Turn on thread page Beta

chemistry- grignards... watch

Announcements
    • Thread Starter
    Offline

    2
    ReputationRep:
    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..


    Offline

    0
    ReputationRep:
    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.
    • Thread Starter
    Offline

    2
    ReputationRep:
    ah cheers thanks.
    but y in the case of water are H+ needed?

    i assume the intermediate is the C2H5O?
    Offline

    0
    ReputationRep:
    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.
    • Thread Starter
    Offline

    2
    ReputationRep:
    tiz very strange ..
    http://www.chemguide.co.uk/organicpr.../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!
    Offline

    0
    ReputationRep:
    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.
    Offline

    2
    ReputationRep:
    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.
    Offline

    0
    ReputationRep:
    i personally think ur eqs involving hcl are wrong as it is hydrolysis and it reacts with h2o and not hcl
    Offline

    2
    ReputationRep:
    heehee! what a silly name for a reagent!!
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by 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
    Offline

    0
    ReputationRep:
    ergh..gee i've never heard of this, is this nuffield edexcel?! or just edexcel?
    Offline

    2
    ReputationRep:
    (Original post by yipee)
    ergh..gee i've never heard of this, is this nuffield edexcel?! or just edexcel?
    It's not on AQA!

    Ben
    Offline

    0
    ReputationRep:
    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 before i shoot myself with a macheti! :eek:
    Offline

    0
    ReputationRep:
    (Original post by yipee)
    ... before i shoot myself with a macheti! :eek:
    machete
    • Thread Starter
    Offline

    2
    ReputationRep:
    lol...
    its plain old edexcel! nothing nuffield about it
 
 
 

University open days

  • University of the Arts London
    MA Performance Design and Practice Open Day Postgraduate
    Thu, 24 Jan '19
  • Brunel University London
    Undergraduate Experience Days Undergraduate
    Sat, 26 Jan '19
  • SOAS University of London
    Postgraduate Open Day Postgraduate
    Sat, 26 Jan '19
Poll
Are you chained to your phone?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.