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Chapter 3 Outline- alkane – hydrocarbon that only contains single bonds- saturated: a hydrocarbon is said to be saturated when it has the maximum number of bonded hydrogens- another way to describe alkanes is “saturated hydrocarbons”- methylene group: – CH2 –- a series of compounds, (like the unbranched alkanes) that only differ by the number of CH2 groups are called a homologous series, with each of the members themselves known as homologs- butane is a homolog of propane- IUPAC (International Union of Pure and Applied Chemistry) nomenclature:1. find the longest continuous chain of carbon atoms, and use the name of thischain as the base name of the compound2. Number the longest chain, beginning with the end of the chain nearest a substituent3. Name the substituent groups attached to the longest chain as alkyl groups. Give the location of each alkyl group by the number of the main-chain carbonatom to which it’s attached4. When 2 or more substituents are present, list them in alphabetical order. When 2 or more of the same alkyl substituent are present, use the prefixes di-, tri-, tetra- (etc) to avoid having to name the alkyl group twice. Include a position number for each substituent, even if it means repeating a number more than once.- degree of alkyl substitution: the number of alkyl groups bonded to a carbon atom in a compound or in an alkyl group- common uses for alkanes – fuels, solvents, lubricants- alkanes are hydrophobic, meaning they are “water hating” and do not dissolve in water- they’re good lubricants and preservatives for metals because they keepwater from reaching the metal surface and causing corrosion- as alkane molecules increase in size (unbranched) their boiling point increases- branched alkanes boil at lower temperatures than unbranched because branched alkanes are more compact, with less surface area for London force interactions- USES OF ALKANES- C1 – C2 : gases at room temp; difficult to liquefy.. usually handled ascompressed gases. After cooling they will liquefy though, and liquefiednatural gas (mostly methane) can be transported in special refrigerated tankers more easily than it can be transported as a compressed gas- C3 – C4 : gases at room tem, can be easily liquefied at room temp under modest pressure. Stored in low pressure cylinders of liquefied petroleum gas (LPG); good fuels both for heating and for internal combustion engines. Propane and butane have replaced Freons in aerosol cans, because those Freons were said to damage the ozone layer.- C5 – C8 : free flowing, volatile liquids; primary constituents of gasoline. Gas must be volatile and also resist the potentially damaging explosivecombustion known as knocking. Anti-knock properties are rated by octane number, that is assigned by comparing the gasoline to a mixture of n-heptane (which knocks badly) and isooctane (which isn’t prone to knocking_.The octane number assigned to the gasoline is the percentage of isooctane in an isooctane/heptane mixture that begins to knock at that same compressionratio.- C9 – C16 : higher-boiling liquids that are somewhat viscous; kerosene is the lowest-boiling of all these fuels- C16 and up : most often used as lubricating and heating oils; sometimes called mineral oils because they come from petroleum, which was once considered a mineral; paraffin “wax” is a purified mixture of high-molecular-weight alkanes with melting points above room temperature- catalytic cracking: converts less valuable fractions to more valuable products; - when cracking is done in the presence of hydrogen it’s calledhydrocracking.. cracking without hydrogen gives a mix of alkanes analkenes- natural gas is about 70% methane, 10% ethane, and 15% propane- methane hydrate: an ice-like substance consisting of individual methane molecules trapped inside cages of water molecules- paraffins: another term for alkanes- combustion: rapid oxidation that takes place at high temperatures, converting alkanes to carbon dioxide and water- halogenation: (usually initiated by heat or light); the reaction of alkanes with halogens, in the presence of heat or light, to give products with halogen atoms substituted for hydrogen atomsETHANE CONFORMATIONS- Newman projections: a way of drawing a molecule looking straight down the bond connecting two carbon atoms- Sawhorse structures:- dihedral angle: the angle between the C – H bonds on the front carbon atom and the C – H bonds on the back carbon in the Newman projection- eclipsed conformation:- staggered conformation: (lowest energy… with the electron coulds in the C – H bonds separated as much as possible)^ anything between these 2 is a skew conformation- as ethane rotates toward an eclipsed conformation, its potential energy increases, and there is resistance to the rotation- resistance to twisting is called torsional strain, and the energy required is called torsional energy- conformational analysis: the study of the energetics of different conformationsBUTANE CONFORMATIONS- totally eclipsed: everything is right behind one another…. There are a lot more conformations draw here^ STUDY CONFORMATIONS- steric strain:- higher alkanes – gauche conformation make kinks in the zigzag structure… we frequently draw alkane chains in this structure to represent the most stable arrangement- cycloalkanes: alkanes that contain rings of carbon atoms- resemble the acyclic, open-chain alkanes in their physical properties and in their chemistry- nonpolar- cis: having two similar groups directed toward the same face of a ring or double bond- trans: having two similar groups directed toward opposite faces of a ring or double bond- geometric isomers: cis/trans isomers- angle strain (sometimes called Baeyer strain): the strain associated with distorting bond angles to smaller (or larger) angles- ring strain: the extra strain associated with the cyclic structure of a compound, as compared with a similar acylic compound; composed of angle strain and torsional strain- heat of combustion: the amount of heat released when a compound is burned with an excess of oxygen in a sealed container called a bomb calorimeter- torsional strain: the resistance to twisting about a bond- chair conformation: the most stable conformation of cyclohexane, with one part puckered upward and another part puckered downward- boat conformation: the less stable puckered conformation of cyclohexane, with both parts puckered upward. The most stable boat is the


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DREXEL CHEM 241 - Chapter 3 Outline

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