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Chapter 2 Alkanes. Hydrocarbon: Hydrocarbon: a compound composed of only carbon and hydrogen. Alkanes: Alkanes: hydrocarbons containing only carbon-carbon.

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Presentation on theme: "Chapter 2 Alkanes. Hydrocarbon: Hydrocarbon: a compound composed of only carbon and hydrogen. Alkanes: Alkanes: hydrocarbons containing only carbon-carbon."— Presentation transcript:

1 Chapter 2 Alkanes

2 Hydrocarbon: Hydrocarbon: a compound composed of only carbon and hydrogen. Alkanes: Alkanes: hydrocarbons containing only carbon-carbon single bonds.

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4 The first two alkanes are methane and ethane.

5 Table 2.1 The first 10 alkanes with unbranched chains

6 The IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts:  (1) a prefix: the number of carbon atoms in the chain. -ane  (2) the suffix -ane: shows that the compound is a saturated hydrocarbon.

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8 The name of an alkane with a branched chain of carbon atom consists of:  a parent name: the longest chain of carbon atoms  substituent names: the groups bonded to the parent chain

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10 Alkyl group: Alkyl group: a substituent group derived from an alkane by removal of a hydrogen atom.  commonly represented by the symbol R-. ane yl  named by dropping the -ane from the name of the parent alkane and adding the suffix -yl.

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12 If there is one substituent, number the parent chain from the end that gives the substituent the lower number.

13 If the same substituent occurs more than once:  Number the parent chain from the end that gives the lower number to the substituent encountered first.  Indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa- and so on.  Use a comma to separate position numbers.

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15 If there are two or more different substituents:  list them in alphabetical order.  number the chain from the end that gives the lower number to the substituent encountered first.  If there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number.

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17 Do not include the prefixes di-, tri-, tetra-, and so on, or the hyphenated prefixes sec- and tert- in alphabetizing;  alphabetize the names of substituents first, and then insert these prefixes

18 Halogens as substituent groups  Same priority as alkyl groups  Fluoro, chloro, bromo, iodo

19 Common names; in this older system, The number of carbon atoms determines the name. The first three alkanes are methane, ethane, and propane.

20 All alkanes of formula C 4 H 10 are called butanes, all those of formula C 5 H 12 are called pentanes, etc. iso For alkanes beyond propane, iso shows that one end of an otherwise unbranched chain terminates in (CH 3 ) 2 CH- For more complex alkanes, use the IUPAC system.

21 Cycloalkanes Cyclic hydrocarbon: Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring.

22 Cycloalkane: Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated.  Cycloalkanes of ring sizes ranging from 3 to over 30 carbon atoms are found in nature.

23 Five-membered (cyclopentane) and six- membered (cyclohexane) rings are especially abundant in nature.

24 Nomenclature cyclo-,  To name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring.

25 If there is only one substituent on the ring, there is no need to give it a location number. If there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

26 Conformation: Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.

27 following are three conformations for a butane molecule.

28 Cyclopentane The most stable conformation of a cyclopentane ring is an envelope conformation.

29 Cyclohexane The most stable conformation of a cyclohexane ring is the chair conformation.  all bond angles are approximately 109.5°.

30 In a chair conformation, equatorial.  six C-H bonds are equatorial. axial.  six C-H bonds are axial.

31 The more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial.

32 Cis-trans isomers Cis: Cis: on the same side. Trans: Trans: across from each other. Same molecular formula, same connectivity—different orientation of atoms in space. Stereoisomers.

33 Viewing a cyclopentane ring edge-on:

34 View from above:

35 View of the cyclohexane ring as a planar hexagon:

36 Physical Properties The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity.

37 The electronegativity difference between carbon and hydrogen is = 0.4 on the Pauling scale. Given this small difference, we classify a C-H bond as nonpolar covalent. Alkanes are nonpolar compounds and the only interaction between their molecules are the very weak London dispersion forces.

38 Melting and boiling points  Boiling points of alkanes are lower than those of almost any other type of compound of the same molecular weight.  In general, both boiling and melting points of alkanes increase with increasing molecular weight.

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40 Alkanes that are constitutional isomers are different compounds and have different physical and chemical properties.

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42 Solubility: a case of “like dissolves like”.  Alkanes are not soluble in water; they are unable to form hydrogen bonds with water.  Alkanes are soluble in each other.  Alkanes are also soluble in other nonpolar organic compounds, such as toluene and diethyl ether.

43 Reactions Oxidation (combustion)  Oxidation of hydrocarbons, including alkanes and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel).

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45 Reaction with halogens (halogenation)  Halogenation of an alkane is a substitution reaction.

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47 Summary of Topics: Chapter 2 Nomenclature  Cis-trans isomers Conformations—particularly cyclohexane Properties (mp/bp; solubility) Reactions:  Combustion  Radical halogenation


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