EM 261 Oct 18, 2018 Photosynthesis and Related Reactions O 2 2 O 6 12 O 6 2 O N 3, S, Fe, u, o, other Natural Products D-Glucose R O R OR Ionic substitution S N 1 & R X X 2 hv Petroleum/ Alkanes R N 2 R S R 1 Burning Addition Elimination E 1 & E 2 Elimination E 1 & E 2 arboxylic acid derivatives O 2 2 O Alkenes 2 2 Alkynes 2 2 Oxidation O O R R R 1 Aldehydes and ketones -Plants consume 4x10 11 metric tons of O 2 per year and convert it to D-Glucose Sn Example: I I!- O O 3 Forward rxn I will not occur O!- Weakest bond Much stronger acid than MeO Two possibilities O!- O ydrogen iodide is a strong acid and will drive the reverse reaction, meaning the forward reaction will not occur.
In order to make the above reaction occur, a stronger base (such as sodium methoxide) must be used to drive the forward reaction. I O Na Works Well O 3 Na I Reactive Stonger Base Weaker Base Example: NaO N pk a = 9 2 SO 4 ighly Toxic Na N Sodium cyanide Toxic I N Acetonitrile Na I The above reaction will not occur unless hydrogen cyanide is converted into sodium cyanide using NaO. The product is acetonitrile, a common laboratory solvent. REVIEW: yes or no? Examples Ex #1) Will NOT proceed no NaO No tert-butyl bromide 2-omo-2-methyl propane O Na Tertiary halide Ex #2) NaO No O Na alide attached to a double bond
Ex #3) This one can work in principle 2-omobutane Na N 2 2 N Na Ex #4) The one belo does NOT work O, OR, NR 2 are never good leaving groups O No Na N 2 2 N NaO Acid/base rxn occurs O Na N 2 In this reaction, the N 2 species will pull off the most acidic proton (the one on the alcohol) to from an alkoxide instead of undergoing an reaction, S N 1 reactions - Substitution Nucleophilic Unimolecular - Rate depends on 1 concentration (concentration of the starting material) - Not concerted has a carbocation intermediate - Not stereospecific - Works if leaving group is tertiary (not primary, slow on secondary) Example: Tertiary alide -No possible, sterically crowded does work by S N 1 X 3º halide X = No Adding water drives equilibrium forward (Le hatelier's Principle) O O Rxn is reversible S N 1 occurs
Mechanism: The bromine group leaves with its electrons from the covalent bond, leaving behind a reactive carbocation intermediate arbocation intermediate arbocation Stability: Tertiary Secondary Primary Methyl ation 3 3 > > > 3 3 Most stable The 3 alkyl groups donate e - density into the positive charge Least stable (no S N 1) Mechanism of Nucleophilic Attack on arbocation Nu 50% 3 3 50% Nu = 2 O If attack is from the top O 3 3 Planar carbocation intermediate sp 2, bond angle 120 Due to the 50/50 chance of attcking from the top or bottom, the reaction is not stereospecific and stereochemistry is lost. O 3 3 sp 3, tetrahedral, bond angle 109 The result of an S N 1 mechanism is often a racemic mixture or mix of diastereomers
S N 1 and Examples: Ex #1) O Nal No Rxn Ex #2) O is never a good leaving group O 2 SO 4 l l eat O arbocation intermediate Mechanism: O O 2 O l l Ex #3)!! - NaO3 OMe Na Secondary Alkyl alide
Ex #4) NaO3 Sterically crowded - reaction is very slow Tertiary Alkyl alide 3 O 2 SO 4,! OMe OMe Diastereomers sp 2, planar Alkene and Alkyne Nomenclature Alkene = double bond = olefin (oleum facere = to make oil) Alkyne = triple bond = acetylene (as functional group, not compound) Alkene Alkyne Simplest Alkene and Alkyne Possible Ethylene Acetylene Alkene Nomenclature Find longest chain, number from end to contain both ends of = and give lowest number to 1 st of = hange ane to ene precede with number to indicate first double bond position
ethylene OR ethene 3 propylene OR 1-propene prop-1-ene 2 = 2 butylene OR 1-butene but-1-ene Below are two structural isomers of 1-butene 3 trans-2-butene diastereomers 3 cis-2-butene