Spectroscopy Flashcards
Absorption
Electron is excited to a high-energy orbital (HOMO to LUMO)
Emission
Electron relaxes back to a lower-energy orbital (LUMO to HOMO)
UV-Vis Spec is good to visualize _____?
Pi Electrons. Will promote an electron on a pi-bonding orbital to a pi antibonding orbital.
Spectroscopy
The interaction between light and matter
IR Spectroscopy induces _?
Symmetric and asymmetric stretching and bending of bonds
What bond is seen at 2840-3000cm^-1?
saturated C-H on alkane
What bond is seen at 3200-3650cm^-1
broad and rounded O-H stretch
What bond is seen at 1630-1780 cm^-1
C=O stretch (narrow and pointed)
On IR Spectrum, what does the x-axis and y-axis display?
X-axis - wavenumber
Y-axis - transmittance
In HNMR looks at?
Chemical shift
Integration
Splitting Patterns
What is the reference peak on NMR spetra?
Tetramethylsilane (TMS) at 0 ppm.
What happens if a proton is close to an electronegative atom?
It will have its electron density withdrawn or deshielded. Peak will be futher downfield (to the left)
Integration values
Relative to a reference peak
The higher the peak, the more chemically equivalent protons.
When evaluating splitting patterns
Neighboring protons will create singlets, doublets, and so on. Each resonance is split into (n +1) peaks n = # of neighboring protons.
Shifts for benzene rings in HNMR
Will be jumbled together usually seen between 7-8ppm.
How do you calculate degrees of unsaturation in hydrocarbons?
(max #H atoms - actual # H atoms)/2
fully saturated hydrocarbons: CnH2n+2
How to calculate the degrees of unsaturation in molecules with heteroatoms
DOF = (2C + 2 + N - X - H)/2
Value for hydroxyl/amino protons in HNMR?
0.5-5.5ppm
Value for sp3 C protons in HNMR?
1-5ppm
Value for sp2 C protons in HNMR?
4-6ppm
Value for aromatic protons in HNMR?
6-8ppm (identifies 4 DOF)
Value for aldehyde/phenol protons in HNMR?
~10ppm
Value for carboxyl/amide protons in HNMR?
12-14ppm
Value for esters/amides/carboxylic acid carbons in 13CNMR?
160-180 ppm
Value for aldehydes/ketone carbons in 13CNMR?
200-280ppm
Value for sp3 alkane carbons in 13CNMR?
10-50ppm
Value for sp2 alkene carbons in 13CNMR?
100-155ppm
Value for aromatic carbons in 13CNMR?
120-180ppm
Chromatography separates substances on the basis of differing:
Polarity
The (blank) polar a substance, the more (blank) it will travel up a TLC plate.
More; Slowly
Rf value must be between what values?
0;1
Extraction separates liquids on the basis of differing:
Solubility
Which substance would be most useful in separating benzoic acid and napththalene?
acqueous base like NaHCO3
In Gas Chromatograhy, what factors cause a lower retention time?
Lower BP
Lower solubility
Hotter column
Distillation separates liquids on a basis differing in:
Boiling points
After recrystallization, melting point analysis can be performed. How do you know you have a pure substance based on melting values?
Narrow range clsoe to tabulated value.
What kind of solvent is ideal for recrystallization?
Low solubility at room temperature and high solubility at high temperatures. Impurity should have high solubility at room temperature or insolubility at high temperature to remove while boiling
What is Tollen’s test?
Mainly used to test for aldehydes. In the reaction, silver ammonia complex ions react with aldehydes to form solid silver and carboxylate ions. The indicator of a positive result is a silver layer along the inside of the test tube.
