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CHEM 2331H
T.Andrew Taton

Chapter 3 Book Notes Structure and Stereochemistry of Alkanes • Alkane: hydrocarbon that contains only single bonds 3 o simplest + least reactive class of organic compounds b/c only have H and sp hybridized carbon & no reactive functional groups o no double/triple bonds o no heteroatoms (atoms other than C or H) o poor acids, bases, electrophiles, and nucleophiles o undergo cracking and combustion @ high temperatures, but still relatively less reactive 3.1) Classification of Hydrocarbons (Review) • Alkane: hydrocarbon w/ all single bonds btwn C and C • Alkene: contains double bonds btwn C and C • Alkyne: contains triple bond btwn C and C • Aromatic hydrocarbons: hydrocarbons that represent benzene rings • Saturated: having no double/triple bonds o  alkanes = saturated hydrocarbons 3.2) Molecular Formulas ofAlkanes • Methylene groups: chains of —CH — 2roups • General formula for alkanes (branched and unbranched): C Hn n+2where n is # carbon atoms • Homologous series: compounds (like unbranched alkanes) that differ by # methylene groups o homologs: individual members of series  ex: butane = homolog of propane (is a part of propane) 3.3) Nomenclature ofAlkanes • 3.3A) Common Names o Common names (trivial names)  n-alkane or just alkane = normal hydrocarbon chain  isoalkane = 1 branch off chain  neoalkane = 2 branches off main chain o But for compounds with many more complicated types of isomers, need other system of nomenclature • 3.3B) IUPAC (International Union of Pure orApplied Chemistry) or Systematic Names: names devised from IUPAC rules (system of names) o Rule 1: The Main Chain  Find longest continuous chain of carbons  name of this chain is base name of compound  Substituents: groups attached to main chain  When there are 2 equal-length longest chains, use one w/ more substituents o Rule 2: Numbering the Main Chain  Number of each C in main chain starting from end closest to a substituent  keeps numbers as low as possible  If both ends have a substituent at equal distance, look at how close 2 substituent is o Rule 3: NamingAlkyl Groups  Name the alkyl group coming off and give # carbon it comes off from  Naming: take the equivalent alkane, take off –ane, and add –yl • As a result, you lose one H and have an empty single bond  *side note: amyl is archaic term for pentyl  For alkyl groups w/ less than 4 carbons, also called n-alkyls  Isoalkyles have “iso” groups of CH 3HCH (wi3h empty single bond coming off CH)  Degree of alkyl substitution: how many branches come off that C • primary (1 ) Carbon: contains 1 alkyl group coming off • secondary (2 ) Carbon: contains 2 alkyl groups coming off (groups are calledosec-alkyls) • tertiary (3 ) alkyl: contains 3 alkyl groups coming off (tert-alkyl)  Haloalkanes: have halogens as substituents • use prefix fluoro-, chloro-, bromo-, or iodo- before main chain name o Rule 4: Organizing Multiple Groups  If more than 1 substituent, list in alphabetical order  Use di-, tri-, tetra-, penta-, hexa-, hepta- if same multiples of same substituent are there  Include position #, even if have to repeat 3.4) Physical Properties of Alkanes • Used for fuels, solvents, & lubricants (natural gas, gasoline, kerosene, heating oil, lubricating oil, paraffin “wax”  different properties b/c of different molecular weights) • 3.4A) Solubilities and Densities ofAlkanes o Nonpolar  dissolve in nonpolar solvents o Hydrophobic (water hating): don’t dissolve in water   good lubricants & preservatives for metals b/c keep water from touching surface o Density of 0.7 g/mL: less dense than water (1 g/mL)  alkanes sit on top of water • 3.4B) Boiling Points of Alkanes o # carbons ↑ and molecular weight ↑  boiling pts ↑  b/c larger molecules  larger surface area  increased van der Waals forces  needs more energy to vaporize o Generally, branched alkane has lower boiling pt than its isomer n-alkane b/c branched = more compact  less surface area  less IMF interaction • 3.4C) Melting Points of Alkanes o Molecular weight ↑  melting pts ↑ but not smooth increase like boiling pts o Alkanes w/ even # C pack better into solid structure  needs higher temp to melt them o Alkanes w/ odd # C don’t pack as well  melt @ lower temp o  jagged graph for increase 3.5) Uses and Sources ofAlkanes • Can distill petroleum  separate alkanes into fractions w/ similar boiling pts  used for different things based on physical properties (volatility, viscosity) • 3.5A) Major Uses of Alkanes o C -C1(M2thane and ethane)  gases @ room temp and atmospheric pressure  difficult to liquefy  handled as compressed gases  liquids @ cryogenic (very low) temp.s  liquefied natural gas (mostly methane) transported in special refrigerated tankers more easily than transported as compressed gas o C -C3(P4opane and butane)  Gases @ room temp and pressure but easily liquefied @ room temp under modest pressure  Often obtained w/ liquid petroleum and stored in low-pressure cylinders of liquefied petroleum gas (LPG)  Good fuels for heating & internal combustion engines  Burn cleanly  pollution-control equipment rarely necessary  More cost-effective tractor fuels than gasoline/diesel fuel in agricultural areas  Have largely replaced Freons as propellants in aerosol cans  Unlike alkanes, chlorofluorocarbon Freon propellants = implicated in damaging earth’s protective ozone layer o C -C5(P8ntane, hexane, heptane, and octane, and their isomers)  Free-flowing, volatile liquids  Primary constituents of gasoline  Volatile  burns more efficiently as gasoline  higher milage  Gasoline must resist knocking (potentially damaging explosive combustion)  Octane number: rates antiknock properties of gasoline • assigned to mixture of n-heptane (knocks badly) and isooctane (2,2,4-trimethylpentane; not prone to knocking)  # = percentage of isooctane in mixture of these two that begins knocking @ same compression ratio (info comes from table) • tested gas used in test engine w/ variable compression ratio • compression ratio increased until knocking begins (higher compression ratios induce knocking) o C -C9(n16anes through hexadecanes)  Higher-boiling liquids, somewhat viscous  Used in kerosene, jet fuel and diesel fuel • Kerosene = lowest boiling; used to be widely available, not anymore; less volatile than gasoline & less prone to forming explosive mixture o Was used in kerosene lamps & heaters  use wicks to allow this heavier fuel to burn • Jet fuel = similar to kerosene but more highly refined & less odorous • Diesel fuel not = volatile (doesn’t evaporate in intake air) o  in engine, sprayed directly into cylinder @ top of compression stroke o hot, highly compressed air in cylinder  fuel burns quickly, swirls and vaporizes o some alkanes in this have high freezing pts  solidify in cold weather o partial solidification  turns into waxy, semisolid mass o  those who use diesel mix bit of gasoline in for winter  dissolves frozen alkanes  dilutes slush can be pumped into cylinders o C an16up  most often used as lubricating and heating oils  called “mineral oils” b/c come from petroleum which one = considered mineral  paraffin wax not = true wax  purified mixture of high-molecular-weight alkanes w/ melting pt higher than room temp. • true waxes = long-chain esters • 3.5B) Alkane Sources; Petroleum Refining o Alkanes = derived from petroleum (aka crude oil) and petroleum by-products  petroleum = pumped from wells that reach pockets containing prehistoric plant remains  crude oil = made of alkanes, aromatics, and undesirable S and N compounds…different amts of contaminants in different types of petroleum o Refining petroleum  1) careful fractional distillation  get alkane mixtures w/ useful boiling pt ranges  2) Catalytic cracking: converts less valuable fractions into more valuable ones • heat alkanes in presence of materials that catalyze cleavage of large molecules into smaller ones • converts higher boiling fractions  mixtures that can blend w/ gasoline • hydrocracking: cracking in presence of hydrogen o removes S and N impurities • 3.5C) Natural Gas; Methane o Used to be considered waste of petroleum production  destroyed by flaring it off; but now considered as valuable as petroleum o Natural gas = 70% methane, 10% ethane, 15% propane…can include other hydrocarbons/contaminants o Found above pockets of petroleum/coal or places where you can’t recover petroleum/coal as well o Primarily used as heating fuel for buildings/generating electricity/starting material to make fertilizers o Methane we use as natural gas = millions of years old but can be made by microbes in herbivore’s stomach & mud under sea floor o Under sea, cold, high pressure  might form methane hydrate (individual methane molecules trapped inside water molecule cages  when comes to surface, melts  methane escapes into atmosphere  global warming o Hard to capture microbial methane/methane hydrate to use 3.6) Reactions of Alkanes • Alkanes = least reactive organic compound class •  Aka paraffins: “too little affinity” • Don’t react w/ strong acids, bases, or reagents • Most useful rxns need energetic/high temp conditions  need special equipment & hard to control in laboratory • Form hard to separate product mixtures (but commercially important in industry, where they can separate & sell products)
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