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Classification of Organic Molecules Functional Groups

Classification of Organic Molecules Functional Groups. Hydrocarbons - molecules that possess only hydrogen and carbon a1. Alkanes – noncyclic hydrocarbons with only C-C single bonds. Formula C n H 2n+2 where n is an integer. .

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Classification of Organic Molecules Functional Groups

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  1. Classification of Organic Molecules Functional Groups • Hydrocarbons - molecules that possess only hydrogen and carbon • a1. Alkanes – noncyclic hydrocarbons with only C-C single bonds. • Formula CnH2n+2 where n is an integer. Example: Butane CH3CH2CH2CH3 C4H10 a2. Cycloalkanes – cyclic hydrocarbons with only C-C single bonds. Formula CnH2n where n is an integer. Example: Cyclobutane C4H8 b1. Alkenes – noncyclic hydrocarbons with C=C double bonds. Formula CnH2n where n is an integer. Example: 2-Butene CH3CH=CHCH3 C4H8

  2. b2. Cycloalkenes – cyclic hydrocarbons with C=C double bonds. Formula CnH2n-2 where n is an integer. Example: Cyclobutene C4H6 c. Alkynes – noncyclic hydrocarbons with C=C triple bonds. Formula CnH2n-2 where n is an integer. Example: Butyne CH3C=CCH3 C4H6 d. Aromatic Hydrocarbons – benzene and its derivatives or C6H6

  3. 2. Compounds containing Oxygen a. Alcohols R-OH Functional Group OH hydroxyl group CH3OH methyl alcohol HOCH2CH2OH ethylene glycol (antifreeze) b. Ethers R-O-R CH3CH2-O-CH2CH3Diethylether c. Aldehydes and Ketones Functional Group C=O carbonyl group

  4. Carboxylic Acids • Functional Group -CO2H -COOH -COH Carboxyl Group O Acetic Acid e. Derivatives of Carboxylic Acids Ester Acid Chloride Amide

  5. 3. Nitrogen-Containing Molecules a. Amines R-NH2 Functional Group -NH2 amino group Amines are Lewis and Bronsted bases because of the nonbonded pair of electrons .. b. Amides (see above) c. Nitriles Functional Group Cyano Group

  6. Use of Wedges and Dashed Lines Solid Wedge indicates “coming toward you” Dashed Line indicates “going away” Narrow Line indicates “in the plane of paper”

  7. Single versus Double Bonds Ethane The carbon-carbon single bond in ethane freely rotates at room temperature. Pi bond broken Ethylene The carbon-carbon double bond in ethylene does not freely rotate at room temperature. Why not? Answer: The Pi bond would have to be broken. It will not break at room temperature

  8. Isomers – different compounds with the same formula • Constitutional (Structural) Isomers – isomers that differ in their bonding sequence C5H12 C5H12 C2H2BrCl C2H2BrCl b. Stereoisomers – isomers that differ in their spatial orientation C2H2BrCl C2H2BrCl Cis isomer Trans isomer

  9. Alkyl Groups R is the general symbol for an alkyl group. For example R-OH represents an alcohol where R is an alkyl group. R-Cl is an alkyl chloride. Alkyl Groups – know the following

  10. Carbon Substitution a. Primary carbon (1o) - carbon attached to one other carbon. R-CH3 1o b. Secondary carbon (2o) - carbon attached to two other carbons. R-CH2-R 2o c. Tertiary carbon (3o) – carbon attached to three other carbons 3o R-CH-R R d. Quaternary (4o) – carbon attached to four other carbons. 4o

  11. Alkane Nomenclature • Find the parent hydrocarbon • a. Find the longest continuous chain of carbon atoms and name it accordingly. • Number CarbonsName • 1methane • 2ethane • 3 propane • 4 butane • 5 pentane • 6 hexane • 7 heptane • 8 octane • 9 nonane • 10 decane Named as a hexane

  12. b. If two different chains of equal length are present, choose the one with the larger number of branch points. Named as hexane with 2 substituents not hexane with 1 substituent 2. Number the atoms in the main chain a. Begin at the end nearer the first branch point. b. If there are branch points at equal distances away, begin at the end nearer the second branch point. Named as a nonane with branch points at carbons 3, 4, 7.

  13. 3. Identify and number the substituents. 3-ethyl 4-methyl 7-methyl 4. Write the name as a single word. a.Use prefixes such asmono, di, tri, tetra, etc. to indicate multiple substituents. b. List substituents alphabetically, ignoring prefixes. c. Use dashes to separate substitutents, use commas to separate numbers. The above example would have the name 3-ethyl-4,7-dimethylnonane Name this alkane: Answer: 5-sec-butyldecane

  14. Physical Properties of Alkanes 1. Combustion - Alkanes are inflammable, i.e. they burn. 2 2 2. Boiling and melting points a. Both bp and mp increase with increasing carbon number for straight-chain alkanes with formula CnH2n+2 Carbon Number Physical State C1- C4 gases C5 – C16 liquids C17 – C30 oils and greases C30 – C50 paraffin waxes >C50 plastics (polyethylene) b. Branching tends to raise the melting point and lower the boiling point.

  15. b. Branching tends to lower the boiling point and raise the melting point BP 60oC 58oC 50oC MP -154oC -135oC -98oC Explanation: MP Branching reduces the flexibility of the molecule which reduces the entropy term S in the equation Tmp = H/S. Since S is in the denominator, Tmp increases. BP Branching reduces surface area (more compact structure), and therefore London dispersion forces which control boiling point for these molecules. 3. Solubility – alkanes are nonpolar molecules and therefore insoluble in water, which is polar. Alkanes are hydrophobic. 4. Densities – alkanes are less dense than water, with densities near 0.7 g/mL. Therefore they float on water, e.g. Exxon Valdez oil spill.

  16. 1. Conformation of Ethane Rotate 60o Sawhorse Newman projection Staggered Conformation Eclipsed Conformation Lower Energy Higher Energy More Stable Less Stable The staggered conformation is more stable by 3.0 kcal/mol

  17. Dihedral Angle – the angle defined by the C-H bond on the front carbon and the C-H bond on the back carbon in a Newman projection. 600 00 3.0 kcal/mol 0.0 kcal/mol Eclipsed Staggered Eclipsed Staggered Eclipsed Staggered Eclipsed

  18. Why is the staggered conformation more stable than the eclipsed conformation? In the eclipsed conformation, the C-H bonds are closer, resulting in arepulsion of the electron clouds. This effect is referred to torsional strain. Torsional strain exists anytime C-H bonds are eclipsed. 1.0 kcal/mol 1.0 kcal/mol Total Energy: 3.0 kcal/mol 1.0 kcal/mol

  19. 2. Conformations of Propane Rotate 600 Rotate 600 More stable Less stable 0.0 kcal/mol 3.3 kcal/mol 1.0 kcal/mole 1.0 kcal/mol The additional 0.3 kcal/mol energy is due to steric strain. Steric strain – electronic repulsion that occurs when two atoms or groups are forced together. 1.3 kcal/mol

  20. 3. Conformations of Butane 60o 60o Anti Conformation Staggered 180o dihedral Eclipsed Anti Gauche Conformation Staggered 60o dihedral

  21. There are two staggered conformations of different energy. There are two eclipsed conformations of different energy. Remember, staggered conformations are more stable than eclipsed conformatiions Quicktime Movie

  22. Petroleum Refining

  23. 1. The first step in petroleum refining is fractional distillation.

  24. 2. Upstream processing of the distillates. a. Catalytic hydrocracking – produces small alkanes from large alkanes by adding hydrogen. b. Catalytic cracking – produces small alkenes and alkanes by cracking in the absence of hydrogen. c. Catalytic Reforming – the alkanes and cycloalkanes are upgraded to higher octane number by conversion into aromatic compounds.

  25. Octane Number Straight-chain hydrocarbons have low octane and make poor fuels. Octane Number = 0 heptane Branched Alkanes burn more slowly and reduce the knocking in the engine. Octane Number = 100 2,2,4-trimethylpentane

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