organic chemistry midterm

Defintion of Melting point

the temperature at which the substances changes state from soild to liquid

when do solid and liquid phases exist in equilibrium

1 atompsheric pressure (100 kPa)

Melting point and boiling point are __________ properties easy to measure that can be used to help identify a compound

intensive

Definition of capillary melting point

A range of temperatures over which a minute amount of the solid in a walled capillary tub first visibly softens and finally liquifies completely

When does actual melting begin

when the first drop of liquid becomes visible, and the melting range continues until the temperature at which all solid has been converted into liquid

What is the melting range for pure compounds

pure compounds have a sharp melting point and a narrow range of 0.5-1

what is the effect of impurities on melting point

depressed or lowers the melting point and widens the range at which the compound melts

what is the effect of impurities on boiling point

increases boiling point due to increased solution-phase entropy that make it more difficult for liquid molecules to escape into the vapor phase

sublimation

state change from solid to gas

Defintion of boiling point

the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, and the liquid changes into a vapor

Theory of microboiling point determination

• heat sample until rapid bubbles merge from the microbell

• turn off heat

• when bubbles are leaving the bell, the vapor pressure is larger than atmospheric pressure which is pushing down on the surface of the liquid

• the bubbling slows down and liquid enters the microbell from capillary tube

• at the time the vapor pressure is just below atmospheric pressure, liquid enters the bulb because of lower pressure

• the boiling point is when the sample vapor pressure equals the atmospheric pressure. we are obtaining bp when the vapor pressure is just below the atmospheric pressure

What is extraction used for?

to remove a solute from a solution by mixing in an immiscible solvent. rarely yields a pure product

Extraction solvent should be

• readily dissolves the substance to be extracted

• low bp

• unreactive with solute or other solvent

• inflammable

what is the salting out effect

adding salt decreases the solubility of most organic solutes and of extraction solvents in aqueous layer. the salt makes the aqueous phase more polar

what does pH change in solute

pH changes protonation of solute which changes its solubility in water

what is distillation used for

a method of separating two miscible liquids. used for purification of liquids

Distillation theory

the vapor obtained from a boiling liquid, once cooled, will re-condense to a liquid known as distillate

what would an ideal curve show

a vertical rise with an infinite slope

pure substances exhibit a ________ bp

constant boiling point

mixtures exhibit _________ in bp

changes in boiling point

A compound with lower boiling point is a ______ proportion of the vapor over the heated mixture than its proportion in the liquid

larger proportion

Vapor is condensed and collected to give liquid with a ________ composition of the lower boiling point

higher composition

what is simple distillation

uses one vaporization-condensation cycle to affect separation

definition of residue

the non-volatile material left in the distilling flask

definition of superheating

a metastable condition when a small section of liquid rises to a temperature above the bp, turns to gas, causing rapid expanding bubble to occur in liquid. the bubble then explodes out of the solution causing liquid to fly into condenser. can lead to contamination

what is fractional distillation

multiple boiling, condensation cycles and results in better separation

what is each cycle of boiling, condensation called

theoretical plate

theoretical plates depend on column efficiency…

• length

• packing material

• reflux ratio

• temperature control

what is reflux ratio

amount of condensate returned to column divided by amount of condensate removed from column

what is an azeotropic mixture

a mixture of two or more liquids whose proportions cannot be altered by simple distillation. when azeotrope is boiled, the vapor has the same proportions of constituents as the unboiled mixture

what carries a mixture of compounds through a long column coated with a solid support that has an adsorbent in GC

gas (in our experiments helium is used)

GC focuses on what

separating mixtures of compounds

over the course of traveling through column, compounds in mixture are allowed to different extents by _____ and exit column at different times

adsorbent

definition of adsorbent

a relatively non-volatile liquid such as silicone oil

what is the goal of GC

separation of mixture into individual groups consisting of one compound that exit the column at different times after injection

how do compounds interact with the stationary phase

intermolecular forces

what is retention time? provide an example

the elapsed time necessary for a component to pass from the injection port to the detector. A interacts with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus, B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A

what are variables that affect retention time

• nature of the stationary liquid phase

• length of column

• temperature of column

• flow rate of the carrier gas

what is resolution

the separation of compounds in the mixture; incomplete resolution has different compounds overlapping as they exit the column

what factors can affect resolution

• column temperature

• carrier gas flow rate

• column length

• size of injection

more opportunities for sample to interact with column caused by lower temperature, slower He flow rate, and longer column increase resolution

separation of non-polar column coating

• separates based on boiling point

• more volatile component has greater affinity for mobile phase and comes off the column faster

separation of polar column coating

• separate by polarity but only when the differences in polarity are wide otherwise these will also be separated by boiling point

• all polar compounds move slower on the polar columns, but polar compounds show a larger effect

Thin Layer Chromatography (TLC) used for…

separation of a mixture of two or more different compounds or ions by distribution between two phases, one of which is moving (mobile) and one of which is stationary

what is the stationary phase

adsorbent on a plastic or glass plate

what is the mobile phase

solvent

what is the general rule for types of interactions on stationary phase

the more polar the function group, the stronger the bond to silica gel

the more polar compound travels the _________ distance on silica gel because of stronger interacts

shortest

what does a smaller Rf indicate

more polar compound

what does a large Rf indicate

less polar compound

what is happening in the mobile phase

solvent pushes the compound the plate. as solvent ascends the plate, the sample is partitioned between the moving liquid phase and the stationary solid phase

what is dielectric constant

a measure of substance’s ability to insulate charges from each over. higher means higher polarity and greater ability to stabilize charges

if solvent is too __________ spots tend to not to move up plate

non-polar

if solvent is too _______ spots tend to move too far up plate

how is TLC preformed?

compounds are put as small spots on plate and travel up the plate depending on affinity for solvent system. plates placed in shallow amount of solvent which moves a plate through capillary action and carries sample up plate

what is the purpose of using Rf

to measure how far a compound moves up a plate with a certain solvent system to allow for comparison between trials using the same solvent system

factors affecting Rf values

• quality of adsorbent

• thickness of adsorbent layer

• developing chamber saturation

• sample concentration

• environmental conditions

• solvent system