The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

can someone help me with this chem question on mass spectrometry

unnamed.jpg
I have gotten up to finding:
peak 1 = c4h10+
peak 2 = c3h7+
peak 3 = c2h5+

but idk what to do with these answers or what the question is fully asking me to do
so you know from I that the alkane has the formula C4H10

Peak 2 has formula C3H7, which can either be a: CH3CHCH3 or b: CH3CH2CH2
(a) implies a structure fro the alkane being identified as c: CH3CH(CH3)CH3 (or 2-methylpropane)
(b) implies a structure of d: CH3CH2CH2CH3 or butane

Peak three has formula C2H5, which must have structure CH3CH2, and as you can see from (c) and (d), this fragment can only come from d. There is no way to get this structure from c by only breaking one bond. Therefore the final answer is butane CH3CH2CH2CH3. DO you understand? Plz say if not and I'll explain in more detail
Original post by Darth_Narwhale
so you know from I that the alkane has the formula C4H10

Peak 2 has formula C3H7, which can either be a: CH3CHCH3 or b: CH3CH2CH2
(a) implies a structure fro the alkane being identified as c: CH3CH(CH3)CH3 (or 2-methylpropane)
(b) implies a structure of d: CH3CH2CH2CH3 or butane

Peak three has formula C2H5, which must have structure CH3CH2, and as you can see from (c) and (d), this fragment can only come from d. There is no way to get this structure from c by only breaking one bond. Therefore the final answer is butane CH3CH2CH2CH3. DO you understand? Plz say if not and I'll explain in more detail


aha thank you but I don't really understand why the fragment can only come from D
SO you know how fragments work right? They can (at this level) only come from the breaking one bond in the original specimen. C2H5 has to have the structure CH3CH2, there is no other s structure it could have. Therefore can you think of a way to, by breaking on bond on c or d, get CH3CH2? It might help to draw out the molecules using lines for bonds and thinking about cutting each line in turn.
Original post by Darth_Narwhale
SO you know how fragments work right? They can (at this level) only come from the breaking one bond in the original specimen. C2H5 has to have the structure CH3CH2, there is no other s structure it could have. Therefore can you think of a way to, by breaking on bond on c or d, get CH3CH2? It might help to draw out the molecules using lines for bonds and thinking about cutting each line in turn.


ohhh okay so the fragment C2H5 can only come from CH3CH2CH2CH3 because when you break the c-c bond in the middle the mass of both fragments are 29 which is peak 3? oh and you can't get C2H5 from CH3CH(CH3)CH3
This makes sense thank you so much
do you know how I would have to structure my answer? It's 5 marks
Basically how I did. Maybe a bit more explanation as to how you can draw the first conclusion about the overall formula. And then a comment at the end about why the students got different answers
Original post by Darth_Narwhale
Basically how I did. Maybe a bit more explanation as to how you can draw the first conclusion about the overall formula. And then a comment at the end about why the students got different answers


okay, thank you!

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you