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Grignard Solvent

  • 21-02-2008 10:43pm
    #1
    backhand bertie
    Closed Accounts Posts: 6 ✭


    Is there any particular logic or rule about using THF instead of ether in a grignard reaction (other than perhaps the boiling point)? And generally which is the better solvent for this reaction?


Comments

  • #2
    Darando
    Registered Users, Registered Users 2 Posts: 732 ✭✭✭Darando


    Just a case of making sure they are dry. And reaction is carried out in dry glassware and under nitrogen/argon (again dry).

    THF probably more common becasue its safer to dry in a lab, also not as volatile.


  • #3
    backhand bertie
    Closed Accounts Posts: 6 ✭backhand bertie


    ah good...thanks. Just to add to your remark about needing anhydrous conditions. According to my practical org teacher, sodium drying is a waste of good sodium because sieves are just as good or better. Its a continuing practice out of habit rather than necessity.


  • #4
    SOL
    Registered Users, Registered Users 2 Posts: 1,155 ✭✭✭SOL


    MTBE would probably do either?


  • #5
    Darando
    Registered Users, Registered Users 2 Posts: 732 ✭✭✭Darando


    I always used sodium wire when drying THF. Sieves need to be dried at a high temperature (used to go to 200C to dry them, the cooled in desiccator etc...) Sodium was just handy and worked well. We used to dry THF in a still that contained benzophenone, which went drak blue when dry. We used "dryish" THF in the still so we didnt destroy the sodium (if you get me?).....plus sodium isnt expensive and doesnt need to be changed everytime.

    I think still smight be on their way out for safety reasons but I still know many labs that have them (PhD labs)..

    You can get good dry THF from Sigma that contaisn sieves already but remeber to remove under nitrogen....never used it in a grignard though so can't comment on how well it works.


  • #6
    backhand bertie
    Closed Accounts Posts: 6 ✭backhand bertie


    Missed it first time...is the inert atmosphere really necessary? surely grignards are just moisture sensitive? MTBE (methyl tert-butyl right?)...seen a few procedures with dibutyl ether...I guess it all depends on what the b.p. of the product is (for separation by distillation) when the more obscure ethers are used


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  • #7
    Mucco
    Registered Users, Registered Users 2 Posts: 717 ✭✭✭Mucco


    MTBE is used on plant scale as it's less volatile the Et2O. I think ether is generally best, and OK on small scale. Stills are on their way out - they're not used in industry, and universities are phasing them out, so best to get some anhydrous stuff from a supplier.
    This new 'green' reagent, 2-methylTHF is supposed to be rubbish for Grignards, though I've no experience with this solvent (and Aldrich recommend it for Grignards).

    I use an inert atmosphere for all reactions, if there's no oxygen around, there's less chance of a fire. Plus how are you going to get your dry atmosphere?

    M


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