According to the French orthography page on Wikipedia, -om becomes ɔ̃ before a consonant (m in this case). So why is ɔ̃ not allowed? Is it because of some syllable breakdown rule that I am not aware of or is it because while m is technically a consonant, the French orthography rule doesn't treat it like one?
This is French. Rules have exceptions. A vowel followed by N or M which is followed a different consonant or at the end of the word becomes a nasal vowel. There are a few exceptions:
-mn does not lead to nasalization, the final
-ums) is pronounced /ɔm/, recent imports from foreign languages are not nasalized, and probably a few more I can't think of right now.
A double consonnant does not nasalize the preceding vowel except in the few cases where it does. The only exception I can think of is when the prefix
-en was added to a word beginning with
m (in the latter case, the word is spelled
emm-), with some fluctuation when
en- was derived from the Latin prefix
in-. Here are a few examples which are nasalized:
But in other words the usual rule applies (
e + double consonant is pronounced with an open /ɛ/ (è sound), occasionally a closed /e/ (é sound)):
emmental /ɛ.mɛ̃.tal/ or /e.mɛ̃.tal/
Emmanuelle /ɛ.ma.nɥɛl/ or /e.ma.nɥɛl/
emm is pronounced /am/ in femme and in adverbs that end in
This is the expected pronunciation, along with [co.mɑ̃.dɑ̃] and variations in the way /ɑ̃/ is realized.
Most nasal vowels directly followed by a nasal consonant (i.e. N or M) lost their nasality several centuries ago.