Lab Techniques - Lecture notes MCAT Week PDF

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MCAT Organic Chemistry...

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Ch 5 Physical Properties Tuesday, November 9, 2021

9:19 AM

Physical Properties of Chemicals -

Melting point and boiling point are measures of how well molecules interact with each other

Nonpolar Compounds -

Branching of hydrocarbons is the biggest effect on IMFs - branching weakens the Van Der Waals forces between molecules (lower boiling point) Molecular weight - larger hydrocarbons have more Van Der Waals interactions and a higher boiling point

Hydrogen Bonding -

Not all electronegative-atom-containing compounds can hydrogen bond (ex. Diethyl ether) ○ They must have H bound to those electronegative atoms

Distillation -

Raising the temperature of a liquid until it can overcome the IMFs that hold it in the liquid phase, then condensing the vapor in another container ○ Simple distillation is done to remove trace impurities or to separate compounds with significantly different boiling points ○ Fractional distillation is done to separate compounds with similar boiling points § Uses a solid material in the column like beads or sponge that make the compound evaporate and condense repeatedly as it moves through the column

Spectroscopy -

General principles: ○ Molecules are excited by light and then release photos that are recorded and analyzed to determined their chemical/structural features

Mass Spectrometry -

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Used to determine the mass of the compound in a sample ○ Bombards a compound with high energy electrons then subjects it to a magnetic field Know how to read mass spec and determine: ○ Molecular weight (base peak) ○ Carbons ○ Presence of Br (100% height of base peak) and Cl (about 1/3 the height of the base peak)

UV/ Vis Spectroscopy -

Similar to IR spec, but excited the electrons more Used to monitor complexes of transition metals Used in organic chemistry to study highly conjugated systems ○ Move extensive conjugation = longer wavelength of maximum absorption § Using different wavelengths of light can help you tell how conjugated a compound is ○ Ex. If a compound absorbs only UV radiation, it appears white (reflects all visible wavelengths) ○ Ex. If a compound absorbs blue light, it appears as orange

IR Spectroscopy -

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Causes the bonds in organic molecules to vibrate at distinct energy levels (wavenumbers) that are the reciprocal of the wavelength ○ Higher wavenumber = higher frequency = greater energy Important IR patterns to know for MCAT

Functional Group Wavenumber (cm-1)

Shape

Carbonyl (C=O)

1700

Strong, sharp

Alkene (C=C)

1650

Strong, sharp

Alkyne (C=C)

2260-2100

-

Nitrile (C=N)

2260-2100

-

Alcohols (-OH)

3600-3200

Strong, broad

sp3 C-H

3000-2850

-

sp2 C-H

3150-3000

-

sp C-H

3300

-

Amine (N-H)

3150-2500

moderate

H-NMR (Proton NMR) Spectroscopy Tells us the number of equivalent protons in the molecule (# of peaks) ○ Protons in equivalent electronic environments - Tells us the number of protons in that set (splitting pattern) ○ Spin-splitting phenomenon- when nonequivalent hydrogens interact, causing the magnetic field felt be each proton to be altered ○ Splitting follows a pattern of (n+1) where n is the number of adjacent nonequivalent protons ○ Integration- done by the NMR machine mathematically, tells you the relative number of protons giving rise to each peak (sometimes does not give it to you) - Tells us the environment of the protons (chemical shift value) ○ Chemical shift- location of resonance ○ The more a protons is de-shielded, the farther it is to the left (downfield) § Protons near an electronegative group are deshielded ○ Hybridization effect- protons on differently hybridized carbons have different shift values § Alcohol protons can vary a lot in shift value Important Shifts to know for H-NMR Spec -

Proton Type

Shift value (ppm)

Aldehyde proton (RCHO)) 9-10 Carboxylic acid (RCOOH)

10-13

Aromatic (C-H)

6.5-8

Alcohols (-OH)

2-5

sp2 C-H (vinyl alkene)

5-6

Benzylic (RCH2C6H5)

2.5-3.5

RCH2X (X= NR2, OR, X)

2.5-3.75

Alkyne (C-H)

2

Alkyl (C-H)

0-2...