lecture 4 regular - CHEM 231 Properties and nomenclature of...

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Unformatted text preview: CHEM 231 Properties and nomenclature of HYDROCARBONS LECTURE 4 LECTURE 4 1 Alkanes Ethane, a simple alkane Carbons and hydrogens only All carbons are sp hybridized Lecture 4 2 Relative stability is determined by comparing the heat produced by isomers upon combustion Physical Properties of AlkanesStability based on structure Least stable Produces most heat Most stable Produces least heat Lecture 4 Branching increases stability of alkane 3 Lets talk about isomers The simplest alkanes do not have isomers CH4 H3C CH3 H2 C H3C CH3 C H is long enough to have isomers H2 C CH3 H C H3C CH3 4 H3C C H2 CH3 Lecture 4 As we increase in carbons the number of isomers increases too C H isomers are shown here H2 C H2 C H CH3 H3C CH3 C C H2 CH3 CH3 CH3 C H3C CH3 H3C C H2 Lets figure out all the isomers for C H Lecture 4 5 Here are the cycloalkane isomers of C H Here are the alkene isomers of C H alkene Lecture 4 6 Physical Properties of Alkanesboiling points Boiling points are related to the shape and size of the molecule For molecules made of similar substances, size matters An alkane with 2 carbons will have a different boiling pt from one that has 3 carbons Common sense allows us to predict how number of carbons affects bps Lecture 4 7 Physical Properties of Alkanesboiling points Boiling points of alkanes with the same empirical formula will have different bp based on structure Molecules that approach a spherical shape have fewer opportunities for intermolecular attractions The fewer intermolecular attractive events the easier it is to enter the vapor phase from the liquid Therefore the more spherelike in shape the lower the bp The more stringlike in shape the higher the bp Lecture 4 8 Physical Properties of Alkanesboiling points Adding a functional group to an alkane will change the molecule's bp For example: exchange out a methyl group and add in a polar functional group like a hydroxyl or carbonyl dipoledipole interactions must be overcome prior to entering the vapor phase. More simply: polar groups increase the boiling point Lecture 4 9 Physical Properties of Alkanessolubility `Like dissolves like' is a good rule of thumb Alkanes are soluble in other alkanes, and similar hydrocarbons These similar hydrocarbons include alkenes and some aromatic compounds Water and very polar organic compounds like CH Cl do not dissolve alkanes Lecture 4 10 Physical Properties of AlkanesAcidity The sp hybridized CH bond is strong and not very polarized The sp hybridized carbon anion is very unstable These 2 factors make alkanes very weak acids Only a strongly basic metal like lithium or another Lecture 4 11 Physical Properties of Alkanes: Basicity Alkanes have no lone pairs of electrons Alkanes do not have electrons in pi bonds Alkanes are not basic. Lecture 4 12 Nomenclature of alkanes Number of carbons 1 2 3 4 5 6 7 Lecture 4 Name Number of carbons 8 9 10 11 12 13 20 Name 13 Nomenclature of carbon substituents Number of carbons 1 2 3 Picture of group CH3 C CH3 H2 Name of group C H2 C H2 C H2 H2 C CH3 H2 H2 C C CH3 H2 H2 H2 C C C CH3 4 5 6 C H2 H2 H2 H2 H2 C C C C CH3 Lecture 4 14 Nomenclature of carbon substituents Number of carbons 3 Picture of group C H Name of group H3C CH3 CH3 4 C C H2 H CH3 4 H3C H C H2 C CH3 4 H3C C CH3 CH3 Lecture 4 15 Nomenclature of halogen substituents Halogen compounds are treated as a subgroup of whatever type of compound to which they are attached Halogen Substituent name Fluorine Chlorine Bromine Iodine Lecture 4 16 Nomenclature rules of alkanes We use IUPAC nomenclature methods IUPAC = International union of pure and applied chemists Lecture 4 17 Nomenclature rules of alkanes We find the longest chain in the molecule Lecture 4 18 Nomenclature rules of alkanes List the substituents (groups) in alphabetical order CH3 CH3 H 3C CH2 H3C CH2 Lecture 4 19 Nomenclature rules of alkanes Number the substituents so that the lowest number is the lowest number possible CH3 CH3 CH3 Lecture 4 20 Nomenclature rules of alkanes Number multiple groups to have the lowest overall numbers Lecture 4 21 Nomenclature rules of alkanes If you can number 2 different ways, number so that lower number goes with the alphabetically first group Lecture 4 22 Nomenclature rules of alkanes If there are 2 different ways to find the longest 'main chain' choose the one that has the most substituents coming off of it this simplifies naming Lecture 4 23 Practice nomenclature of alkanes Lecture 4 24 Naming alkanes with halogen groups Prefixes used in nomenclature for halogen groups Fluoro Chloro Bromo Iodo Lecture 4 25 Rudimentary nomenclature rules of alkenes The last name of alkenes is `ene' The prefixes in alkenes are just what they are in alkanes There are 2 additional things that we need to worry about in alkene nomenclature The locant for the start of the pi bond and the relative positions of groups on the alkene bonds Lecture 4 26 Nomenclature rules of alkenes With 2 and 3 carbon alkenes nomenclature is simple Once the chain is longer than 3 carbons we must use locators As with alkenes we need to indicate where the alkyne bond is located , alkyl group and halogen groups operate just like they do in alkanes Lecture 4 27 Rudimentary nomenclature rules of alkynes The suffix in alkynes is `yne' As with alkenes we need to indicate where the alkyne bond is located , alkyl group and halogen groups operate just like they do in alkanes. Lecture 4 28 ...
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