Ir inactive problem

[super112]

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If between two answer choice where one is an alkane and the other IR inactive alkane. Which would have the weakest IR signal? Does the IR inactive even show up on the graph or do we ignore it and choose the regular alkane? Thanks

 

[DAT Destroyer]

If between two answer choice where one is an alkane and the other IR inactive alkane. Which would have the weakest IR signal? Does the IR inactive even show up on the graph or do we ignore it and choose the regular alkane? Thanks

A molecule that is IR inactive still gives an IR spectrum...but not for the functional group ! Alkanes give no signals beyond 3000.There are ways to discern alkanes in IR graphs.....however this is well beyond the scope of the DAT exam. The best bet is to look at the FINGERPRINT region.....this the area around 1400-600cm-1. Just like human fingerprints......alkanes have different fingerprints ! Thus,,,,,we can make a positive ID by comparing the fingerprint regions. For the DAT exam,,,,,know numbers for functional groups such as alkenes (1650)....carbonyls 1720.....nitriles and alkynes...2200........alcohols 3200 to name the lions share of them ! Also know that the alcohol signal is usually broad,,,,,,and a primary amine is 2 small spikes, while a secondary amine...1 spike,,,,,not broad.......all at around 3200. Also know that conjugation LOWERS THE IR numbers..... (Consult an orgo text to see these shapes....alcohols vs. amines)....

Hope this helps !

Dr. Romano