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Chapter 2 The Nature of Organic Compounds: Alkanes.

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Presentation on theme: "Chapter 2 The Nature of Organic Compounds: Alkanes."— Presentation transcript:

1 Chapter 2 The Nature of Organic Compounds: Alkanes

2 Functional Groups The structural features that make it possible to classify compounds by reactivity are called functional groups A given functional group behaves almost the same way in every molecule it’s a part of

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8 Carbon-Carbon Multiple bonds

9 Carbon singly bonded to an electronegative atom

10 Carbon-Oxygen Double Bond (carbonyl groups)

11 (-al)(-one) (-oic acid)

12 Alkanes and Alkyl Groups: Isomers HydrocarbonHydrocarbon: a compound composed only of carbon and hydrogen Saturated hydrocarbonSaturated hydrocarbon: a hydrocarbon containing only single bonds AlkaneAlkane: a saturated hydrocarbon whose carbons are arranged in an open chain –General molecular formula, C n H 2n+2 –Homologous series, -CH 2 - (methylene) group Aliphatic hydrocarbonAliphatic hydrocarbon: another name for an alkane

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15 Constitutional Isomerism Constitutional isomersConstitutional isomers: compounds with the same molecular formula but a different connectivity (order of attachment of their atoms) Constitutional isomersMolecular formula Constitutional isomers CH 4 1 C 2 H 6 1 C 3 H 8 1 C 4 H 10 2 C 5 H 12 3 C 10 H C 15 H

16 Straight-chain alkanes (normal alkanes) –n-butane Branched-chain alkanes –Isobutane (2-methylpropane)

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18 Substituent group Alkyl groups (R-); suffix -yl

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20 Classification of C & H Primary (1°) C Primary (1°) C: a carbon bonded to one other carbon –1° H: a hydrogen bonded to a 1° carbon Secondary (2°) C Secondary (2°) C: a carbon bonded to two other carbons –2° H: a hydrogen bonded to a 2° carbon Tertiary (3°) C Tertiary (3°) C: a carbon bonded to three other carbons –3° H: a hydrogen bonded to a 3° carbon Quaternary (4°) C Quaternary (4°) C: a carbon bonded to four other carbons

21 Naming Branched-chain Alkanes IUPAC system of nomenclature (International Union of Pure and Applied Chemistry)

22 Naming Branched-chain Alkanes Step1 Find the parent hydrocarbon 1.Find the longest continuous carbon chain in the molecule 2.If two chains of equal length are present, choose the one with the larger number of branch points as the parent

23 Step1 Find the parent hydrocarbon Step2 Number the atoms in the main chain –If there is one substituent, number from the end of the chain that gives it the lower number –If more than one substituents is attached to the longest continuous chain, the chain number in the direction that will result in the lowest possible number

24 Step1 Find the parent hydrocarbon Step2 Number the atoms in the main chain Step3 Identify and number the substituents –Assign a number to each substituent according to its point of attachment on the parent chain –If there are two substituents on the same carbon, assign them both the same number

25 Naming Branched-chain Alkanes Step1 Find the parent hydrocarbon Step2 Number the atoms in the main chain Step3 Identify and number the substituents Step4 Write the name as a single word –Use hyphens to separate the various prefixes and commas to separate numbers –If two or more different side chains are present, cite them in alphabetical order –If two or more identical side chains are present, use one of the di-, tri-, tetra-, and so forth. Don ’ t use these prefixes for alphabetizing

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29 Properties of Alkanes Physical Properties Low-molecular-weight alkanes (methane....butane) are gases at room temperature Higher-molecular weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature High-molecular weight alkanes (paraffin wax) are semisolids or solids at room temperature

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32 Dispersion force Intermolecular forces of attraction Ion bonding (188 kcal/mol) Hydrogen bonding (2-10 kcal/mol) Dispersion force ( kcal/mol)

33 Physical properties Constitutional isomers have different physical properties bp: more branch, lower mp: branch, decrease; but symmetry, increase

34 Reactions of alkanes: –The reaction of an alkane with O 2 occurs during combustion in an engine or furnace when the alkanes is used as a fuel – Carbon dioxide and water are formed as products, and a large amount of heat is released CH O 2 → CO H 2 O KJ (213 Kcal)

35 Conformations of Ethane The 3-dimentional arrangements of atoms that result from rotation around a single bond are called conformations They interconvert too rapidly for them to be isolated

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37 Staggered conformationStaggered conformation: a conformation about a carbon- carbon single bond where all six C-H bonds are as far away from one another as possible Eclipsed conformation: a conformation about a carbon- carbon single bond where the six C-H bonds are as close as possible Torsional strain (~12 KJ/mol) (99%)(1%)

38 (2.9kcal/mol)

39 Decane

40 Drawing Chemical Structures Skeletal structure

41 Cycloalkanes General formula: (CH 2 ) n or C n H 2n

42 Step 1 Find the parent chain Step 2 Number the substituents, and write the name

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45 Cis-Trans Isomerism in Cycloalkanes Cycloalkanes are less flexible than the open-chain alkanes No rotation around a C-C bond can take place in cycloalkane without breaking the ring

46 Because of their cyclic structure, cycloalkanes have two sides: a “top”side and a “bottom”side Isomerism is possible in substituted cycloalkanes

47 Constitutional isomers –Have different connections among atoms Stereoisomers –Have the same connections –Differ in three-dimensional orientation

48 Cis-trans isomers have –a subclass of stereoisomers –the same molecular formula –the same connectivity –an arrangement of atoms in space that cannot be interconverted by rotation about single bonds under ordinary conditions Cis: A prefix meaning on the same side Trans: A prefix meaning on opposite side

49 Conformations of Some Cycloalknes Cyclopropane – is a flat, triangular molecule with C-C-C bond angles of 60 o –All six C-H bonds have an eclipsed arrangement with their neighbors

50 Intramolecular strain: –Torsional strain: arises when atoms not bonded to each other are forced abnormally close to each other; e.g., eclipsed hydrogens in ethane –Angle strain: introduced into a molecule when a bond angle is deformed from its ideal value (109.5 o ) –Steric strain: arises when two groups are too close together and try to occupy the same space

51 Cyclohexane – is a not flat –All C-C-C bond angles are near 109 o –All adjacent C-H bonds are staggered –Three-dimensional shape called a chair conformation

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53 Axial and Equatorial Bonds in Cyclohexane

54 There are two kinds of positions for hydrogens on the chair conformation of cyclohexaneThere are two kinds of positions for hydrogens on the chair conformation of cyclohexane –Axial position is perpendicular to the ring –Equatorial position is in the plane of the ring Each carbon atom has one axial and one equatorial position Each side of the ring has 3 axial and 3 equatorial positions

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57 Conformational mobility of Cyclohexane There are two equivalent chair conformations Different chair cyclohexane conformations readily interconvert, resulting in the exchange of axial and equatorial positions –all C-H bonds equatorial in one chair are axial in the other, and vice versa

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59 For monosubstituted cyclohexane, the substituent is always more stable in an equatorial position than in an axial position A steric interference occurs in the axial conformation in methylcyclohexane The 1,3-diaxial interaction introduces 7.6 kJ/mol of steric strain into the methylcyclohexane because the axial methyl group and the nearby axial hydrogen are too close together

60 (more stable) (1,3-diaxial interaction)

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