Organic Chemistry Zumdahl Chapter 22 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 Normal vs Branched Alkanes • NORMAL alkanes consist of continuous chains of carbon CH2 CH2 CH3 CH2 CH3 atoms • Alkanes that are NOT continuous chains of carbon atoms contain branches CH3 CH2 CH CH3 • The longest continuous chain of carbons is called the CH3 parent chain 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” CH2 CH2 CH3 CH2 CH3 n-pentane, C5H12 CH3 CH2 CH3 CH CH3 2-methlbutane, C5H12 The First 10 “Normal” Alkanes • • • • • • • • • • Name Formula M.P. B.P. Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 -183 -172 -187 -138 -130 -95 -91 -57 -54 -30 -162 -89 -42 0 36 68 98 126 151 174 # Structural Isomers 1 1 1 2 3 5 9 18 35 75 C1 - C4 are Gases at Room Temperature C5 - C16 are Liquids at Room Temperature 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 Example 1: 2,2-dimethylpentane 2 4 • The parent chain is indicated by CH2 5 CH2 3 1 the ROOT of the name CH3 CH2 “pentane”. This means there are CH3 5 carbons in the parent chain. • “dimethyl” tells us that there are TWO CH3 methyl branches on the parent chain. A methyl branch is made of a single carbon atom. CH2 C CH3 CH2 CH3 • “2,2-” tell us that BOTH methyl CH3 branches are on the second carbon atom in the parent chain. 4 1 3 5 Example 2: 3-ethyl-2,4-dimethylheptane 2 4 • The parent chain is indicated by CH2 5 CH2 3 1 the ROOT of the name CH2 CH2 CH3 “heptane”. This means there are 7 carbons in the parent chain. H2C 6 CH3 CH3 CH CH CH3 CH3 7 CH3 • “2,4-dimethyl” tells us there are TWO methyl branches on the parent chain, at carbons #2 and #4. CH CH2 CH2 • “3-ethyl-” tell us there is an ethyl CH2 branch (2-carbon branch) on CH3 carbon #3 of the parent chain. 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. 3 5 4 2 6 7 1 8 CH3 2 1 3 5 4 6 7 • “2,3,3-trimethyl” tells us there are CH3THREECmethyl branches CH2 - one on CH #2 andCH CH2#3. carbon two on carbon 8 CH3 CH3 • “4-propyl-” tellCH us there is CH a propyl 2 2 branch (3-carbon branch) CH2 on CH3 carbon #4 of the parent chain. CH3 Example 4: Name the molecules shown! CH3 • parent chain has 5 carbons “pentane” CH CH2 • two methyl branches - start CH3 CH CH3 counting from the right - #2 and #3 • 2,3-dimethylpentane 3 4 5 CH3 • parent chain has 8 carbons - “octane” • two methyl branches - start counting from the left - #3 and #4 • one ethyl branch - #5 • name branches alphabetically 5-ethyl- 3,4-dimethyl octane 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 Alkanes Review - Cycloalkanes • A cycloalkane is made of a hydrocarbon chain that has been joined to make a “ring”. H2 C 109.5° bond angle CH2 CH3 CH3 n-propane C3H8 H2C 60° bond angle unstable!! CH2 cyclopropane C3H6 •Note that two hydrogen atoms were lost in forming the ring! •What is the general formula for a cycloalkane? Other Cycloalkanes Angle (ring) Strain - results from compression of cyclobutane the bondbond angles.angles Cyclopropane has the C4internal H8 - ~90° greatest angle strain (60° bond angles) while the strain is eliminated in cyclohexane. cylcopentane C5H10 ~109.5° bond angles 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 cyclohexane too close together in a particular conformation of 6H12 = 109.5° bond angles aCmolecule. Cycloalkanes: Cis-Trans Isomerism • Consider The Trans-isomer 1,2-dimethylcycloalkane is the molecule with - abranches moleculeon thatOPPOSITE illustrates GEOMETRIC sides of the ring ISOMERISM - compounds with the same molecular and connectivity but differ their geometries. • formula The Cis-isomer is the molecule within branches on the SAME SIDE • of The themolecule ring. on the left shows the two methyl branches on SIDES ofisthe ring. molecule on the right shows • OPPOSITE Cis-Trans isomerism one typeThe of geometric isomerism the two methyl branches on the SAME SIDE of the ring. Trans-1,2-dimethylcyclopentane Cis-1,2-dimethylcyclopentane Cis-Trans Isomers - Examples cis-1,3-dimethylcyclobutane Cl 1-chloro-1-methylcyclohexane cis-1,2-dichlorocyclohexane 2 CH2 6 H3C CH 3 CH2 5 Cl Cl CH 1 CH3 4 trans-1-ethyl-2-methylcyclopropane 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 Alkenes & Alkynes • 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: Examples H2C CH CH2 H2C H C HC ethene CH3 ethyne propyne 1-butene H2 C CH C CH3 H3C propene 2-pentyne C C CH2 16 H3C H2C C H CH3 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 cis-2-butene H C H C H3C CH3 trans-2-butene H C H3C CH3 C H Name those alkenes... H C H C cis-4-methyl-2-pentene HC CH CH 3 2-methyl-2-hexene 3 H3C trans-2-bromo-3-methylBr 2-pentene H2 C H2cyclopentene C CH2 C H C H cis-3-heptene 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