proton NMR question

[capn jazz]

Which compound shows a 2H quartet in its 1HNMR spectrum?

A) 2-chloropentane
B) 3-chloropentane
C) 2,2-dichloropentane
D) 3,3,-dichloropentane

I picked A because the 3rd carbon has 2 hydrogens adjacent to 1 H on the 2nd carbon and 2 H on the 4th carbon, for a total of 3 adjacent Hs, which makes a quartet.

The answer is apparently D. "A 2H quartet results from a CH2 group adjacent to a CH3 group on one side and a carbon with no hydrogens on the other side. In 2-chloropentane there is no isolate CH2 group, so choice A is eliminated.......

......In 3,3-dichloropentane, there are 2 CH2 groups adjacent toCH3 groups, and each is adjacent to a CCl2 group (which has no Hs). The result is that the two CH2 groups are isolated quartets, making D the correct answer."

If there are two EQUIVALENT CH2 groups in 3,3-dichloropentane (it's symmetrical) then would that result in ONE peak on 1HNMR, and it would be a 4H quartet? And am I crazy or is there a 2 H quartet in 2-chloropentane?

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[Doodl3s]

the NMR for D will yes have ONLY ONE PEAK!... but it will be double the height of the peaks for the other compounds (rather have double area under the curve)
on the other hand, for A the hydrogens you were using to get your answer are not all equal, so the hydrogen you looked at will be split into a doublet by the one hydrogen and a triplet by the 2H on the other side, So the Hydrogens you're looking at will actually be a Doublet of a triplet, not a quartet.... A quartet means the H you're looking at is next to 3 IDENTICAL hydrogens, hope that helps...

 

[capn jazz]

Thanks! I vaguely remembered something about doublets of triplets and things like that.

 

[phEight]

Doodl3s, I'm still not understanding why it wouldn't be a 4H quartet. Shouldn't the integration indicate 4H instead of only 2H?

 

[loveoforganic]

In order of the IUPAC numbering

for A, you'd have a doublet with an integral of three, a doublet of quartets with an integral of one, a triplet of quartets with an integral of two, and a triplet with an integral of three.

for B, you'd have a triplet with an integral of six, a doublet of quartets with an integral of four, and a triplet with an integration of one

for C, you'd have a singlet with an integration of three, a triplet with an integration of two, a doublet of quartets with an integration of two, and a triplet with an integration of three

for D, you'd have a triplet with an integral of three, and a quartet with an integral of two

The reason for the integrations in D is the symmetry of the molecule.

 

[Doodl3s]

Doodl3s, I'm still not understanding why it wouldn't be a 4H quartet. Shouldn't the integration indicate 4H instead of only 2H?

I mean, you COULD look at it that way... but When have you seen a carbon bonded to four hydrogens AND a carbon with three hydrogens on it (thats five bonds)...

Either way theres a hint to tell you it cant be 4H, and so 2H must be what it is... even if it does happen to be TWO 2H quartets, the question didnt say there was only one