Proton NMR/IR Spec signal values we need to know

[GomerPyle]

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Regarding NMR/IR signals, it has been said that you only need to know that aldehyde protons are at 9-10 ppm, COOH protons are at 11-12 ppm, and the wide OH infrared signal is at 3300 sec^-1 (N-H around that same area).

EK states that this is all you need to know, but this is wrong. I know this because I have taken the MCAT multiple times. You need to have memorized other important signal points. Anybody care to share which ones are the important ones?

 

[milski]

Carboxyl groups at 1700 cm^{-1 and OH at wide band around 3300 cm -1. I cannot remember any AAMC material which could not have been solved with what you listed plus these two.}

 

[GomerPyle]

We do need to know alkyl, vinyl, and benzyllic groups...just letting you guys know!!

 

[jakernewt]

Does it go as follows?

Benzyllic - 7-8
Vinylic - 4-5
Alkyl - 2-3 (unless pulled downfield by e neg atom)

 

[gettheleadout]

IR:

C-O : Low 1000's (1300 cm-1)
C=O : High 1000's (1700 cm-1)
C≡N : Low 2000's (2250 cm-1)
O-H : Low 3000's (2500 [acids] - 3500 [alcohols] cm-1)

H-NMR:

Almost All Values Are A little Arbitrary

Acid: 10-12 ppm
Aldehyde: 9-10 ppm
Ar-H: 7-8 ppm
Vinylic: 5-6 ppm
Alkoxy: 3.5-4 ppm
Alpha: 2-2.5 ppm

🙂

 

[BerkReviewTeach]

IR:

C-O : Low 1000's (1300 cm-1)
C=O : High 1000's (1700 cm-1)
C?N : Low 2000's (2250 cm-1)
O-H : Low 3000's (2500 [acids] - 3500 [alcohols] cm-1)

H-NMR:

Almost All Values Are A little Arbitrary

Acid: 10-12 ppm
Aldehyde: 9-10 ppm
Ar-H: 7-8 ppm
Vinylic: 5-6 ppm
Alkoxy: 3.5-4 ppm
Alpha: 2-2.5 ppm

This is EXACTLY what you should know. Not too much to memorize and it flows. Great job GtLO!

 

[jakernewt]

I also recall working a few passages that required us to know the IR values for an alkane, alkene, alkyne C-C stretch.

Alkyne = about 2100- 2200
Aklene = about 1650
Alkane = somewhere lower around 1100 or so I believe

 

[BerkReviewTeach]

I also recall working a few passages that required us to know the IR values for an alkane, alkene, alkyne C-C stretch.

Alkyne = about 2100- 2200
Aklene = about 1650
Alkane = somewhere lower around 1100 or so I believe

You can estimate an alkene from a carbonyl. C=O is a stronger bond than C=C, so it takes a little more energy to stretch (1700 cm^-1 > 1600 cm^-1). Because a triple bond is stronger than a double bond, an alkyne absorbance is going to be above 1700 cm^-1, and 2000⁺ cm^-1 is a good enough guess for any MCAT question.

The point being that memorizing a little and learning to extrapolate from that small pool of facts is incredibly helpful on the MCAT. Know a little; think a lot!

 

[GomerPyle]

This is great - thanks guys!

 

[MTHFR]

IR:

C-O : Low 1000's (1300 cm-1)
C=O : High 1000's (1700 cm-1)
C≡N : Low 2000's (2250 cm-1)
O-H : Low 3000's (2500 [acids] - 3500 [alcohols] cm-1)

H-NMR:

Almost All Values Are A little Arbitrary

Acid: 10-12 ppm
Aldehyde: 9-10 ppm
Ar-H: 7-8 ppm
Vinylic: 5-6 ppm
Alkoxy: 3.5-4 ppm
Alpha: 2-2.5 ppm

🙂

This is cool.

 

[TotalDomination]

IR:

C-O : Low 1000's (1300 cm-1)
C=O : High 1000's (1700 cm-1)
C≡N : Low 2000's (2250 cm-1)
O-H : Low 3000's (2500 [acids] - 3500 [alcohols] cm-1)

H-NMR:

Almost All Values Are A little Arbitrary

Acid: 10-12 ppm
Aldehyde: 9-10 ppm
Ar-H: 7-8 ppm
Vinylic: 5-6 ppm
Alkoxy: 3.5-4 ppm
Alpha: 2-2.5 ppm

🙂

Sorry for the almost all values, is the V coming one too early?

 

[Deadlifts]

Sorry for the almost all values, is the V coming one too early?

He did descending values for the letters. If you read it in increasing ppm values (start on the Alpha side), it reads correctly with the mnemonic,

A,A,V,Ar,Al,A

 

[GomerPyle]

According to a question in kaplan, we also need to know the C-H stretch (which is a sharp, intense) peak at 2800 cm-. Anybody care to chime in as to why it is at 2800?

Oh, and that alcohols on the NMR are around 5-6 ppm....

 

[TotalDomination]

This is EXACTLY what you should know. Not too much to memorize and it flows. Great job GtLO!

sorry again, what is alpha??? i'm so lost on this

 

[gettheleadout]

sorry again, what is alpha??? i'm so lost on this

Hydrogens on a carbon that is adjacent (think adjacent = alpha) to a carbonyl.