Organic Chemistry Experiments 1-8

Exam 1

how to calculate yield

1. Convert grams to moles to get Actual Yield

• use molar mass to go from g to mol

• use density to go from mL ot g

2. Theoretical Yield

• use moles of reactants to determine moles of product

• determine limiting reagent:

  • use mole fraction to find out which reactant gives you the least amount of product

  • this is your Theoretical Yield

3. % Yield = Actual Yield of Product / Theoretical Yield * 100

• theoretical yield should be higher

• wetness or contamination error can mess up % yield

Sig Fig Rules

• * and / = lowest number of sig figs

• + and - = fewest decimal places

Fold filter paper to…

increase surface area, making it faster to filter

Water is good for heating because…

it has a high heat capacity

Be careful with magnesium solid because…

it could start a fire

What can smother a fire?

sand bath

Volatile

• low boiling point to vaporize

• fume hood

What is melting point?

temperature at which phase transitions from solid to liquid

Water Phase Diagram

supercritical fluid next to liquid above gas

It’s melting not liquification because…

liquification can be any phase → liquid

petrification

gas → solid

What factors affect melting point?

• size of molecule/compound

  • butane (mp = -138 degrees C) vs. octane (mp = -57 degrees C)

  • larger molecule = more LDFs = stronger IMFs

• branching

  • branched molecules have a lower mp and bp

    • more stacking if not branched + more LDFs and IMFs = higher mp, higher bp

• intermolecular interactions

• contamination/mixtures

  • weakens crystal lattice, causing mp to decrease

Contamination/Mixtures

• pure crystal solid has proper orderly crystal lattice which would take a specific heat to melt

• contaminant → chaos (increased entropy) → crystal lattice is weaker and will break down with less heat

• melting point depression

If you have pure A or B and introduce B or A, mp will…

decrease

Eutectic temperature/point

• minimum melting point for a mixture

• true lowest temperature A & B can reach

• as more of another component is introduced, melting point of mixture decreases from original melting points of A and B

Calibration Curve

• gives very precise curve without having to run the experiment

• range is between 2 points, mp isn’t always instantaneous

The mp should be lower for the mixture than pure, so less than 55-58 degrees C, but not significantly because it’s 92%.

Pure/Impure/Glucose Sample mp ranges

• pure sample: small mp range (± 1-2 degrees C)

• impure sample: larger mp range (> ±2 degrees C)

  • takes longer

• pure glucose: mp = 146 degrees C (pure = 145-147)

How to take mp?

• need a fine powder and capillary tube

  • solid grounded down

• turn capillary tube upside down and tap into powder

• flip over and tap to get into closed part

• program the temperature you want and observe sample

• first liquid drop = solid → liquid phase has begun = beginning of melting point/range

Recrystallization is a… in which you…

• purification technique

• isolate a target compound as pure crystals from other components in a crude sample (mixture)

Recrystallization simple overview steps

1. dissolve solid to release impurities

2. cool down, pure solid reforms

Recrystallization more in depth steps

1. crude crystals are dissolved in hot solvent to dissolve unwanted contaminants

2. during cooling, crystal formation begins (nucleation) and proliferation of crystals (growth) continues

3. crystals are then collected with filter flask/Buchner Funnel and vacuum, and usually washed with cold solvent

4. dry product and weigh crystals to determine yield, mp analysis to check purity

Why is it relevant that recrystallization is slow?

• it is thermodynamically controlled

  • slow process = most yield because crystals have time to form crystal lattice

Recrystallization Graph

• nucleation: crystal seed is planted

• end saturation/crystal formation

Recrystallization Solvent Rules

• solute must be relatively insoluble in solvent at room temperature but more soluble in solvent at higher temperature

• impurities that are present but be soluble in solvent at room temperature OR insoluble in solvent at high temperature

• typically, a solvent with a similar polarity to solute being dissolved will be good

• ideal:

  • unreactive with solutes

  • inexpensive

  • low toxicity

• solvents with low boiling points (diethyl ether, acetone, low boiling petroleum ether) are highly flammable and can be difficult to work with because they evaporate quickly

• good options: high boiling petroleum ether/ligroin, methanol, hexanes, athyl acetate, ethanol, water, toluene 

How to determine if Recrystallization was successful

• melting point of recrystallized product should be higher than that of the original crude sample

• mixtures have lower overall mps

If no crystals form in recrystallization…

• glassware may be dirty, try scratching to induce nucleation

• too much solvent can cause your crystals to not recrystallize

If you use hot solvent to wash the collected crystals in recrystallization…

• some of the crystals will be lost

• this means lower yield

• always use cold solvent

What are colligative properties?

• affected by the amount and number of components in the solution

  • vapor pressure

  • boiling-point elevation

  • freezing-point depression

Freezing Point Depression

• lowers freezing point of a liquid when another substance is dissolved in it

• deltaT = iKf_fm 

  • i = van’t Hoff factor (number of components)

  • K_f = molal freezing point depression constant

  • m = molality (amounts of each component)

• pure water → salt water

  • freezing point lowers

Heterogeneous vs Homogeneous Mixtures

• heterogeneous: not uniform in appearance (a and c)

• only homogeneous mixtures can be called solutions (b)

  • solutions = solids dissolved in liquids or liquids dissolved in liquids

Distillation: what is it and why do we perform it?

• purification technique

• allows us to separate the components of a mixture or purify a liquid based on their different boiling points

Solution mixtures

• homogeneous

• composed of individual components

• we can use distillation if they have different boiling points

• ex: acetone (57 degrees C) and water (100 degrees C)

Types of Distillation

• simple distillation

• fractional distillation

Simple Distillation

• used when difference in bp of components of a solution mixture is >70 degrees C

  • larger gap

• usually at least 75 degrees C difference

• Dalton’s Law

Fractional Distillation

• used when simple distillation of a mixture would be inefficient (difference in bp of mixtures < 70 degrees C)

  • smaller gap

• fractionating column

Dalton’s Law

• total vapor pressure of a solution is equal to the sum of the individual vapor pressures of the components

Fractionating Column

• allows for multiple equilibrations of the volatile mixture to occur along the height of the column

• vapor will rise and condense multiple times in the column

• each vaporization/condensation event is called a theoretical plate

• more theoretical plates = better separation

Label: 

• Bunsen burner

• round-bottom flask

• fractionating column

• thermometer

• condenser

Phase Diagram for a general 2-component liquid mixture

• middle section = liquid and gas

A 50%/50% mixture of two components whose boiling points differ by only 20-30 degrees C would require at least __________ to obtain a distillate with >95% purity.

• 3 theoretical plates to obtain a distillate with >95% purity

The best chance of obtaining purity via fractional distillation is when…

there is very little impurity to begin with

Azeotropes

• certain liquid mixtures that are very hard to separate using fractional distillation

  • methanol and toluene

• occurs because vapor pressure above the solution mixture has the same composition as the liquid mixture

• two types:

  • low-boiling

  • high-boiling

low-boiling azeotrope example

• 95.6% ethanol in water

• will always have a bp of 78.62 when mixed

  • boil together like one substance with a shared bp

What type of distillation should be used to purify this solution mixture? Which of the two components should be isolated first?

• <70 degrees C difference → fractional distillation

• acetone should be isolated first because it has a lower boiling point

• A = single pure component (good)

• B = simple distillation used on two components of similar boiling points (not good)

• C = fractional distillation used on two components with very different boiling points (good)

What would a successful distillation look like on a graph of temp vs. fraction collected?

• flat temp plateau while one component is boiling off (collecting first substance)

• sharp temperature jump to higher plateau, showing next substance starting to distill

• c

What fractions are the most pure in distillation?

• the fractions closest together in temperature (the plateaus)

Extraction

• used when distillation is not possible

• some compounds have a really high bp

What factors affect a molecule’s solubility?

• IMFs/Vanderwaal’s Forces

  • H bonding

  • electrostatic interaction (ionic)

  • dipole-dipole

H-bonding criteria

• H-bond donor (partially + H NOT on carbon), FON

• H-acceptor (anything with a lone pair)

Which are soluble in water?

• B in its enol form → double bond O becomes single bond OH and double bond C-C

• D

Organic molecules are soluble in water when…

• they are either ionic or very polar

  • positively charged ion or negatively charged ion

• water is polar

Neutral organic molecules are generally soluble in…

• organic solvents

• not ions

Ions are __________ in water.

• very soluble

What is liquid-liquid extraction?

• compounds in one liquid layer can be extracted into another liquid layer

• liquid in separatory funnel → add immiscible solvent → shake layers → add saturated NaCl to deal with emulsion (foamy liquid when you mix too vigorously) → open valve to collect individual layers (in beaker)

In liquid-liquid extraction, which layer is which?

• top layer = organic solvent

• bottom layer = aqueous, water is more dense than organic solvents

How do we represent the preference of molecules for organic/aqueous solvents?

• Partition Coefficient (K)

• we want K > 1 so we can isolate the majority of the target component in the organic layer (organic/top layer is favored)

• K = [S]₂/[S]₁ = organic/aqueous

  • S2 = Phase 2 = solute in organic phase (top layer)

  • S1 = Phase 1 = solute in aqueous phase (bottom layer)

K is essentially…

• the same regardless of the amount of organic solvent used

• volume of organic is unimportant

What determines a successful extraction?

• n

• the number of times that you do the extraction

• more times = better

What do variables mean?

• W = original weight of compound

• W1(n) = weight of compound remaining in Phase 1 after “n” extractions

• n = number of extractions

• V1 = volume of phase 1

• V2 = volume of phase 2 (extracting phase)

• Kp = (solubility of W in phase 2) / (solubility of W in phase 1)

The efficiency of extraction is strictly dependent on the…

number of extraction events performed (n)

• W = 5.2 g

• W1(n) = ?

• n = 1

• V1 = 60 mL

• V2 = 50 mL

• K = 8.1

• W1(n) = 5.2 (60 / (8.1×50) + 60)^1 = … g leftover

• How much was extracted = W - W1(n)

• in this instance, should be really high because Kp = 8.1

The conjugate base is much more ______and, therefore, more
soluble in the _________

• polar

• aqueous phase

• sodium benzoate = conjugate base of benzoic acid

• red = partial negative/e- rich

• yellow = neutral/np

• dark blue = partial positive/e- deficient

• red and blue = polar

• sodium benzoate is more polar!

Converting a compound to its conjugate base

• very acidic

• + bicarbonate (weak base)

• forms carboxylate (just make H negative)

Applications of Extraction

• isolation of genomic DNA from human blood

• analysis of isolated DNA using agarose gel electrophoresis

• extracting vanilla beans in solution of alcohol and water

What is TLC?

• detection technique for purity of samples

TLC Phases

• competing attractive forces provide an environment for molecules to display their relative polarities

• Stationary phase: silica gel plate

  • polar OH groups

  • molecules attracted more/less strongly relative to their own polarity

  • polar molecules grab on tight, nonpolar don’t

• Mobile phase

  • solvent system/liquid

  • drawn up the plate by capillary action / by solvent

    • b/c plate is porous, surface area surface tension pulls liquid up

  • if polar, it will pull polar compounds along more; if np, polar compounds stick to silica gel

What IMFs are happening between polar molecules and silica gel?

• hydrogen bonding

• dipole-dipole

How far compounds travel up the TLC plate depends on…

• how strongly they are attracted to silica gel (how polar they are)

• the polarity of the solvent system

• Green is most polar because it grabs onto silica gel

• Green, Blue, Red

Different solvent systems have different eluting powers, and changing the ratio of the solvents…

• changes the results

• ethyl acetate:hexane ratio

  • ethyl acetate is more polar

• Left plate: solvent system is less polar (3:1)

• Right plate: solvent system is more polar (1:3)

Retention Factor

• R_f = distance sample traveled/distance solvent system traveled = d1/d2

• values range from 0-1

• small Rf = most polar

• largest Rf = least polar

• same Rf = same compound

2 factors that affect Rf values

• polarity of molecules

• polarity of solvent

Lipophilicity

• comparing non-polar vs polar moieties in molecules

• ratio of polar to np (more ch = less polar, lipid-like chain)

What is more polar?

• b, less symmetrical, more func. groups

• b, more func groups = more polar

Hexane vs. Ethyl Acetate Solvent TLC

• hexane = non-polar, sample will not interact strongly

• ethyl acetate = polar, sample will move far

Solvent front

• point where mobile phase (solvent) had climbed when removing TLC plate (top line)

TLC uses

• separating components of mixture

• assessing purity of compound (one spot = pure, multiple spots = impure)

• identifying components of a mixture, using a cospot

• monitoring progress of a reaction (starting material vs. product)

relative stabilities of alkenes

• more substituted = more stable

• because uses sp²-sp³ bonds for bonding rather than sp³-sp³

  • sp²-sp³ is stronger which decreases heat of formation

Electrophilic Addition Reactions

• carbon-carbon double bond (pi bond) of alkene is broken and X-Y bond of reagent is broken

• new C-X and C-Y bonds are formed

• pi electrons and lone pair electrons are nucleophile (Lewis base)

  • nucleophiles attack partial positive or formally positively charged species (H+)

• in solvent like Br2 (not H2O)

Electrophilic Addition Reactions Reaction Profile

• rate-limiting step is highest climb

• transition state of highest free energy/activated complex = peak

• intermediate present in valley (ie bromonium ion)

• exothermic reaction (reactants have higher energy than products)

  • deltaG rxn = deltaG products - deltaG reactants

  • deltaG rxn <0 → spontaneous or favorable

Meso Compounds

• 2+ stereocenters

• achiral with at least 2 chiral centers with opposite stereochemistry

• have symmetry along center of molecule

• symmetrical

1. R, S; E or trans

2. S, S; Z or cis

• 1 is meso

Halogenation

• alkene + Br2

• Br lp attacks alkene, alkene attacks Br back

  • forms bromonium ion intermediate: triangle

• Br- lp attacks a C, which breaks bond between Br+, forming Br addition on either side adding in anti-addition fashion (antiperiplanar/staggered)

• trans-dibromo product

Bromine (Br2) is pretty…

toxic and volatile, so we use pyridinium hydrobromide perbromide instead

• Br2 + N cyclohexene + HBr

• trans-addition of Br

Halogenation Reactions - Alcohol Mechanism

• H+ from H-X attacks OH

• O becomes positive, H2O leaves

• Carbocation forms (strong electrophile)

• Now negatively charged X attacks carbocation

  • not a great nucleophile (wean anion) but good w carbocation

• Markovnikov = most substituted (carbocation can move)

Why is hydroxyl (R-OH) such a bad lg?

• strong base/nucleophile

• = reactive/unstable, will go right back to R group of H+

Why is R-X a better leaving group than R-OH?

• X = halogen

• X^- = weak Lewis base + R⁺ = conjugate acid → H-X (strong conjugate acid)

• lower pKa of <2 (acidic)

• halide = weak base = solid

Is the OTs a better LG than OH?

• Yes

• it’s stable because of resonance (O^-) making it a good LG with a low pKA (acidic)

What mechanism for this reaction?

What kind of alcohol would be best?

What solvent would be best?

• SN1 (carbocation)

• tertiary → tertiary carbocation

• protic solvent (polar) - favors ion formation and stabilizes ions

How can we check success of SN1 alcohol reaction?

• use IR

• E = hv = hc/wavelength

• want to see OH bond 3400-3200 disappear

Dehydration of Alcohols makes…

• alkenes

• strong acids (H2SO4 or H3PO4) catalyze a beta-elimination reaction in which water is lost from a secondary or tertiary alcohol → alkene

carbocation rearrangement

• conversion from alcohol to halide could undergo rearrangement

y = 10^x

solve for x

log y = log(10x)

log y = x log10

x = log y / log10 = log y

SN1 vs. E2

• SN1: steps, intermediate (carbocation)

• E2: concerted, no intermediate