2.12 Drawing Chemical Structures Representations of Structural Formulas Dot formulas are more cumbersome to draw than dash formulas and condensed formulas Lone-pair electrons are often (but not always) drawn in, especially when they are crucial to the chemistry being discussed McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Dash formulas Each dash represents a pair of electrons This type of representation is meant to emphasize connectivity and does not represent the 3-dimensional nature of the molecule McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Condensed structures: C-H and C-C and single bonds aren't shown but understood If C has 3 H’s bonded to it, write CH3 If C has 2 H’s bonded to it, write CH2; and so on. The compound called 2-methylbutane, for example, is written as follows: Horizontal bonds between carbons aren't shown in condensed structures—the CH3, CH2, and CH units are simply but vertical bonds are added for clarity McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Condensed Structural Formulas In these representations, some or all of the dash lines are omitted In partially condensed structures all hydrogens attached to an atom are simply written after it but some or all of the other bonds are explicitly shown In fully condensed structure all bonds are omitted and atoms attached to carbon are written immediately after it For emphasis, branching groups are often written using vertical lines to connect them to the main chain McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Three-Dimensional Formulas The conventions for this are: Bonds that lie in the plane of the paper are indicated by a simple line Bonds that come forward out of the plane of the paper are indicated by a solid wedge Bonds that go back out of the plane of the paper are indicated by a dashed wedge McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Classes of Compounds Classification based on functional group Three broad classes Hydrocarbons Compounds containing oxygen Compounds containing nitrogen => McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Hydrocarbons Alkane: single bonds, sp3 carbons Cycloalkane: carbons form a ring Alkene: double bond, sp2 carbons Cycloalkene: double bond in ring Alkyne: triple bond, sp carbons Aromatic: contains a benzene ring => McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Compounds Containing Oxygen Alcohol: R-OH Ether: R-O-R' Aldehyde: RCHO Ketone: RCOR' => McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Cyclic ethers and ketones THF (tetrahydrofuran) cyclopentanone McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Carboxylic Acids and Their Derivatives Carboxylic Acid: RCOOH Acid Chloride: RCOCl Ester: RCOOR' Amide: RCONH2 => McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Cyclic esters Cyclic ester (cyclic ether and cyclic ketone) McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Compounds Containing Nitrogen Amines: RNH2, RNHR', or R3N Amides: RCONH2, RCONHR, RCONR2 Nitrile: RCN => McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Summary of Important Families of Organic Compounds McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Summary (cont.) McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

3.2 Alkanes and Alkane Isomers Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups) The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n Alkanes are saturated with hydrogen (no more can be added They are also called aliphatic compounds McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Alkane Isomers CH4 = methane, C2H6 = ethane, C3H8= propane The molecular formula of an alkane with more than three carbons can give more than one structure C4 (butane) = butane and isobutane C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Constitutional Isomers Isomers that differ in how their atoms are arranged in chains are called constitutional isomers They must have the same molecular formula to be isomers McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 3.4 Naming Alkanes Compounds are given systematic names by a process that uses Prefix-Parent-Suffix Follows specific rules Named as longest possible chain Carbons in that chain are numbered in sequence substituents are numbered at their point of attachment Compound name is one word (German style) Complex substituents are named as compounds would be McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Names of Small Hydrocarbons No. of Carbons Formula Name (CnH2n+2) 1 Methane CH4 2 Ethane C2H6 3 Propane C3H8 4 Butane C4H10 5 Pentane C5H12 6 Hexane C6H14 7 Heptane C7H16 8 Octane C8H18 9 Nonane C9H20 10 Decane C10H22 McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Nomenclature of Branched-Chain Alkanes (IUPAC) Locate the longest continuous chain of carbons; this is the parent chain and determines the parent name. Number the longest chain beginning with the end of the chain nearer the substituent Designate the location of the substituent When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain Substituents are listed alphabetically McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc. Commas are used to separate numbers from each other The prefixes are used in alphabetical prioritization When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents When branching first occurs at an equal distance from either end of the parent chain, choose the name that gives the lower number at the first point of difference McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 3.3 Alkyl Groups Alkyl group – remove one H from an alkane (a part of a structure) Naming: replace -ane ending of alkane with -yl ending CH3 is “methyl” (from methane) CH2CH3 is “ethyl” from ethane See Table 3.4 for a list McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Physical Properties Boiling points and melting points increase as size of alkane increases Forces between molecules (temporary dipoles, dispersion) are weak McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 3.6 Cycloalkanes Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds) Simple cycloalkanes rings of CH2 units, (CH2)n, or CnH2n Structure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure) cyclobutane cyclopentane cyclohexane cyclopropane McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Complex Cycloalkanes Naturally occurring materials contain cycloalkane structures Examples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane) McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

6. Alkenes: Structure and Reactivity

Alkene - Hydrocarbon With Carbon-Carbon Double Bond Also called an olefin but alkene is better Includes many naturally occurring materials Flavors, fragrances, vitamins McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 6.3 Naming of Alkenes Find longest continuous carbon chain containing the double bond and name the compound using the suffix “ene” Number carbons in chain so that double bond carbons have lowest possible numbers Rings have “cyclo” prefix McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Names of Small Alkene No. of Carbons Formula Name (CnH2n) 1 2 Ethene C2H4 3 Propene C3H6 4 Butene C4H8 5 Pentene C5H10 6 Hexene C6H12 7 Heptene C7H14 8 Octene C8H16 9 Nonene C9H18 10 Decene C10H20 McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Many Alkenes Are Known by Common Names Ethylene = ethene Propylene = propene Isobutylene = 2-methylpropene Isoprene = 2-methyl-1,3-butadiene McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

6.3 Degree of Unsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for saturated a acyclic compound is CnH2n+2 Each ring or multiple bond replaces 2 H's McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Example: C6H10 Saturated is C6H14 Therefore 4 H's are not present This has two degrees of unsaturation Two double bonds? or triple bond? or two rings or ring and double bond McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I) Halogen replaces hydrogen C4H6Br2 and C4H8 have one degree of unsaturation Oxygen atoms - if connected by single bonds These don't affect the total count of H's McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Degree of Unsaturation and Variation Compounds with the same degree of unsaturation can have many things in common and still be very different McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

If C-N Bonds Are Present Nitrogen has three bonds So if it connects where H was, it adds a connection point Subtract one H for equivalent degree of unsaturation in hydrocarbon McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Summary - Degree of Unsaturation Count pairs of H's below CnH2n+2 Add number of halogens to number of H's (X equivalent to H) Don't count oxygens (oxygen links H) Subtract N's - they have two connections McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

6.4 Cis-Trans Isomerism in Alkenes The presence of a carbon-carbon double can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer similar groups on opposite sides Each carbon must have two different groups for these isomers to occur McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Cis, Trans Isomers Require That End Groups Must Differ in Pairs 180°rotation superposes Bottom pair cannot be superposed without breaking C=C X McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

6.6 Sequence Rules: The E,Z Designation Neither compound is clearly “cis” or “trans” Substituents on C1 are different than those on C2 We need to define “similarity” in a precise way to distinguish the two stereoisomers Cis, trans nomenclature only works for disubstituted double bonds McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Develop a System for Comparison of Priority of Substituents Assume a valuation system If Br has a higher “value” than Cl If CH3 is higher than H Then, in A, the higher value groups are on opposite sides In B, they are on the same side Requires a universally accepted “valuation” McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

E,Z Stereochemical Nomenclature Priority rules of Cahn, Ingold, and Prelog The Cahn-Ingold-Prelog convention is used to assign the groups of highest priority on each carbon Compare where higher priority group is with respect to bond and designate as prefix E -entgegen, opposite sides Z - zusammen, together on the same side McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority Br > Cl > O > N > C > H In this case,The higher priority groups are opposite: (E )-2-bromo-2-chloro-propene McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003 Extended Comparison If atomic numbers are the same, compare at next connection point at same distance Compare until something has higher atomic number Do not combine – always compare McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003

Dealing With Multiple Bonds Substituent is drawn with connections shown and no double or triple bonds Added atoms are valued with 0 ligands themselves McMurry Organic Chemistry 6th edition Chapter 3 (c) 2003