Grignard reagents Flashcards
What is a Grignard reagent?
Magnesium bonded to both a halogen on one side and an alkyl/aryl group on the other side
How are Grignard reagents made?
Add halogenoalkane (with halogen and alkyl group) to magnesium under conditions of dry ether and reflux the flask over a hot water bath
Conditions for making a Grignard reagent?
Dry ether
Reflux flask using a hot water bath
What are the bonds in a Grignard reagent?
Covalent bonds between alkyl group to magnesium and magnesium to bromine
Polar bonds in a Grignard reagent
Slight negative charge on carbon the Mg bonded to = C d-
Mg is d+
Because electrons in bond is pulled toward more electronegetaive carbon
Why can Grignard reagents act as a nucleophile?
Slight negative charge on carbon atom so acts as electron pair donor by breaking C-Mg bond and attract electrophiles
General formula of a Grignard
RMgX
Why is Grignard made under condition of dry ether
Because grignard reacts readily with any traces of water present to form an alkane and magnesium hydroxide halide salt
Grignard + water makes
Alkane from alkyl group on grignard, H from alcohol group accepted
Magnesium hydroxide halide salt, Mg Br react with remaining OH from water
What type of reaction is Grignard + water and why?
Acid base because water acts as acid and donates a H+ to alkyl chain on Grignard = alkane
Grignard acts as base, C d- can attract H+ to accept it and break carbon -Mg+Br bond
Grignard + alcohol makes
Alkane from alkyl group on grignard, H from alcohol accepted
Magnesium alkoxide halide salt, Mg Br reacts with O+ carbon chain from alcohol
Example of magnesium hydroxide halide salt
Mg(OH)+ Br-
Example of magnesium alkoxide halide salt
Mg(OCH3)+ Br-
What type of reaction is Grignard + alcohol?
Acid base because alcohol acts as acid and donates a H+ to alkyl chain on Grignard = alkane
Grignard acts as base, C d- can attract H+ to accept it and break carbon -Mg+Br bond
Organometallic compound
Describes a Grignard reagent
Organic compound (CH) with a metal (Mg)
All Grignard reactions to INCREASE THE CARBON CHAIN
React with CO2
React with methanal
(increase carbon chain by 1)
React with aldehyde —> secondary alcohol
React with ketone —> tertiary alcohol
What are Grignard reagents useful for?
Increasing the carbon chain of a compound in synthesis
React starting reagent with Grignard
Convert that to molecule with longer carbon chain with 2nd reagent
Once grignard + second reagent reacts then what happens?
Dilute acid is added
Grignard Reagent + primary/secondary amine makes
Alkane (from Grignard carbon chain)
Magnesium alkoxide bromide
Reaction of GRignard + primary/secondary amine mechanism
Hydrogen on amine is donated (acts as acid) to the alkyl group on Grignard = alkane formed
Amine becomes alkylammonium magnesium 1+ ion to form salt with Br- anion
alkylammonium magnesium 1+ ion
Mg(RNH)+
Reaction of Grignard + secondary/primary amine type
Acid/base reaction
Why is Grignard + secondary/primary amine an acid base?
Because 1 H+ bonded to the nitrogen on amine is donated to alkyl group of grignard which accepts H+
Amine = acid
Grignard = base
Can tertiary amines react with Grignard?
No they lack N-H so no H+ can be donated to alkyl group of Grignard
Why does Grignard react as a base?
C d- can attract H+ and accept it
Wash the magnesium alkylammonium salt with dilute acid forms
Just the alkylammonium salt
Carbon dioxide + Grignard makes?
RC=O
|
OMgBr
Then: Carboxylic acid + magnesium hydroxide bromide
Hydrolyse product of CO2 + Grignard with water in dilute acid forms?
Carboxylic acid with 1 more carbon than Grignard
And Mg(OH)Br
Where does CO2 add onto grignard?
1 Double bond in CO2 breaks and bonds to last carbon on Grignard between magnesium and bromine, which bond to the single bonded oxygen
And 2nd C=O is on added carbon
Grignard + methanal
Makes primary alcohol with 1 more carbon than grignard
Grignard + longer chain aldehyde
Makes secondary alcohol with number of carbons in aldehyde added on to number of carbons in Grignard
Grignard + ketone makes
Tertiary alcohol with number of carbons in ketone added on to grignard
