of other elements also bond strongly to carbon, and a tremendous variety of compounds can result
• The number of covalent substances is far larger than the number of ionic compounds, largely because of the ability of one element, carbon, to form strong bonds with itself. Hydrogen, oxygen, nitrogen, and a number of other elements also bond strongly to carbon, and a tremendous variety of compounds can result
• In the early days of chemistry such compounds were obtained from plants or animals rather than being synthesized by chemists, and so they came to be known as organic compounds. This distinguished them from the inorganic compounds available from nonliving portions of the earth’s surface.
• the study of carbon compounds is still referred to as organic chemistry. Since organic compounds all involve covalent bonds, we will describe a number of them as we discuss covalent substances.
• In the case of molecules whose atoms are connected by covalent bonds, such intermolecular forces may be of three types
• All molecules are attracted together by weak London forces. These depend on instantaneous polarization and increase in strength with the size of the molecular electron cloud
• When a molecule contains atoms whose electronegativities differ significantly and the resulting bond dipoles do not cancel each other’s effects, dipole forces occur. This results in higher melting and boiling points than for nonpolar substances.
• The third type of intermolecular force, the hydrogen bond, occurs when one molecule contains a hydrogen atom connected to a highly electronegative partner.
• The other molecule must contain an electronegative atom, like fluorine, oxygen, or nitrogen, which has a lone pair
• Carbon normally forms four bonds, and carbon-carbon bonds are quite strong, allowing formation of long chains to which side branches and a variety of functional groups may be attached.
• Some covalently bonded substances do not consist of individual small molecules. Instead, giant macromolecules form. Examples include most of the rocks in the crust of the earth as well as modern plastics.

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• Carbon is unique among the elements of the periodic table because of the ability of its atoms to form strong bonds with one another while still having one or more valences left over to link to other atoms. The strength of the carbon-carbon bond permits long chains to form:

  
• This behavior is referred to as catenation.
• The hydrocarbons contain only hydrogen and carbon.
• Single bonded hydrocarbons are called alkanes. A example of an alkane is butane
• 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
• Hydrocarbons can also form ring structures, which are referred to as cycloalkanes. An example is 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.
• 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.
• A special class of multiple bond hydrocarbons are the aromatic hydrocarbons, which all take the form of hydrocarbon ring structures with double bonds between the carbons. Benzene is an example
• The fossil fuels, coal, petroleum (or crude oil), and natural gas consist primarily of hydrocarbons
• A schematic of the fractional distillation of crude oil used in petroleum refining. The mixture is separated into Gases, Gasoline, Kerosene, Fuel oil, Lubricating oil, and Residue(asphalt).

  • See diagram and descripitions
• The gas fraction contains hydrocarbons containing 1 to 4 carbon atoms
• Gasoline consists of hydrocarbons with 5 to 12 carbon atoms
• Kerosene consists of hydrocarbons containing between 12 and 16 carbon atoms
• Lubricating oils consist of 16 to 20 carbon atoms
• 20 carbon atoms per molecule. These hydrocarbons can be used as asphalt

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• Most of the hydrocarbons in petroleum belong to a family of compounds called the alkanes, in which all carbon  atoms are linked by single bonds.
• The methyl group, CH₃—
• methylene group, —CH₂—
• Consequently projection formulas, which indicate how the atoms are connected but do not show tetrahedral angles of 109.5°, are commonly used.

• normal alkanes or straight-chain alkanes, indicating that all contain a single continuous chain of carbon atoms and can be represented by a projection formula whose carbon atoms are in a straight line.
• Note that all the projection formulas in the table have an initial hydrogen atom followed by a number of CH₂ groups. The chain ends with a second single hydrogen atom. The general formula H(CH₂)_nH, or C_nH_2n+2, may be written
• branched chain, in which some carbon atoms are linked to more than two other carbons,
• Nevertheless, you can verify from the projection formulas or from the ball-and-stick drawings that both normal butane and isobutane have the same molecular formula, C₄H₁₀. The two compounds are isomers, just like ethyl alcohol and dimethyl ether, hence the prefix iso in the name for one of them
• One should note from this table, that of all the organic compounds to consider, alkanes, both straight chained and branched, have the lowest boiling points comparatively.
• As the number of carbon atoms in an alkane molecule increases, so do the possibilities for isomerism of this kind.
• molecules cannot stop wriggling, and most alkane structures are not rigid. Groups on either side of a carbon-carbon single bond are able to rotate freely with respect to each other.
• free rotation around the carbon bond, certain positions are more stable than others

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• the general formula for a cycloalkane is C_nH_2n

• Other things being equal, the more carbon  atoms, the higher the boiling point of an alkane. As can be seen from the table of straight chain alkanes,
• The alkanes are rather unreactive and do not combine readily with other substances. When heated sufficiently, however, they burn in air, a process known as combustion:
• atural gas is mainly (approximately 85 percent) methane.
• Better-quality gasoline usually contains a higher percentage of branch-chain alkanes than the regular grade.
• The variation of boiling point with chain length in the alkanes provides a simple method for partially separating them from each other in petroleum. When petroleum is heated, the shorter-chain compounds begin to boil off initially. These can be collected by cooling the vapor until it recondenses to a liquid
• 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

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• Two important families of hydrocarbons which are not found in petroleum are the alkenes (also called olefins) and the alkynes (also called acetylene).
• Alkene molecules are similar to alkane molecules, except that they contain a carbon-carbon double bond () and two fewer H atoms. They thus have the general formula C_nH_2n
• Alkyne molecules contain triple bonds () and have four H atoms less than the corresponding alkane. Their general formula is thus C_nH_2n–2.
• Compounds containing double or triple bonds are often referred to collectively as unsaturated compounds. Because of their multiple bonds, alkenes and alkynes are usually more chemically reactive than alkanes and aromatic hydrocarbons.
• Thus while there are only two alkane molecules possible with four carbon atoms, four alkene molecules are possible
• Although alkenes are not present in crude petroleum, they are produced in large quantities in petroleum refining
• The most important petroleum product, gasoline, requires molecules containing from 6 to 12 carbon atoms. These can be obtained by heating the longer-chain compounds. These big molecules writhe around so fast at higher temperatures that they “crack” or break into smaller fragments. Usually a catalyst is added to speed up the reaction, which is called catalytic cracking. When cracking occurs, one alkene and one alkane molecule are produced:
• Among the alkynes only the two simplest are of any industrial importance. Both ethyne (acetylene), and propyne (methyl acetylene), , are used in welding and steel cutting where they are burned together with pure oxygen  gas in an oxyacetylene torch

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