3. Alkanes and Cycloalkanes

3. Alkanes and Cycloalkanes - 2010, Department of...

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© 2010, Department of Chemistry, The University of Western Ontario 3.1 3. Alkanes and Cycloalkanes (text 3.1 – 3.9) Alkanes and cycloalkanes are generally unreactive due to the strength non-polarity of C-C and C-H bonds. However, they react exothermically with O 2 to form CO 2 and H 2 O by combustion (oxidation). Despite their low reactivity, alkanes and cycloalkanes are important in because they act as molecular scaffolds – three-dimensional frameworks that can be used for the attachment of other functional groups. HO H H H cholesterol A. Structure and Physical Properties Alkanes are straight- or branched-chain compounds of C and H. They have the general formula C n H 2n+2 .
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3.2 Formula Name BP °C CH 4 methane 162 C 2 H 6 ethane 89 C 3 H 8 propane 42 C 4 H 10 butane 0.5 C 5 H 12 pentane 36 C 6 H 14 hexane 69 B. Constitutional (Structural) Isomerism Constitutional isomers are compounds that have the same molecular formula but different bonding sequence (atom connectivity). The number of possible isomers rapidly increases with molecular size, since there would be more possible combinations of bonding sequences. This structural diversity is key to the centrality of organic chemistry in biological systems. CH 4 1 C 8 H 18 18 C 3 H 8 1 C 9 H 20 35 C 4 H 10 2 C 10 H 22 75 C 5 H 12 3 C 20 H 42 366,319 C 6 H 14 5 C 40 H 82 62,481,801,147,341 C 7 H 16 9
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3.3 Constitutional isomerism results in compounds of the same formula, but with different chemical and physical properties, e.g. the boiling points of various pentanes (C 5 H 12 ). CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 3 CH 3 CH 3 CCH 3 CH 3 CH 3 BP (deg C) = 36 28 9 Self-study: review constitutional isomers from first-year chemistry and practice drawing them. C. Nomenclature Self-study: learn the IUPAC rules in sections 3.3 and 3.5 of the textbook. Cl Br 5-bromo-6- sec -butyl-1-chloro- 8-ethyl-3,3,4-trimethylundecane
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3.4 D. 3-D Structure Recall the common conventions used for drawing a 3-D molecule on a 2-D surface. For example, ethane can be represented as: 1. Dot-Line-Wedge is most versatile and widely used.
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3. Alkanes and Cycloalkanes - 2010, Department of...

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