Lab Quiz 1

Melting point and Boiling Point

Physical properties such as melting point, boiling point and solubility are largely determined by intermolecular attractive forces
London attractive forces (Van Der Waals)- organic molecules that contain only carbon and hydrogen are weakly attracted to each other by London forces

- These attractive forces increase as molecular size increases

- Larger molecule, greater the attractive forces increase for neighboring molecules and greater energy required tog et 2 molecules to move apart

Dipole dipole attractive forces- attractive forces between molecules increase when functional groups containing electronegative atoms (Cl, O, N) are present because these atoms are more electronegative than carbon.

- Bond with slight charge superstation termed a polar bond, polar atomic bonds often give a molecule a dipole: slightly positive and negative ends symbolized by arrow in the direction of negative charge

- Dipole-dipole attractive forces are stronger than London forces

Hydrogen bonding- even stronger intermolecular attractive force

- Occurs with organic molecules containing O-H groups (alcohol, carboxylic acids) or N-H groups (amines or amides).

- Hydrogen in these groups attracted to the unshared pair of electrons on the O or N of another molecule, forms hydrogen bond

- Major role in the special physical behavior of water and is major determinants of chemistry of proteins and nucleic acids in living systems

Ionic attractive forces - usually solid with melting points (>300 C) due to strong attractive forces between positive and negative ions

- Most organic molecules contain only covalent bonds and have no ionic attractive forces between them

- 3 important exceptions involving acidic or basic functional groups that can form ionic structures as the pH is raised or lowered

Hydrogens on the -OH carboxyl group in carboxylic acids (acetic acid), acidic and reacts with bases such as KOH and NaHCO3, salt. Process is reversed by adding acid to lower pH.
Hydrogens on -OH group attached to aromatic ring is weakly acidic and reacts with strong bases (NaOH) and forms high melting ionic salts. Reactions revered by addition of an acid
Amines are basic and will react with acid to form ionic anime salts with elevate melting points

Pure compounds containing identical molecules, total attractive force between molecule is the sum of all attractive forces listed, bot weak and strong
Solubility- integration of 2 different molecules, may have different types of attractive forces
the majority of organic molecules contain other elements, in functional groups that can be polar, exhibit hydrogen bonding, show ionic tendencies or have any combination thereof.
In addition to water, many other liquid solvents are used in lab to dissolve substances (acetone, dichloromethane, ethanol, diethyl ether, hexane, methanol and toluene)
“Like dissolves like”
The melting point of a pure solid orangey compound is one of its characteristic physical properties, along with molecular weight, boiling point, refractive index and density
Pure solid will melt reproducibly over a narrow range of temperatures typically less than 1C.
Process of determining melting point is done on microscale using less than 1mg of material
Melting point is determined for 3 reasons

Compound is a known one, the melting point will help characterize the sample
If the compound is new then the melting point is recorded to allow future characterization by others
The range of the melting point is indicative of the purity of the compound- impure compound will melt over a wide range of temperatures

Recrystallization of the compound will purify it and decrease the melting point range, entire range will be displaced upward
Solid is considered pure of the melting point does not rise after recrystallization
Most organic molecules are covalent in nature and melt at temperatures below 300C.
Inorganic compounds are ionic and have much higher melting points
Ionic molecules often decompose before melting as do compounds having many strong hydrogen bonds (sucrose)
Equal larger molecules melt at higher temperatures than smaller molecules
More symmetrical isomer will have higher melting point
R and S enantiomers will have the same melting points but the racemate will usually posses different melting points
Molecules that can form hydrogen bonds will usually posses higher melting points than their counterparts of similar molecular weight
The melting point of a pure compound is the temperature at which the vapor pressure of the solid and liquid are equal
The melting point range of a mixture of compounds is generally broad and the breadth of the range is indication of purity
Recrystallization will enrich the predominant compound while excluding the impurity and will decrease the melting point range
Impurity must be soluble in the compound, so an insoluble impurity will not depress the melting point
If X and Y are identical then the mixture of 2 will have the same melting point and if not identical then a small amount of either will reduce the melting point
Boiling point of molecules depends on the hydrogen bonds that can be formed
Distillation allows the building temperature range to be determined thus giving an indication of purity

Recrystallization

Recrystallization is the most important method for purifying solid organic compounds
Pure, crystalline organic substance composed of 3d array of molecules held together by london forces

Fairly weak forces; most organic solids melt at 22-250C

Impure solid will not have well defined crystal lattice because impurities do not allow the crstalline strucute to form
Goal of recrystallization is to remove the impurities from a solid to allow a perfect crystal lattice to grow
4 important concepts for recrystallization

solubility
saturation level
exclusion
nucleation

Recrystallization involves dissolving the material to be purified (solute) in appropriate hot solvent to yield solution (solubility).
Soluble impurities remain in solution because they are not concentrated enough to saturate the solution and therefore crystallize
Recrystallization of solute is initiated at a point of nucleation which can be a seed crystal, dust or scratch
7 steps of recrystallization

choosing the solvent and solvent pairs
dissolving the solute
decolorizing the solution with pelletized norit
filtering suspended solids
recrystallizing the solute
collecting and washing the crystals
drying the crystals

Hydrocarbon solvents such as hexane will dissolve hydrocarbons and other nonpolar compounds, hydroxylic solvents such as water and ethanol will dissolve polar compounds
Best recrystallization solvent will dissolve the solute when the solution is hot but not when the solution is cold; it will either not dissolve the impurities at all or will dissolve them well
Solvents adjacent to each other in the list will dissolve in each other- that is, they are miscible with each other and each solvent dissolve substances similar to its chemical structure
Water (boiling point 100C) best choice because it will dissolve a large variety of polar organic molecules
Acetic Acid not commonly used as it will react with alcohols and amines and is difficult to remove from crystals
Methanol (boiling point is 64C) is a common solvent because its high boling point
A solution can become superheated and explosive voilence can happen, process is called bumping

Distillation

IN ethanol and water mixture the liquid boils at 87C and the vapor above the mixture is not 100% ethanol
A liquid contains closely packed but mobile molecules of varying energy
When a molecule of the liquid approaches the vapor liquid boundary and possesses sufficient energy, it may pass from the liquid phase to the gas phase
Heating the liquid causes more molecules to enter the vapor phase; cooking the vapor reverses this process
Closed system is at equilibrium, many molecules are escaping into the vapor phase from the liquid phase, an equal number are returning from the vapor phase to the liquid phase

Equilibrium is maintained because the number moving from the vapors phase to the liquid phase also increases

the number of molecules in the vapors phase increases which increases the vapor pressure
The number of molecules in the vapors phase phase depends on the volume of the system, temperature, combined pressure of all gaseous components and strength of the intermolecular forces exerted in the liquid phase
Simple distillation-boiling liquid in a vessel and directing the resulting vapors through condenser which the vapors cooled and converted to a liquid that flows down into o a collection vessel
Simple distillation used to purify piqued mixtures by separating one liquid that different in boiling point by at least 75C
Closer the boiling points of the components of a liquid mixture, the more difficult they are to completely separate by simple distillation
Fractional distillation differnt from simple in that fraction in column is placed between the distilling flask and condenser
Fractional distillation produces much better separation between liquids with boiling points closer than 75C
fractional Distillation. Multiple condensations and vaporizations occur as the vapors scent the column
Condensing of higher boiling vapor releases the heat, causes vaporization of the lower boing liquid on the packing so that the lower boiling component moves up while the higher boiling component moves down
Successive condensation-vaporization cycle is called theoretical plate- produces vapor that’s richer in the more volatile fraction
If the column packing has larger surface area it will absorb much of the material being distilled
Most important variable that contributes to good fractional distillation is the rate at which the distillation is carried out
As a purification method, fractional distillation requires larger amounts of material than recrystallization, liquid/liquid extraction or chromatography
Not all liquids form ideal solutions and conform to raoults law, ethanol and water is an example
A pure liquid has a constant boiling point, change in boiling point during distillation indicates impurity
2 mischible liquids of similar chemical structure that boil at the same temperature individually will have nearly the same boiling point as a mixture

Extraction

Reactions are typically homogenous liquid mixtures and can be extracted with either an organic or aqueous solvent
Organic reactions often yield a number of by-products- some inorganic and some organic
Since some organic reactions do not go to 100% completion, small amount of starting material is present at the end of the reaction
Liquid/liquid extraction and neutral and acid/base extraction
Liquid/liquid extraction involves 2 layers: organic and aqueous layer
Solvent used for extraction should possess properties including

Readily dissolve the substance to be extracted at room temperature

Should have low boiling point so that it can be removed readily

Should not react with the solute or other solvent

Should not be flammable or toxic

Should be relatively inexpensive

Some common liquid/liquid extraction solvent pairs are water-ether, water-dichloromethane, and water-hexane
The densities of the solvents will predict the identities of the top and bottom layers, nonhalogenated organic solvents are less than 1.0g/ml and those of halogenated solvents are greater than 1.0g/ml
Very concentrated amount of solute dissolved in either layer can rever the order
Micro scae separations, mixing and separating the layers with a properties incurs very little product loss
2 solvents typically in a reaction tube the 2 layers can be mixed by drawing up and rapidly expelling them with a pipette
Solvents such as dichloromethane do not dissolve nearly as much water and therefore dried over a chemical drying agent
Good but specialized and reactive drying agents are potassium hydroxide, anhydrous potassium carbonate, sodium metal, calcium hydride, lithium aluminum hydride and phosphorus pentoxide
Work up technique of liquid/liquid extraction has 4 steps

Mixing layers
Separating the layers
Drying the organic layer
Removing the solvent