CHEM 262 FINAL

Melting point

the point where the solid is in equilibrium with the liquid (precise)

melting range

span of temperatures where the solid just becomes a liquid to when it is fully liquid (narrow)

How do impurities affect the melting point?

they can lower the melting point and broaden the melting range

What does the critical point refer to?

It's where the boundary between the liquid and gas disappears

What does the triple point refer to?

Refers to the point at which the compound will do all these things (sublimation, deposition, vaporization, condensation, melting, and freezing) at the same time.

What happens when a compound changes states (ex: from a solid to a liquid)?

It breaks the IMF (dipole dipole, H bonding, and London Dispersion)

What is the eutectic composition?

Unique for a mixture of two compounds and is the lowest possible melting point for a mixture. It's a sharp melting point

Will impure compounds have higher melting points than the pure compound and the eutectic composition?

Impure compounds have lower melting points than pure compounds and higher than eutectic composition

What is the purpose of a mixed melting point?

Since melting point is a characteristic of a compound, oftentimes it can be used to identify an unknown. However, many compounds have the same melting point, so by using a mixed melting point, we can ascertain the identity of unknown compounds.

Orientation melting point

An approximation of the melting point

Sublimation

Solid to a vapor

Why is recrystallization done for a compound that is solid at room temperature?

It's a method of purifying the compound (sharp melting point)

How is solubility of a compound affected with increasing temperature?

Solubility increases

How is solubility of a compound affected as the solution cools down?

Solubility decreases, leading to crystals

How do you choose a solvent to dissolve a compound in?

Like dissolves like. The boiling point of the solvent also has to be lower than the melting point of the compound that is being crystallized

When does the compound oil out?

When the boiling point of the solvent is greater than the melting point of the compound since the compound is insoluble above the melting point in the solvent

For insoluble impurities, what method would need to be used?

Gravity filtration?

How to remove colored impurities?

Activated carbon is used

What happens if a lot of activated carbon (charcoal) is used?

It would remove a lot of the pure product

What type of filter paper is used for filtration of insoluble impurities?

Fluted filter paper has a higher surface area, which maximizes the filtration

How can crystallization be induced?

By scratching

What is a Büchner funnel used for?

To recover crystals

% recovery formula

(mass of final/mass of initial) x 100%

What is distillation used for?

Purifying a compound

Does being a volatile liquid affect boiling points and how?

Being a volatile liquid lowers the boiling point, causing higher vapor pressures

Materials used for distillation: pot

where the liquid is boiled

Materials used for distillation: water-cooled condenser

Liquefies the vapor produced by boiling

Materials used for distillation: receiver

Collects the liquefied compound called the distillate

When does evaporation occur in relation to the boiling point?

It occurs below the boiling point

When is conversion of a liquid to vapor faster?

It's faster with boiling than just evaporation

What does increasing the heating do at the boiling point?

Doesn't change the temperature, only the rate at which the vapor is produced increases

Raoult's law

Ptotal = Pa + Pb (ideal mixture)
Pa: xaPa (where xa is mol fraction)

When do two liquids form a homogenous mixture?

When they are completely miscible

In the vapor form of a homogenous mixture, how is the composition different from the liquid composition?

The vapor has more of the volatile component

Where is the boiling point of a miscible mixture in comparison to pure liquids?

Between the boiling points of the pure liquids

When would you use simple distillation over fractional distillation?

If one component is significantly more volatile than the other and is a single vaporization step

When would you use fractional distillation over simple distillation?

Used if the boiling points have a difference greater than 40C and involves many simple distillations via a fractionating column

How do boiling chips work?

They agitate the liquid, helping the air expand, creating the bubbles

Why can't boiling chips be reused?

The pores inside the boiling chips become filled with liquid on cooling

What is done in solvent extraction?

Separation technique that dissolves one or more compounds in a solvent. The solution is called an extract

Why is solvent extraction used?

To remove soluble materials from insoluble ones (they have to be immiscible)

How can you tell whether the upper layer is aqueous or organic solvent?

The organic solvent is more dense, so the upper layer is aqueous

Washing technique

The impurities are removed in a second solvent, leaving the desired compound in the first

What is used for larger scale extractions?

Separatory funnel

What is an emulsion?

Fine dispersion of small droplets of one liquid in another where it is not soluble/miscible

When can an emulsion occur?

During an extraction when the solvents are not miscible

What does emulsion look like?

Small droplets of one solvent in the other, but they can often be mistaken for a third layer since they are opaque/cloudy

Ways to get rid of the emulsion "layer"

gentle swirling, addition of brine, addition of some ethanol, addition of more solvent, filtration

What is brine?

saturated solution of NaCl in water.

What is salting out?

Addition of NaCl

What is salting out used for?

When organic compounds are polar enough to be soluble in water, they become hard to extract. By adding NaCl (water soluble), it reduces the solubility of the organic compound in water and forces the organic compound to dissolve in the organic solvent

What can acidic compounds be converted into?

Acidic compounds (carboxylic acids and phenols) can be converted into the conjugate bases. Carboxylic acids have lower pKa than phenols (more acidic)

Converting carboxylic acids into carboxylate

Converting phenol into phenoxide

Converting amines to the salts, the conjugate acids

What are common drying agents?

Na2SO4, MgSO4, CaCl2

What is the purpose of chromatography?

separating compounds

What is chromatography?

A method of separating compounds that involves equilibrium of the compound/compounds between two phases

What are the two phases in chromatography?

Stationary and mobile phases

In column chromatography, what is the stationary phase?

A solid or contains another liquid

In column chromatography, what is the mobile phase?

Liquid or gas

How does the compound move in column chromatography?

It's carried by the liquid or gas (mobile phase) over the stationary phase

How can you separate compounds in a mixture with column chromatography?

In a mixture, two compounds can interact different with each phase and compounds that interact more strongly with the mobile phase end up moving faster than those that interact strongly with the stationary phase

What is the solid adsorbent used in column chromatography?

usually silica gel (SiO2.xH2O) or alumina

What is the organic solvent used as a mobile phase in column chromatography known as?

Eluent

Column chromatography: Dry packing method

glass wool/cotton to plug the bottom, then sand to provide a level base on top, dry adsorbent, sample-adsorbent powder (ferrocene/acetylferrocene), dry adsorbent (silica gel). the eluting solvent is poured ontop

Column chromatography: slurry packing/wet packing method

glass wool/cotton to plug the bottom, sand to provide a level base, solvent, slurry of adsorbent in the eluent

How do the compounds move in column chromatography?

As bands

What is the stationary phase in GC?

non-volatile liquid

What is a non-volatile liquid?

a polymer with a high boiling point

What is the mobile phase in GC?

Inert gas/carrier gas

What is used in the inert gas/carrier gas?

Helium or Nitrogen

Which compounds are retained longer in the GC?

Compounds that interact more strongly with the stationary phase

What is the relationship between retention time and interaction with the stationary phase?

Longer retention time, more stronger interaction with the stationary phase

What can affect retention time?

Structure of the compounds, type of stationary phase, column temperature, length of the column, rate of flow of carrier gas

How to determine area under peaks: Triangulation method

area = h x w1/2
h=height of peak
w1/2 = width at half height of peak

% of compound A

(Area of compound a/ sum of all areas) x100

What can be used to measure column efficiency?

Number of theoretical plates

What is the number of theoretical plates? (formula)

N = 16 (tR/w)^2
where tR is equal to the height of the peak and w is the width of the peak

What is resolution? (Definition)

separation between the two peaks

What is resolution? (formula)

R= (tB-tA) / (Wa + Wb/2)
where tB is height of peak B and WB is the width of peak B

What is the stationary phase in TLC?

Adsorbent (silica gel/alumina) on a backing/sheet of plastic, glass, or aluminum

What is the mobile phase called in TLC?

The eluent/eluting solvent

Which compounds are retained longer on a TLC plate?

Compounds that interact more strongly with the stationary phase are retained longer

What is TLC used for?

Identifying compounds, determining how many compounds are in a mixture, monitoring course of reactions, finding suitable solvents for column chromatography, separating and isolating compounds

What are the steps in performing TLC?

Spotting a TLC plate, developing a TLC plate, visualizing the spots on a TLC plate, calculating Rf values

How to calculate Rf (retention factor)

Distance moved by compound/distance moved by solvent front

Which types of compounds have higher Rf vaues?

Less polar compounds have higher Rf values (interact less with the stationary phase)

Which compounds will have a greater distance moved by the compound?

The least polar since they interact less with the stationary phase)

What type of dihalides are produced when alkenes react rapidly with Br2 and Cl2?

Vicinal dihalides

How do alkenes react with I2 and F2?

I2 generally reacts sluggishly and F2 reacts explosively

What type of reaction happens when alkenes are brominated?

Electrophilic addition reaction

How to calculate the number of stereoisomers

2^n
where n = number of chiral centers

What is the chemical structure of PBP (pyridinium tribromide or pyridinium bromine perbromide)?

How can you test the presence of alkyl Cl or Br atoms?

Using an ethanolic solution of AgNO3 (silver nitrate)

From left to right on the infrared spectroscopy, how does frequency and wavelength change?

Frequency increases and shorter wavelengths

From left to right on IR spectroscopy, what are the waves?

Radiowaves, microwaves, infrared, visible, ultraviolet, x-rays, gamma rays

What is the range for IR radiation?

400-4000 cm-1

What is the output for IR?

A graph with wavenumber (v) in cm-1 in the x-axis and % transmittance on the y-axis

What are specific adsorption bands diagnostic of?

Specific functional groups, giving structural information