CHM 2211L exam 1
Terms in this set (108)
which type of organic compounds boil at lower boiling points?
more branched, weak IMFs
hydrogen bond acceptor
typically an electronegative atom with a lone pair
hydrogen bond donor
hydrogen directly bonded to a N, O, or F atom
why do the trends in dipole moments and dielectric constants exist?
the electronegativity differences of the individual atoms within each molecule
benzene derivative polarity rankings
para (least)< meta < ortho (most)
petroleum ether is another organic solvent that is a mixture of simple hydrocarbons obtained by the distillation of petroleum. Would it be classified as polar or non-polar? would you expect it to be miscible or immiscible with water?
It would be classified as non polar which would make it immiscible with water. This is because water is very polar and like dissolves like.
which 4 solvents in the table are denser than water? explain why this might be the case.
CCl4, CH2Cl2,CHCl3, and Acetic Acid. This is the case because density partially depends on molecular weight and chlorine has a high molecular weight which makes it denser.
Organic liquids A, B, and C have densities of 0.691, 0.955, and 1.126 g/mL. A and C are low polarity solvents, while B is a high polarity solvent. When each is added to water, how would you expect them to behave?
Organic liquids A and C are immiscible with water due to low polarity and will form separate layers. Liquid A will form a layer above water because it is less dense and Liquid C will form the layer below water because it is more dense. Organic Liquid B will be miscible with water because it has high polarity.
why does ethyl acetate have a higher boiling point than hexanes, even though they are approximately the same molecular weight?
because ethyl acetate is polar and has dipole-dipole forces which are stronger forces than the IMFs that hexanes have.
why does methanol have a lower boiling point than ethanol even though ethanol is more polar?why does ethanol have a higher boiling point than ethyl ether even though ethyl ether has a greater molecular mass?
methanol has a lower boiling point than ethanol because ethanol has a longer carbon chain and is heavier which increases boiling point. Ethanol has a higher boiling point than ethyl ether because ethanol has hydrogen bonding.
why must you allow recrystallized samples to dry completely before determining the MP?
if you do not allow recrystallized samples to dry completely before determining the MP, the water will act as an impurity and lower the MP.
give two reasons for knowing the literature solubility data when doing a recrystallization.
1. so you know if the two substances will mix
2. the solvents need to be able to dissolve the solutes at high temperatures but not at low temperatures in order for the crystals to form
did the literature values for the solubility of acetanilide in H2O hold true? which solvent system (20mL or 40 mL) worked better for yield? which for purity?
yes the literature values held true because it didn't dissolve at room temperature but it did at higher temperatures. the 20mL H2O worked best for yield and purity.
IUPAC name of acetanilide
IUPAC name of trans-cinnamic acid
consider acetanilide and cinnamic acid samples. which one appeared to benefit the most from recrystallization? how can you tell?
for acetanilide, the 20 mL system benefited most. For TCA the sample that contained ethanol and H2O benefited the most. you can tell by comparing MP values to lit values.
2-methylbenzamide and 3-nitrobenzoic acid both have a melting point of 140.1-140.4 C. however, if you mixed them together and took the melting point, it would not be 140.1-140.4 C. why not?
they would contaminate each other and act as impurities which would disrupt the IMFs and lower the melting point.
why does dichloromethane have a higher dielectric constant than acetic acid?
DCM is moderately polar but the two dipole moments do not cancel out. Acetic Acid has more carbons which neutralizes its polarity.
in the reaction of salicylic acid and caffeine, which is the Lewis Acid and Lewis Base? why does caffeine protonate at N-9? what is the hybridization of the non-bonded pair at N-9?
Lewis Acid: Salicylic Acid
Lewis Base: Caffeine
caffeine protonates at N-9 because that position is the least hindered. the hybridization of the non-bonded pair at N-9 is sp2.
In TLC, what will happen if the initial spot is very large when preparing a plate?
it could reduce the resolution and make results blurry or it could overlap with another spot.
In TLC, what will happen if the initial spot is below the level of the solvent in the developing chamber?
the spot will dissolve into the solvent instead of being carried up the plate.
Which solvent system appears to give the best spread of Rf values? can you explain why this might be so?
the 10:1 ethyl acetate:hexane give the best spread because it is primarily ethyl acetate with is moderately polar
(a more polar solvent will always give the higher Rf values for all solutes)
All TLC experiments have three important factors that must be considered in making any generalizations about the behavior of the solutes being analyzed, what are they?
1. the polarity of the TLC plate
2. the polarity of the solvent(s) used to develop the plate (mobile phase)
3. the polarity of the compound/solute itself-how polar or non-polar is it?what dissolves it well or poorly?
How do you calculate Rf values?
Rf=distance of compound/distance of solvent
always a positive fraction/decimal less than 1
what is the polarity level of the silica gel on the TLC plates?
very polar (more polar than most organic compounds and solvents)
why do non polar compounds have higher Rf values?
In general, polar compounds tend to "stick" on the polar silica gel while non-polar compounds prefer the solvent. Thus for TLC run on silica gel plates, non-polar compounds have higher Rf values.
Given a mixture of caffeine and benzoic acid, what would happen if they were added to a separators funnel with CH2Cl2 and 1M HCl? which compound would go into which layer and why?
HCl would protonate the caffeine and make it water soluble. caffeine would go into the aqueous layer and benzoic acid would dissolve in the DCM and go into the organic layer.
HBA's: 3 (the two O's and N-9)
Function as a Base: will deprotonate salicylic acid to form salt (proton added to most basic nitrogen)
-not basic enough to take a proton from acetaminophen
Hybridization of orbital that contains the non-bonded pair on nitrogen? (p when nitrogen has no pi bonds , sp2 when directly attached to double bond)
We used the bromine in DCM method to test for unsaturation. We also took an IR spectrum of the compound. However, neither method could really give a direct answer for the number of C=C bonds present. What methods could we use to discern this information?
1. baeyer test: allow us to test if compounds have C=C bonds, test is positive if the reagent (potassium permanganate) yields a brown precipitate
2. H-NMR: allow us to determine the structure based on absorption
Why do we not perform a simple distillation on lemon grass oil at atmospheric pressure to separate the components? what's the advantage of steam distillation? can you imagine another distillation method that might serve the same purpose?
Simple distillation is only effective when attempting to separate two non volatile substances. Citral is a moderately volatile compound, and immiscible in water (good for steam distillation). An advantage of steam distillation is that the lower pressure reduces the boiling point, you need less heat in return to achieve distillation. Vacuum distillation would also be effective because it also reduces pressure.
what are two properties of a good recrystallization solvent?
1. dissolves the compound in hot solvent but not cold
2. dissolves impurities in hot and cold
Dalton's Law of Partial Pressures
-total vapor pressure of mixture is the sum of the vapor pressure of the water plus vapor pressures of each of the organic components in mixture
-total vapor pressure greater than any component's individual vapor pressures
-boiling point of liquids is inversely proportional to vapor pressure, result is that the boiling point of immiscible liquids must be lower than that of the lowest boiling component
-boiling point of water drops below 100 C, which allows compounds with very high boiling points to codistill at temperatures less than 100 C
Endo vs. Exo Adduct
Endo adduct: less stable product, steric hindrance, kinetic control, faster forming (more stable transition state)
exo adduct: more stable, thermodynamic control, slower forming
2, 4 DNP Test & Semicarbazone Test
converts liquid aldehydes/ketones into solids (test is positive if the ppt. is formed)
4 1/2 Carbon Rule
once non-polar surface area, or the ratio of total surface area to polar area exceeds a certain value, solubility in H2O becomes negligible; there are insufficient attractive forces to associate solute with polar solvent
C=O stretch (ketones)
C=O stretch (aldehyde/esters)
C=O stretch (amides)
C3-H stretch (alkanes)
C2-H stretch (alkenes)
C=C aromatic stretch
with caffeine will act as a base
-test for unsaturation (presence of C=C bonds)
-uses the C=C bonds to add bromine across the double bond
-color change=positive result (if negative, remains red/brown= saturated, no C=C bonds)
chemical reaction when benzoic acid reacts with NaOH/H2O. HCl added to now basic aqueous solution. Why is this sequence of reactions necessary?
deprotonate to dissolve then protonate to separate from mixture.
NMR: chemical shift of an aldehyde?
NMR: chemical shift of a benzene ring?
NMR: chemical shift of C=C-CH3?
NMR: chemical shift of C=C-H?
NMR: Chemical shift of carboxylic acid?
NMR: chemical shift of C-CH3?
NMR: chemical shift of O-CH3?
Ferric Chloride Test
-tests for phenols
-Fe(III) forms colored complex
-positive result=red/blue/violet color change
Furan & Maleic Anhydride (Diels-Alder reaction)
-furan (pictured)= diene, Maleic anhydride= dienophile
-preference for exo product
-chemical shift of aldehyde hydrogen 4.2 ppm
-splitting= doublet (one neighbor)
-gives positive result for methyl carbonyl
-if under reaction iodoform is insoluble, yellow precipitate forms (evidence of methyl ketone)
Jones Oxidation Test
-tests for alcohols
-strong oxidation converts primary alcohols to carboxylic acid
-ketones and tertiary alcohols will not react
- positive for secondary/primary= opaque suspension, green/blue color
-positive for tertiary=no visible reaction, remains orange in color
-test for primary, secondary, and tertiary alcohols
-converts alcohols to alkyl chlorides (primary alcohols will not react)
-rel. energy different from geranial because of steric hindrance interaction between vinyl hydrogens and aldehyde group
two methods for developing a TLC plate
1. iodine vapor
2. UV light
steam distillation is a process of codistilling a compound with water. It takes advantage of the ___________of most organic compounds in water to provide a route for separation based on differing _________. In summary, steam distillation involves a multiphase system with each compound contributing to the overall _______.
2. boiling points
3. vapor pressure
-tests for aldehydes
-positive result= Ag plates on glassware creating a "silver mirror"
What influences a chemical shift?
1. electronegativity of nearby atoms
2. hybridization of adjacent atoms
3. diamagnetic effects from adjacent pi bonds
What is one advantage and one disadvantage that results from using too much solvent during a recrystallization?
-more solvent produces less recrystallized product
-yet results in less impurities present in the recrystallized solid
what is the disadvantages of boiling off your solvent in order to isolate your product?
-crystals often stick to the bottom on the flask
-can burn/overheat too easily which leads to more impurities
what is the advantage of using a roto vap?
facilitates the collection of the liquid from the solute and prevents decomposition
what was the purpose of using NaOH in the chemical extraction lab?
-to deprotonate benzoic acid so it becomes soluble
-when reprotonated with HCl, becomes insoluble and can be separated out as a solid
when performing liquid-liquid extractions, what are the general names of the two immiscible layers? which layer is typically on the bottom?
-organic and aqueous layer
-typically the organic layer is on the top unless DCM is used, then the organic layer would be on the bottom
you are performing a recrystallization, but no crystal product is observed. What are three techniques you can use to facilitate crystal growth?
-use a starter crystal
-ice water/cold water bath
-scratch inside of erlenmeyer flask with glass stir-rod
When hydrogen adds to the carbon-carbon double bond of an unsymmetrical alkene, hydrogen adds preferentially to the carbon that already has more hydrogens
when hydrogen is removed from a species to form an alkene, the hydrogen is lost from the carbon atom that has fewer hydrogens
what are the variables that influence the rate of an Sn1 reaction?
2. alkyl halide structure
3. leaving group ability
unimolecular reactions that involve a stepwise mechanism and occur for tertiary and some secondary alkyl halide through the formation of an intermediate alkyl cation
one step bimolecular process for primary and secondary alkyl halides
-the rates of Sn2 reactions depend on steric hindrance around the reacting center and to an extent
why does benzyl bromide react under both Sn1 and Sn2 conditions?
benzyl bromide is a primary alkyl halide so naturally it would occur under Sn2 conditions. However, benzyl bromide can form a stable carbocation based on resonance so it has the option to undergo Sn1 conditions.
why is bromobenzene unreactive under both Sn1 and Sn2 conditions?
the bond between the bromine and the carbon is stronger due to the double bonding in the ring and backside or front side attacks dont work
to promote the Sn1 mechanism we used AgNO3 in a polar protic solvent, why?
Ag+ coordinates with halide ion facilitating the formation of carbocation. Further polar protic solvent will favour SN1 mechanism by stablizing the carbocation
trans-cinnamic acid structure
salicylic acid structure
add quiz questions
what would taking an IR of your unknown tell you about its structure? suggest how you would use an IR to facilitate the "process of elimination" to converge on the correct unknown.
the IR would let us know what functional groups our unknown has and would help us create an in and out table which could help us predict the structure
propofol, the anesthesia-induction medication implicated in the death of singer michael jackson, does not give a positive result when tested with ferric chloride in water. look up the structure of propofol and suggest why. how would you modify the standard procedure to address the difficulty in performing this test?
Propofol does not yield a positive ferric chloride test result because the isopropyl groups make the -OH group hindered. A way to see if propofol has an -OH group instead of the ferric chloride test is to use IR or H-NMR spectroscopy and look in the absorption ranges for -OH groups.
what are the two "tickets" you need to be permitted to attend lab and to perform an experiment?
1. pre lab
T/F: dizziness, nausea, or diplopia can be the result of breathing chemical vapors, and should be reported to your TA
T/F: lab coats do not need to be worn when you are just taking melting points
T/F: spoiled weight boats or TLC plates may simply be discarded in the trash after use
T/F: water spilled on the floor and left unattended is potentially as serious a safety hazard as overexposure to many chemicals we routinely use (acetone, ether, hexanes, DCM)
T/F: chemical burns only need to be rinsed under cold water for 5 minutes
T/F: the inward flow of air in a fume hood creates a dynamic barrier that minimizes the movement of vapor/gases out of the hood and into the lab. The best way to preserve this safety feature is to lift the sash up rather than sliding the doors of a hood
A completed experimental lab report written by a student consists of three major parts (sections), what are they?
1. pre lab
3. post lab
give three resources we have in lab to address fires and chemical burns or chemicals splashed in the eyes/on the face.
2. eye wash
3. fire extinguisher/blanket
5 keys to working safely in fume hoods
1. work behind the red line
2. clean up all messes (spills) in the hood
3. turn everything off in the hood when done
4. never lift the sash, use sliding doors
5. keep sliding doors closed when not working
besides being classified by their acid-base properties, organic waste may be classified into which major categories before disposal?
you pick up an erlenmeyer flask in your drawer and notice a chip in the lip of the top. You need to mix a solution and this flask is just the right size. what do you do next?
dispose of broken flask in glass disposal bin and ask TA to get you a new flask
if a classmate cracks a thermometer in half and appears to have pushed the thermometer into their hand, what should you do prior to applying direct pressure to stop the bleeding?
Tell TA, check for glass in hand, if no glass, rinse the wound with soap and water. If blood is spurting, elevate and apply pressure.
what is the purpose of recrystallization
to get a pure substance from an impure one
is it possible to have a melting point that is higher than the expected/literature value?
no, if the substance is pure, the expected MP is the highest possible. If it is impure that would only lower the MP.
which attractive intermolecular force is generally considered the strongest?
what atomic property is considered in determining if a covalent bond is polar or non polar?
longer straight chain means
more surface area
infrared spectroscopy focuses primarily on which of the following in a molecule?
how many IR active modes does Cl2 (g) have?