Organic Chemistry

Organic Chemistry Course Number: PCH 1120-217 Lecture # Tuesday September 24, 2013 Geometric Isomerism in Cyclic Compounds & Alkenes, and Conformations of Open-Chain Compounds Prof Oludotun A Phillips Room # 2-81, 2nd Floor Pharmacy Building Email: dphillips@hsc.edu.kw Tel: 24986070 Learning Objectives At the end of the class students should be able to:  describe geometric isomerism  identify and assign configuration of geometric isomers in alkenes and cyclic compounds (cis / trans designations)  assign configuration of geometric isomers (E / Z nomenclature) using the sequence rules  discuss conformation in open-chain compounds Geometric Isomerism in cyclic compounds and alkenes  Geometric Isomerism: Results from rigidity, due to restricted rotation in molecules  It occurs in two classes of compounds, namely, the Alkenes and Cyclic Compounds  Generally, the atoms and groups in molecules that are attached to single bonds (sigma bonds) can rotate such that the molecular structure is in a state of continuous change  However, atoms or groups attached to a double bond cannot rotate around the C=C double bond without the pi bond being broken Geometric Isomerism in cyclic compounds and alkenes H H H C C Cl Cl H H Cl H C OR C sigma (single bond) bond free rotation double bond restricted rotation pi orbitals H C Cl H Cl H is different from C Cl (A) Chloro atoms groups are on same side of db Cl C C Cl H (B) groups are Chloro atoms on opposite sides of db The double bonds are rigid hence the chloro groups attached to the sp2-hybridized carbons are fixed in relation to one another Therefore, molecule (A) is not readily inter-convertible with molecule (B) Geometric Isomerism in cyclic compounds and alkenes  The molecule is designated as cis (Latin, “on the side”) when the two groups are on the same side of the double bond (pi-bond)  The molecule is designated as trans (Latin, “across”) when the two groups are on opposite sides of the double bond (pi-bond)  The cis or trans designation is therefore incorporated into the Nomenclature as italized Prefix: H C Cl H H (A) C is different from C Cl cis-1,2-Dichloroethene bp 60 oC Cl Cl C (B) H trans-1,2-Dichloroethene bp 48 oC Geometric Isomerism in cyclic compounds and alkenes  cis or trans stereoisomers are two different compounds, having different physical properties, such as b.p, m.p etc  They are classified as “Stereoisomers” because they differ in the arrangement of the atoms in space, and belong to the specific category of “Geometric Isomers” (designated: cis-trans isomers)  Geometric Isomers: are Stereoisomers that differ by groups being on the same side or opposite sides in a rigid molecule Geometric Isomerism cis or trans Designation  A requirement for Geometric Isomerism in Alkenes is that each Carbon Atom involved in the pi bond (double bond) have Two different groups attached to it, such as H and Cl or CH3 and Cl H3C H3C CH2CH3 C C H is Diffrent from H cis-2-pentene Cl H C C H cis-1-chloropropene CH2CH3 H CH3 C is Diffrent from H C trans-2-pentene CH3 C H Cl C H trans-1-chloropropene Geometric Isomerism cis or trans Designation  However, if one of the Carbon atoms of the double bond has two identical groups, such as two H atoms, or two CH3 groups, geometric isomerism is not possible e.g the following molecules are not Geometric Isomers: H C C H is Same as Cl Cl C is Same as CH2CH3 Cl C C H CH2CH3 H CH2CH3 C H H CH2CH3 CH2CH3 C Cl C CH2CH3 Geometric Isomerism E / Z System of Nomenclature  The cis/trans designation becomes somewhat difficult to assign when there are Three or Four Different Groups attached to the Carbon Atoms of a Double Bond: e.g Br F C I C Cl  In the structure above, we can say that Br and Cl are trans to each other or that I and Cl are cis to each other  It is difficult to designate the structure as either cis or trans isomer  To resolve this ambiguity, the E / Z System of Nomenclature is used Geometric Isomerism E / Z System of Nomenclature  The E / Z System of assigning the configuration of an isomer is based on Assignment of priorities to the atoms or groups attached to each Carbon of the Double Bond  The Isomer is (E) if the Higher-priority Atoms or Groups are on Opposite Side of the Double Bond  While the Isomer is (Z) if the Higher-priority Atoms or Groups are on the Same Side of the Double Bond  The Letter (E) is from the German word entgegen, (“across”); and (Z) is from the German zusammen, (“together”) Sequence Rules in Geometric Isomerism E / Z System “Sequence Rules” for Order of group priority: for Atoms and group of atoms: Atom: Atomic Number: F Cl 17 Br 35 I 53 Increasing Priority Atom No: Atoms: (1) (6) (7) (8) (16) (17) (35) (53) -H -CH3 -N H2 - OH - SH - Cl - Br -I Increasing priority for Isotopes: Isotope: 1H or H hydrogen 1H or D deuterium Sequence Rules in Geometric Isomerism E / Z System “Sequence Rules” for Order of group priority: if Atoms are identical, the Atomic Number of the Next Atoms are used for Priority Assignment: Three H's: lower priority H H H3C CH3 C H C CH3CH2 H H (E)-3-Methyl-2-petene -pentene H3CH2C CH2CH2CH2Cl C CH3 C CH3 C H C H H C C H H Two H's and One C: gives this group the higher priority Two H's and One C: gives this group the higher priority C CH2CH2CH3 (Z)-isomer (Z)-7-Chloro-4-propyl-3-methylhept-3-ene Sequence Rules in Geometric Isomerism E / Z System “Sequence Rules” for Order of group priority: Atoms attached by double bond or triple bond are given single bond equivalencies: O O -CH=CR2 < < < < -CN [...]... (E)-1-Bromo-2-chloro2-fluoro-1-iodoethene Sequence Rules in Geometric Isomerism E / Z System  Note! that the Determination of Priorities by Atomic Number alone cannot resolve / handle all the cases encountered in Organic Chemistry H3C  For e.g for naming: CH3 C CH3CH2 C H  Priority Order can be determined using the “Sequence Rules” according to the Cahn-Ingold-Prelog Nomenclature System (discussed under the R/S System