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Properties of Alkanes Long, unbranched alkanes tend to have higher melting points, boiling points, and enthalpies of vaporization than their branched isomers Cycloalkanes: ring alkanes; made up of CH2 groups General formula: CnH2n cyclohexane “boat” “chair” Unsaturated Aliphatic Hydrocarbons Alkenes: carbon-carbon double bond (sp2 hybridized) Alkynes: carbon-carbon triple bond (sp hybridized) CHCH (acetylene) CH2=CH2 (ethylene) IUPAC Nomenclature for Aliphatic Hydrocarbons Straight-chain alkanes - name ends in -ane Branched alkane - side chain is a “substituent” name the substituent formed by the removal of one H atom from an alkane by changing the ending from -ane to -yl name of the alkane is derived from the longest continuous carbon chain to indicate the position of the substituent, the C atoms in the longest chain are numbered, starting at the end that will give the lowest number for the position of the first attached group use prefixes di-, tri-, tetra-, penta-, etc. to indicate the number substituents are listed in alphabetical order (disregard the prefix) CH3 H 3C CH 5 4 CH3 CH2 3 C 2 CH3 2,2,4-trimethylpentane 1 CH3 H 3C CH3 CH2 CH3 2-ethyl-1,1-dimethylcyclohexane (sum of the numbers is lowest) Alkenes and Alkynes Double bonds - change the “ane” suffix to “ene” Triple bonds - change the “ane” to “yne” Position of the multiple bond is given by the number of the first C atom in the multiple bond CH3-CH2-CH=CH-CH3 2-pentene CH3-CH2-CH2-CCH3 1-pentyne CH2=CH-CH=CH2 1,3-butadiene Reactions of Alkanes Alkanes are not very reactive Strong C-C and C-H bonds mean bond enthalpy (kJ/mol) C-C 348 C-H 412 1) Oxidation Reactions CH4(g) + 2 O2(g) CO2(g) + 2H2O(g) DHo = -890 kJ Break the strong C-H bond, but replaced by two C=O bonds (mean bond enthalpy of C=O is 743 kJ/mol). Also O-H bond is strong (463 kJ/mol) 2) Substitution Reactions light or heat CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) light or heat Cl-Cl 2 Cl Cl + CH4 CH3 + HCl initiation step propagation steps CH3 + Cl2 CH3Cl + Cl Cl + Cl Cl2 CH3 + CH3 CH3CH3 CH3 + Cl CH3Cl termination steps Alkenes Prepration - Elimination Reactions 1) From alkanes by dehyrogenation CH3CH3(g) catalyst CH2=CH2(g) + H2(g) 2) From haloalkanes - dehydrohalogenation CH3CH2Cl + KOH CH2=CH2 + KCl + H2O H2C Cl CH2 + K OH H2C CH2 + KCl + H2O H 3) Dehydration of alcohols CH3CH2OH H2SO4 CH2=CH2(g) + H2O http://www.whfreeman.com/chemicalprinciples/con_index.htm?18 Reactions 1) Addition reactions Double bonds are more reactive than single bonds Br C C + Br Br CCl4 C C Br C C + H H catalyst C C H H http://www.whfreeman.com/chemicalprinciples/con_index.htm?18 H 3C CH2 CH2 Cl 1-chloropropane H H C H 3C + HX C H H 3C CH CH3 Cl 2-chloropropane 2-chloropropane is the product The H atom always goes to the C atom of the double bond that already has the most H atoms - Markovnikov addition H 3C CH2 CH2 OH 1-propanol H H C H 3C C + HOH H H 3C CH CH3 OH 2-propanol Markovnikov’s rule holds - 2-propanol is favored Polymerization reactions n CH2=CH2 catalyst -[CH2-CH2]-n CH3 H 2C CH3 C H 2C CH C CH2 CH CH2 isoprene (2-methyl-1,3-butadiene) H 3C H C H 2C n H 3C C H C H 2C C H2 H 3C C H C H 2C C H2 rubber cis geometry C CH2 H2 C H3C C H2C CH2 C C H H3 C H H3 C C H2 C CH2 C C H2 gutta-percha trans-geometry C H Aromatic Hydrocarbons Parent compound of aromatic hydrocarbons - benzene (C6H6) C is sp2 hybridized, ring is planar As a substituent - phenyl (C6H5) OH Phenol (C6H5OH) CH3 CH3 O2 N NO2 NO2 Toluene 2,4,6-trinitrotoluene (TNT) Resonance Stablization p-bonding electrons are delocalized over all C atoms Resonance imparts stability to benzene with respect to hydrogenation and oxidation cyclohexane Addition (Br2) none cyclohexene rapid benzene none Substitution Reactions - p-bonds in the ring are left intact; substituent replaces an H atom Br + Br 2 FeBr3 + HBr NO2 Nitration + HNO 3 H2SO4 + H 2O + H 2O SO3 H Sulfonation + H2SO4 SO3 Akylation CH3 + CH3Br AlBr3 + HBr