IR help

[Zahta]

okay i am trouble with IR  spec i  can identify the peaks  c=o is  present or o-h ,c-h aswell as  c-c  but how do  i know which type of carboxylic acid it is,  how many carbons would be present?

[Hellrocks]

Are you asking how do you know the specific molecule?
The answer is you can't. You often require other analytical techniques to give you clues about different features of a molecule.
IR is mainly used for the identification of functional groups.

[Zahta]

yeah whether it is ethanoic acid or methanoic acid

[thushan]

you'd differentiate the two using the fingerprint region. you'd have to compare to known samples of HCOOH and CH3COOH though if you wanted to use the fingerprint region

[pi]

you'd differentiate the two using the fingerprint region. you'd have to compare to known samples of HCOOH and CH3COOH though if you wanted to use the fingerprint region

btw, the fingerprint region is like between 500 to 1500 cm-1 (it varies per textbook definition, but this is what I used) for those unsure

[Zahta]

yeah i know the finger region thanks  okay could you guys help me ith this question,  so i can  identify the molecule. also can i buy the  atar notes book online.     

[pi]

Methanoic acid?

[Zahta]

but how do you know its methanoic acid . im so confused

[Ravit]

The heading of the IR spectrum states it is "for a carboxylic acid" this identifies the homologous series to which the unknown molecule belongs to.
In the question stem it states that the molecule "only has one carbon atom in its molecular structure" hence the molecule must be meth- something.
And you have identified 3 peaks outside the fingerprint region, O-H, C=O, C-H, using the knowledge of the presence of these three peaks, which are characteristic of carboxylic acids and that the molecular structure has only one carbon atom.
You should see that the unknown molecule is methanoic acid.
Hope this helps.

[pi]

^Yep, generally, it is best to ignore peaks in the fingerprint region (ie. the C-C peak you circled isn't very accurate).

[Zahta]

thank you guys so much i finally get it , took me a while though, but thanks alot.

[thushan]

yup you can buy it online - go to eStore