HNMR Flashcards
What are exchangeable protons?
Name a few of the most common:
hydrogen atoms in a molecule that can readily swap with other hydrogen atoms from the solvent
–> Hydrogen in alcohol (bonded to O)
–> Hydrogen in amine (bonded to N)
–> Hydrogen in carboxylic acid (bonded to O)
What is the issue with exchangeable protons?
They are unreliable!
Sometimes won’t show up on the NMR spectra OR will appear in a very broad range!
What are the components of an H-NMR spectrum that we can get information from?
1) Resonance (# of peaks)
2) Integration (area under the peaks –> Gives a ratio)
3) Chemical shift (location of the peaks)
4) Peak splitting
What does H-NMR resonance tell us?
Resonances = # of peaks present
represents the # of UNIQUE hydrogen environments!!
What does the integration in H-NMR tell us?
**Tells us the relative # of hydrogens in a given environment **–> Gives a RATIO
–> Depending on the chemical equation, that ratio may represent the exact # of hydrogens in each environment
OR the ratio may be utilized to calculate the exact # of H’s in each environ.
What does peak splitting tell us?
The number of splits in a peak tells us the # of adjacent hydrogen atoms to the hydrogens in the environ that the peak represents
Follows the (n + 1) rule
# of splits = # of adjacent hydrogens + 1
SOOOO, the # of adjacent hydrogens = # of splits MINUS 1
If a peak is a quintet, how many adjacent hydrogens does that environment have?
FOUR
If a hydrogen in one environ. has two adjacent hydrogens, how many splits will the peak have?
3 peaks!
(1 more than the number of adjacent hydrogens)
What does chemical shift tell us?
Chemical shift values for a given peak can tell us what might be in the vicinity of a given hydrogen
–> Gives hints for the TYPE of environ. the hydrogens might be in for a given peak!
–> More downfield (deshielded) = LEFT (higher #s)
–> More upfield (shielded) = RIGHT (lower #s)
What can impact chemical shift?
electronegative substituents!
–> Typically, the closer a hydrogen is to a more electronegative atom, the more downfield (LEFT) its chemical shift will be!
How does chemical shift change with electronegativity?
The more electronegative an atom near or bonded to a hydrogen is, the more downfield the peak will be!
–> Additionally, the > # of electroneg substituents, the greater the deshielding effect!
How does distance from electronegative atoms impact chemical shift of a hydrogen environ?
The farther away a hydrogen is from an electroneg atom, the less it is deshielded (feels the effects less) and is therefore more to the RIGHT
