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chemistry- grignards...

For the reactions of grignard reagents with water, carbon dioxide and carbonyl compounds, I have to 'understand that the intermediate in these reactions needs to be destroyed by the addition of acid' ..
anyone got a clue? its not it any books I have.
this is edexcel btw..

:redface:

Reply 1

minkailin

wen the grignard adds to carbonyl/carbon dioxide, u get the anion formed, e.g. C2H5O-. so u need to add acid to protonate the O-, to get OH to make the alcohol.

Reply 2

misty

ah cheers thanks.
but y in the case of water are H+ needed?

i assume the intermediate is the C2H5O?

Reply 3

minkailin

i havent got a clue....
when u add water or acid to a grignard, i think u get the alkane
e.g.
CH3MgBr+H2O gives CH4+MgBrOH
i.e. the water protonates the grignard.
so i guess u add acid to neutralise the OH- generated?
i'm really unsure.

Reply 4

misty

tiz very strange ..
http://www.chemguide.co.uk/organicprops/haloalkanes/grignard.html
talks alot about it but still doesnt make sense..
hmm. im gunna go sort it out. ill come back if i find out heh
cheers for ur help!

Reply 5

minkailin

no probs.
i was thinking the following mechanism might be possible (i just guessed this):
R-CO2MgBr, H+ adds to this grignard giving
R-CO2H and MgBr+
MgBr+ + H2O gives MgOHBr and H+

hence the acid is a catalyst, regenerated at the end.

Reply 6

Sahir

Grignard + Water --> Alkane (dry, RTP)

CH3CH2MgBr + H20 --> CH3CH3 + MgBrOH

Grignard + CO2 --> Carboxylic Acid (-76C as CO2 has to be solid, then RTP on addition of dilute acid to decompose the intermediate)

CH3CH2MgBr + CO2 --> CH3CH2COOMgBr

CH3CH2COOMgBr + HCl --> CH3CH2COOH + Mg^2+ + Br^- + Cl^-

Grignard + Gaseous Methanal --> Primary Alcohol (dry, RTP)

CH3MgBr + HCHO --> CH3CH2OMgBr

CH3CH2OMgBr + HCl --> CH3CH2COH + Mg^2+ + Br^- + Cl^-

Grignard + Gaseous Aldehyde --> Secondary Alcohol (dry, RTP)

CH3MgBr + CH3CHO --> CH3CH(CH3)OMgBr

CH3CH(CH3)OMgBr + HCl --> CH3CH(CH3)OH + Mg^2+ + Br^- + Cl^-

Grignard + Gaseous Ketone --> Tertiary Alcohol (dry, RTP)

CH3MgBr + CH3COCH3 --> (CH3)(CH3)(CH3)COMgBr

(CH3)(CH3)(CH3)COMgBr + HCl --> (CH3)(CH3)(CH3)COH + Mg^2+ + Br^- + Cl^-

And Grignards always react in dry conditions (ether such as ethoxyethane) because when they're produced from haloalkanes, this is too in dry conditions.

Reply 7

ravs

i personally think ur eqs involving hcl are wrong as it is hydrolysis and it reacts with h2o and not hcl

Reply 8

presebjenada

heehee! what a silly name for a reagent!! :tongue:

Reply 9

misty

ravs

i personally think ur eqs involving hcl are wrong as it is hydrolysis and it reacts with h2o and not hcl

its def HCL, i think thats a main error of candidates

Reply 10

yipee

ergh..gee i've never heard of this, is this nuffield edexcel?! or just edexcel?

Reply 11

Ben.S.

yipee

ergh..gee i've never heard of this, is this nuffield edexcel?! or just edexcel?

It's not on AQA!

Ben

Reply 12

yipee

i dont do AQA, i do nuffield edexcel course! which has got me worried now cus on the first post it said edexcel... but nuffield edexcel and edexcel are different. so i would appreciate it if someone could clarify which board it is :biggrin: