S20-Week06-Lecture25-29

S20-Week06-Lecture25-29 - GP25-01 Amines Examples: H 3 C N...

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Unformatted text preview: GP25-01 Amines Examples: H 3 C N H H H 3 C N H H 3 C N CH 3 H 3 C CH 3 H 3 C N H H CH 3 H 3 C Primary amines Secondary amine Tertiary amine Spectra: H 3 C N H CH 3 N–H stretch ~ 3200-3350 cm-1 H 3 C N H CH 3 H H ~ 1.0 ! ~ 1.0 ! ~ 2.6 ! H 3 C N H CH 3 ~ 44 ! ... and all 'regular' amines are basic: Representative pK a 's It might be thought surprizing that we use pK a 's - what we use is the pK a of the conjugate acid : Example: H 3 C N H CH 3 + H H 3 C N H CH 3 H pKa ~ 11 Primary, secondary, and tertiary amines , R 3 N where R = alkyl or H pK a ~ 9-12 If the lone pair is involved in a conjugated system (i.e. it's delocalized), this conjugation (= stabilization) is not possible in the conjugate acid (there's no lone pair!). This will move the equilibrium further over to the right – the pK a goes down, the amine is less basic: N H H H N H H + H pK a ~ 4.6 and you can take this to extremes! N H H H N H H + H pK a ~ – 9.3(!!) NO 2 NO 2 O 2 N O 2 N N N O O O O Lone pair can be delocalized on to all three NO 2 groups 2 Strong intramolecular H–bonds 1 H nmr i.r. 13 C nmr Watch out for this feature of amine nomenclature! primary amine RNH 2 secondary amine R 2 NH tertiary amine R 3 N Lower value for the pK a = weaker base The lone pair is the donor - if this is stabilized then the basicity is reduced (lower pK a ): ( similar to HCl ) GP25-02 Amines Amines are basic - and like most bases they can act as nucleophiles. This can cause problems when we need to synthesize amines. For example: If you attempt to prepare a primary amine by an S N 2 reaction using ammonia as the nucleophile and an alkyl halide this will not (usually) be a good synthesis: H 3 N X R H 3 N R X + H 2 N R + H 4 N H 3 N As soon as any A is formed, ammonia will react as a base to set up this equilibrium- it's a proton transfer so it's rapid, and the equlibrium constant will be ~ 1. A B The primary amine B will react with the alkyl halide to generate another ammonium salt, and then the whole process can happen again, and again... X R H 2 N R R H 3 N H N R R X R NH R R H 3 N R N R R R X R N R R R R X D a quaternary ammonium salt Reacting equimolar amounts of ammonia and the alkyl halide is likely to give a mixture of primary amine B , secondary amine C , tertiary amine D , and the quaternary ammonium salt. NOTE: This means that reaction of a primary amine and an alkyl halide is not a good reaction for the preparation of a secondary amine, nor is reaction of a secondary amine with an alkyl halide likely to be a good method for the preparation of tertiary amines. X = leaving group C In the 'real world'... ...sometimes it is possible to get reasonable yields of secondary and tertiary amines using this approach, but these are usually 'special cases' where the amine substituents or the alkyl halide are rather bulky (so the sequential S N 2 reactions get progressively much slower) or if reaction conditions are such that the first-formed ammonium salt (such as E & F above) precipitate from solution. But be warned - these above) precipitate from solution....
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This note was uploaded on 01/25/2011 for the course CHEM S20ab taught by Professor Mccarty during the Summer '10 term at Harvard.

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S20-Week06-Lecture25-29 - GP25-01 Amines Examples: H 3 C N...

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