CHEM 2510 Prelim 1 Real

How to assess purity?

melting point and apperance

What to look for in compound apperance?

color, crystal form, winking

What to look for in MP analysis?

If there is a broad, lower range, the compound is impure because crystalline compounds tend to have a sharp characteristic melting point

What is the SN1 Mechanism?

leaving group departs first —> form a flat carbocation intermediate —> then attacked by a nucleophile

Best in polar protic solvents

rate = k[substrate]

How to calculate percent yield?

% Yield = (Actual Yield ÷ Theoretical Yield) × 100

Theoretical Yield = moles of limiting reactant × mole ratio × molar mass of product.

Thin Layer Chromatography (TLC) Basics

Compounds partition between stationary (polar) and mobile phases, traveling at different rates based on polarity

Polar interact with silica & travel less; less polar travel farther

Why do Thin Layer Chromatography (TLC)?

understand seperation & identification of a mixture of organic compounds (compare to standards, follow rxn in real time, asses purity)

Why use IR? What are the key groups?

Good for identifying functional groups

Alcohol (smooth tongue); Sp3- CH3 stretches (no smooth tongue); Alkene: sp2 C-H stretched

Why is it important to wash the product with copious amounts of water? What are you removing?

To remove water-soluble impurities, such as residual reagents, inorganic salts, acids/bases, and solvents, while leaving the desired water-insoluble product behind

Stationary phase vs Mobile phase

Stationary phase: Polar silica gel (H-bond donor/acceptor) adhered to plastic; does not move.

Mobile phase: Solvent that moves up the plate via capillary action.

TLC Procedure

1. Make sample: Dissolve small amount in solvent.

2. Prep chamber: Add 0.5–1 cm solvent + filter paper. Close lid.

3. Draw line: Pencil line 1.5 cm from plate bottom.

4. Spot sample: Tiny dot on line using capillary. Let dry.

5. Run plate: Put in chamber, solvent below pencil line. Close lid.

6. Remove: When solvent near top, take out. Mark solvent front ASAP.

7. Visualize: Dry plate → circle visible spots → check UV → circle more spots.

TLC visualization

How does UV light help us see spots on a TLC plate?

TLC plate glows green under UV . Some compounds absorb UV light and block glow, making dark spots appear.

Which compounds can be seen under UV light on TLC?

Compounds with conjugated DB (pi system)/ absorb 254 nm are usually visible —> absorb UV light

Compounds without UV-absorbing groups are often invisible, require chemical stain

How does iodine vapor help visualize TLC spots?

Iodine sticks to (forms complexes with) many organic compounds, making them appear as brown spots.

What does potassium permanganate (KMnO4), “Olefin dip”, stain detect on TLC?

Compounds that can be oxidized, (alkenes, alkynes, alcohols, aldehydes)

Appear yellow/brown against magenta

What does phosphomolybdic acid (PMA) stain detect on TLC?

Many different organic compounds. It is a general stain that makes spots appear green/brown.

What does p-anisaldehyde stain detect on TLC?

Nucleophiles and oxygen-containing compounds. Spots appear in different colors against a pale pink background

Retention Factor =

distance traveled by a compound / distance traveled by solvent

good value is around 0.2-0.6

2 spots with different Rf values ____ be the same compound

cannot

2 spots with the same Rf values ____ be the same compound

might

In normal TLC, how does compound polarity affect Rf?

More polar compound → sticks more to polar silica → travels less → LOWER Rf

Less polar compound → sticks less → travels farther → HIGHER Rf

How does increasing solvent polarity affect Rf in TLC?

More polar solvent → competes with compounds for silica → compounds travel farther → HIGHER Rf

Less polar solvent → compounds stick more → LOWER Rf

What is the polarity order of common TLC solvents?

Hexanes (nonpolar) < Hexanes + 10% ethyl acetate (somewhat polar) < Ethanol/Methanol (very polar; H-bonding)

How do you adjust the TLC solvent if spots travel too far or barely move?

Spots barely move (low Rf) → increase solvent polarity

Spots travel too far (high Rf) → decrease solvent polarity

Column Chromatography Procedure

1. Pack column: Cotton at bottom → silica gel (80% full) → thin sand layer on top.

2. Wet column: Add running solvent to soak silica. Never let silica dry.

3. Load sample: Add dissolved mixture carefully on top of silica.

4. Run solvent: Add more solvent; use gentle air pressure to push through column.

5. Collect fractions: Collect liquid in separate numbered tubes (2–5 mL each).

6. Run TLC

Column Chromatography Mechanism

Silica (stationary phase) = polar, attracts polar compounds

Solvent (mobile phase) flows down column, carrying compounds.

LESS polar compounds stick weakly to silica → move faster → exit column FIRST.

MORE polar compounds stick strongly to silica → move slower → exit column LAST.

More polar solvent competes with compounds for silica → helps compounds move faster.

Why is TLC useful both before and after column chromatography?

Before: Find best solvent system to separate compounds (different Rf values)

After: Check if compounds were successfully separated and are pure (one spot per compound)

How does a steam bath work, and why does hose placement matter?

Steam enters top → heats bath → condenses into water.

Water exits bottom → drains away.

Air/N₂ gently blows over sample → helps solvent evaporate faster.

Wrong hose placement → water builds up → flooding and hot water splashing.

What is the general procedure for liquid-liquid extraction?

Prepare: Add mixture + 2 immiscible solvents (organic solvent and water) to separatory funnel.

Stopper, invert, shake gently, and vent frequently to release pressure.

Let layers settle. Identify organic vs. aqueous layer (Water drop test)

Remove bottom layer, then collect top layer separately.

Extract unwanted layer again with fresh solvent to recover more product.

Combine layers containing desired product.

Wash organic layer to remove impurities.

Add anhydrous MgSO₄ to remove leftover water.

Filter: Remove drying agent.

Evaporate: Remove solvent to isolate product.

Purpose of liquid-liquid extraction?

To separate desired product from impurities based on differences in solubility between two immiscible liquids (organic solvent and water).

- Organic layer: contains desired organic product.

- Aqueous layer: contains water-soluble impurities (salts, acids, bases).

Reduction

Gain C–H bonds, lose C–O bonds

Example: Aldehyde → Primary alcohol (C=O becomes C–OH).

Oxidation

Lose C–H bonds, gain C–O bonds

Example: Primary alcohol → Aldehyde → Carboxylic acid

Which organic solvents are miscible with water, and why is this important for extraction?

- Most nonpolar organic solvents: Do not mix with water → form 2 layers.

- Small, polar solvents: Often mix with water → form 1 layer.

- Water-miscible examples: Acetic acid, acetone, acetonitrile, DMSO, DMF, dioxane.

What is the partition coefficient (Kᴅ) in liquid-liquid extraction?

Measures how a compound distributes between two immiscible solvents at equilibrium

Tell you which layer the compound prefers

Kᴅ =

[Compound]organic / [Compound]aqueous

- High Kᴅ: More compound in organic layer.

- Low Kᴅ: More compound in aqueous layer.

Extraction

COI moves from solvent 1 to solvent 2 (all contaminants stay in solvent 1)

Washing

COI stays in solvent 1, impurities move from solvent 1 to solvent 2

Classic Oxidizing Agents

Jones reagent, PCC → often have Cr or Mn which are often toxic and are stoichiometric

Stoichiometric oxidation

Oxidant directly performs oxidation and is consumed in stoichiometric amounts

Catalytic oxidation

Oxidizing catalyst performs substrate oxidation and is regenerated by a terminal oxidant, which is consumed stoichiometrically

What does the octanol-water partition coefficient (P) measure?

how a drug distributes between fat-like octanol (represents membranes) and water (represents aqueous fluids like blood plasma).

What is the formula for the octanol-water partition coefficient?

P = [Drug]octanol / [Drug]water

High P = prefers fat.

Low P = prefers water

What does a low vs. moderate log P mean for a drug?

log P < 0: Very hydrophilic → prefers water → poor passive membrane crossing

log P = 1–3: Balanced fat/water solubility → often favorable for drugs

What does a very high log P (>4–5) mean for a drug?

Very lipophilic → prefers fat → poor water solubility → may accumulate in fatty tissues and remain longer

Why does organic solvent-water extraction efficiently separate aldehydes from impurities, even though the starting alcohols and products have similar structures?

Separation depends on solubility, not just structural differences.

- Aldehydes and alcohols: Both prefer the organic layer (pentane/MTBE) because of their large nonpolar portions.

- Water-soluble impurities: Salts, catalysts, and other polar residues prefer the aqueous layer.

- Result: Organic layer contains the product, while water removes impurities.

Why is acetone good for the oxidation reaction but bad for liquid-liquid extraction?

Acetone (oxidation): Polar solvent that dissolves alcohol, helps mix the reaction components.

Acetone (extraction): Mixes completely with water → cant form 2 separate layers.

Pentane/MTBE (extraction): Don’t mix with water → form separate organic and aqueous layers, allowing product separation.

What is the purpose of recrystallization? (temp)

To purify an impure solid using differences in solubility at different temperatures.

- Hot solvent: Dissolves the desired compound.

- Cooling: Desired compound forms pure crystals as solubility decreases.

- Impurities: Stay dissolved in solvent or are removed by hot filtration.

Why is recrystallization inherently lossy?

Cannot recover 100% of the compound because some stays dissolved.

What makes an ideal solvent for recrystallization?

Hot: Dissolves lots of desired compound.

Cold: Dissolves very little → crystals form.

Impurities: Stay dissolved or remain insoluble (can be filtered out).

Nonreactive: Does not react with desired compound.

Safe/practical: Easy and reasonably safe to use.

solubility =

mass/volume

Use ____ solubility to calculate how much solvent is needed to dissolve the compound, and ___ solubility to calculate how much compound remains dissolved after cooling

hot, cold

SN2 mechanism

nucleophile attacks the carbon from the backside while the leaving group leaves simultaneously, causing inversion of stereochemistry

What’s the point of protonation?

To make a bad leaving group a good leaving group

Reflux Procedure

Add reactants + solvent + 2–3 boiling chips to round-bottom flask.

Set up: Attach vertical condenser with cooling water IN bottom, OUT top.

Heat: Gently boil mixture; vapor rises, condenses, and drips back into flask.

Maintain: Keep refluxing for required time without letting flask boil dry.

Cool: Remove heat and allow mixture to cool before removing condenser or adding reagents.

What is reflux used for?

To heat a reaction at the solvent's boiling point for a long time without losing solvent.

- Solvent evaporates → condenser cools vapor → liquid drips back into flask.

- Allows reaction to proceed faster at high temperatures.

Why do we pour the reaction mixture onto ice instead of cold water during SN2 Williamson ether synthesis?

Adding the hot reaction mixture to water releases heat (exothermic).

- Ice: Absorbs heat as it melts → prevents overheating.

- Cold water: Can heat up quickly → dangerous boiling and splashing.

What is atom economy in organic chemistry?

Measures how efficiently reactant atoms are incorporated into the desired product instead of becoming waste.

Atom Economy (%) = (Molar mass of desired product ÷ Total molar mass of reactants) × 100

- High atom economy: More atoms become product → less waste.

- Low atom economy: More atoms become unwanted byproducts → more waste.

What is the purpose of acid-base extraction?

To separate acidic, basic, and neutral compounds by changing their charge and water solubility using pH.

- Charged (ionic) compounds: Prefer aqueous layer (water).

- Neutral compounds: Prefer organic layer.

- Adjust pH: Convert compounds between charged and neutral forms to separate and recover them.

What is the mechanism of acid-base extraction?

1. Dissolve mixture in organic solvent → put in separatory funnel.

2. Pick acid or base:

   • Have acidic compound (COOH)? Add BASE.

   • Have basic compound (NH₂)? Add ACID.

3. Shake + vent → compound gets charged → goes into water.

4. Separate water & organic layers.

5. Organic layer: Brine wash → MgSO₄ to dry → filter → evaporate solvent.

6. Water layer: Add opposite chemical (acid if used base, base if used acid) → compound becomes neutral.

7. Collect: Crystals? Vacuum filter. No crystals? Extract into organic solvent.

Remember: Make charged → Goes to water → Separate → Make neutral → Get compound back.

Filter paper waste disposal

rinse with water, squeeze dry, place in trash

Organic Waste waste Disposal

down organic waste bottle

Why does aspirin's solubility change with pH during acid-base extraction?

High pH (base): Aspirin loses H⁺ from COOH → becomes charged (COO⁻) → dissolves in water.

Low pH (acid): Aspirin gains H⁺ → becomes neutral (COOH) → less water-soluble → crystallizes.

Remember: Charged = water. Neutral = organic/solid.

Why is ethyl acetate a good solvent for acid-base extraction? What other solvents could work?

Ethyl acetate dissolves neutral organic compounds (aspirin and caffeine), forms a separate layer from water, and evaporates easily.

Other options: MTBE or dichloromethane.

Requirements: Dissolve desired compounds, mostly immiscible with water, chemically unreactive, easy to remove.

What is brine, and how is it made?

Brine = saturated saltwater (NaCl dissolved in water until no more dissolves).

Why is brine added during liquid-liquid extraction?

Brine pulls some water out of the organic layer, helps break emulsions, and makes layers separate more cleanly.

Aqueous waste disposal

Down sink

carbonyl IR

1650–1750 Very strong, sharp peak around 1700

sp2 CH stretch IR

3000–3100 Weak–medium, sharp peaks above 3000

sp3 CH stretch IR

2850–2960 Medium, sharp peaks below 3000

Primary Amine stretch IR

3300–3500 Two medium, relatively sharp N–H peaks

Secondary Amine stretch IR

3300–3500 One medium, relatively sharp N–H peak

Alcohol Stretch IR

3200–3600 Broad, rounded peak

Carboxylic Acid stretch IR

2500–3300 and ~1710 Extremely broad O–H peak + strong C=O peak

Aldehyde stretch IR

~1720 and 2720–2820 Strong C=O + two weak C–H peaks near 2720 and 2820