OAT Organic Chemistry Spectroscopy and Lab Techniques

Infrared Spectroscopy

Commonly used to identify the functional groups present in a molecule

How does infrared spectroscopy work?

Every bond has a unique vibration frequency in the infrared region of the EM spectrum. Bonds absorb radiation (in the form of light) that has the same frequency as their frequency of vibration. However, for vibrations to occur for a given bond, it must have a net dipole moment. Transmittance levels are used to determine unique functional groups

_______________ bonds such as in O2 and N2, as well as the carbon-carbon bonds in C2H2 and C2H4 do not have a dipole moment and therefore do not show up on an IR spectrum

Symmetrical

On an IR spectrum, which axis is the wavenumber on?

X-axis

On an IR spectrum, which axis is the percent transmittance on?

Y-axis

What does it mean if the transmittance is very low?

It means that light did not make it to the detector and was absorbed by the sample

What is the absorption frequency and the appearance of -OH on an IR spectrum?

Absorption Frequency: 3200-3500

Appearance: Large, broad peak

What is the absorption frequency and the appearance of NH on an IR spectrum?

Absorption Frequency: 3200- 3500

Appearance: Sharp peak (secondary amine has 1 peak while a primary amine has 2 peaks)

What is the absorption frequency and the appearance of -COOH on an IR spectrum?

Absorption Frequency: 2500-3500

Appearance: Broad jagged peak

What is the absorption frequency and the appearance of a nitrile (C triple bond N) on an IR spectrum?

Absorption Frequency: 2200-2250

Appearance: Medium peak

What is the absorption frequency and the appearance of C=O on an IR spectrum?

Absorption Frequency: 1700-1750

Appearance: Large, sharp peak

What is the absorption frequency and the appearance of aromatic (C=C) on an IR spectrum?

Absorption Frequency: 1450-1600

Appearance: Medium peak

NMR Spectroscopy

Used to determine the molecular structure of a compound

What are the two most important types of NMR spectroscopy?

1. H NMR (proton NMR)

2. C NMR (carbon NMR)

When are protons equivalent?

When they exsist in the same chemical environment

When are protons non-equivalent?

When they exist in different chemical environment

In H NMR spectroscopy, do equivalent or non-equivalent protons in a molecule generate a distinct signal?

Non-equivalent protons generate a distict signal.

What can make protons chemically equivalent even if they are bonded to different carbon atoms?

Internal lines of symmetry

Will a proton with less electron density around it be considered de-shielded or shielded?

De-shielded (it will also be downfield/to the left on an NMR spectrum)

Why are protons next to electronegative atoms considered de-shielded?

Because the electronegative atom will withdraw electron density

What does the multiplicity of a proton's H NMR signal indicate?

The number of non-equivalent neighboring protons

What rule can be used to predict the multiplicity?

the n+1 rule (n = # of nonequivalent neighboring protons)

What is integration for a NMR signal?

A number that is above each NMR signal that indicates the relative number of protons giving rise to each signal

In C NMR spectroscopy, do chemically equivalent or nonequivalent carbons generate a distinct signal?

Nonequivalent carbons generate a distict signal

Are carbons near electronegative atoms considered deshielded or shielded?

Deshielded

A compound's molecular formula can be used to determine how many __________________ it has

Degrees of unsaturation

A fully saturated hydrocarbon contains the maximum number of ___________ atoms with the molecular formula of ____________

hydrogens; CnH2n+2

Each degree of unsaturation indicates ___ less hydrogens are present compared to the maximum

2

A pi bond or ring will cause ______ degree(s) of unsaturation. Why?

One because they both reduce the number of hydrogen atoms in a compound by 2

What can be determined by finding the degree of unsaturation?

If there is a presence of pi bond or ring

Formula for degree of unsaturation

Degrees of Unsaturation = (2C + 2 + N - X - H)/2

Mass Spectroscopy

Used to determine the mass of a compound

What happens to the sample before mass spectroscopy?

The sample is vaporized, ionized, and smashed into molecular fragments and then sent through a long tube where they are deflected to varying degrees by a magnetic field and separated according to their mass to charge (m/z) ratio

What does a charge of +1 mean in correlation with mass spectroscopy?

it means that its mass to charge ratio is represented of the fragment's mass

What is the molecular ion peak?

The tallest peak in the rightmost section of the spectrum that represents the compound's molecular mass.

Ex: the molecular ion peak of acetone is generated by acetone that is ionized but intact. The peak is located at 58 because that is the molecular mass of acetone.

What is the base peak?

The tallest peak in the entire spectrum which corresponds to the most stable molecular fragment (designated as 100% intensity, and the intensities of all other peaks are reported relative to this value)

What is UV-Vis spectroscopy used for?

To analyze molecules with conjugated pi bonds (which absorb light in the UV or visible light spectrum

What does higher absorption mean?

That the molecule has more conjugated pi bonds

How does conjugation stabilize a molecule?

By reducing its overall energy

Molecules with _________ ___________ will absorb longer wavelengths of light

greater conjugation

pigment beta-carotene (a highly conjugated molecule) has a maximum absorption of _______________

400- 500nm

What is the apparatus used for liquid-liquid extraction?

Separatory funnel

How does a separatory funnel work?

The funnel is filled with two solvents that are immiscible (they cannot mix). In most cases, one solvent will be hydrophobic and will form the organic layer while the other will be hydrophilic and form the aqueous layer. The compounds between layers will be shaken within the funnel to allow migration of the compounds between layers and then they are allowed to separate again. The stopcock on the funnel is opened to drain the bottom layer of solvent. The top layer is poured out through the top of the separatory funnel.

What are the 3 common applications of liquid-liquid extraction?

1. Used to separate two neutral compounds with different polarities

2. Used to separate two compounds based on acidity

3. Used to separate two compounds based on basicity

Thin Layer Chromatography

Separates compounds based on differing polarity/intermolecular forces

Is thin layer chromatography used for small or large samples?

Small

How does the solvent move up the TLC plate?

Capillary action

Do non-polar or polar compounds move up the TLC plate faster?

Non-polar

Why do non-polar compounds move up the TLC plate faster?

Because the solid absorbent on the TLC plate is polar (the more polar the compound, the more it will interact with the silica on the TLC plate and the slower it will travel)

Hydrogen bond donors move especially _______ because they interact with the silica gel

slow

What is the retention factor (Rf)

it is equal to the distance the compound traveled divided by the distance the solvent traveled

The compound that moves the furthest in TLC (the least polar compound) will have the __________ retention factor?

largest

The Rf value is inversely proportional to the _____________________

polarity

Column chromatography

Uses the same principles as TLC but functions to separate components in a mixture rather than monitor a reaction over time

Less polar compinds with weaker IMFs come out of the column _______

First

Gas-liquid chromatography

Used to determine the relative abundance of compounds in a liquid mixture

Components with ________ boiling points travel slower. Why?

higher boiling points move slower because they are more likely to condense in the column

What is retention time?

The time it took a compound to pass through the column

A compound with a higher boiling point will have a _______ retention time

longer

Distillation

A technique used to separate two miscible liquids with differing boiling points

Recrystallization

A method used for purifying solids