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Organic Chemistry

Organic Chemistry. Zumdahl Chapter 22. Alkanes: Saturated Hydrocarbons. Hydrocarbons are molecules composed of carbon & hydrogen Each carbon atom forms 4 chemical bonds

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Organic Chemistry

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  1. Organic Chemistry Zumdahl Chapter 22

  2. Alkanes: Saturated Hydrocarbons • Hydrocarbons are molecules composed of carbon & hydrogen • Each carbon atom forms 4 chemical bonds • A saturated hydrocarbon is one where all C - C bonds are “single” bonds & the molecule contains the maximum number of H-atoms • Saturated hydrocarbons are called ALKANES

  3. Normal vs Branched Alkanes • NORMAL alkanes consist of continuous chains of carbon atoms • Alkanes that are NOT continuous chains of carbon atoms contain branches • The longest continuous chain of carbons is called the parent chain

  4. Structural Isomerism • Structural isomers are molecules with the same chemical formulas but different molecular structures - different “connectivity”. • They arise because of the many ways to create branched hydrocarbons. • a.k.a. “Constitutional Isomers” n-pentane, C5H12 2-methlbutane, C5H12

  5. The First 10 “Normal” Alkanes C1 - C4 are Gases at Room Temperature Name Formula M.P. B.P. # Structural Isomers • Methane CH4 -183 -162 1 • Ethane C2H6 -172 -89 1 • Propane C3H8 -187 -42 1 • Butane C4H10 -138 0 2 • Pentane C5H12 -130 36 3 • Hexane C6H14 -95 68 5 • Heptane C7H16 -91 98 9 • Octane C8H18 -57 126 18 • Nonane C9H20 -54 151 35 • Decane C10H22 -30 174 75 C5 - C16 are Liquids at Room Temperature

  6. IUPAC Rules for Naming Branched Alkanes • Find and name the parent chain in the hydrocarbon - this forms the root of the hydrocarbon name • Number the carbon atoms in the parent chain starting at the end closest to the branching • Name alkane branches by dropping the “ane” from the names and adding “yl”. A one-carbon branch is called “methyl”, a two-carbon branch is “ethyl”, etc… • When there are more than one type of branch (ethyl and methyl, for example), they are named alphabetically • Finally, use prefixes to indicate multiple branches

  7. 2 4 1 3 5 Example 1: 2,2-dimethylpentane • The parent chain is indicated by the ROOT of the name - “pentane”. This means there are 5 carbons in the parent chain. • “dimethyl” tells us that there are TWO methyl branches on the parent chain. A methyl branch is made of a single carbon atom. • “2,2-” tell us that BOTH methyl branches are on the second carbon atom in the parent chain.

  8. 2 4 1 3 5 6 7 Example 2: 3-ethyl-2,4-dimethylheptane • The parent chain is indicated by the ROOT of the name - “heptane”. This means there are 7 carbons in the parent chain. • “2,4-dimethyl” tells us there are TWO methyl branches on the parent chain, at carbons #2 and #4. • “3-ethyl-” tell us there is an ethyl branch (2-carbon branch) on carbon #3 of the parent chain.

  9. 3 5 4 6 2 7 1 8 5 3 2 4 6 7 1 8 Example 3: 2,3,3-trimethyl-4-propyloctane • The parent chain is indicated by the ROOT of the name - “octane”. This means there are 8 carbons in the parent chain. • “2,3,3-trimethyl” tells us there are THREE methyl branches - one on carbon #2 and two on carbon #3. • “4-propyl-” tell us there is a propyl branch (3-carbon branch) on carbon #4 of the parent chain.

  10. 4 3 5 5-ethyl- 3,4-dimethyl octane Example 4: Name the molecules shown! • parent chain has 5 carbons - “pentane” • two methyl branches - start counting from the right - #2 and #3 • 2,3-dimethylpentane • parent chain has 8 carbons - “octane” • two methyl branches - start counting from the left - #3 and #4 • one ethyl branch - #5 • name branches alphabetically

  11. HOMEWORK ASSIGNMENT • Read Zumdahl section 22-1 • make notes on REACTIONS OF ALKANES and on CYCLIC ALKANES • Copy table 22.2 on page 1040 • Pay attention to sample exercises! • Answer end-of-chapter problems: • 15, 16, 17, 18

  12. Alkanes Review - Cycloalkanes • A cycloalkane is made of a hydrocarbon chain that has been joined to make a “ring”. • Note that two hydrogen atoms were lost in forming the ring! • What is the general formula for a cycloalkane?

  13. Other Cycloalkanes Angle (ring) Strain - results from compression of the internal bond angles. Cyclopropane has the greatest angle strain (60° bond angles) while the strain is eliminated in cyclohexane. Torsional Strain - a barrier to free rotation around single bonds, due to the eclipsing of atoms in a molecule. This results when atoms are brought too close together in a particular conformation of a molecule.

  14. Cycloalkanes: Cis-Trans Isomerism • The Trans-isomer is the molecule with branches on OPPOSITE sides of the ring • The Cis-isomer is the molecule with branches on the SAME SIDE of the ring. • Cis-Trans isomerism is one type of geometric isomerism • Consider 1,2-dimethylcycloalkane - a molecule that illustrates GEOMETRIC ISOMERISM - compounds with the same molecular formula and connectivity but differ in their geometries. • The molecule on the left shows the two methyl branches on OPPOSITE SIDES of the ring. The molecule on the right shows the two methyl branches on the SAME SIDE of the ring. Trans-1,2-dimethylcyclopentane Cis-1,2-dimethylcyclopentane

  15. Cis-Trans Isomers - Examples cis-1,3-dimethylcyclobutane 1-chloro-1-methylcyclohexane cis-1,2-dichlorocyclohexane trans-1-ethyl-2-methylcyclopropane

  16. Cyclohexane - Boat & Chair Conformations • Cyclohexane is NOT a planar molecule. To achieve its 109.5° bond angles and reduce angle strain, it adopts several different conformations. • The BOAT and CHAIR (99%) are two conformations

  17. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond The suffix for the parent chains are changed from “ane” to “ene” and “yne” e.g. ethene, propyne Where it is ambiguous, the BONDS are numbered like branches so that the location of the multiple bond may be indicated Alkenes & Alkynes

  18. Alkenes & Alkynes: Examples ethene ethyne propene propyne 1-butene 2-pentyne

  19. Cis-Trans Isomerism…Again! • Like rings, alkenes and alkynes show restricted rotation - this time about the multiple bonds • Because of the 120° bond angles in alkenes, cis-trans isomerism is possible • If one of the carbons in the double bond have two of the same substituents, there is NO cis-trans isomerism! • Remember that “trans” means opposite sides of the double bond and “cis” means the same side of the bond! • Use molar mass to prioritize the substituents to decide cis-trans isomerism

  20. Name those alkenes... cis-4-methyl-2-pentene 2-methyl-2-hexene trans-2-bromo-3-methyl- 2-pentene cyclopentene cis-3-heptene

  21. Homework Assignment • READ sections 22.2 (Alkenes & Alkynes), 22.3 (Aromatic Hydrocarbons) and 22.4 (The Petrochemical industry) • Don’t get hung up on “s and p bonds” • Copy figure 22.11 and 22.12 • Summarize the use of ortho, meta, para prefixes when naming benzene derivatives • Make notes on section 22.4 - the Petrochemical Industry • Complete Questions 19 - 25, 27, 29, 31, 32, 54 - 56

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