Organic Chemistry – Focus on Isomers Green & Damjii – Chapter 10 Chang - Chapter 24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Types of Isomerism - I Structural – same molecular formula different bonding –Hydrocarbon chain (different HC skeletons) Straight-chain vs branched alkane Alkene vs. cycloalkane Alkyne vs. diene (or alkene & cycloalkane) –Positional (location of functional group) –Functional group (different functional group) Alcohol vs. alkoxyalkane (ether) Aldehyde vs. ketone Carboxylic acid vs. ester

24.2 What type of isomerism is shown here? What is the IUPAC name for isobutane?

What are the structural isomers of hexane, C 6 H 14 ? 24.2

Show a pair of structural isomers (hydrocarbon chain) that exemplifies each of the relationships below for the compound C 5 H 10. Alkene vs. cycloalkane Alkyne vs. diene (or alkene & cycloalkane) 24.2

Show a pair of structural isomers (positional) that exemplifies the relationship below for an alcohol with the molecular formula C 4 H 10 O Show a pair of structural isomers (functional group) that exemplifies the relationship below for the compound C 4 H 10 O. Alcohol vs. alkoxyalkane (ether)

Show a pair of structural isomers (functional group) that exemplifies the relationship below for the compound C 4 H 8 O. Aldehyde vs. ketone 24.2

Show a pair of structural isomers (functional group) that exemplifies the relationship below for the compound C 4 H 8 O 2. Carboxylic acid vs. ester 24.2

Types of Isomers - II Stereoisomers –same molecular formula, structural formula, general name –different orientation in space; shape  Geometric (cis, trans)  Enantiomers

Types of Isomers - II Stereoisomers - Geometric (cis, trans) Due to the lack of rotation around a double bond (or rigid ring structure) –cis  groups on the same side of the double bond –trans  groups on the opposite sides of the double bond What type of bonding is involved? (sigma? pi?) –Draw and name the two geometric isomers of 2-butene:

Types of Isomers - II –Draw and name the two geometric isomers of 1,2-dichloroethene: –Draw and name the two geometric isomers of 1,2 dichlorocyclobutane: [NOTE: a ring structure has the same lack of rotation as a double bond]

Types of Isomers - II Stereoisomers - Geometric (cis, trans) generally similar in chemical properties –(functional groups determine chemical properties) often different physical properties –Ex: boiling point »cis-1,2-dichloroethene 60.3ºC »Trans-1,2-dichlorethene 47.5ºC –Why?

What factors influence the physical properties of organic compounds? Intermolecular forces –Dipole-dipole forces »electrostatic attraction between permanent dipoles between polar molecules –Hydrogen bonding »electrostatic attraction between permanent dipoles between polar molecules that include a H attached to a highly electronegative and small N, O, or F atom NOTE the bp of 3 molecules with ~ the same Mr (44,44,46) Propane (231K), ethanal (294K), ethanol (352K)

What factors influence the physical properties of organic compounds? INTERMOLECULAR FORCES (Weak  Strong) –Van der Waals (aka London forces, dispersion forces) »electrostatic attraction between temporary induced dipoles between all molecules »Influenced by molar mass, surface area » methane (bp: 111K) vs hexane (bp: 341 K) » pentane (bp: 309K) vs. dimethyl propane (bp: 283K)

Types of Isomers - II Stereoisomers - Geometric (cis, trans) often different physical properties –Ex: melting point »Cis-but-2-ene-1,4-dioic acid ºC »Trans-but-2-ene-1,4-dioic acid 286ºC –Why? generally similar in chemical properties… but –Ex: lose water to form anhydride? »Cis-but-2-ene-1,4-dioic acid YES »Trans-but-2-ene-1,4-dioic acid NO –Why?

Types of Isomerism – II cont. Stereoisomers - Enantiomers (aka Optical Isomers) Requires a molecule with a chiral center –An atom (usu C) with four different groups attached to it –aka an asymmetric carbon atom Two forms are mirror images of one another that cannot be superimposed on each other (like gloves)

24.2 achiralchiral

Types of Isomerism – II cont. Stereoisomers – Enantiomers Common example are amino acids –only glycine does not have an enantiomeric form

Types of Isomerism – II cont. Stereoisomers – Enantiomers Have similar physical properties Have similar chemical properties –except when they interact with other optically active substances – which happens often in the human body –general, only one enantiomer of a drug, agrochemical (herbicide, pesticide), flavoring agent, or other molecule (when asymmetric) has the desired biological effect, while the other enantiomer has very different effects or, at least, places a metabolic burden on the body.

Types of Isomerism – II cont. amino acid asparagine one form tastes bitter, the other tastes sweet each of the two enantiomers binds differently to chemoreceptors in the tongue. Source -

Types of Isomerism – II cont. Thalidomide a drug once prescribed to counteract pregnancy-related morning sickness, is an effective sedative as the R enantiomer (left ), but the S enantiomer (right) is a potent teratogen (it causes birth defects). Source -

Types of Isomerism – II cont. Stereoisomers – Enantiomers have different effects on polarised light one form is said to be optically active in a polarimeter if a mixture has both enantiomers it is said to be a racemic mixture and is not optically active

NAMING ENANTIOMERS (not required by IB) Since two enantiomers are different compounds, there is a nomenclature which distinguishes them from each other. The convention which is used is called the (R,S) system because one enantiomer is assinged as the R enantiomer and the other as the S enantiomer. What are the rules which govern which is which?? –Priorities are assigned to each of the four different groups attached to a given stereogenic center (one through four, one being the group of highest priority). Priority is based upon atomic number, i.e., H has the lowest priority, O over C, F over O, and so on. –Orient the molecule so that the group of priority four (lowest priority) points away from the observer. –Draw a circular arrow from the group of first priority to the group of second priority. –If this circular motion is clockwise, the enantiomer is the R enantiomer. –If it is counterclockwise, it is the S enantiomer.