Naming alkyl halides

[kov82]

Hi guys, having some problems with "assigning" priority for names, I have no problem naming, BUT numbering sometimes messes me up when there are two different halogens attached, so one molecule 2-bromo-1,1,1-trichloroethane I got right on my practice but naming another one I got wrong, the book said the answer was...... cis-1-bromo-2-chlorocyclobutane I switched the 1 and 2 (so I said cis-2-bromo-1-chlorocyclobutane), but the book said thats wrong, is it the same? or in the first problem is it because there were three Cl's that it was given carbon #1 (more substituents)? what if theres one bromo on one end and one Cl on the other then bromo gets number 1? because its based on weight?

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[peyman2002]

ok here's how it goes:

for the first one that you got right, you number them starting from the most substituted carbon, so triCl gets 1.
for the cyclobutane, bromo gets 1, because both bromo and chloro carbons have only 1 hydrogen attached and each is also attached to 2 other carbon, thus carbons of bromo and chloro have the same number of substituents, so now since bromo starts with B and chloro starts with C, you should give the bromo carbon 1 and chloro carbon 2 resulting in cis-1-bromo-2-chlorocyclobutane.
for your last question bromo gets 1 because it starts with B

 

[kov82]

wow I can't believe they just based it on alphabetical order, thanks man, I had forgotten and I couldn't find it in the book, I realized the first one had to do with the number of substituents but couldn't figure the other out, so something like 1-iodo-2-methylcyclopentane cannot be called 2-iodo-1-methylcyclopentane? either way you would draw out the correct molecule, but is the second one considered wrong because its not numbered in alphabetical order? or professors/tests accept both?