Lecture 16 More arbonyl hemistry March 8, 2018 Victor Grignard Georg Wittig
Exam II Wed March 21 from 7pm to 9pm In this room overs through todays lecture Know the reactions Play cards Please practice.old exams are posted Exam includes at least one homework problem No phones or calculators. Do a good job!
eaction Theme The most common reaction of a carbonyl group is addition of a nucleophile to form a tetrahedral addition compound Nu - ere it is an acid or electrophile - Nu Tetrahedral carbonyl addition compound
Grignard eagents X = l, Br, I Diethyl ether Et 2 Tetrahydrofuran TF
rystal Structure of 3 2 MgBr Br Mg L.J. Guggenberger and.e. andle, J. Amer. hem. Soc. 90(20) 5357 (1968)
Grignard eagents Addition of a Grignard reagent to formaldehyde followed by 3 gives a 1 alcohol 3 2 - MgBr 3 3 2 - MgBr 3 2 TF dil. Mg 2 This sequence (mechanism) is general and important!
Grignard eagents Addition to any other gives a 2 alcohol 3 2 - MgBr 3 3 2 - MgBr 3 2 TF dil. Mg 2 You may change decorations at will read pages 630-636
Grignard eagents Addition to a ketone gives a 3 alcohol 3 2 3 2 3 2 2 2-3 3 2 MgBr 3 2 - MgBr TF dil. 3 3 2 3 2 3 Mg 2 Please try this with other Grignard reagents and other ketones
Grignard eactions 3 2 - MgBr 3 3 2 - MgBr 3 2 TF dil. Mg 2 3 2 - MgBr 3 3 2 - MgBr 3 2 TF dil. Mg 2 3 2 3 2 3 2 2 2-3 3 2 MgBr 3 2 - MgBr TF dil. 3 3 2 3 2 3 Mg 2
Grignard eagents Addition to 2 gives a carboxylic acid - 3 3 2 MgBr 3 2 - MgBr TF dil. 3 2 Mg 2 This is a great way to add a carbon
Grignard eactions MgBr TF - MgBr 3 Dilute This is a great way to add two carbons These are very valuable and important reactions Please add to your card stock!
Grignard reagents also react with esters d ' MgX d ' diethyl 3 ether 3 MgX but species formed is unstable and dissociates under the reaction conditions to form a ketone
Grignard reagents react with esters d ' MgX d ' diethyl 3 ether 3 MgX this ketone then goes on to react with a second mole of the Grignard reagent to give a tertiary alcohol ' 3 MgX
Example 2 3 MgBr ( 3 ) 2 3 1. diethyl ether 2. 3 ( 3 ) 2 3 3 (73%) Two of the groups attached to the tertiary carbon come from the Grignard reagent
Grignard reagents react with: formaldehyde to give primary alcohols aldehydes to give secondary alcohols ketones to give tertiary alcohols esters to give tertiary alcohols 2 to give acids epoxides give primary alcohols
Grignard eagents Problem: 2-phenyl-2-butanol can be synthesized by three different combinations of a Grignard reagent and a ketone. Show each combination - 2 3 3
Practice Problem 1. Starting from benzene, write a synthetic path to the structures below. You are free to use any reagents or reactants you choose, but you must start with benzene. l 0 Br l
xygen Nucleophiles " ' 2" " ' 2
Alcohols eact with Aldehydes and Ketones in two steps first ' " " ' Product is called a hemiacetal.
emiacetal reacts further in acid to yield an acetal " ' This product is called an acetal., " ' This hemiacetal reacts further.
Example 2 3 2 l ( 2 3 ) 2 2 Benzaldehyde diethyl acetal
Diols Form yclic Acetals 3 ( 2 ) 5 2 2 2 2 benzene acid catalyst 2 ( 2 ) 5 3
Mechanism of Acetal Formation
Mechanism esonance stabilized cation
Mechanism
Mechanism
Mechanism
Mechanism atalyst regenerated! emi acetal!
Mechanism of Acetal Formation Second stage is hemiacetal-to-acetal conversion involves carbocation chemistry
emiacetal-to-acetal Stage These are not separate reactions this is all one big equilibrium
emiacetal-to-acetal Stage
emiacetal-to-acetal Stage ere is the water!
emiacetal-to-acetal Stage
emiacetal-to-acetal Stage
emiacetal-to-acetal Stage The acetal egeneration of catalyst
Note that EVEY step is an equilibrium Therefore, the reaction can be pushed forward or backward by appropriate choice of conditions The forward reaction is synthesis The backward reaction is hydrolysis
ydrolysis of Acetals " ' 2 2" ' " mechanism: reverse of acetal formation hemiacetal is intermediate. application: aldehydes and ketones can be "protected" as acetals.