HL Physical Organic Chemistry: Supplementary Material ELIMINATION & CONDENSATION REACTIONS and STEROISOMERISM

Reaction Pathways

Many Organic Reactions are carried out using Distillation and/or Reflux

10.6 & 20.5: Reaction Pathways Many times desired reaction products can not be produced in a single step ….. Examples: Preparation of propanal from 1-iodo-propane

10.6 & 20.5: Reaction Pathways Many times desired reaction products can not be produced in a single step ….. Examples: Preparation of butanone from 2-butene

10.6 & 20.5: Reaction Pathways Many times desired reaction products can not be produced in a single step ….. Examples: Preparation of ethylamine from ethene

10.6 & 20.5: Reaction Pathways

10.6 & 20.5: Reaction Pathways

Elimination Reactions

20.3: Elimination Reactions By changing rxn conditions, an “elimination rxn” is favored rather than a nucleophilic substitution In this specific example, ethanol reacts with hydroxide ion to form the ethoxide ion:

20.3: Elimination Reactions The ethoxide ion is a stronger base (weaker nucleophile) than hydroxide ion, so at high OH-1 concentration and high temperature, the elimination rxn is favored The ethoxide ion removes the H from the bromoethane, causing the elimination of bromide ion and the formation of an alkene If the halogen is not primary, the double bond can form in either direction, making a mixture of products

Condensation Reactions

20.4: Condensation Reactions (esters) Condensation reactions involve the elimination of water The most common reaction is between an alcohol and a carboxylic acid to produce an ester The protons from the sulfuric acid act as a catalyst to increase the rate of reaction, react with the water produced to form hydronium ions, which shifts the reaction to right via LeChatelier’s Principle

20.4: Condensation Reactions (amides) Ammonia and primary amines react with carboxylic acids to form the acid’s salt, but upon further heating condense to form an amide For example, ethanoic acid + methylamine form methylammonium ethanoate, which then forms N-methylethanamide:

20.4: Condensation Reactions (polymers) Addition polymers simply add monomers across a double bond with no byproducts. For example: Condensation polymers usually remove water during the linking of monomers The two most common condensation polymers occur between alcohols & carboxylic acids (polyesters) or between amines & carboxylic acids (polyamides)

20.4: Condensation Reactions (polyesters and polyamides)

Stereoisomerism

20.6: Stereoisomerism Stereoisomers are molecules with the same molecular formula, but with different arrangements of atoms in space Geometric isomers have structures stabilized by double (or triple) bonds. This prevents free rotation about the C-C bond, and thus unique spacial molecular arrangements. Typically, geometric isomers have different physical properties (i.e. polarity, boiling point, etc.) and can be separated using distillation, chromatography, etc. The chemical properties of geometric isomers are usually similar, but sometimes the isomers behave differently in reaction pathways where proximity of the side groups influences mechanisms

20.6: Stereoisomerism The cis geometric isomer has a plane of symmetry (for our purposes either the double bond or the plane of a ring), with two R groups on the same side of the plane The trans geometric isomer similarly has a plane of symmetry, but with two R groups on the opposite side of the plane of symmetry

20.6: Stereoisomerism

20.6: Stereoisomerism An example of the difference in chemical properties resulting from geometric isomerism can be seen in the dehydration reaction of butenedioic acid trans - butenedioic acid sublimes at 200º C, and will only form its anhydride at much higher temperatures since the carboxyl groups must be rotated about the double bond in order for the reaction to take place At 200º C, cis - butenedioic acid easily dehydrates to form butane anhydride (see the diagram below)

20.6: Stereoisomerism Another type of stereoisomer are the optical isomers (enantiomers) Enantiomers are mirror images of one another, and can not be superimposed without breaking and remaking bonds In order to qualify as enantiomers, one of the carbon atoms in the molecule must have 4 different subsistent groups attached, and is said to be a “chiral center” There are 2 enantiomers for each chiral center in a molecule. So, for example, a molecule with 4 chiral carbons would have 8 enantiomers

20.6: Stereoisomerism

Summary of IB Required Reaction Pathways