ORGANIC CHEM LAB FINAL VIRGINIA TECH 2545

Are staggered or eclipsed conformations more stable?

staggered, eclipsed are higher energy

What factors influence hazards of an acid?

Strength and concentration

Strong Acids Examples

Nitric acid (HNO3), Concentrated sulfuric acid (H2SO4), and concentrated hydrochloric acid (HCl)

Weak Acids Examples

Acetic acid (vinegar), citric acid (fruits)

Strong Bases Examples

Sodium hydroxide (NaOH), Potassium Hydroxide (KOH)

Weak Bases Examples

Baking soda (NaHCO3), Ammonia (NH3)

_____ was a major ingredient in fingernail polish remover

Acetone

What modeled skin?

egg white and egg yolk

what modeled contact lenses?

polystyrene/styrofoam

what modeled metal jewelry?

aluminum foil

____ are a major component of skin and tend to be reactive with certain acids.

Sugars

physical property def.

one that can be observed without changing the identity of the pure substance

intensive property

independent of amount of sample used

extensive property

depend on amount present (mass and volume)

physical property ex

color, density, melting point, boiling point, solubility, molecular weight, refractive index

boiling point

temperature where vapor pressure of liquid equals applied pressure of system

vaporization

change from liquid to vapor; endothermic (must add energy to system to increase number of molecules in vapor phase)

What if the boiling/melting point differs from accepted values?

most likely impurity

Increase pressure..._____ b.p.

increase (exponential)

______ intermolecular forces = higher b.p.

stronger

forces from weak to strong

vdw, dipole-dipole, h bond, and ionic

dipole-dipole forces are present in ...

polar molecules

acetone

(ch3)2c=o

ionic forces are present in many ...

salts

What occurs to b.p if two miscible liquids are mixed?

b.p. between two liquids depending on concentration of each in solution

density formula

D=m/v (g/mL or g/cm3)

Uses for refractive index

identify liquid and assess purity

closer RI measurement to accepted value =

more pure sample

Refractive index def

ratio of velocities (n=velocity of light in air/velocity of light in liquid = sin(angle of incidence)/sin(angle of refraction))

What does a liquids RI depend on?

temperature and wavelength

melting point

temperature where molecules in a crystal gain necessary energy to overcome forces holding them together and move freely

melting

similar to vaporization; endothermic

the presence of an impurity ____ m.p range.

widens

phases of melting point

1. first signs of change 2. first signs of liquid (T1) 3. formation of meniscus 4. completely clear liquid (T2) T1-T2 = range

Steps for recrystallization

1. choose appropriate solvent 2. dissolve impure compound 3. decolorize solution 4. remove suspended solids 5. recrystallize pure compound 6. collect, wash, and dry crystals

best recrystallization solvent dissolves ____ amount desired solute at high temps. and ____ amount at low.

large; minute

greater difference in solubility of solute at high/low temps

more efficient recrystallization

A good recrystallization solvent is...

inert, inexpensive, nontoxic, and easily removed (low b.p.)

how do you prevent oiling out?

bp below mp so solid does not melt in liquid and not dissolve

in recrystallization, too much solvent =

difficult for pure crystals to form

in recrystallization, too little solvent =

crystallization may occur when not desired

how is a colored impurity removed?

small amount of active charcoal (absorption)

Two ways to remove suspended solids in recrystallization:

gravity filtration (more common) or vacuum filtration

as the solution cools the solubility of the solute ____

decreases

if crystallization proceeds too fast,

impurities become trapped in the lattice of the solid

how can crystallization be maximized?

place in ice bath during last minutes to further decrease solubility

Is boiling point or melting point more sensitive to Impurities

melting point

what technique is used to collect and begin to dry solid?

vacuum filtration

why are crystals washed with cold solvent during filtration?

remove absorbed impurities

why must solids be completely dry before mp can be determined?

remaining solvent acts as an impurity and leads to mp depression

on the ABBE refractometer, what is used to clean the glass in the top prism case?

swab of cotton dampened with ethanol

on the ABBE refractometer, what is used to introduce the sample on the prism?

plastic pipette

equation to correct RI temperature

n = n(at T) + 0.00045(T-20)

percent weight recovery equation

% weight recovery = (mass of recrystallized compound, g/mass of impure compound used, g)*100%

or mass of compound after extraction/starting mass of compound

List two things that the lab manual tells you to do if you get an acid or base on your skin

immediately flush the area

with a large amount of water and notify your teaching assistant

Describe what happened when you added the H2SO4 to the sugar

e white crystals of sugar became a yellow/green color, and after

several minutes, the color changed to black. Additionally, the area where the drop of acid was placed

started off small but expanded to include a larger area of the sugar over time. It could also be observed

that the area of sugar that interacted with the acid became stuck together and hardened.

Describe what happened when you added sodium hydroxide to the aluminum foil

the bottom of the aluminum foil cup that was

covered with the sodium hydroxide dissolved into the base. This resulted in a hole in the bottom of the

foil cup and the NaOH to become a darker color as it mixed with the dissolved aluminum foil.

Describe what happened when you added HCl to the cotton

the edges of the cotton square began to roll towards the middle of the

cotton, and the cotton changed from a white color to a darker, off-white color.

Describe what happened when you added HNO3 to nylon

nylon became partially dissolved after being stirred with the acid.

The part of the nylon that dissolved into the acid mixed to create a yellow liquid while some of the nylon

fabric remained mostly solid.

Describe what happened when you added bleach to the eggs

the eggs remained liquid. The egg whites were observed to change

from a clear liquid to having a slight yellow tint, and the egg yolks remained yellow after the addition of

bleach

Describe what happened when you added acetone to the Styrofoam packing peanuts

Styrofoam that was hit by the acetone

was dissolved immediately, and the remainder of the packing peanut dissolved quickly as well.

Mixed melting point is a technique used to determine the identity of an unknown solid. 10% of a(n) ------ compound is added to a(n) ----- sample. If the melting point of the unknown solid by itself matches the mixed melting point, there is strong support that the unknown solid and the known solid are the same.

known; unknown

In liquid chromatography, components of a mixture are separated by partitioning between a ____ liquid phase and a solid ______.

moving; adsorbant

examples of solid adsorbents used in liquid chromatography.

silica gel, cellulose, and alumina

What is TLC used for?

to identify a compound by comparing to a standard

to test the completeness of a reaction

to get a rough estimate of the purity of a product

What is the role of capillary action in TLC?

move liquid phase up the plate

If a compound does not appear visible to the naked eye, they can be visualized if they absorb ______ . The plates we use are coated in a _______ chemical. When a plate is exposed to ______ , the compounds will appear as ________ spots because they ________ the light.

uv light; fluorescent; uv light; dark; absorb

On a TLC plate, _____ polar substances generally advance further than the ______ polar substances.

less; more

A compound's Rf value is _______under a precise set of conditions. An Rf value is calculated by dividing the __________.

constant;distance the compound traveled by the distance the solvent traveled .

examples of good TLC techniques:

Spots on the spotting line should be small; The TLC chamber should be left closed when developing a plate; Solvent level in the TLC chamber should be below the spotting line

Tailing on a TLC plate can result from

a compound slowly decomposing on the plate

application of too much compound on the spotting line

TLC is primarily a tool for rapid ______ analysis.

qualitative

The ______ the compound's affinity for the solid phase relative to the liquid phase, the faster the compound will move through the adsorbent.

lower

A good separation is achieved when each compound

has a different Rf value

A student is making TLC solvents using hexane and ethyl acetate. The student decides the solution being used is not polar enough. Which solvent does the student need to increase in the mixture?

ethyl acetate

The Bronsted-Lowry definition of an acid is a species that

donates a proton

The ______ the pH, the lower the hydrogen ion concentration and the ______ acidic the solution.

higher; less

The pKa of benzoic acid is 4.17. The pKa of HCl is -7. This means that HCl is a ______ acid than benzoic acid.

stronger

The pKa of cyclohexyl amine is 10.7. This means that at a pH of 10.7, cyclohexyl amine is

50% protonated and 50% neutral

In a liquid-liquid extraction, an extracting solvent must:

be immiscible with the original solvent

dissolve the solute better than the original solvent

When two _____ solvents come into contact, they separate into two layers according to their densities. The top layer will be ____ dense than the bottom layer.

immiscible; less

T/F Neutral compounds (compounds with no acidic or basic functional groups) are soluble in ether.

true

In general, salts are soluble in _______ and neutral (uncharged) organic molecules are soluble in ______ .

water; organic solvents

_____ will deprotonate _____ to form salts of organic acids. These salts are soluble in _______.

bases; acids; water

T/F Acid-base reactions require both an acid and a base. If an acid is added to an acid, there will be no reaction.

true

Why do you need to mark the solvent front?

this line is used to determine the distance the solvent travels which is a necessary component in calculating the retention factor. The solvent front needs to be marked immediately upon removing the TLC plate from the chamber because it will evaporate after being removed from the chamber.

swirl the centrifuge gently after adding water to your organic solvents. Why should you swirl it gently instead of shaking it?

this prevents emulsions from forming.

During the extraction of the spinach leaf, you will add sodium sulfate. Why?

it acts as a drying agent. it dries the extract

When spotting your compound on the TLC plate, you don't want to add too much of your sample. What would happen to your plate if you add too much?

tailing of the sample. It also causes the plate to become overloaded resulting in smears.

When spotting your compound on the TLC plate, you don't want to add too little sample. What would happen if you add too little sample?

not being able to visualize the sample.

who is the father of chromatography?

Russian botanist, M.S. Tswett

Who received the Noble Prize in 1915 for analyzing pigments?

R.M. Wilstatter

Refraction equation

rf=distance substance travels/distance solvent travels

what occurs when spots are not placed high enough on the TLC plate and solvent level is above spotting line?

compounds diffuse into solvent and do not climb the plate

solute

that which is dissolved

pH indicates the concentration of

positively charged protons (H ions)

pKa indicates

strength of acid

smaller pKa =

stronger acid

Henderson-Hasselbach equation

pH = pKa + log [A-]/[HA]

when pH equals pKa

concentration of acidic and basic form are equal, half material is present as acid and half as the conj. base