A prominent (M + • -18) peak suggeststhat the compound might be a(n): a. Primary

Question

A prominent (M+• -18) peak suggeststhat the compound might be a(n):

a. Primary amine

b. Alkane

c. Ether

d. Alcohol

e. Ketone

Please explain why. Thank you so much.

Explanation / Answer

Mass spect measures the charge to mass ratio m/z. Thepositive charge is generated by the loss of an electron when thecompound is ionized. What combination of atoms adds up to 18? a.) Primary Amine: NH2 = 16, but in mass spect after fragmetationthe nitrogen can pick up another hydrogen giving a m/z ratio of17. This is not the answer. b.) Alkane: smallest fragment ---> CH4 = 16. This would not be especially prominent and is too small. Thisis not the answer. c.) Ether: Relevant fragment = -OCH3R. We canalready see that this is too big. This is not the answer. d.) Alcohol: if the -OH group of the alcohol is cleaved, thanthe oxygen picks up another hydrogen and becomes HOH, or H2O whichis equal to 18. O + H + H = 16 + 1 + 1. This is thecorrect answer. In your book, it should tell you thatprominent peaks at 18 represent a water molecule which is atell-tale sign of an alcohol group that has been cleaved... e.) Ketone: in its simplest/lightest form it would beacetone --> CH3(CO)CH3 = 58. Much too big. Additionally, formaldehyde H(CO)H is even smaller but wouldregister as bigger than 18. This is not the answer.

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