Experiment 4: The Grignard Reaction

who discovered the Grignard reagent

Victor Grignard 1901; nobel prize in 1912

Why don't we use acetone when reacting with Grignard

acetone is ketone; will react with Grignard and form a by product

what were the 4 possible ketones

acetone, butanone, 2-pentanone,3-pentanone

what do Grignards react with to give alkanes

acids

how is the tertiary alcohol formed

after the first reaction the intermediate loses alkoxide to form a ketone; ketone then reacts with a second mole of Grignard to form tertiary alcohol

what IR groups are expected for the product

alcohol OH, sp3 stretching, C-O bond stretch; if the ketone did not fully react C=O stretch would be present

Grignard + epoxides

alcohols

what are the products of Grignard reagents

alcohols

in this experiment the Grignard reagent was formed from

alkyl bromide and magnesium

what is another reason ether solvents are a good choice

aprotic and do not contain any acidic protons that would result in a compound that wasnt an alcohol like an alkane

in general any organic halide can be used to make Grignard reagent but exceptions are

aryl chlorides; chlorine attached to the ring doesnt react with magnesium

what else to Grignard reagents react with

atmospheric oxygen; but the reaction is so slow that it is not necessary to run the reaction under inert conditions; atmospheric moisture is a bigger problems

how did we identify the staring alkyl halide and ketone

boiling point and IR

it is safe to distill most organic compounds

boiling up to about 150

what SHOULD happen as soon as the alkyl halide to the magnesium turnings

bubbling

define pyrophoric

burst into flames upon exposure to air

What is an organometallic compound

carbon-metal bound

Grignard + CO2

carboxylic acids

why are grignard reagents a good choice

cheap and low toxicity

what should happen in the first minute of sonification

cloudiness

what ether solvents are typically used

diethyl ether or THF; used for both synthesis and subsequent reaction

why are grignard reagents strong carbon nucleophiles

electronegativities of carbon and magnesium; because carbon has a greater electronegativity it pulls electrons toward it increasing its negative charge

what does the negative charge of the carbon allow for

enables alkyl group to attack carbonly carbon of ketone of aldehyde it is reacting with; allows full negative vharge on oxygen when the bond flips

what is a key requirement for the synthesis of the Grignard reagent

ether solvent

grignard reagents are prepated by mixing

ethreal solution of alkil halide with mageniusm turnings

why is diethyl ether a good solvent

extremely volatile; low miscibility with water; if it is not possile to run the reaction under an inert atmosphere, since it is heavier that air it acts as a barrier and prevents water from entering

what is required if a much higher boiling point is used in simple distillation

remove water from condensor; might thermally fracture condensor; consider thermal stability, product could decompose

reaction with other aldehydes

secondary alcohol

What methods did we use in the exp

simple distillation and extraction to allows for isolation and purification

organomagnesium (GRIGNARD REAGENT) and organolithium reagents are sufficiently ionic enough to be

strong nucleophiles, strong bases, but covalent enough to be soluble in many organic solvents

What is metalation

terminal acetylenes and acidic hydrocarbons are transformed into Grignard reagents by reaction with a preformed Grignard reagent

reaction with ketones

tertiary alcohol

why is ether a required solvent

the electron pairs on the ether coordinate with the magnesium of the Grignard reagent and form a soluble complex

what were the 3 possible alkyl bromides

1-bromopropane,butane,or pentane

once the reagent is formed however

the halide ion has no affect on the rate of formation or the yield of the reaction

esters react with how many molecules of Grignard reagent

2

how do alkyl halides affect the rate of the reaction

I>Br>Cl

Why do we use an ammonium chloride solution rather than only water in the work up of the Grignard reaction?

NH4Cl is better at hydrolyzing magnesium alkoxide than water alone, but keeps the acidity lower than if HCl as used

What is the objective of the Grignard experiment?

to prepare and use the Grignard reagent as a method of studying the utility of organometallic reagents in synthesis and to identify starting materials using retrosynthetic analysis

how did we help facilitate the reaction

used a sonicator; high frequency sound eaves shake off oxide coating on the magnesium to produce small molecules that provide a vast surface area for the reaction to start on

when is it best to use simple distillation

used to separate volatile liquid from soluble nonvolatile; acceptable separation of 2 miscible liquids with boiling point difference greater than 25, bigger is better

why is THF less ideal that diethyl ether

very soluble with water and will absorb it so it doesnt protect from atmospheric moisture

what do these reagents react explosively with

water

what will protonate a Grignard reagent to give the corresponding alkane

weak acids like water alcohols and phenols

when do you need to use two equivalents of grignard reagent

when esters are reacting

what happens if the reaction proceeds uncontrolled

wildcat; need ice bath

want to slow the reaction NOT stop it

will be hard to reinitiate

why is using a heat gun useless to remove this trace amount of water

will condense upon cooling; better to rinse with acetone and use the heat gun

if made without ether

yield is very very low

Grignard + nitries

yield ketones

if you react a grignard with an unprotected alochol, phenol, or terminal acetylene

you have to use at least 2 equivalents of grignard since 1 equivalent will be destroyed by the proton source

methyl iodide used instead of

methyl chloride or methyl bromide because they are hazardous

simple distillation

minimum level of separation; fastest

why would the solution not bubble when the alkyl halide is added

moisture; even a little trace of water will coordiate with the surface metal and passivate the magnesium

what is the reactivity of an organimetallic dependent on

nature, ionic or covalent, of the carbon-metal bond

what is the workup step in the sythesis of grignards

oxygen with fully negative charge will pick up a proton to become tertiary alcohol

reaction with formaldeyde yields

primary alcohol

how are Grignard reagents prepared

reaction of alkyl or aryl halides with magnesium

when do you know its time to use an icebath

gas coming out of condensor

WHEN is THF the solvent of choice

higher boiling and less flammable ether is desired

ether safety hazard

highly flammable; readily react with atmospheric oxygen to form peroxides which undergo explosive decomposition

organosodium and organopotassium reagents are

highly ionic and the carbon acts like a powerful base

after the reaction is initiates the alkyl halide

is added at a rate to maintain the rate of the reaction

risk of explosion of an ether left out to react with oxygen

is directly proportional to the concentration of the peroxide

the formation of the Grignard

is exothermic and the rate of the reaction increases with temperature