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Hydrocarbons

Hydrocarbons. Straight Chain Alkanes aren’t “Straight”. C – C bonds are sp 3 hybridized. Butane, C 4 H 10. Structural Shorthand. Explicit hydrogens (those required to complete carbon’s valence) are usually left off of drawings of hydrocarbons. C 1. C 2. C 3. C 4. C 1. C 3. C 4. C 2.

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Hydrocarbons

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  1. Hydrocarbons

  2. Straight Chain Alkanes aren’t “Straight” C – C bonds are sp3 hybridized Butane, C4H10

  3. Structural Shorthand Explicit hydrogens (those required to complete carbon’s valence) are usually left off of drawings of hydrocarbons C1 C2 C3 C4 C1 C3 C4 C2 Line intersections represent carbon atoms

  4. Structural Isomers Isomers are molecules with the same chemical formula, but different organization of atoms (different bonding) n-Pentane, C5H12 Isopentane, C5H12 Neopentane, C5H12

  5. Cyclic Alkanes Cyclopropane, C3H6 Cyclobutane, C4H8 Cyclopentane, C5H10 Cyclohexane, C6H12 Cycloheptane, C7H14 Remember, explicit hydrogens are left out

  6. Rules for Naming Alkanes (Nomenclature) • For a branched hydrocarbon, the longest continuous chain • of carbon atoms gives the root name for the hydrocarbon 1 2 3 4 4 carbon chain = butane

  7. Rules for Naming Alkanes (Nomenclature) • When alkane groups appear as substituents, they • are named by dropping the -ane and adding -yl. —CH3 Methyl —CH2CH3 Ethyl —CH2CH2CH3 Propyl —CH2CH2CH2CH3 Butyl Methyl

  8. Rules for Naming Alkanes (Nomenclature) • The positions of substituent groups are specified • by numbering the longest chain of carbon atoms • sequentially, starting at the end closest to the • branching. 1 2 3 4 Methyl

  9. Rules for Naming Alkanes (Nomenclature) • The location and name of each substituent are • followed by the root alkane name. The substituents • are listed in alphabetical order (irrespective of any • prefix), and the prefixes di-, tri-, etc. are used to • indicate multiple identical substituents. 1 2 3 4 Name: 2-methylbutane Methyl

  10. Nomenclature Practice Name this compound 1 9 carbons = nonane 2 4 3 5 • Step #1: For a branched hydrocarbon, the longest continuous chain of carbon atoms gives the root name for the hydrocarbon 6 7 8 9

  11. Nomenclature Practice Name this compound 9 carbons = nonane 1 2 4 3 5 6 CH3 = methyl 7 chlorine = chloro 8 9 Step #2: When alkane groups appear as substituents, they are named by dropping the -ane and adding -yl.

  12. Nomenclature Practice Name this compound 9 carbons = nonane 1 2 4 3 5 • Step #3: The positions of substituent groups are specified by numbering the longest chain of carbon atoms sequentially, starting at the end closest to the branching. 6 CH3 = methyl 7 chlorine = chloro 8 9 1 9 NOT 9 1

  13. Nomenclature Practice Name this compound 9 carbons = nonane 1 2 4 3 5 • Step #4: The location and name of each substituent are followed by the root alkane name. The substituents are listed in alphabetical order (irrespective of any prefix), and the prefixes di-, tri-, etc. are used to indicate multiple identical substituents. 6 CH3 = methyl 7 chlorine = chloro 8 9 2-chloro-3,6-dimethylnonane

  14. Properties of Hydrocarbons • Made up of mostly C and H • Relatively nonpolar • Low solubility in polar solvents (e.g. water) • Good solvents for other nonpolar molecules • Mostly london-dispersion forces (weak) • Low boiling and melting points

  15. Reactions of Hydrocarbons • Combustion: • Hydrocarbons burn readily in air to produce carbon dioxide and water. C3H8(g) + 5 O2(g) --> 3CO2(g) + H2O(g)

  16. Reactivity of Hydrocarbons • Alkanes are generally less reactive than alkenes or alkynes • Aromatic compounds are more reactive than alkanes, but less reactive than alkenes and alkynes. Alkanes < aromatics < alkenes < alkynes

  17. Reactions of Alkanes • Single bonds between carbon atoms are difficult to break. (This is why alkanes are relatively unreactive) • Can undergo combustion reactions

  18. Reactions of Alkanes • Substitution Reactions: • Hydrogen atoms may be substituted by a halogen. • The product is a halogenated alkane (alkyl halides)

  19. Reactions of Alkenes and Alkynes • Addition Reactions: • Reactions in which a molecule is added to a double or triple bond. • No loss of hydrogen atoms from the hydrocarbon

  20. Hydrogenation Halogenation

  21. Hydrohalogenation Hydration

  22. Markovnikov’s Rule • When molecules with two identical atoms (e.g. H2) are added to a double bond, only one possible product is formed. • When molecules of nonidentical atoms (e.g. HBr) are added, two different products are theoretically possible

  23. Markovnikov’s Rule • However, experiments show that only one main product is formed. • This product can be predicted by Markovnikov’s rule • “The rich get richer” • When a hydrogen halide is added to an alkene or alkyne, the hydrogen atom bonds to the carbon atom that already has more hydrogen atoms. Carbon 1 has 2 H’s Carbon 2 has 1 H

  24. Benzene Ring • Does not act as 3 single bonds and 3 double bonds • It’s 6 identical bonds of intermediate length • Due to hybridization (delocalized, shared electrons)

  25. Reactions of Aromatic Hydrocarbons • Less reactive than alkenes and do not undergo addition reactions unless under conditions of extreme temperature or pressure • Do undergo substitution reactions (more reactive than alkanes)

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