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Cherrie Baby:

The method mentioned by Rhodium is by Hartman & Dreger. Org. Syn. Coll. vol. 1, 
pp 357-360. and Org, Syn. vol 10.

It is possible to make CH2Br2 with just DCM and Br- salt or HBr gas - I'm not 
sure of the details. There are US patents from the '50's or '60's that detail 
it. I couldn't find the refs. in my notes. 

Looking at "Phase Transfer Catalysis" by Starks, Liotta and Halpern (Chapman & 
Hall, 1994) I open book on page 349 to find three pages of info on the subject 
of "Bromide-Chloride Exchanges" with 21 references. I read [you may hear more 
about this later.]

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Profuse apologies for the lost US patent. I never photocopied it 'cos it was on 
microfilm. Now I've lost my notes. The Merck gives the patent number as 
US 2553518 (1951 to Dow). I think I remember that they passed HBr gas into DCM 
to get CH2Br2 and CH2BrCl

As far as phase-transfer catalysis goes I can find no specific mention of the 
reaction needed, but it looks like it would work with DCM, LiBr and Aliquat 336.

Summary of relevant part (from :"Phase Transfer Catalysis" by Starks (1994)

...An equilibrium is established using a phase-transfer catalyst:

RCl (neat org) + M+Br- (aq) <=> RBr + M+Cl-

Cation dependence results from unequal (or non-proportional) solubilities of 
MCl and MBr for different kinds of M+ in the aqueous phase. The degree of anion 
hydration affects both the transferability of halide from aqueous to organic 
phase and the reactivity after transfer.

The equilibrium position of the PTC reaction of a primary alkyl chloride with 
M+Br- (M = Li, Na, K, Ca) to give the alkyl bromide was found to depend 
strongly on the nature of the cation M+, and the amount of water present [1]. 
RCl was converted to RBr in 80-94% yield by treatment with slight excess of 
LiBr containing 1% water and 5 mol% Aliquat 336 at 85°-110°. RBr was converted 
to RCl by treatment with 25% aq. solutions of metal chlorides containing 
Aliquat 336 at 95°. Equilibria data is available [2]. Bulky quaternary salts 
are the preferred catalysts[3,4,5] although even Cetrimide will work[6]

References:
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[1] Sasson et al. Israel J. Chem. Soc., Chem. Commun. (16) 1250-2 (1986)
[2] Yonovich-Weiss & Sasson. Israel J. Chem. 26, 243-7 (1985)
[3] Forster, J. Chem. Soc. Chem. Comm. 918 (1975)
[4] Starks & Gordon. US pat. 3899542 (1975)
[5] Gordon US pat. 3696084 (1972)
[6] Jursic & Branko, Chem. Res., Synop. (4), 104-5 (1989)

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