[sashikanth]

Dear friends,
I have aquestion to all. I am doing a condensation reaction between 1-bromo-2-chloro ethane and cyclic secondary amine. In result i am getting dimer instead of monomer i.e.,chloro derivative.
can any one suggest a good reaction conditions to get single chloro derivative compound but not dimer.

[kiwi]

what do you mean by "dimer", are you getting attack from an amine at each end to give a bis(tertiary amine), or two attacks by the same N to give a quaternary ammonium salt? and what ratio of amine/electrophile are you using?

[kremar]

amine synthesis by aliphatic nuclephilic sustitution has this downside: they usually go all the way to the quaternary ammonium salt.

if you insist on going this way, i think that unless you change your reactives to make them less reactive once the tertiary amine is formed (like adding electron withdrawing groups to the amine, the alkane or both, and removing them on a later step).

cant you do a reductive amination?

[Custos]

In this case it's even worse. Your first product is a chloroethyl tertiary amine - and it will suffer increased electrophilicity from the neighbouring group effect (see below - I've used piperidine for the cyclic secondary amine). That's why mustard gas is so reactive. It will be very hard to recover this product using the synthesis you have proposed. Even if you make it, the product will be very susceptible to hydrolysis and attack by any nucleophile.