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Chapter 2

Chapter 2. The Nature of Organic Compounds: Alkanes. 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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Chapter 2

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  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

  3. Carbon-Carbon Multiple bonds

  4. Carbon singly bonded to an electronegative atom

  5. Carbon-Oxygen Double Bond (carbonyl groups)

  6. (-al) (-one) (-oic acid)

  7. Alkanes and Alkyl Groups: Isomers • Hydrocarbon: a compound composed only of carbon and hydrogen • Saturated hydrocarbon: a hydrocarbon containing only single bonds • Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain • General molecular formula, CnH2n+2 • Homologous series, -CH2- (methylene) group • Aliphatic hydrocarbon: another name for an alkane

  8. Constitutional Isomerism • Constitutional isomers: compounds with the same molecular formula but a different connectivity (order of attachment of their atoms) • Molecular formula Constitutional isomers • CH4 1 • C2H6 1 • C3H8 1 • C4H10 2 • C5H12 3 • C10H22 75 • C15H32 4347

  9. Straight-chain alkanes (normal alkanes) • n-butane • Branched-chain alkanes • Isobutane (2-methylpropane)

  10. Substituent group Alkyl groups (R-); suffix -yl

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

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

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

  14. 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

  15. 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

  16. 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

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

  18. Dispersion force Intermolecular forces of attraction • Ion bonding (188 kcal/mol) • Hydrogen bonding (2-10 kcal/mol) • Dispersion force (0.02-2 kcal/mol)

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

  20. Reactions of alkanes: • The reaction of an alkane with O2 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 CH4 + 2 O2→ CO2 + 2 H2O + 890 KJ (213 Kcal)

  21. 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

  22. (99%) (1%) • Staggered conformation: a conformation about a carbon-carbon single bond where all six C-H bondsare as far away from one another as possible • Eclipsed conformation: a conformation about a carbon-carbon single bond where the six C-H bondsare as close as possible • Torsional strain (~12 KJ/mol)

  23. (2.9kcal/mol)

  24. Decane

  25. Drawing Chemical Structures Skeletal structure

  26. Cycloalkanes • General formula: (CH2)n or CnH2n

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

  28. 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

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

  30. Constitutional isomers • Have differentconnections among atoms • Stereoisomers • Have the same connections • Differ in three-dimensional orientation

  31. 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

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

  33. 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 (109.5o) • Steric strain: arises when two groups are too close together and try to occupy the same space

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